As usual, I'm sure I've missed something. If so, let me know, and if I'm not too sick and tired of working on this, I'll go ahead and add it in.
In the 1980s Daryl Domning and Christian de Muizon reported
a small sirenian rib from the Pisco Formation of Peru, one of the few records
of dugongid sea cows from the Pacific coast of South America.
Years later it was more precisely identified as a rib of the small sea cow Nanosiren,
but considered to be strange in comparison to most other sea cows. This new
study by Eli Amson and others took a histological slide from the rib and
compared it with sirenians and the aquatic sloth Thalassocnus. Sea cows
typically have an unremodeled layer of sheet-like cortical bone with an inner
zone consisting of remodeled "secondary osteons"; Thalassocnus lacks
this parallel-sheet like zone and instead the entire cross section consists of
remodeled bone. The mystery rib matches the histological pattern of Thalassocnus,
and is reidentified by the authors as an aquatic sloth rib. The authors point
out that the most highly specialized aquatic sloths now do not overlap
stratigraphically with sea cows in the Pisco Formation, suggesting that the sea
cow niche was occupied by Thalassocnus after sirenians went extinct in
the western south Atlantic.
This new study reports a fragmentary mysticete mandible from
the Azores Islands
off the coast of North Africa from Pleistocene strata.
This mandible is not complete enough to identify to the family level, but is
significant owing to its geographic location and strange bone modifications. It
has a series of large pits on one surface, which appear to be anatomically
genuine (e.g. ante-mortem rather than post-mortem) and thus not taphonomic in
origin. The authors point out that relatively few fossil cetaceans have been
reported from small oceanic islands, and hypothesize that such occurrences are
likely underreported. However, it's worth noting that small islands are
unlikely to have large sedimentary basins and that most small islands have
Cenozoic strata formed as "bathtub ring" deposits where the abundance
and ease of discovery for fossil cetaceans is likely going to be rather low in
comparison to continental margin deposits. Then again, cetacean bones are not
small (quite the opposite) so there's always a trade off.
Desmostylians are some of the most wonderfully bizarre of
all marine mammals. Well known Miocene forms like Desmostylus, Paleoparadoxia, and the recently
split-off Archaeoparadoxia and Neoparadoxia, are so derived that it's
difficult to pick out exactly which group of plodding semiaquatic herbivores
they belong to. Tradition dictates that they're tethytheres, most closely
related to sirenians and proboscideans - but the possibility remains that
they're perissodactyls. More fossils of early desmostylians are needed, and in
1986 Daryl Domning, Clayton Ray, and Malcolm McKenna published some late
Oligocene specimens from Oregon
that my hero Doug Emlong had collected - some mandibles and other elements,
upon which they erected two species - Behemotops proteus and Behemotops
emlongi - the "Behemoth face". None of the skull was known, but
the more primitive teeth permitted the authors to link desmostylians with the
anthracobunids, hippo-like relatives of early elephants. Later finds of Behemotops
led the same authors to reevaluate the more fragmentary species B.
emlongi and synonymize it with B. proteus. Still, no skull was known
- until Brian Beatty and Thomas Cockburn reported this new specimen of Behemotops
cf. proteus with most of a skull, some teeth, and quite a bit of the
postcranial skeleton. The new specimen indicates that the rostrum was unusually
elongate and narrow, similar to Cornwallius. The rather wide
"shovel jaw" of B. emlongi is proportionally much wider than
the rostrum of Behemotops cf. proteus, and so these authors
erected the new genus Seuku to house the other species - Seuku
emlongi. These Oligocene desmostylians indicate that several different
desmostylians inhabited the same area at the same time - Cornwallius
sookensis, Behemotops proteus, and Seuku emlongi - apparently
coexisted in the Pacific Northwest, paralleling
ecological diversity in sea cow assemblages.
This textbook is the best edition yet - the first edition
came out in 1999, and the second edition came out in 2006. This text deals with
most aspects of the biology and evolution of marine mammals, and gives a
comprehensive summary of the phylogeny, anatomy, and adaptations of all marine
mammal groups (modern & extinct), serving as an excellent introduction to
graduate students interested in marine mammal paleontology. I got a copy of the
second edition at the SVP benefit auction in Ottawa
(2006) and read the entire front half - the second half, focusing more on soft
tissues, diving behavior, and ecology and conservation is less relevant towards
paleontologists. The third edition has nearly completely revamped the phylogeny
and skeletal anatomy chapters, and many up-to-date references have been added.
To my surprise, many citations of my own work were included - I'm young enough
to still suffer from a bit of impostor syndrome, so seeing my papers referenced
in a textbook is quite surreal. Several of my illustrations have been included
as well, which I was quite pleased with! I highly recommend this for any
"student" (in the broad sense) of marine mammal evolution.
The fossil record of pinnipeds was dominated by taxonomic
confusion for nearly a century because most fossil pinnipeds - particularly the
phocid seals - were named based on isolated postcranial bones and referred to
the same species based on dubious criteria (see below, Koretsky et al.).
Pinniped paleontology began in earnest in the North Sea,
principally with true seals (Phocidae) and the overly confusing taxonomy of
walruses (see my series on the walrus fossil record, hereXXX). Pliophoca etrusca,
a Pliocene phocid from northern Italy,
is a notable exception as the holotype is a partial skull with associated
cervical, thoracic, lumbar, sacral, and even caudal vertebrae, as well as
forelimb and hindlimb elements. However, it was originally reported in the
1940's and has been needing a modern "treatment" - which is exactly
what this study provides. Pliophoca is very similar to extant Monachus
(Hawaiian, Mediterranean, and the extinct Caribbean
monk seals) but differs in several cranial, dental, and hindlimb features -
such as having narrow, compressed incisors. This study reports many new
specimens of Pliophoca cf. P. etrusca including mandibles,
isolated teeth, and ankle bones, from Italy,
France, and Spain.
Previously, other material from the early Pliocene Yorktown Formation of North
Carolina was referred to Pliophoca etrusca, but the authors
rightly point out that no comparisons were made and no anatomical features
linking the two were identified. Cladistic analysis fails to corroborate
identification of this east coast USA
material, which appears to represent an unnamed monachine instead; true Pliophoca
is closely related to Monachus, sharing common ancestry and indicating
origin of the Pliophoca-Monachus clade in the Mediterranean
during the Plio-Pleistocene. This was followed by dispersal to the Caribbean,
and later to the tropical Pacific.
The Pisco Formation is a spectacular Miocene shallow marine
deposit in coastal Peru
with excellent exposures of diatomite, sandstone, and mudrocks with skeletons
of marine vertebrates littering the desert. Preservation is generally quite
comparable to that of the Monterey Formation in central California in terms of
preservation (i.e. abundant well-preserved skeletons with high rates of
articulation), one of the only other stratigraphic units in the world where
fossilized baleen has been reported - yet central California is not desert and
is instead well-vegetated, relegating most exposures to difficult to access
coastal outcrops. These two studies report highly detailed maps showing the
occurrence of vertebrate fossils at two of the more important localities in the
Pisco Formation: Cerro Los Quesos (cheese hills) and Cerro Colorado
(red hills). The study at Cerro Los Quesos indicated that an earlier study
conducted by young earth creationists failed to identify many smaller
non-baleen whale fossils (small odontocetes, pinnipeds, birds, bony fish,
sharks) calling into question the rigor of their field methods. At Cerro Los
Quesos, the sheer majority of marine vertebrates are present within a 160 m
thick section of the Pisco exposed at the tops of these hills. Again, at Cerro Colorado,
marine vertebrates are concentrated into a narrow stratigraphic band (nearly
90% of all marine vertebrates from Cerro Colorado
were found in a 35 m section near the base of their column). Both of these
carefully executed studies fill in a desperately needed baseline for basic data
in one of the world's premier marine vertebrate lagerstätten which, until the
past year or so, was being supplied entirely by studies published by young
earth creationists.
In the late 19th century a local found a large
cetacean skull eroding from a cliff of the late Miocene Monterey Formation near
Santa Barbara in southern California.
It slowly eroded out over 30 years before being excavated in 1909. It was named
Ontocetus oxymycterus by Remington Kellogg in 1925, who recognized it to
be a very small part of a very large sperm whale. The tip of the snout is
preserved with about ten upper alveoli, and the anterior tips of both mandibles
with several poorly preserved teeth. The type species of Ontocetus, Ontocetus
emmonsi, is not a sperm whale but is in fact a walrus and a senior synonym
of Alachtherium and Prorosmarus. Because of this, O. oxymycterus
needed a new genus name, and these authors provide a much needed
redescription of this fragmentary but fascinating whale and rename it Albicetus
oxymycterus. The teeth of Albicetus are enormous – 8 cm in diameter,
roughly ¾ the size of the giant sperm whale Livyatan melvillei from Peru.
I always assumed that the snout of this gigantic whale was incomplete, but
these authors interpret the rostrum to be more than ¾ complete, and when
plugged into body size calculations for odontocetes, a body size of 6 meters
(~20 feet), small in comparison to the 10-14 meter length estimate of Livyatan
despite the rather large teeth. The teeth of Albicetus, like Livyatan
and other members of the “Scaldicetus” tooth grade, has enamel caps
on the teeth. Given its size and robust enamel-capped teeth, Albicetus is
probably another large macroraptorial apex predator like Livyatan –
though in my opinion perhaps somewhat underestimated in terms of body size.
The fossil record of eared seals (Otariidae) is limited and
mostly consists of a few primitive fur seal-like species from the latest
Miocene and Pliocene of the Pacific basin in comparison to their higher modern
diversity and wider geographic range. The oldest known fur seals, Pithanotaria
and Thalassoleon, are only known from the late Miocene of the North
Pacific (California, Japan)
and are no older than about 10 Ma in age; otariids evolved from an enaliarctine
ancestor similar to Pteronarctos, but the youngest enaliarctines are
much older - early middle Miocene, about 17-16 Ma. This ~7 million year gap in
the mid Miocene begs the question "where the heck did otariids come
from?" and "where were they in the middle Miocene?" Morgan and I
published this paper after I discovered a partial mandible from the early
middle Miocene Topanga Formation (~17.5-15 Ma) hiding in collections at the Cooper
Center in Orange
County. This specimen was
misidentified as the small walrus Neotherium, but differed in having
greatly simplified teeth and being much smaller - it was almost on its way
towards being a typical late Miocene otariid, but retained an extra cusp on the
lower molar lost in all other otariids (the metaconid) as well as primitively
retaining a second lower molar. We named this new transitional pinniped Eotaria
crypta, referencing its early age, and also its rarity - no specimens of
true otariids have yet been reported from the ridiculously oversampled
Sharktooth Hill bonebed, nor from well-sampled middle Miocene rocks in Japan.
The earliest otariids may have been primarily pelagic, rarely straying into
shallow shelf waters - a hypothesis originally proposed by our Japanese
colleague Naoki Kohno.
The “river” dolphin Parapontoporia
is widely known from the late Miocene and Pliocene of California, known from
three species, and found exclusively in marine rocks. Its relationships have
been elusive, and when first described originally thought to be similar to the La
Plata River
dolphin Pontoporia and placed in the
Pontoporiidae – though similarities with the earbones of the now-extinct Yangtze
River dolphin Lipotes
were noted by the author. More recent studies utilizing cladistic methods have
consistently identified Parapontoporia
as a lipotid dolphin with no close relation to the pontoporiids (though the
family is known from Pliocene rocks of the Atlantic states, and widely within
South American fossil assemblages). Lipotes
was completely riverine, which begs the question: why, when, and where did the
lipotids become adapted to freshwater? In 2011 I found a single earbone,
originally misidentified as a delphinid dolphin, hiding in fossil collections
at UCMP in Berkeley.
This earbone – the petrosal – is very distinctive, and closely matched those of
Parapontoporia. However, this earbone was recovered from the non-marine
Tulare Formation, which is late Pliocene in age, from the Kettleman Hills in
the San Joaquin Valley of California. The valley was an ocean basin with a
narrow connection until about 2 Ma when uplift of the Sierra Nevada
introduced more and more sediment into the basin, along with uplift of the
coast ranges which closed off the connection to the sea. At the time this
individual of Parapontoporia died,
the inland sea had transformed into a large lake or body of brackish water fed
by rivers. This discovery not only indicates that palaeontologists in
California should more closely search Pliocene terrestrial deposits for marine
mammal remains, but that freshwater living may have characterized Parapontoporia in addition to Lipotes, heralding modern behaviour and
distribution as far back as the late Pliocene.
[This is the third publication of my Ph.D. thesis - and one
of the biggest chapters.] The history of study of eomysticetids is a bit
convoluted - first formally recognized in 2002 with the publication of Eomysticetus,
the most primitive toothless baleen whale. Eomysticetus was reported
from the Oligocene of South Carolina, right here in the Charleston area - it
has a mix of archaeocete-like features (e.g. tiny braincase, primitive
earbones, large sagittal and nuchal crests, anteriorly placed blowhole, large
fan-shaped coronoid process, "pan bone" on the mandible) and many
derived features unique to modern baleen whales (rostral kinesis, flattened
palate, toothlessness, beam-like mandibles without a fused symphysis). As it
turns out, the story of eomysticetids really begins 70 years earlier with the
discovery of an unassuming mysticete braincase in the Milton Lime Quarry in
south Otago, about a half hour's drive from Otago Campus. It was named Mauicetus
parki by Prof. Benham in the late 1930s; 20 years later, Prof. B.J.
Marples discovered several mysticete skeletons in the Kokoamu Greensand and
Otekaike Limestone of north Otago - and named all these as species of Mauicetus.
One of these, Mauicetus waitakiensis, was reidentified as an
eomysticetid and placed in the new genus Tohoraata last year
(Boessenecker and Fordyce 2014). The most complete of these, Mauicetus lophocephalus,
had an Eomysticetus-like skull; more recently, additional preparation
and new specimens of Mauicetus parki shows that it is actually in
the stem Balaenopteroidea - a "Kelloggithere", or cetothere sensu
lato - poorly known baleen whales that are structurally similar to Parietobalaena.
Because Mauicetus parki (the type species) and M. lophocephalus belonged
to different families altogether, a new genus name was needed for the latter -
but, always a complication: sometime in the 1960s an enterprising mover
involved in moving the Otago Zoology dept. collections threw away the holotype
skull of M. lophocephalus in the garbage, and is now likely in a landfill
within 20 km of campus somewhere. R.E. Fordyce started collecting eomysticetid
specimens from the Kokoamu Greensand and Otekaike Limestone in the early 1990s,
and found at least one specimen (OU 22235) that is congeneric but with some
tympanoperiotic differences (the tympanic bullae and postcrania of the type
specimen were indeed spared by the ever-so-talented university movers) and
another specimen (OU 22081) that was morphologically inseparable from the
remaining elements of the type specimen. So, we named the first specimen as the
holotype of Tokarahia kauaeroa, a beautiful Eomysticetus-like
skull with a significant amount of postcrania, identified the second skull as a
referred specimen of Mauicetus lophocephalus, and referred lophocephalus
to Tokarahia, recombining it as Tokarahia lophocephalus. Tokarahia
is structurally similar in terms of skull morphology to Eomysticetus but
principally differs in having a longer occipital shield and more derived
postcrania; Tokarahia kauaeroa has a beautiful mix of basilosaurid-like and
modern mysticete-like features in the well-preserved tympanoperiotics and
postcranial skeleton, and is a spectacular example of a transitional fossil.
Most significantly, a single tooth with a flattened was found near the maxilla
of the referred T. lophocephalus, matching the size and shape of
maxillary tooth alveoli in other eomysticetids - suggesting that eomysticetids
like Tokarahia retained a vestigial dentition, now representing an
additional intermediate stage between the tooth/baleen bearing aetiocetids and
the completely toothless crown Mysticeti.
Fossils of oceanic dolphins (Delphinidae) are not exactly
common in late Neogene rocks of the eastern North Pacific. This family is the
most diverse and widely distributed modern group of cetaceans and can be found
in every ocean basin. Delphinids are widely known from north Atlantic
and Mediterranean Plio-Pleistocene fossils, but are virtually unknown from the
late Miocene except for a few fragmentary specimens from Japan
(Eodelphinus) and California
(unpublished). Given this background, I'm very interested whenever evidence
turns up in California of fossil
delphinids, since they are quite rare in Pacific margin sediments. In 2009,
photos of a rather enormous skull which I at first thought was a monstrous
beluga turned up in my email inbox; within a few days I had the collector on
the line and he agreed to donate the specimen to UC Berkeley. Sometime later
that summer I looked at a private collection and noticed two rather large
pilot-whale like earbones, which the collector agreed to donate to UCMP as
well. Most odontocetes from the late Miocene and Pliocene of California are
small - porpoises, the "river dolphin" Parapontoporia, small
delphinid dolphins, with only occasional evidence of early belugas and sperm
whales. This rarity of large odontocetes, especially in the Purisima Formation
of California - makes me quite interested whenever I stumble across any fossil
evidence. After some preparation with an airscribe at Museum of the Rockies
in Bozeman, Montana
(where I was a student at the time), I found that the skull was actually rather
similar to pilot whales (Globicephala) and false killer whales (Pseudorca)
rather than extinct belugas (Denebola). The skull was found as float,
but is almost certainly younger than 5.3 Ma and older than 2.47 Ma based on
associated matrix and its likely stratigraphic position; the earbones
(petrosals) were found in situ within a bonebed dated to 3.5-2.5 Ma. I
initially thought the petrosals were from the same species as the skull, but a
linear regression of petrosal size and skull size amongst delphinoids indicates
that the petrosals are too small to belong to the same species as the skull -
potentially indicating that two species of globicephalines inhabited the California
coastline during the Pliocene. These new fossils, in concert with published
fossils, indicates that globicephaline dolphins were already widely spread
around the world by the early Pliocene.
Desmostylians are a bizarre but progressively more publicly
beloved group of extinct marine mammals with hypothesized affinities to sea
cows and proboscideans. Vaguely hippo-like with a conveyor-belt like tooth
replacement seen in elephants and sea cows, and hippo-like tusks, desmostylians
are inferred to have fed on sea grasses and kelp. The clade was never diverse,
but new specimens are always being found and new material from the lower
Miocene Unalaska Formation of Unalaska Island in the Aleutian island
archipelago of Alaska appear to represent a new species. The new material
includes a rather gigantic mandible similar to Desmostylus and Cornwallius,
and several more fragmentary specimens. Based upon some minor differences with Cornwallius and Desmostylus, the new genus and species Ounalashkastylus tomidai was named.
Another gigantic mandible known only as the Sanjussen specimen from Hokkaido
is reidentified as a western Pacific occurrence of Vanderhoofius, formerly reported from the middle Miocene of
California. The separation of Vanderhoofius
from Desmostylus has been questioned
before, but a distinguishing feature suggested by these authors is the loss of
lower incisors during postnatal ontogeny; indeed, the upper incisors (but not
lower) are lost by Desmostylus and Cornwallius during postnatal ontogeny.
These desmostylines also bear large bony prominences on the medial side of the
mandible but do not house unerupted teeth, and their function has remained
unclear; this study suggests that either 1) the dense bone serves as ballast to
keep the head negatively buoyant during feeding or 2) the bony prominences help
buttress the mandible as the animal “clenched its teeth” together during
suction feeding.
The diet of pinnipeds is well-established for modern
species, but it's difficult to determine the diet of fossil pinnipeds. For
modern species we can go watch them eat – or we can cut them open when they die
and look at what's in their stomach. For example, before we observed leopard
seals filter feeding like crabeater seals, dead leopard seals with bellies full
of krill had been found. Diet in fossil pinnipeds is difficult because we
certainly cannot do the former, and the latter is rare – only one fossil
pinniped with preserved gut contents has been recovered, the phocid seal Kawas
from Patagonia. Diet, or at least feeding behavior, can be inferred some
cases from feeding adaptations. Many pinnipeds feed in a similar fashion on
fish (sea lions), whereas some others are primarily suction feeders that don't
really use their teeth (walruses). Can diet be inferred from feeding morphology
in extinct pinnipeds? This new study attempts to answer this question by using
principal components analysis (PCA), hierarchical cluster analysis (HCA), and
discriminant function analysis (DFA) to examine trends in tooth spacing and
crown size, and diet. The DFA only reported a weak relationship with diet, and
a stronger correlation between tooth spacing/crown size and feeding behavior
(e.g. prey capture strategy. Tooth size and spacing were most strongly
correlated with how important teeth were in prey capture, with narrowly spaced
large teeth present in “biting” pinnipeds, and smaller, spaced out teeth
present in “sucking” pinnipeds. Smaller tooth spacing and larger crowns also
characterized pinnipeds that rip prey into pieces or filter feed (e.g. leopard,
crabeater seals). This study applied these features to the extinct pinnipeds Desmatophoca
and Enaliarctos, and recovered both as being similar to modern
otariids – generalist feeders like sea lions and fur seals.
Pinnipeds – seals, sea lions, and walruses – are a group of
mammalian carnivores that evolved from dog or bear-like ancestors (or possibly
otter-like – e.g. Puijila darwini). Modern pinnipeds are all either
suction feeders or pierce feeders – teeth are used only to capture prey, but
prey is swallowed whole instead of being chewed (masticated). Enaliarctine
pinnipeds – the earliest known seals – have carnassials like terrestrial
“fissiped” carnivores. Modern carnivores chew their food, and the carnassials
shear through bone and flesh during mastication. Did enaliarctines chew their
food like their terrestrial ancestors? And if so, did they have to leave the
water to masticate after prey capture? To address these questions, data similar
to that collected for modern and fossil pinnipeds as in Churchill and Clementz
(2015: see above) was analyzed using principal components analysis (PCA) and
phylogenetic independent contrasts (PIC) to see where Enaliarctos plotted
within 2 dimensional "morphospace". Enaliarctines occupied an
intermediate morphospace between terrestrial carnivores and pinnipeds,
retaining close tooth spacing of "fissipeds", but with reduced
heterodonty of pinnipeds. PCA indicated that Enaliarctos grouped with
other pinnipeds as a pierce feeder - and that it likely did not masticate; this
indicates that pierce feeding likely arose as a common feeding behavior of
pinnipeds early during their evolution. Lastly, this suggests that Enaliarctos
did not need to return to land after catching fish in order to feed, as
suggested by some earlier studies. This article also gives a fantastic review
of dental evolution of pinnipeds, supplementing earlier discussions by
Boessenecker (2011) and Boessenecker and Churchill (2013). There's a common
theme here: Morgan and I really like seal teeth!
On rare occasions vertebrate skeletons will get preserved
with the remnants of their last meal. The identity of the gut contents associated
with the Triassic dinosaur Coelophysis has been debated to death: are
smallish bones found within the ribcage of an adult Coelophysis skeleton
bones of a different species, or bones of a juvenile? Because the latter would
make Coelophysis a cannibal. Fossilized examples of gut contents are
rare, but provide pretty powerful data on trophic relationships - in other
words, "who ate who". Examples of fossil marine reptiles with gut
contents abound - from the Cretaceous of Kansas alone there is evidence of
virtually every imaginable trophic relationship amongst large marine
vertebrates. However, until this year there was very little evidence of gut
contents for marine mammals. Two basilosaurids have been discovered with gut
contents (Dorudon - published; Basilosaurus, not published) and
two pinnipeds (phocid seal, New Zealand
- private collection, not published; Kawas, published). Within Neoceti
(baleen whales and toothed whales) there were no known examples. This new study
reports fossilized gut contents of a late Miocene cetotheriid baleen whale from
the Pisco Formation of Peru consisting of a mass of fractured fish bones tucked
between the ribs of a partial baleen whale skeleton. The entire skeleton was
not excavated by the authors, but the mass of fish bones was documented in situ
and removed. The fish material consists of a single skeleton of a sardine, Sardinops;
while the bones are fractured and disarticulated, they show no evidence of
partial acid digestion. The authors interpreted the fish remains as being
within the forestomach at death (whales are artiodactyls and thus have
chambered stomachs). The authors interpret this as indicating that cetotheriid
mysticetes were adapted to feed upon fish as opposed to soft bodied planktonic
crustaceans (e.g. krill). A few minor problems exist with this study - such as
the fact that the gut contents consist of a single individual - which is not a
problem for the gut contents of a large macrophagous predator like a mosasaur,
but for a filter feeder that would consume thousands of fish this size a day,
it makes you wonder if it's an example of accidental ingestion. Another issue
is that the taphonomically informative skeleton was left in the field (I
suspect owing to a storage problem at the host museum). Regardless, it's a
solid advance and I was pleased to see it published. Truth be told, I wondered
when fossil odontocetes and mysticetes would be found with gut contents - and I
always assumed they would be discovered in the Pisco Formation of all places.
It's nice when predictions are verified!
Modern true porpoises (Phocoenidae) are amongst the smallest
of all cetaceans, and few surpass 2.5 meters in length; they share a common
ancestry with oceanic dolphins (Delphinidae) and white whales (Monodontidae)
sometime during the middle or late Miocene, perhaps arising from the
"kentriodontid" dolphins. The sheer majority of fossil porpoises are
from the north Pacific with a few important specimens from the west coast of South
America, a possible periotic from New
Zealand, and an extinct genus Septemtriocetus
from Belgium.
In contrast, phocoenids are currently nearly worldwide in distribution (within
subtropical/temperate waters, anyway). This study reports a second porpoise
from the North Sea, Brabocetus gigaseorum, based
on a partial braincase from the early Pliocene Kattendijk Formation of Belgium.
In many regards this genus is Phocoena-like (harbor porpoise) with a
similar facial region but differs by possessing some archaic features. At first
glance, I would assume that this would be one of the closest morphological
matches to modern phocoenids, which typically form a crown clade without any
extinct genera in cladistic analyses of porpoises, with all extinct genera of
porpoises falling outside this group. However, their analysis shows Brabocetus
forming a clade with Septemtriocetus, Haborophocoena, Salumiphocoena,
Archaeophocoena, Miophocaena, and strangely, Semirostrum.
I'm skeptical of the placement of Brabocetus, and I strongly suspect
that the Phocoenidae is taxonomically oversplit - but more fossils are the only
way to cure this issue and this paper is a fine contribution to porpoise
evolution. Because Brabocetus and Septemtriocetus are in the
eastern North Atlantic, they suggest an early Pliocene
dispersal of phocoenids through the Arctic shortly after
the opening of the Bering Strait - followed by a second
dispersal through the arctic during the middle or late Pleistocene by extant
harbor porpoise (Phocoena phocoena).
The giant sea cow Hydrodamalis gigas was discovered
by the shipwrecked crew of the Svyatoy Pyotr in the Komandorsky
Islands in 1741 and named the Steller's
sea cow after the Russian expedition's German naturalist Georg Wilhelm Steller.
Within 30 years this giant kelp-feeding sirenian was extinct; for over two
centuries it was assumed that Hydrodamalis was hunted to extinction
because of how easy it was to kill. Indeed, stories about this source of food
circulated amongst fur traders in the Kamtchatka region, and it has always been
suspected that subsistence by fur traders drove the last population of Hydrodamalis
to extinction. Another hypothesis that has gained traction in recent years,
but has been notoriously difficult to actually test - is the possible influence
of sea otter hunting rather than direct hunting of Hydrodamalis. After
all, Hydrodamalis lived pretty much from Japan
to the Aleutians and down to Baja
California during the late Pliocene, long before
humans ever made it to the Pacific coasts; whatever snuffed out the last
remaining populations of Hydrodamalis in the subarctic was perhaps a
long time coming. The idea is simple and elegant: sea otters tend to keep sea
urchin populations down, and in areas where sea otters have been removed from
the environment by overzealous hunting, sea urchins completely consume and
destroy kelp forests (within 5-8 years of sea otter extirpation). The diet of Steller's
sea cow was entirely based on kelp - and the crew of Bering's expedition noted
abundant sea otters in the Komandorsky
Islands. Because of this
relationship, sea otters are a keystone species and help maintain kelp forests.
Whereas sea cows were demonstrably extinct by 1768 at Bering
Island, sea otters had been
extirpated in the Komandorsky Islands
by 1753; in Alaska, sea urchin
populations did not really "explode" until a few years after sea
otters began to decline, followed by kelp forest collapse. This lag matches
rather well with the earlier reported extirpation date of sea otters (1753) and
the sea cows (1768). [Note that the Bering expedition discovered the sea otter
as well, and within a year or so of being discovered the word got out about their
luxurious fur and the sea otter fur trade began; sea otters were hunted
mercilessly from west to east, with the Russians pushing the fur trade into
Northern California by the early 19th century.] The authors go one step further
and used population modeling and simulation of starvation to show that sea
otter hunting alone, even without any direct hunting of sea cows, would have
driven extinction of the doomed giant sirenian by itself.
This paper is a review of archaeocete evolution, and since
it is a review paper, will only get brief treatment here. The review is
intended to provide a comprehensive summary of trends in archaeocete evolution
- indeed, nearly 2/3 of the paper consists of a family-by-family discussion of
what we know about each archaeocete "family". While the reference
list is quite good in terms of inclusion regarding papers published prior to
2005, many more recent advances such as the discovery of good postcrania for
North American protocetids (Natchitochia, Georgiacetus),
protocetids with excellent skeletons and fetuses (Maiacetus), and also fails to include
any citations of recent excellent work produced by Julia Fahlke and colleagues.
The paper casts doubt on the hippo-whale link, consistently claiming that the
cetacean-mesonychid hypothesis is always identified when molecular data is
excluded - which may have a grain of truth, but the most comprehensive analysis
to date by Michelle Spaulding et al. (2009: PLoS One) showed that tree topology
of the artiodactyls (and thus cetaceans and mesonychids) is very sensitive
towards which taxa are included/excluded from the matrix, which seems to be a
bigger problem than just molecules v. morphology/combined analyses. Ultimately,
this paper doesn't really present any new ideas, and since we've already had on
average one review paper on archaeocete evolution ever two years, I'm not sure
why we needed another - particularly considering that many important studies of
archaeocete evolution have been omitted in this study. The authors could have
just consulted my 2012, 2013, and 2014 marine mammal paleontology summary
posts!
Marine vertebrate paleontologists studying fossils from the
west coast of North America are well-acquainted with concretions - extremely
hard carbonate (and occasionally phosphatic) nodules that form around marine
vertebrate remains. Concretions are much harder than surrounding rock, and will
often erode out of cliffs but keep the entombed fossil in good condition as it
slowly worries away by wave action. Concretions are an absolute pain to work
with, as they often require mechanical preparation with pneumatic tools or
chemical preparation with acids to remove the bones. Fossil marine vertebrates
in the Pisco Formation of Peru are often entombed within dolomite concretions,
and may be linked with the excellent preservation of fossils. This new study is
yet another excellent contribution towards the geological context of
spectacular marine vertebrate assemblages from the Pisco Formation, and reports
new data from the field and petrographic results to investigate the formative
processes involved in Pisco concretions. These authors note that a higher
proportion of mysticete (baleen whale) fossils are associated with concretions,
suggesting that they are more likely to form with larger carcasses. Specimens
in concretions are also more likely to be complete and articulated. Based on
the distribution of dolomitic matrix, soft tissues must have already been
decayed prior to formation of dolomitic cement, and bones must have been at
least partially buried. In some cases, dolomitic matrix forms only within the
bones - an ideal situation, as it has
prevented burial compaction and deformation of the bones. Dolomite is commonly
assumed to be a diagenetic or metamorphic "sequel" to original
limestone - but in these cases, it appears that dolomite was directly
precipitated without a limestone precursor (a similar process affects Purisima
Formation vertebrate-bearing concretions at Point
Reyes National Sea
Shore). This study proposes that
dolomite was precipitated as a response to the decay of organic matter of the
whale after skeletonization and burial, thereby forming nodules and infilling
bones with cement that contribute to their preservation and recovery. An
earlier study by young earth creationists claims that excellent vertebrate
preservation in the Pisco Formation is caused by extremely fast sedimentation (these
rates, by the way, they use in the non-scientific literature and extrapolate to
the entire Pisco basin, claiming that the entirety of the strata would have
been deposited in 20,000 years rather than 20 million, supposedly meaning that
scientific dating methods do not work). Instead of requiring bizarre claims
about ultra-fast (shall we say, biblical?) sedimentation rates, this new study
finds a far more logical solution: that the organic matter whales possess at
the time of death and burial contributes to dolomitic cementation and
ultimately their preservation.
The toothy basilosaurids are easily some of the most iconic
of all cetaceans owing to their fearsome jaws and large size, and are also the
largest and most completely known of all the archaeocetes. Many are quite large
(Basilosaurus, Basilotritus) but others are smaller - including Dorudon
and Zygorhiza. The former is easily the most well-known archaeocete
(South Carolina, Egypt)
and the latter is well-known from the southeastern USA
- but this paper was more or less the first treatment of Zygorhiza since
Remington Kellogg's seminal masterpiece "Review of the Archaeoceti"
published 80 years ago, a fact which Gingerich laments. There are many new
specimens of Zygorhiza now, but for some strange reason very few of them
have been published upon (the same can be said for Basilosaurus cetoides,
in my opinion). A "new" specimen (collected in a large plaster jacket
in the 1960s, and shopped around on loan for forty years until Gingerich took
it to Michigan to get prepped
out) includes part of a skull and much of the vertebral column, allowing
reevaluation of vertebral numbers and morphology. Further, the skull was
scanned to produce a digital endocast, which was compared with a digital
endocast for a separate specimen published a decade ago by Lori Marino and
others. Relative brain size is on the small size for a terrestrial mammal of
comparable body size, but intermediate between mysticetes and odontocetes.
Lastly, Gingerich discusses the archaeocete fauna of the Eocene of the Gulf
Coast, remarking that there is
always one small basilosaurid (Zygorhiza kochii), one large basilosaurid
(Basilosaurus cetoides), and a medium size basilosaurid. Pontogeneus
priscus was originally based on an isolated, medium sized cervical
vertebra, in the 19th century when standards for paleocetological holotypes
were not yet "evolved". This taxon was later declared a nomen nudum
by Mark Uhen, who named the new genus and species Cynthiacetus maxwelli based
on a partial skeleton including a nearly complete (but incompletely figured)
skull. Gingerich interpreted Cynthiacetus as a junior synonym of Pontogeneus,
and discussed what we can and should do with ICZN rules for types. In other
words: a type need not survive to the present day (see below) or even be
diagnostic, so we can get away with using shitty Georgian or Victorian era
types. My opinion: if there's no preserved autapomorphic features (e.g. the fossil
is a cetacean vertebra), it should be declared a nomen dubium as it is
impossible to unambiguously diagnose the taxon. A name, then, is only as good
as its type. We'll see how this one pans out.
This new paper provides a long-needed redescription of the
problematic "cetothere" Mesocetus argillarius, originally
named by Flemming Roth in the late 1970s. This specimen is from the upper
Miocene Gram Formation of Denmark, which historically has also yielded the Pelocetus-like
baleen whale Uranocetus and poorly preserved pontoporiid dolphins.
Differing from other Mesocetus (which similarly deserve modern
re-treatment), the authors assign it to the new genus Tranatocetus. The
skull is broadly similar with some poorly known "cetotheres"
(occasionally regarded as true Cetotheriidae or "cetotheres" sensu
lato) such as Mixocetus, "Aulocetus" latus,
"Cetotherium" vandelli, and "Cetotherium"
megalophysum,, but has a very primitive lower jaw with an enormous
mandibular foramen. Cladistic analysis places these poorly known
"cetotheres" including Tranatocetus as sister to the
Balaenopteroidea (gray whales + rorquals), and the authors erect a new family,
Tranatocetidae, based on derived features of the tympanic bulla and some other
features of the braincase. "Cetotheres" in this study have been split
into the Cetotheriidae, sister to the Neobalaenidae similar to the provocative
hypothesis of Fordyce and Marx (2013), on the neobalaenid/cetotheriid
"stem", within the Tranatocetidae, and on the tranatocetid +
balaenopteroid "stem". Further testing of this interesting hypothesis
of mysticete relationships will require redescription and reanalysis of poorly
known "cetotheres" like "Cetotherium" megalophysum
and "Cetotherium" vandelli.
Bone histology is a useful way to study how vertebrates
grow. In the case of many terrestrial vertebrates, bone growth can even be
studied at a level where annual growth lines may be counted like tree rings -
many friends of mine who were in graduate school when Sarah and I were at
Montana State involved this sort of study - Holly Woodward, Julie Reizner,
Alida Bailleul, John Scannella, and even our friends Liz Freedman-Fowler and
Laura Wilson-Brantley (taphonomists originally!) couldn't avoid histology. Marine
vertebrates on the other hand do not preserve these sorts of growth lines quite
so well (though counting growth lines in marine mammal teeth is a commonly used
method for modern species). However, various marine tetrapods have adapted
their terrestrial skeletons to problems of swimming and buoyancy in various
ways, and many possess peculiar patterns of dense bone growth, hypothesized by
some to be bone ballast. Histology is destructive, and this study sought to
sample many postcranial bones of archaeocete whales including precious
vestigial hindlimbs - so high resolution CT imaging was used instead to study
changes in bone microstructure across the terrestrial-marine transition in
early whales - remingtonocetids, protocetids, and basilosaurids. All exhibited
bone mass increase in their ribs, but the vertebral column consists chiefly of
spongy bone. The humerus of protocetids retains thick cortex (an adaptation for
locomotion on land or paddling), but within basilosaurids the humerus becomes
more strongly spongy and porous - indicating a transition from forelimb
paddling to use of the foreflipper as a simple hydrofoil like modern cetaceans.
On the contrary, the femur becomes very dense - even in the vestigial hindlimb
of basilosaurids, which remains unexplained. The pattern of bone mass increase
in Basilosaurus was originally interpreted to be an adaptation for control of
"trim" - orientation of the vertebra column with respect to the
horizontal plane (e.g. "pitch" in an aircraft) - sampling additional
bones from the skeleton now rules out this hypothesis, but the authors indicate
that no other existing explanation is sufficient. Regardless, patterns of bone
microstructure are consistent with remingtonocetids and protocetids being
shallow diving, semiaquatic swimmers with a limited capability of terrestrial
locomotion like pinnipeds and sea otters, whereas basilosaurids are broadly
comparable with modern cetaceans, consistent with interpretations of
basilosaurids being the earliest oceangoing (pelagic) cetaceans. This study
also showed that microstructure can change dramatically along the long axis of
a rib or other bone, demonstrating the importance of taking numerous sections.
How did the skull of aquatic carnivores evolve after making
the land to sea transition? Was it a passive process, or did pinnipeds undergo
an adaptive radiation? This new study by Katrina Jones and others investigates
this by using 3D morphometrics of modern and extinct terrestrial
"fissiped" and pinniped carnivorans within a phylogenetic context. Several
fossil pinnipeds were included such as Enaliarctos emlongi, Allodesmus,
Pontolis, Piscophoca, and Acrophoca. Overall pinnipeds
exhibit a greater variation in skull shape (disparity) than terrestrial
carnivores. However, there is no increase in evolutionary rate at the base of
the pinniped tree, indicating passive evolution of skull shape (e.g.
"Brownian motion") and indicating that an adaptive radiation model
does not fit very well. Within later groups of pinnipeds evolutionary rates
sped up, perhaps associated with ecological specialization (e.g. walruses and
suction feeding). In the context of the late Oligocene-early Miocene pinniped
fossil record, this does make sense as nearly all pre-middle Miocene pinnipeds
are enaliarctines that look fairly similar and share similar body sizes despite
belonging to different lineages (Enaliarctos, Pteronarctos, Pinnarctidion,
Prototaria, Proneotherium) - Desmatophoca being a notable
exception.
This article is entirely in Japanese, and while it does have
an English abstract and good photos, the fossil described is incomplete and the
abstract short – so my summary will be as well! This specimen is from the
middle Miocene and consists of a partial posterior part of a mandible. This
specimen has a small, laterally projecting coronoid process, an enormous and
anteriorly expanded mandibular foramen a tiny mandibular condyle, and a
ventrally deflected angular process. These features are all consistent with
this specimen belonging to a “Kelloggithere” - an informal name for
“cetotheres” sensu lato like Parietobalaena, Pelocetus, and Diorocetus.
This group is well-represented by a large collection of terribly understood
fossils. Other specimens with a similar angular process have also been reported
from Japan, but
belong to an unknown mysticete – a similar mandible is present in Mauicetus
parki from New Zealand.
This article is also entirely in Japanese with the exception
of the abstract, so my summary is going to be brief! It does have an English
abstract and good figures, so I'll communicate what I can. Japan has an
excellent fossil record of cetaceans including my favorite group - baleen
whales. However, many of these are not yet described, and many late Miocene and
Pliocene mysticetes from Japan have been under study for years without any
resulting publications, a frustrating situation for some. This new paper
reports a balaenopterid whale, more commonly known as a rorqual (humpbacks,
minke, fin, blue whales are all rorquals), from upper Miocene rocks of Miyako
Island which is located pretty far south and relatively close to Taiwan. This
specimen consists of a partial skull lacking a rostrum, is somewhat fractured,
and has some adhering concretionary matrix. The specimen cannot be identified
to any existing balaenopterid genus owing to its braincase morphology - and is
too incomplete to designate as a type specimen, so the authors simply identify
it to the family level. In my opinion, this whale is most similar to the
archaic taxon Protororqualus cuvieri, but it's anybody's guess at this
point; more preparation would be instructive, as the critical earbones are
still in situ. A tantalizing find, and according to Felix Marx more
research is being done on this specimen.
Many of the world's oldest well-preserved phocid seals
(true, or earless seals) - and many of the earlier reports of fossil true seals
in general - come from marine deposits of Paratethys, a former sea that
occupied a foreland basin to the north of the Tethys sea (the Black, Caspian,
and Aral seas are the remaining deep pockets of this former sea). Paratethyan
deposits stretch from Austria to Kazakhstan; biostratigraphy is often
rudimentary, but marine mammal fossils are plentiful. One of the earliest
phocids known by a good skull was named Devinophoca claytoni in 2002;
this early seal has features of both the Monachinae (southern seals) and the
Phocinae (northern seals), potentially indicating ancestral relationships. This
new article by Irina Koretsky and Sulman Rahmat names a second species of Devinophoca
based on another well preserved skull, isolated teeth, and mandibles. The skull
is similarly generalized, but has a monachine-like number of incisors; the new
mandible differs from monachines and is very similar to the gracile mandible of
the early phocine Leptophoca. Interestingly, Devinophoca
inhabited subtropical waters with abundant corals, and was likely a shallow
diver.
Unlike the above mentioned Devinophoca, the sheer
majority of the phocid (true seal) fossil record is constituted by
disarticulated, non-associated postcranial bones. This problem has plagued
pinniped paleontology in general since J.P. Van Beneden began studying
Mio-Pliocene true seals from North Sea deposits in Belgium and the Netherlands,
and named a bunch of problematic species & genera based on disparate
material, many of which have been suspected to be chimaeric assemblages of
postcrania. With the exception of European & Paratethyan taxa based on
crania (Devinophoca, Praepusa, and Pliophoca - see Berta
et al., above) and New World taxa based on associated skeletons (Monotherium
wymani, Leptophoca, Piscophoca, Acrophoca, Hadrokirus,
and now Australophoca - see below) it's unclear how many of these
postcranial taxa are real. How much do pinniped postcrania vary within a
population, or how similar are they between taxa? Sexual dimorphism is also a
huge problem. These are valid but also very basic questions that have not
really been addressed, yet the study of fossil phocids has been plagued by
these problems for over a century while students of true seal anatomy continue
to ignore them. This new study reports several isolated postcranial elements
from the including two humeri and a sacrum - one complete female humerus, and a
partial male humerus. These remains do reflect an extraordinarily tiny seal -
smaller even than the newly described Australophoca (see Valenzuela-Toro
et al., below). The authors make favorable comparison to postcrania of the
Miocene Paratethyan seal Praepusa, and base the new species Praepusa
boeska on the complete female humerus and refer the other bones to it.
Dating of the locality is poor - late Miocene to "mid" Pliocene,
11.5-3.5 Ma. The authors discuss the fossil record of Praepusa, and point out
that the earliest fossils (P. vindobonensis) originate from the middle
to late Miocene (16.5-11.2 Ma) of Kazakhstan and Austria, the somewhat younger
species P. pannonica is from the late Miocene (12.3-11.2 Ma) of Moldova
and Hungary, P. magyaricus is similarly from the late Miocene (13.6-12.3
Ma) of Austria, and the new North Sea species, P. boeska, is from the
late Miocene-Pliocene of the eastern North Atlantic (11.6-3.2 Ma). This does
paint a rather interesting picture of seal dispersal out of the Paratethys westward
into the north Atlantic. Future discoveries of cranial material and associated
skeletons are needed to assess whether or not Praepusa is monophyletic.
The early Miocene is an intimidating time for students of
toothed whale evolution – quite a bit is going on, and there are a zillion
different types of long-snouted dolphins living alongside the early ancestors
of modern groups (some of the earliest sperm whales, the earliest possible
delphinoids, purported early beaked whales). Some of these speciose families of
longirostrine dolphins either originating or diversifying during the early
Miocene include the Eurhinodelphinidae, Squalodelphinidae, Platanistidae,
Allodelphinidae, Eoplatanistidae, and the “Dalpiazinidae”. Early Miocene
odontocetes suffer from several problems – many are known from good skulls but
not well-prepared or figured earbones; some taxa are almost certainly
oversplit, and there is likely an overemphasis on the definition of family
level taxa, some of which are likely para- or polyphyletic. Lastly, because of
these issues, the phylogenetic relationships of these problematic dolphins are
poorly known – and these reasons are why the early Miocene scares the shit out
of me. This study daringly reports a long snouted dolphin, Chilcacetus
cavirhinus from the lower Miocene Chilcatay Formation of Peru. This dolphin
has a long snout and a homodont dentition – in other words, the teeth are all
identical in shape and all are single rooted. Chilcacetus uniquely has a
deep cavity between the nasal bones and the mesethmoid. It is similar to the
giant dolphin Macrodelphinus kelloggi from the lower Miocene Jewett Sand
of California, which has been classified in the past as a giant
eurhinodelphinid. However, Chilcacetus – which has part of a mandible as
opposed to the fragmentary Macrodelphinus type specimens – has an
unfused mandibular symphysis, unlike all eurhinodelphinids. The taxonomically
informative earbones were unfortunately lost between collection and
publication, but somewhat detailed line drawings were prepared before they were
lost. Other features preclude assignment to any other odontocete family.
Cladistic analysis of Chilcacetus and other odontocetes actually
supports a clade including Chilcacetus, Macrodelphinus, Argyrocetus
from Argentina, and “Argyrocetus” (two species from California) – which
could be named as a new family. The authors stop short of this, highlighting
low statistical support for the grouping and lack of unambiguous
synapomorphies. However, the grouping does consist of mostly eastern North Pacific
and South American species; anatomical features supporting this clade are
mostly primitive features that set them apart from later diverging odontocetes.
As per usual, more fossils and more character evidence is needed to make sense
of these challenging taxa.
Gut contents is widely reported for fossil skeletons of
marine reptiles – skeletal remains of a larger animal's last meal. Mosasaurs,
plesiosaurs, ichthyosaurs, sharks, and large fish are all known with
well-preserved gut contents. For some reason, however, gut contents of marine
mammals is much more rare. Ironically, little is known about the feeding
behavior of modern marine mammals since it is difficult to directly observe
them out at sea – so much of our knowledge of the diet of modern marine mammals
is recorded from necropsies and gut contents. The difference is that in modern
specimens, soft bodied prey can be observed – but in the rock record, only prey
items with hard tissues become preserved, with some rare exceptions (e.g.
cephalopod beaks, which are keratinous). Prior to this study the only marine
mammal with gut contents was the Miocene seal Kawas from Argentina. I
have wondered when the first fossil cetacean with gut contents would be
discovered, and figured that if it were to be discovered anywhere it would be
from the Pisco Formation of Peru. My gut instinct (no pun intended) was
vindicated this year with the discovery of a Messapicetus specimen with
abundant sardine skeletons preserved around it. However, it is unclear what
exactly was collected as opposed to left in the field (see Collareta et al.,
above). Regardless of these issues, this study indicates that an early beaked
whale (Ziphiidae) had a gut full of shallow water fish (Sardinops) when it died. Modern beaked whales are suction feeding,
deep diving squid specialists. Assuming that this individual died after
consuming a typical meal for its species, this fossil may indicate that Messapicetus was not a deep diving squid
specialist, perhaps indicating that deep diving “teuthivory” is a more recent
feature of beaked whales. Surely, the Pisco Formation will produce more
treasures that will expand our knowledge of ancient food webs.
This new study is the product of Felix Marx's Ph.D. thesis
at the University of Otago, and I was fortunate to see quite a bit of this long
before it came out. The core of this study is a new phylogenetic analysis of
baleen whales (Mysticeti) and is a successor to an earlier cladistic matrix
produced during Marx' master's program at Bristol. This new phylogenetic
analysis includes the most number of baleen whales (though with 3/4 the
character evidence as the largest published analysis). These authors again
recovered an aetiocetid-mammalodontid toothed mysticete clade, a monophyletic
Cetotheriidae including the pygmy right whale, possible resolution amongst
"kelloggitheres", and gray whales (Eschrichtiidae) deeply nested
within the rorquals (Balaenopteridae). This new analysis is essentially a
semi-comprehensive study of mysticete evolution, and also used the
morphological dataset to study disparity (anatomical diversity) through time.
Disparity peaked during the Oligocene and plateaued during the Neogene, whereas
taxonomic diversity was highest during the middle and late Miocene and dropped
off during the Plio-Pleistocene (perhaps an artifact of the rarity of published
accounts of Pliocene mysticetes). Alternatively, evolutionary rates were
highest during the Oligocene and flatlined thereafter - suggesting early
settling into "modern" filter feeding niches. Mysticete diversity
seems to drop as soon as modern gigantism appears, suggesting an influence of
Plio-Pleistocene glacial influence on baleen whale evolution.
Marx, F.G., C.H. Tsai, and R.E. Fordyce. 2015. A new early Oligocene toothed ‘baleen’ whale (Mysticeti: Aetiocetidae) from western North America: one of the oldest and the smallest. Royal Society Open Science2:150476.
Modern river dolphins were formerly thought to constitute a
single group, the Platanistoidea - until it became obvious in the 1980s from
skeletal anatomy and fossils that each modern river dolphin (Inia, Lipotes,
Platanista, Pontoporia) likely had separate origins, later
confirmed by molecular work in the early 2000s. Two of these may in fact be
somewhat closely related - the Amazon river dolphins, Inia,
and the La Plata river dolphin, aka
Franciscana, Pontoporia (which is actually mostly marine). The recently
extinct Yangtze river dolphin Lipotes is now
thought to be closely related to Parapontoporia from California
(see Boessenecker and Poust, above), but has no other near relatives in the
fossil record. The origins of the totally bizarre Ganges/Indus river dolphins Platanista
have been more contentious, possibly involving the Squalodelphinidae,
Squalodontidae, and even the Oligocene Waipatiidae. The evolution of the South
American river dolphins is less controversial, and many fossils have been
reasonably identified as extinct marine (and freshwater) of Inia (Ischyorhynchus,
Saurocetes, Goniodelphis, Meherrinia) and Pontoporia
(Brachydelphis, Pliopontos, Protophocoena, Auroracetus,
Stenasodelphis). A new fossil discovered by J. Velez-Juarbe and others
during the Panama Canal Project (PCP-PIRE) from the late Miocene of Panama
includes a large, well-preserved skull and mandible with associated teeth,
scapula, and carpal elements and is named in this study as Isthminia
panamensis. It is large (nearly 3 meters in length based on skull size) with
relatively robust teeth and an elongate, narrow rostrum like extant Inia.
Unfortunately, anatomically informative tympanoperiotics are unknown. Because Isthminia
was recovered from marine sediments, the authors interpret it to be a marine
rather than a freshwater iniid. However, it is important to note that
terrestrial animals frequently become entombed in marine sediments (likely
carried out to sea during floods) and in general tetrapods are terrible paleodepth
indicators. Regardless, the cladistic analysis and inferred environmental
preference of modern and extinct inioids indicates that in South America, a
patchwork pattern of freshwater invasion of river basins occurred, paralleling
the recently reported occurrence of a platanistid from the Peruvian Amazon
(Miocene; Bianucci et al., 2013) and the possible freshwater invasion of
California's San Joaquin Valley by Parapontoporia (Boessenecker and
Poust 2015, see above) - indicating significant adaptability among modern and
extinct river dolphins and their marine relatives.
This is another review article, so this will be brief. Most
of this review is concerned with the ecology and evolution of the invertebrate
fauna inhabiting modern whale falls - which, for the uninitiated, are whale
carcasses that have sunk down to the deep sea floor and host a very distinctive
fauna of marine invertebrates also seen at deep see vents and methane seeps.
Occasionally fossil cetaceans have been recovered with trace fossils or
associated/attached body fossils of these deep sea specialists, such as
vesicomyid clams and the bone eating worm Osedax (which doesn't have a
mineralized skeleton, but produces distinctive borings in whale bones which do
preserve). This summary concludes that most elements of modern whale fall
communities had their origins during the Oligocene, corresponding to the
diversification of the Neoceti - one third of all extant genera of cold seep
mollusks appear during the late Eocene and early Oligocene, tightly
corresponding to the diversification and worldwide dispersal of Pelagiceti
(Neoceti + Basilosauridae). These authors suggest that more research into the
evolution of bone lipids (the principal source of nutrients for many whale fall
specialist invertebrates) within extinct cetaceans should be conducted - which may
perhaps be inferred from postcranial bone histology. Lastly, modern evidence of
the "reef stage" - the fourth stage in the evolution of a single
whale fall (after the mobile scavenger, enrichment-opportunist, and sulfophilic
stages) has been criticized by other whale fall biologists, yet has support
from fossils. This stage was hypothesized to exist as a period after which the
nutrients have been completely removed from the bone, but because the bones
still physically extend above the seafloor sessile filter feeding invertebrates
will colonize the bone to take advantage of a higher current. Many examples of
barnacles, serpulid worms, bryozoans, and other sessile filter feeders are
known from marine vertebrate skeletons preserved in deep marine settings.
Basilosaurus is known from two species from eastern
North America (B. cetoides) and northern Africa (B. isis) and
represents the largest basilosaurid archaeocetes known - giant serpentine
whales with quasi-vestigial hindlimbs that lived during the late Eocene (a
third possible species is debated but has been reported from Pakistan - B.
drazindai). All basilosaurids are characterized by fearsome dentitions with
caniniform anterior teeth and large, triangular, cuspate shearing cheek teeth -
and like most archaeocetes, have tiny braincases with enormous jaw muscle
attachments. Smaller archaeocetes from the same deposits as B. isis,
including several juvenile skulls of the small basilosaurid Dorudon atrox,
have been found with large tooth punctures, reasonably hypothesized by Julia
Fahlke to be tooth punctures from B. isis. Does Basilosaurus have
sufficient bite force to cause bite marks like this? What is the bite force of
an archaeocete? We can't go out and put a force gauge into the mouth of an
extinct organism - so computer modeling, specifically finite element modeling
- provides a means by which to estimate
bite force. I won't go into how FEM modeling works, principally because I am
not mentally equipped to do so - but it can be done using CT data or, as in
this case, a 3D surface scan of a 3D object. Using high resolution CT data
permits density/strength values to be placed onto tiny 3D 'cells' (voxels: aka
3D pixels) with the density derived directly from the CT scan. Another method
is to use a surface scan and arbitrarily assign bone wall thickness (or treat
it as a solid) and uniform bone density/strength within. When scaled to the
same size, FEM indicates that Basilosaurus had comparable bite forces
with giant predatory pliosaurs (e.g. Pliosaurus kevani). Although lesser
in magnitude than the highest bite forces measured and estimated for large
crocodylians and dinosaurs, bite forces predicted (16,400 newtons) at the upper
third premolar (P3) of Basilosaurus exceed those of any other mammal and
additionally exceed predictions of force as expected from its relatively narrow
skull. Notably, Basilosaurus was capable of comparably higher bite force
at the tip of its snout than crocodylians. Bite force is indeed consistent with
indenting and breaking bones, and feeding behavior likely consisted of catching
prey with the anterior teeth and mastication (or should we say in this case,
"chopping") of prey items with the cheekteeth.
This paper utilizes newly recovered molecular data from the
extinct Steller's sea cow (see Estes et al., above) to run a comprehensive
phylogenetic analysis - combined with morphological data - of modern and
extinct sirenians. Much of this paper is concerned with new molecular results -
which are interesting, but this post is focusing on paleontological advances so
I'll focus on those. The second sentence of the abstract goes like this:
"The phylogenetic affinities of [Hydrodamalis gigas] to
other members of this clade, living and extinct, are uncertain based on
previous morphological and molecular studies." This raised huge red flags
for me because from everything I had read by two of the foremost experts in the
world on sirenian evolution and anatomy, Daryl Domning and Jorge Velez-Juarbe -
had indicated that Hydrodamalis is closely related to the southwestern
Pacific Dugong dugon, and that giant hydrodamaline sea cows
evolved in situ within the north Pacific during the late Neogene, giving rise
to Hydrodamalis by the late Miocene/early Pliocene - all of this
appeared non-controversial. The paper of course reports similar results, and
both Daryl and Jorge are coauthors - upon further reading, other researchers
have produced some rather odd results with the west Indian manatee coming out
as more closely related to Hydrodamalis - which doesn't make sense for a
number of reasons, such as the shared presence of tail flukes. The article also
provides a brief but handy summary of macroevolutionary trends in sirenian
evolution.
The incompletely preserved dolphin Prosqualodon marplesi was named in
the 1960's from the upper Oligocene-lower Miocene Otekaike Limestone of New
Zealand, originally placed in the squalodontid genus Prosqualodon. R.E.
Fordyce recognized how dissimilar it was to Prosqualodon, and placed it
in the squalodelphinid genus Notocetus instead when he described the
other NZ dolphin Waipatia maerewhenua. Last year, my labmate Yoshi
Tanaka published a reevaluation of "P." marplesi and
assigned it to the new genus Otekaikea - and surprisingly recovered this
specimen in a cladistic analysis as a sister taxon to Waipatia, and
reclassified Otekaikea marplesi as a waipatiid. In this new paper,
Tanaka and Fordyce describe a second species, Otekaikea huata, based on
a much more complete specimen. Otekaikea huata has a similar braincase
and earbones, differing in only a few subtle ways obvious only to students of
whale anatomy too nuanced to repeat here. However, the relatively complete
holotype specimen of O. huata exhibits many notable features that are
interesting from a functional perspective. The rostrum is very elongate, and
the teeth are nearly homodont posteriorly, with simple crowns and single roots
- and transition anteriorly into tusklike apical teeth. The anteriormost tooth
is huge, about 4-5 inches long, and straight - the tusk would have been
procumbent, and probably not functioning for biting prey. The facial region is
strongly dish-shaped, indicating the presence of a melon and associated facial
muscles involved in sound production - clearly indicating that Otekaikea
huata used echolocation. Hearing was specialized like many modern
odontocetes, with large sinuses in place around the earbones, either for soft
tissues or pneumatic sinuses - acoustically isolating the inner ear from
bone-conducted sounds in the skull. Interestingly, most Oligocene odontocetes
to date are known for their comparably modest rostral proportions - whereas
nearly all non-squalodontid odontocetes from the early Miocene have embarrassingly
elongate snouts, like Otekaikea huata (and nobody has really offered a
good solution as to why). Otekaikea thus may represent the first
known member of this functional group of longirostrine dolphins, giving a
preview of future affairs.
During the late 19th and early 20th
centuries a number of fragmentary but anatomically curious fossil cetaceans and
penguins were discovered and named from various Oligocene marine rocks in the
Waitaki Valley region of the South Island of New Zealand. Several other papers
summarized above have also dealt with some of these historical specimens
(Boessenecker and Fordyce 2015, Tokarahia), as well as more recently
collected fossils (Boessenecker and Fordyce, 2015 – Waharoa; Tanaka and
Fordyce, 2015, Otekaikea huata; Tsai and Fordyce, 2015, Horopeta). Early
identifications and efforts to properly interpret these fossils were hampered
by their incompleteness and lack of comparable material. The species Microcetus hectori is one of these,
based on a fragmentary mandible and some isolated teeth collected from the
upper Oligocene Otekaike Limestone in 1881 by notable geologist Alexander
McKay. The teeth are tiny with high crowns, accessory cusps on the posterior
side, and labial and lingual cingulum – in person, I call them “cute” (Yoshi
Tanaka had these on the desk next to me in my office for several months while
working on this chapter of his thesis). This fragmentary specimen was
originally placed in the genus Microcetus
based on its inferred dental similarity with Microcetus ambiguus; however, detailed observations show that the
teeth differ in many regards, and at a gross level are more similar with other
NZ dolphins like Waipatia. A skull in
a block of sediment was also collected and discovered over 100 years later by
R.E. Fordyce. Given that numerous teeth, a partial mandible, and most of a
braincase were now known, the authors included it within a cladistics analysis
– wherein it was allied with Waipatia
maerewhenua. The authors recombined it as Waipatia hectori. This, with the reinterpretation of “Prosqualodon” marplesi as the new genus of waipatiid Otekaikea and the naming of a second species, Otekaikea huata (see above), really shakes up what was formerly
thought of in terms of odontocete diversity in New Zealand as many of these
seemingly different odontocetes were formerly thought to represent other
odontocete families. And, there are more waipatiids to come!
This study marks another contribution by retired fossil
preparator Howell Thomas into the field of paleopathology - the study of
disease in the fossil record. This study surveys osteochondrosis in modern and
fossil marine mammals. Osteochondrosis has an idiopathic origin - idiopathic
roughly translates to "we have no idea what exactly causes it."
Osteochondrosis usually comprises damage to the articular surface of a long
bone, and is thought to be caused by trauma to the joint, like extreme vertical
loading of a human knee; shear loading, avulsions, and continued low-grade
traumas to the same location along with some other problems can cause
osteochondrosis. Trauma upsets normal cartilage growth at the joint and bone
death occurs below the cartilage which manifests as a deep pit on the articular
end of the bone. The authors figure and describe pits in humeri, ulnae,
scapulae, and skulls of extant marine mammals including walrus, monk seals, and
narwhals. Osteochondrosis is present in postcranial bones of the desmatophocid
pinnipeds Allodesmus "kelloggi", Allodesmus
kernensis, the "cetothere" Tiphyocetus temblorensis,
a skull of the sperm whale Aulophyseter morricei, the atlas
vertebra of the dolphin Zarhinocetus errabundus, and postcrania
of isolated odontocetes and the desmostylian Neoparadoxia cecilialina
from the Monterey Formation. This study reports the first occurrences of
osteochondrosis both within modern and fossil marine mammals. It was not found
in any sirenians, but instead was found only within amphibious pinnipeds and
desmostylians (which could become injured when exiting/entering the water) and
cetaceans (which could become injured when breaching or similar behavior).
Modern baleen whales are readily identifiable based upon
their baleen as well as their enormous body size; indeed, their great mass is
perhaps what best captures the imagination of the public. As alluded to above,
baleen whales had rather humble beginnings – the “chonecetine” aetiocetids (e.g.
Chonecetus, Fucaia) were scarcely larger than a harbor porpoise (~2 meters
long). Other aetiocetids, like Aetiocetus
and Morawanacetus, reported from
Japan and the Pacific Northwest – are only slightly larger, perhaps approaching
the size of a large bottlenose dolphin (~3-4 meters). This rather small range
of body sizes contrasts with the somewhat larger (and contemporaneous) early
baleen-bearing eomysticetids, which were about the size of minke whales (5-8
meters). A new aetiocetid fossil from the upper Oligocene of Hokkaido
(northernmost major island of Japan) reported by Tsai and Ando consists of a
squamosal and a periotic similar in morphology to Morawanacetus yabukii – but is approximately twice as large, with a
body length estimate of 8 meters. This body length is in the size range of
eomysticetids, and expands the range of size disparity amongst toothed
mysticetes. Furthermore, because this large morawanacetine is found in the same
deposits as smaller Morawanacetus yabukii, some degree of niche partitioning
must have been present. Future finds preserving the feeding apparatus of the
large, unnamed morawanacetine may reveal how niche partitioning occurred.
One of the earliest fossil baleen whale earbones I ever saw
photos of was a specimen collected by Ron Bushell, formerly of Eureka in
northern California, who had collected it from the Plio-Pleistocene Rio Dell
Formation nearby in Humboldt County. As a high school student interested in
local paleontology, it boggled my mind that nobody could identify it. Years
later I found out that it, and other neat specimens collected by Ron, had been
kindly donated to Sierra College in Rocklin, CA. After spending a couple years
during my Ph.D. staring at mysticete earbones until my eyes felt like they were
going to bleed, I realized it was probably an early record of a gray whale – so
I invited my labmate Cheng-Hsiu Tsai to describe it. Turns out it’s nearly
identical to modern Eschrichtius robustus,
so we identified it as Eschrichtius
sp., cf. E. robustus. This specimen,
consisting of a tympanic bulla and a compound posterior process, is more
similar to modern E. robustus than a
Pliocene specimen published in 2006 from Japan identified as Eschrichtius sp. As it happens,
Bushell’s specimen is from the uppermost Rio Dell Formation, making it early
Pleistocene (~1-2 Ma) in age – a time period nearly completely unrepresented
for marine mammal fossils in the east Pacific. There is a more “primitive”
unnamed genus of gray whale (Eschrichtiidae) from older Pliocene rocks in
California, but no bona fide records of Eschrichtius;
the Pliocene of California is probably well sampled enough to declare that Eschrichtius was not present (but at
least a half dozen other mysticetes were present instead). Given the delayed
occurrence of Eschrichtius in
California relative to Japan, we hypothesized that the modern gray whale
evolved in the western North Pacific during the Pliocene, and dispersed to the
eastern North Pacific during the early Pleistocene – sometime after the Plio-Pleistocene
marine mammal extinction which led to the demise of eastern Pacific walruses (Dusignathus, Valenictus), the bizarre porpoise Semirostrum, and other cetaceans.
The idea of ancestor-descendant relationships has pervaded paleontology
since the 19th century, but with the advent of cladistics and the emphasis on
phylogenetic relationships a bizarre misconception that we cannot identify
ancestors and descendants in the rock record has arisen. Certainly this is an
artifact caused by the fact that cladistics - the dominant method for inferring
phylogenetic relationships amongst modern and extinct organisms - can only
infer "relatedness" but not time. Thus, inability to interpret
ancestors versus descendants is based on a limitation of our current methodological
paradigm - a limitation that this new study seeks to circumvent. This study
investigates the highly problematic and controversial relationships of the
pygmy right whale, Caperea marginata. A very Caperea-like fossil,
Miocaperea pulchra, is known from the late Miocene of Peru (and these
authors suggest that it could even be recombined as Caperea pulchra,
given the similarity). A well-known but underappreciated aspect of anatomy is
that growth of vertebrates roughly parallels the evolutionary history - in an
imperfect sense, not quite as predicted by Ernst Haeckel (ontogeny
recapitulates ontogeny). Generally speaking, in many vertebrate groups,
juveniles will look like ancestors - to the point where juvenile hadrosaur
dinosaurs have been misinterpreted as small adults of late surviving archaic
hadrosauroids. An earlier study by some Canadians and my dear friend Liz
Freedman-Fowler (Hi Liz!) found that when juveniles of known hadrosaur species
within different families were coded into an existing cladistic dataset, the
juveniles all plotted together on the paraphyletic "stem" of the
group. This concept applies to cetaceans as well. In this study, Miocaperea,
adult Caperea, and juvenile Caperea were coded as different OTUs
into two existing cladistic matrices. In both cases, Miocaperea was
phylogenetically bracketed between juvenile and adult Caperea. Given this, and
relatively slow change in the neobalaenid lineage and neoteny within the
ontogeny of modern Caperea, Miocaperea and Caperea could
be end-members of a late Miocene-Holocene anagenetic lineage undergoing
evolutionary stasis. Ultimately, this does raise additional red flags for
interpreting the relationships of cetaceans based on juvenile specimens (e.g. Nannocetus
eremus, Parietobalaena palmeri).
Lunge feeding - otherwise known as gulp feeding - is one of
the more derived means by which baleen whales filter feed for prey. As
discussed above (see Boessenecker and Fordyce 2015: Waharoa) skim
feeding consists of swimming slowly through the water column and continuously
filtering out planktonic prey - this is utilized by modern right whales,
probably Caperea, and inferred in eomysticetids. Gray whales feed by
ingesting large volumes of sediment and filtering out small benthic
crustaceans. Rorquals (humpbacks, blue, fin, minke whales) lunge feed - they
swim fast towards prey and rapidly open the mouth and close it; water is
expelled by the slowly contracting throat pouch. Many fossil baleen whales from
the Oligocene of New Zealand - particularly the Duntroonian stage (27-25 Ma) -
are eomysticetids, but by the Waitakian (25-23 Ma) are much more rare, and
early "Kelloggithere" like mysticetes are present - these are poorly
known, poorly understood whales like Parietobalaena; Mauicetus parki from
the Waitakian (Otekaike Limestone, Milburn Limestone) of NZ is a prime example.
Their relationships are unclear, and do not belong within the true
Cetotheriidae, and a number of other families have been proposed. This new
study reports perhaps the oldest member of this grade, Horopeta umarere,
from the transition between the Kokoamu Greensand and the Otekaike Limestone in
south Canterbury, New Zealand (same locality as one of the juvenile specimens
of Waharoa ruwhenua). This whale has a partial skull that was
disarticulated and bioeroded and thus many of the bones do not articulate, but
the braincase is otherwise well preserved and includes immaculately preserved
earbones - which are weird looking. They resemble the younger Mauicetus
parki, but differ from pretty much all other Chaeomysticeti (except the
more archaic eomysticetids) in lacking fusion of the posterior process of the
earbones, indicating rather archaic status amongst the mysticetes. The
mandibles are huge with a wide cross section and - most importantly - are
laterally bowed like a humpback whale. These mandibular features are consistent
with lunge/gulp feeding, and represent the geochronologically earliest
occurrence of such adaptations. Horopeta also has a rather large, robust
sternum with attachment points for multiple ribs - and is thus more primitive
than the delicate platelike sternum of eomysticetids like Waharoa and Tokarahia.
More strange mysticetes have yet to be described from the Oligocene of New
Zealand (and Washington, U.S.A.) and will certainly complicate the emerging
picture of early mysticete evolution.
Elephant
seals (Mirounga spp.) are the largest members of Carnivora and the most
sexually dimorphic of all mammals, with males weighing and measuring many times
larger/longer than females and having extreme ritualized behavior and bizarre
probosces for display. Elephant seals live in the Antarctic and Southern Ocean
as well as the eastern North Pacific. Despite having well-studied ecology and
behavior, virtually nothing is known of their evolution. Bits and pieces have
been mentioned, but have never been described until this new paper by Ana
Valenzuela and others on middle-late Pleistocene records of Mirounga.
These fossils include skull fragments and a partial mandible and some other
fragments from Mejillones in Chile,
and represents the first described fossil record of elephant seals. The fossils
are not very old – and most other undescribed records of Mirounga are
also Pleistocene, suggestive of a geochronologically shallow history of
elephant seals.
Velez-Juarbe, J., and D.P. Domning. 2015. Fossil Sirenia of the West Atlantic and Caribbean region XI. Callistosiren boriquensis, gen. et sp. nov. Journal of Vertebrate Paleontology 35:1:e885034.
This study is the eleventh (!!!) installment of the series of papers dedicated to fossil sirenians from the west Atlantic and Caribbean, started by reknowned sireniologist Darly Domning - the last few contributions (9-11) have been coauthored by Jorge Velez-Juarbe, and include some spectacular finds from South Carolina, Florida, and Jorge's home territory - Puerto Rico. The holotype of Callistosiren is an impressive skull collected by Jorge back in 2005 - and it has made some appearances on his blog and on SVP posters. It's a large medium sized dugongid from the Oligocene Lares Limestone of Puerto Rico, characterized by mild rostral deflection (nowhere near as vertical as extant Dugong, but not quite as horizontal as the giant hydrodamalines I'm used to in the north Pacific) and large tusks with enamel present only on the medial surface of the tusk. Notably the ribs and vertebrae show substantially less dense bone than other sirenians. This new discovery highlights how diverse the dugongid lineage was in the Oligo-Miocene of the Atlantic and Caribbean basins; in the North Pacific, there tends to be only one or two sirenians present (possibly owing to competition with desmostylians?) whereas earlier work by Jorge has already demonstrated that many Atlantic, Caribbean, and Indian ocean sirenian faunas are characterized by multispecies assemblages with evident niche partitioning. Callistosiren is the first sirenian recorded from the late Oligocene (Eocene and early Oligocene examples are already known - e.g., Pezosiren, Priscosiren), and an undescribed halitheriine dugongid is also known from the coeval Mucabarones Sand in Puerto Rico. Rostral deflection and tusk size indicate that Callistosiren likely fed on rhizomes of relatively large seagrasses, and the authors further hypothesize that the low density of postcrania - virtually unknown in all other post-Eocene sirenians - may be an adaptation for deeper diving and foraging at greater depths.
This is one of the strangest papers on fossil marine mammals
this year, and I do not mean that in a bad way - it's really a good example of
thinking outside the box when it comes to applying fossil vertebrates towards
answering questions outside the realm of vertebrate paleontology. It all begins
in 1964, when a young undergraduate student from Yale interested in
paleoanthropology was on a field expedition with Bryan Patterson in the Turkana
Basin in Kenya and found what everyone assumed to be a weird turtle shell.
Later on, it was prepared and discovered to be a cetacean - and not only that,
but a rare beaked whale (Ziphiidae). Mead's discovery "derailed" his
future in paleoanthropology and drove him towards studying cetaceans - he
produced a spectacular dissertation on dissecting out the facial region of
modern odontocetes in order to investigate the source of echolocation-related
sound production, and quickly became an expert on the anatomy and biology of
beaked whales. One of his first papers (Mead, 1975) focused on the Turkana
ziphiid. Then, some years later, the specimen went missing, and wasn't
rediscovered until somebody cleaned out Stephen J. Gould's old office at
Harvard in 2011, which was temporarily being used for storage. The importance
of this specimen actually lies in its geologic context - the Turkana Basin is
entirely terrestrial, and Mead speculated that it was an individual that swam up
a river and became stranded. Wichura et al. use this in conjunction with data
on how far modern oceanic cetaceans have swam up rivers to put a maximum
elevation of about 30 meters above sea level on this fossil at the time,
indicating it must have swam 600-700 km from the hypothesized shoreline at the
time. During the early middle Miocene, at the time of the stranding, the
coastal plain in this area consisted entirely of tropical rainforest with
significant rainfall. Sometime after, the entire region began to uplift and it
became very arid - leading to the first savannas in east Africa, an event
thought to have driven the earliest human ancestors (e.g. Ardipithecus,
australopithecine hominins) from the safety of the forest and onto the plains.
Timing of this uplift has been poorly constrained, and the occurrence of this
ziphiid so far inland now indicates that uplift must have taken place sometime
after 17 Ma.
2 comments:
I'm just gonna say it: Best marine mammal paleo review of the year.
You're damn right. I'll take the oscar for first, second, and third place.
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