I apologize for the lack of posts over the past month - the last few weeks I have been working furiously in order to complete a long list of manuscript revisions for a long manuscript for the journal Geodiversitas. Additionally, I've been trying to complete a laundry list of collaborations, which are slowly being completed. Now that I have breathing room again, welcome to the 200th post on Coastal Paleo (down under)!
Last week I was pleased to see the publication of not one,
not two, but three different papers on cetaceans in the most recent JVP issue.
I was also amused by the lamentation by some of my former fellow students from
MSU that there weren’t enough dinosaur papers in JVP (one only!). It’s a
welcome change, from my perspective – to be honest, roughly half of JVP issues
have no marine mammal articles, and the majority have multiple (boooring…)
dinosaur papers. To be fair, there are a hell of a lot more dinosaur
researchers than there are marine mammal paleontologists, so it’s to be
expected. On the other hand, despite the disappointment that comes with
scrolling through brand new but marine mammal-less issues of journals, it does
come with the upside that we often don’t have to add citations for numerous
articles coming out while your manuscript is in review (unless it’s a long
article that takes a while for review and revisions; I had a revised article I
recently resubmitted which had taken about eight months from submission to
completion of revisions, and I had to add citations for about a dozen new
articles).
Comparison of Brachydelphis mazeasi (above) and Brachydelphis jahuayensis (below).
From Lambert and Muizon (2013).
The new studies are published by my colleagues Giovanni
Bianucci and Olivier Lambert, and several of their European and South American
colleagues. The first paper is by Olivier and
prominent French paleocetologist Christian de Muizon on a new species of
Brachydelphis from the Pisco
Formation of Peru. The genus was named by Muizon (1988) who described
Brachydelphis
mazeasi, a short-beaked pontoporiid dolphin from the Cerro La Bruja
vertebrate level of the Pisco Formation in Peru,
a late middle Miocene locality. Pontoporiids today are represented only by the
franciscana, aka La Plata River Dolphin,
Pontoporia blainvillei. It’s my
wife’s favorite cetacean, which she just refers to as “francis”. They don’t
really look much like dolphins the public is familiar with – it’s got odd
triangular fins and an elongate beak. It’s also tiny, topping out at 1.8 meters
in length (5’ for American readers). It’s not quite as freaky looking as the
Ganges/Indus river dolphin (
Platanista; more on this one further down),
or the Amazon river dolphin (
Inia), and is
actually pretty cute as far as cetaceans go (so is the Yangtze river
dolphin,
Lipotes). Note for the Californian readers: one of the most
common fossil dolphins in California,
Parapontoporia, was originally considered to be a franciscana relative, but is
most closely likely related to the recently extinct
Lipotes. One thing
all modern river dolphins share in common, aside from generally plesiomorphic
skull morphology, is an elongate and narrow rostrum.
Platanista and
Inia
also retain what are arguably the least homodont dentition of any odontocete;
modern river dolphins are sort of a relic of the Miocene fauna. Note: for the
rest of the article, when you see the term “river dolphins” in quotes, I’m
referring to all modern and fossil platanistids, pontoporiids, iniids, and
lipotids; these taxa do not form a natural monophyletic grouping, and appear to
have independently evolved (a topic for another post, I think).
The holotype (top) and paratype (bottom) of Brachydelphis jahuayensis from the
Pisco Formation of Peru. From Lambert and Muizon (2013).
Up until the description of
Brachydelphis, all fossil
“river dolphins” also shared an elongate rostrum (or, if not complete, the
indication of an elongate rostrum) –
Pontistes,
Pliopontos,
Ischyorhynchus,
Parapontoporia,
Zarhachis,
Pomatodelphis, the list goes on
and on.
Brachydelphis on the other hand bore a tiny little rostrum, to
the point where it must have resembled something like a true porpoise with
weird flippers. A subsequent study reinterpreted the fragmentary
Protophocoena
minima from the Miocene of Belgium as a brachydelphinine (Lambert and Post
2005). The new study by Lambert and Muizon (2013) focuses on the description of
a new species of
Brachydelphis with a less stunted rostrum, and in a way
is a bit more normal-looking. The rostrum of
Brachydelphis jahuayensis,
however, is still quite short relative to other “river dolphins”, and perhaps
in that aspect still evocative of the name
Brachydelphis. They reported
on numerous new skulls which are preserved beautifully. The most fascinating
aspect of the paper is the age of
Brachydelphis jahuayensis – it’s from
the El Jahuay vertebrate level of the Pisco Formation, which is younger than
Cerro La Bruja, and thus younger than the shorter-snouted
Brachydelphis
mazeasi. Assuming an ancestor-descendant relationship (which I consider to
be reasonable) between the two, it implies that
Brachydelphis first
evolved a short rostrum, and was then reversed during the late Miocene. It may
not be surprising, as we already know that the cetacean rostrum is a fairly
plastic feature that is readily adjusted to changes to feeding ecology
(presumably, anyway). I actually was one of the reviewers for this paper, and
I’m pleased to see it published. Interestingly, this younger, longer-snouted
taxon was preliminarily recognized by Gutstein et al. (2009) from similarly
aged strata from the Bahia Inglesa locality in Chile,
which Lambert and Muizon (2013) referred to
Brachydelphis jahuayensis.
Yet another skull of Brachydelphis jahuayensis. From Lambert and Muizon (2013).
The second paper is by the Italian paleocetologist Giovanni
Bianucci on a new genus and species of oceanic dolphin from the Pliocene of
Italy,
Septidelphis morii. At first look, it is similar in overall
proportions to many extant long beaked delphinines such as
Stenella
longirostris, and other Italian Pliocene delphinids such
Etruridelphis
giulii and
Astadelphis gastaldii. It does differ from all other
delphinids in having a fairly wide mesorostral groove, among other subtle
features. Bianucci (2013) included it within a limited phylogenetic analysis
which he used a molecular tree as a “backbone” constraint – in other words,
forcing the extant species on the tree to conform to the molecular tree, and
using the morphological characters to position the fossil taxa relative to the
extant species. The analysis resulted in the recovery of
Astadelphis,
Etruridelphis,
and
Septidelphis as the earliest diverging delphinines, with the
exception of extant
Sousa chinensis.
This information, in concert
with molecular divergence dates, led Bianucci (2013) to suggest that perhaps
extant genera of delphinids may be nearly lacking entirely in the Pliocene, and
that aside from a few cases of extinct species in extant genera (e.g.
Orcinus
citoniensis), most modern lineages probably did not originate until the
Pleistocene. One piece of evidence for this is that the majority of nodes in
the molecular phylogeny of extant delphinids occur in the Pleistocene (again,
based on molecular divergence dating). To summarize, this suggests perhaps an
even more rapid divergence among delphinids than has been previously hypothesized.
The holotype cranium of Septidelphis. From Bianucci (2013).
The last (and shortest) paper is on a single earbone, also
by Giovanni Bianucci, Olivier Lambert, and colleagues. This paper reports a
fossil earbone of a platanistine river dolphin – closely related to the modern Ganges
River dolphin – from the Amazon
river basin in eastern Peru.
A
Ganges River
dolphin relative in
South America. How cool
is that? It’s also only the second reported platanistine fossil ever – the
first is an admittedly less convincing chunk of mandible from the early Miocene
Nye Mudstone of Oregon. This new earbone is fairly strange looking (a
requirement for a platanistine, to be honest), with a long and strangely curved
anterior process. It shares several feature with modern
Platanista. This
specimen is middle Miocene in age, and indicates that platanistines used to
inhabit South America. One of the more interesting
aspects of this earbone is where it was preserved, environment wise: it’s found
with a number of terrestrial mammals, and occurs in fluvial-lacustrine deposits
with occasional marine influence. Fossils of freshwater cetaceans are
notoriously rare, and include some eurhinodelphinids material from Australia
(Fordyce, 1983; possibly platanistoid, according to research presented by Erich
Fitzgerald at the 2010 SVP meeting),
Ischyorhynchus from South
America (Cozzuol, 2010) and some
Pomatodelphis material
from Florida. Bianucci et al.
(2013) proposed the following sequence of events in the biogeographic history
of Platanista: platanistines evolved in the Pacific as primarily marine
odontocetes, with some becoming adapted to freshwater environments of the
Amazon basin during the middle Miocene, and finally dispersing and adapting to
freshwater environments of southern Asia (
Platanista), with all other
platanistines (as yet unknown) becoming extinct. Interestingly, where the
platanistine fossil occurs is now inhabited by the Amazon river
dolphin,
Inia geofrrensis. Bianucci et al. (2013) also noted that an
odontocete historically regarded as an iniid,
Ischyorhynchus, was
recovered as the sister taxon to
Platanista in the Geisler et al. (2012)
cladistic analysis. Further analysis is required to be sure, as many taxa jump
around depending upon the type of analysis and changes in coding, but it is a
tantalizing possibility that two platanistines may have been in Miocene rivers
of South America.
The fossil platanistine periotic (left), compared to a modern Platanista gangetica periotic (right). From Bianucci et al. (2013).
References cited
Bianucci, G. 2013. Septidelphis morii, n. gen. et.
sp., from the Pliocene of Italy: new evidence of the explosive radiation of
true dolphins (Odontoceti, Delphinidae). Journal of Vertebrate Paleontology 33:
722-740.
Bianucci, G., Lambert, O., Salas-Gismondi, R., Tejada, J.,
Pujos, F., Urbina, M., and Antoine P.O. 2013. A Miocene relative of the Ganges
River dolphin (Odontoceti,
Platanistidae) from the Amazonian Basin. Journal of Vertebrate Paleontology
33:741-745.
Cozzuol,
M. A. 2010. Fossil record and the evolutionary history of Inioidea; pp. 193–217
in M. Ruiz-García and J. Shostell (eds.), Biology, Evolution and Conservation
of River Dolphins within South America and Asia. Nova Science Publishers, New
York.
Fordyce, R.E. 1983. Rhabdosteid dolphins (Mammalia: Cetacea)
from the middle Miocene, Lake Frome, South
Australia. Alcheringa 7: 27-40.
Geisler,
J. H., S. J. Godfrey, and O. Lambert. 2012. A new genus and species of late
Miocene Inioid (Cetacea, Odontoceti) from the Meherrin River, North Carolina,
U.S.A. Journal of Vertebrate Paleontology 32:198–211.
Gutstein,
C. S., M. A. Cozzuol, A. O. Vargas,M. E. Su´ arez, C. L. Schultz, and D.
Rubilar-Rogers. 2009. Patterns of skull variation of Brachydelphis
(Cetacea,
Odontoceti) from the Neogene of the southeastern Pacific. Journal of Mammalogy
90:504–519.
Lambert, O., and Muizon, C. de. 2013. A new long-snouted
species of the Miocene pontoporiid dolphin Brachydephis and a review of
the Mio-Pliocene marine mammal levels in the Sacaco Basin, Peru. Journal of
Vertebrate Paleontology 33:709-721.
Muizon
C. de 1988. Les vertébrés fossiles de la Formation Pisco (Perou). Troisieme
partie: Les odontocètes (Cetacea, Mammalia) du Miocène. Editions Recherche sur les Civilisations 78: 1-244.
