Pinniped ancestry: the "Oligocene Seal" from South Carolina

One of the most fascinating aspects of the study of marine mammals are the origins of the land to sea transition in various groups. Often marine mammals are so adapted for marine existence, that it is difficult to identify what group (fossil or modern) they descended from. Such has been the case with the origin of whales, for example. While I'm certainly interested in cetaceans, there has been a ton written about them, and I'm currently more intrigued by pinnipeds.

While modern cetaceans don't necessarily look like their artiodactyl relatives, pinnipeds generally retain enough features that allows them to be readily identified as carnivoran mammals, straight down to the dog-like bark of California sea lions. Certainly, this should make identifying the terrestrial progenitors of pinnipeds an easier task. Or does it?

The prevailing opinion throughout much of the 20th century was that the Pinnipedia are diphyletic; the sea lions and walruses formed the "Otarioidea", who descended from the Ursidae, and the true seals (the Phocidae) descended from mustelid (or musteloid) ancestors. In 1973, Ed Mitchell and Richard Tedford described Enaliarctos mealsi from the Early Miocene Pyramid Hill member of the Jewett Sand in Kern County, California. Enaliarctos still bore an ursid-like shearing dentition, which is absent in all modern pinnipeds (which have generally homodont postcanine teeth). Enaliarctos was purported to be the common ancestor of the Otarioidea.

Later work (which at some point will be detailed on here) by Andre Wyss (UC Santa Barbara) and Annalisa Berta (San Diego State University), proposed a drastically different phylogeny of the pinnipeds, which suggested that pinnipeds were instead monophyletic, and that walruses (Odobenidae) were more closely related to the true seals (Phocidae), forming a new group, the Phocomorpha. Dozens of molecular phlyogenetic analyses have unequivocally supported pinniped monophyly, chucking the old diphyletic view out the window. The major tenets of the new studies are the following: 1) all pinnipeds descended from a common ancestor; 2) Enaliarctos is basal to the Otariidae, Odobenidae, and Phocoidea; 3) the Otarioidea is paraphyletic, as odobenids and desmatophocids are more closely related to the phocids; and 4) pinnipeds were derived from an ursid (bear) like ancestor sometime during the Late Oligocene, probably in the Northeast Pacific, where the oldest pinniped fossils are known.

Putative seal femora from the Late Oligocene of South Carolina, from
Koretsky and Sanders (2002).

In 2002, Irina Koretsky and Al Sanders reported on some partial femora from the Late Oligocene of South Carolina. According to Koretsky and Sanders, these femora are most similar to those of extant phocids, and list several features they share in common. However, they argued that the Late Oligocene age of these, as well as the occurrence of these in deposits of the Atlantic Coastal Plain, indicate that true seals were already present in the Atlantic Ocean when the very primitive Enaliarctos was just appearing in the Pacific Ocean. Thus, the monophyletic origin of pinnipeds was not supported by this fossil, and it appeared that true seals evolved in the Atlantic separately from the Otarioidea, which are (during the Oligocene and the Miocene) restricted to the North Pacific.

There are, of course, some issues with this study and its inherent implications, irrespective of the incomplete nature of the fossils. For starters, the fossil record is notoriously crappy; in fact, the Early Oligocene marine record is the poorest of all with regards to marine mammal fossils. Very few cetaceans are known from the Early Oligocene worldwide, for example, primarily due to the scarcity of marine rocks for this time, due to low sea levels caused by the Eocene-Oligocene climate crash. Pinniped fossils just "show up" after sea levels rise and deposit more marine sediments during the late Oligocene, in both the Pacific (Enaliarctos) and Atlantic (Phocid femora). So - it is entirely possible for even more primitive pinnipeds to be found in earlier sediments (or, as "lazarus taxa" in Late Oligocene rocks).

The completeness of these fossils requires additional scrutiny. Some of Koretsky's other work focuses on the major elements of the fore- and hind-limb of true seals (and lower jaws) but generally placing low importance on cranial material (Koretsky and Ray, 2008). Koretsky and Sanders (2002), however, did not compare these specimens with femora of Enaliarctos, or basal odobenids such as Proneotherium and Neotherium (or an unnamed basal odobenid described by Naoki Kohno early on, ~1990). Additionally, now that the putative stem-pinniped Puijila darwini has been described, it's femora should be compared with these specimens as well. These could very well turn out to belong to something more like Enaliarctos or Puijila that we don't yet have a record of in the Atlantic. However these fossils are interpreted, their Late Oligocene occurrence (if the provenance is accurate) is intriguing, and further field investigation of Late Oligocene sedimentary rocks of the Atlantic Coastal Plain (and elsewhere!) should be considered.

References:

Koretsky, I.A. and A.E. Sanders, 2002. Paleontology of the Late Oligocene Ashley and Chandler Bridge Formations of South Carolina, 1: Paleogene pinniped remains; the oldest known Seal. Smithsonian Contributions to Paleobiology 93: 179-183.

Koretsky, I.A., and Ray, C.E. 2008. Phocidae of the Pliocene of Eastern USA. In: C.E. Ray, D. Bohaska, I.A. Koretsky, L.W. Ward, and L.G. Barnes (eds.), Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virginia Museum of Natural History Special Publication 14: 81-140.

Mystery Pleistocene bones

Hey folks, I've been rather busy preparing for spring break (which I leave for in about 30ish hours), and I've actually gotten quite a bit accomplished - I finished preparing the whale cranium from the aforementioned post (there will be more follow up posts to that soon), and as of this afternoon, finished the second plaster/fiberglass cradle for the skull, which I will eventually be able to sink bolts through to keep the two halves together. Tommorrow I'll be attaching foam to the inside of the cradles, and then lugging the whole monstrosity to my (rather small) car. However, as this thing now weighs around 100 lbs or so, it is still only 1/3 of its original weight, so my car can do it.

Other than that, I submitted a manuscript this week (late monday night), about bite marks on some bones from the Purisima Formation; you'll hear more about it later on down the line.

Mystery marine fossils from the Pleistocene.

Anyway, as part of my mental preparation for spring break and looking at marine Pleistocene "rocks", here are some Middle Pleistocene fossils from a marine unit in California. I haven't been able to ID them, but they were originally ID'd by the collector as sea turtle claws. I'm naturally skeptical of that, but I'd like to see what you folks think, or if anyone can come up with something better (or, validate that ID).

Prep update: Purisima Formation mysticete skull 1

In 2005 I received a permit to collect vertebrate fossils from a locality in the Purisima Formation. This project lasted two summers, and resulted in the collection of dozens of shark teeth and vertebrae, calcified cartilage skate jaws, pinniped bones and teeth, several bird bones, odontocete and mysticete bones, including a handful of tympanics, a complete lower jaw of Herpetocetus, a porpoise cranium, and the big kahuna: a 300+ lb plaster jacket with some sort of mysticete skull inside. I wasn't really sure what it was in the field, although I suspected it was a balaenopterid (i.e. rorqual whale, like a Minke, Humpback, Fin, or Blue whale) on gut feeling alone.

The cranium prior to excavation, June 2005.

I first spotted the skull over thanksgiving break in November, 2004, with a flashlight just after sunset. I thought it was a skull at the time, but hadn't seriously thought about getting a permit yet. After I started the permit application process, I actually forgot entirely about the skull, and hadn't thought seriously about excavating it. When I finally stumbled across it again, I thought "why the hell not?" and up until the last few weeks, I've regretted the decision. Bottom line - I spent five months preparing the soft matrix from the ventral side, and in Spring 2006 I moved it to Museum of the Rockies, where I've been (intermittently, given my school schedule) preparing it with pneumatic airchisels since.

Day 1 of the excavation - we started at 5 in the afternoon, and stopped at about 10pm, digging by lantern, headlamp, and heavy metal.

Four years of preparing it with pneumatic tools - this skull was encased in a ridiculously hard concretion which only gets harder toward the center; concretions from elsewhere in the Purisima Formation are "nice" in that they 1) are of constant hardness throughout, and 2) the rock splits off the bone in these other cases. For this specimen, rock never splits off the bone, and in most cases I've had to prep all but 1-2mm of rock away and then grind it down with the airchisel, so some surfaces have definitely gotten a little scored. No matter, because all the sutures are visible, and it looks pretty nice anyway (unless you look at it with a hand lens). So shoot me; if I'd done it 'better', I'd have another year or two of prepwork to do. As it is, I'm leaving some matrix in the left temporal fossa, and elsewhere.

Vicki Jacklich and Liz Johnson (North Carolina State University) assist with the excavation, on Day 2.

A closeup of the skull, which is upside down. The rostrum is missing, and the vomer is the elongate bone pointing to the left. The frontal is the roughly triangular bone in the middle, the arcuate bone is the lateral crest of the supraoccipital shield, which slightly overhangs the temporal fossae (filled with matrix); the base of the squamosal is preserved at left.

I love surprises; this is a broken, partial basking shark (Cetorhinus maximus) gill raker found associated with the skull. I've found two more during preparation.

Here Vicki and I are putting the finishing touches on a preliminary jacket to protect some exposed bone.

Liz Johnson (NCSU) assisting with plaster jacketing.

The end of day 3: the skull is now trenched and tunneled under (see rock hammer) and ready for the rest of the plaster jacket to be applied.

Another view of the skull bearing pedestal after trenching and tunneling.

The completed top jacket prior to flipping on Day 4; I didn't have much time to take photos after this.

After the top jacket dried (which took over a day, due to the extremely humid air and my lack of jacketing experience), my temporary field assistant (and friend since 1st grade) Matt Berrini and I used what resources we had, and played 'egyptian' for a while: the ancient egyptians used earth for monument construction. For building pyramids, they built an earthen ramp; for raising obelisks, they had a chamber filled with sand; after the obelisk was raised by ropes, the obelisk was propped up by the sand in the chamber, which was slowly removed, allowing the obelisk to become upright with comparatively little further struggle. So - we built a fairly large 'ramp' of beach sand up to the jacket (as the skull was about 1 meter above the beach) so that the jacket did not break apart and collapse during flipping. In retrospect, the incredible density of the concretion probably indicated we could have probably let it fall anyway (although parts of the bones protruded from the concretion's boundaries, and may have broken).

Anyway, that's all for now, but soon I'll have another post about the prep process, and let you in on the last 5 years of my life.

Benthic feeding in basal mysticetes, part 2: Mammalodon, the freak

Sorry about the "short" delay; the last week or so has been pretty rough, between my Paleobiology course, getting sick, and administering/grading my first exam. But, all of that's over now, so I've been able to allot some time to this again.

My last post concerned paleopathology and histology of the "cetothere" Diorocetus hiatus, published by Brian Beatty and Alton Dooley last summer in Jeffersoniana. In it they posited that Diorocetus may have been a benthic feeder based on peculiar pathologies as well as an interesting pattern of rib osteosclerosis.

Comparison of the size of Mammalodon and a blue whale. From http://museumvictoria.com.au

There's been some buzz in the science blogosphere recently about the truly bizarre Australian toothed mysticete Mammalodon, the subject of a new monograph by colleague Erich Fitzgerald. I'll admit I've been anticipating this paper since Erich's talk at SVP in 2008 (the same session as my first ever SVP talk, you know, the one where no one could see any of my text). Some other blogs have covered Mammalodon already, but mostly just some of the stuff you already hear in the news; ya, it was a mysticete with teeth (all toothed mysticetes look pretty weird), and it likely a benthic suction feeder, so what? There's far more interesting 'meat' in the paper, if you will, that someone who wasn't a mysticete worker and therefore didn't have time or interest to read 110 pages might not pick up on. Okay, I didn't read the description, but I've read the in depth discussion.

The skull and dentary of Mammalodon. From http://museumvictoria.com.au

Mammalodon colliveri was originally described in 1939 from the Oligocene Jan Juc Formation of Australia, although it was not identified as a toothed mysticete until the 1980's when Ewan Fordyce (U. Otago, New Zealand) undertook additional preparation of the fossil. In 1966, Doug Emlong (an extremely prolific and gifted amateur collector) described a peculiar toothed cetacean he discovered in the Oligocene of Oregon: Aetiocetus cotylalveus. Emlong thought it was an archaeocete, but Leigh Van Valen soon after recognized several mysticete features. Toothed mysticetes may seem odd at first, but when you think about it - 1) teeth are primitive for cetaceans and indeed mammals, and 2) they had to eat with something before baleen evolved. Mammalodon shares many similarities with Janjucetus, including a short rostrum, short triangular supraoccipital, and large orbits; however, Mammalodon is distinct in having laterally oriented teeth, and a relatively flat rostrum.

Comparison of Mammalodon (left) and Janjucetus (right). From Fitzgerald (2010).

Mammalodon has some strange features - a short, flat snout, large, upward facing orbits, large foramina on the maxilla, upper teeth that project laterally, and peculiar wear facets on the lower dentition that look like they were filed down at the same time (so to speak) - i.e. they share wear facets oriented on one single plane. Some authors have interpreted the denticulate teeth of basal mysticetes like Llanocetus and Mammalodon to have had a role in filter feeding, much like the teeth of the extant crabeater seal (Lobodon carcinophagus). Fitzgerald (2010) rejected this interpretation, as he did earlier (2006) for the tooth morphology of Janjucetus. Based on the probable presence of hyaline cartilage in the jaw joint of Mammalodon (as opposed to fibrocartilage in lunge feeding balaenopterids), Fitzgerald also rules out lunge filter feeding. Bony correlates of baleen, nutrient foramina, are absent on the palate of Mammalodon, suggesting that it did not possess baleen (such as later diverging aetiocetids - I'll talk more about those next time). Taken in full, few features of Mammalodon suggest filter feeding, rather than raptorial/pierce feeding (aka, biting stuff). Aside from this, some of the features unique to Mammalodon such as the very short rostrum, large maxillary and mandibular foramina, and extreme tooth wear suggest an alternative behavior. Typically, large foramina indicate improved blood flow and innervation to a region; for example the tactile whiskers of pinnipeds are correlated with a greatly enlarged infraorbital foramen (a pinniped synapomorphy), relative to "fissipeds". In this case, the short rostrum, probable highly innervated oral region, and wear most likely due to substrate interaction, all point towards benthic suction feeding. Additionally, the up-and-forward facing orbits would have conferred some degree of binocular vision, similar to the walrus and the bizarrer walrus convergent odontocete, Odobenocetops.

Various archaeocetes (top row), mammalodontids (second row) and aetiocetids (bottom two rows). From Fitzgerald (2010).

Most interestingly are the implications of the phylogenetic hypothesis of this study on the evolution of feeding in the mysticetes, relative to other recent hypotheses. Raptorial feeding is primitive for cetacea as a whole, including basal mysticetes. Interestingly, Llanocetus, Janjucetus, and Mammalodon all form a southern toothed mysticete clade, and Fitzgerald (2010) lumped Janjucetus and Mammalodon into the family Mammalodontidae, scrapping the family Janjucetidae he proposed earlier (2006). Llanocetus exhibits no adaptations for suction feeding, although Janjucetus does, suggesting that suction feeding is derived for this clade, but not a typical feature of basal mysticetes in general. Thus, this suggests that whatever feeding mode the mammalodontids employed was likely not representative of early mysticetes, contra Fitzgerald (2006).

Feeding evolution in baleen whales. From Fitzgerald (2010).

What the hell is Llanocetus, anyway? Llanocetus is perhaps the most interesting and strange described/undescribed cetacean out there. Well, what does that mean? Llanocetus was collected from the late Eocene La Meseta Formation of Antarctica. Yes, the Eocene of Antarctica, my friends. Why is that particularly awesome? The Eocene is dominated by archaeocetes, and elsewhere on earth during the late Eocene fantastic critters like Basilosaurus and Dorudon were kicking ass and taking names. And Llanocetus is the earliest known mysticete, let alone record of the Neoceti. The original description of Llanocetus denticrenatus by Ed Mitchell (1989) includes a maxillary fragment, mandible fragment, and an endocast - overall, marginally crappy. Later on, Ewan Fordyce revisted the site and collected what he initially thought was an archaeocete, and it even included (apparently) vestigial hindlimb elements (plural), similar to the condition in basilosaurid archaeocetes (which are not too phylogenetically divorced from Llanocetus, after all). This turned out not only to be a toothed mysticete, but the remainder of the holotype skeleton. My guess is that whoever originally collected the holotype scraps was freezing their ass off and said "to hell with it" and left. However, even though a cast of the skull is now on display at the USNM, the rest of the holotype is not yet described, and the dotted outline provided by Fitzgerald (2010) is the most informative figure ever published of this rather interesting beast. Anyway, I'll be pretty excited whenever the description gets published (much of the above information was presented by Fitzgerald, 2010, in his discussion of taxa used for the cladistic analysis).

The next post will (hopefully) be less-rambling, and will detail the transition from teeth to baleen in archaic mysticetes.

References:

Fitzgerald, E.M.G. 2006. A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proceedings of the Royal Society B: Biological Sciences
273: 2955–2963.

Fitzgerald, E.M.G. 2010. The morphology and systematics of Mammalodon colliveri (Cetacea:Mysticeti), a toothed mysticete from the Oligocene of Australia. Zoological Journal of the Linnean Society. 110p.

Benthic feeding in basal mysticetes, part 1: paleopathology of a Miocene "cetothere"

The last couple years have been relatively good for cetacean paleontology; we saw the description of the protocetid Maiacetus (which I still have to cover...) about a year ago, the rediscription of "Balaenoptera" gastaldii, now recognized to be a fossil gray whale, the fantastic analysis done by T.A. Demere et al. regarding the presence of baleen in toothed mysticetes, Larry Barnes' Albireo monograph, the odontocete cranial lexicon by Jim Mead and Ewan Fordyce, the taphonomic study of the Sharktooth Hill Bonebed by Nick Pyenson and colleagues, Frank Whitmore and L. Barnes' Herpetocetus monograph, Steeman's (2009) paper on Uranocetus, Brian Beatty and Alton Dooley's paper on paleopathology in the Carmel Church Diorocetus, and most recently Erich Fitzgerald's paper on Mammalodon, among many others which I've probably failed to remember.

For the purposes of the next couple posts, I'll be focusing on basal mysticete feeding, and will be discussing Beatty and Dooley (2009), Fitzgerald (2010), and Demere et al. (2008).

The pathologic left dentary of Diorocetus from the Carmel Church Quarry in Virginia, From Beatty and Dooley (2009). The pathologic fracture is directly below the 'c' in 10 cm.

For those of you who pay close attention to Alton Dooley's blog "Updates From the Vertebrate Paleontology Lab", Dooley does quite a bit of fieldwork at the Carmel Church Quarry, an exposure of the middle Miocene Calvert Formation. The Calvert Formation in Maryland and Virginia is famous among amateur paleontologists and fossil collectors for the stunning abundance of easily collected fossil shark teeth. The Calvert Formation is also famous for its incredible cetacean fossil assemblage - primarily chronicled by Remington Kellogg, the father of marine mammal paleontology (although there have been a number of papers recently on Calvert Fm. and Chesapeake Group cetaceans). One of the recent discoveries at Carmel Church is a beautiful skeleton of the archaic mysticete Diorocetus. This skeleton includes a complete skull (which initially was fragmented to hell, but Dooley and the VMNH have managed to put all of the pieces back together, and it looks pretty damn nice), dentaries, anterior vertebral column, and nearly complete set of ribs.

Fracture and pathology in the left dentary, from Beatty and Dooley (2009).

The left dentary was found to have an odd fracture in it; the two parts didn't match up very well, and it did not appear to be a post-depositional fracture, like most of the fractured material at Carmel Church (it is not clear if the jumbling and fracturing of bones is biostratinomic or diagenetic - e.g. peri- or post- burial). Additionally, a callus of bone was identified around this fracture, and can be seen well in x-rays (above). Additional pathologies were noted in the anterior tips of the premaxillae, and the left squamosal, which was significantly less dense and more porous than the right (potentially due to decreased stress during post-injury feeding?). Most interesting is the fact that although the callus formed, the fracture never healed, suggesting repetitive use that kept the bone from healing, i.e., the anterior and posterior portions of the dentary remained as separate elements until death, allowing some motion at the fracture site.

What could cause this sort of a fracture? The authors indicate the most common cause of these sorts of injuries in extant mysticetes are collisions with ships and boats - which obviously did not exist in the middle Miocene. Other possibilities include predation, agonistic (violent) intraspecific behavior, and a collision or impact with seafloor topography. If this is a case of predation, then our friend survived, given the amount of healing (i.e. callus formation). Agonistic behavior among mysticetes is poorly documented, and are largely restricted to injuries on the order of cuts and scrapes. Otherwise, the authors conclude, lies the chance that this injury was caused by an impact with the seafloor, or submarine outcrop (i.e. much of the California coastline I'm used to has rocky points and sea stacks and a topographically complex seafloor with many submarine exposures of rock). Apparently, injuries of this sort are the most commonly observed trauma on dead gray whales, which are benthic suction feeders.

Before I continue, I'll add a quick note about mysticete feeding. Among modern mysticetes, there are three observed modes of feeding: Lunge/engulfment feeding, suction feeding, and skim/ram feeding. Lunge or engulfment feeding is mostly utilized by balaenopterid whales (e.g. Blue, Fin, Sei, Minke, and Humpback whales), and is characterized by the whale opening its mouth and engulfing clusters/'schools' of nektonic organisms (i.e. krill, fish, etc.). The mouth is closed, and water is actively 'pushed' out of the baleen plates (I can't remember if both the tongue and throat are used for this action, or one or the other). Skim feeding is employed by balaenids (and the sole existing neobalaenid, Caperea, the pygmy right whale), and consists of the whale slightly opening its mouth while swimming forward; nekton prey-rich water enters the oral cavity, and during forward movement, water flows passively out of the oral cavity through the baleen, which traps the poor critters inside the mouth; this feeding is often at or near the surface. Benthic feeding, on the other hand, is only observed in the gray whale (Eschrictius robustus), which will filters through muddy substrate; fine sediment is entrained in suspension, and can escape with water through the baleen, trapping benthic organisms (i.e. amphipods) in the oral cavity. The major problem is that the most basal known fossil mysticetes retained teeth, and did not yet have baleen. More on that later, though.

Cross sections of mysticete ribs; A-B is the new Diorocetus specimen; C - undescribed basal edentulous mysticete, Oligocene, Oregon; D-E-toothed mysticete Aetiocetus cotylaveus; F & I - balaenopterids (latter is Eobalaenoptera); G-H - cetotheriid (sensu stricto) Metopocetus; K - Diorocetus hiatus; L- Balaena ricei. All of these have osteoclerotic ribs, with the exception of Eobalaenoptera and Balaena ricei, which are part of the mysticete crown group.

Another interesting feature Beatty and Dooley (2009) noted was the osteosclerotic condition of the ribs in Diorocetus. In contrast, extant cetaceans have postcranial bones that are osteoporotic (yes, like menopausal women). For the purposes of this discussion, there are two types of bone: cancellous, and cortical (spongy and dense, respectively; there are many other types, which I won't go into here; read papers by de Ricqles and Horner for more info on paleohistology). Cortical bone (or the cortex) is the strong, outer portion, while cancellous bone is the very spongy middle part. Osteosclerosis refers to increasing bone density by adding cortical bone toward the center of the bone, making the cancellous inner portion thinner. Pachyostotic bone is where cortex is increased outward, giving the bone an 'inflated' look - sirenians have pachyostotic (and osteosclerotic) bones. Osteoporosis simply refers to bone that is very porous, and generally lense dense - this is simply a condition; in cetaceans it is 'normal', but in adult women it is a bad condition which can lead to fractures. Osteosclerosis, on the other hand, can act as a sort of 'bio-ballast' adaptation for maintaining (or simply attaining) neutral bouyancy - most terrestrial vertebrates are positively bouyant, especially in seawater. Champsosaurs, I just learned in Jack Horner's class, have retained super-dense embryonic bone into the adult stage as a ballast adaptation.

Beatty and Dooley (2009) observe that Diorocetus hiatus is one of the last mysticetes to retain osteosclerotic bone, and that it may be related to bouyancy problems associated with benthic feeding. Indeed, osteosclerotic bone is a plesiomorphic feature among not only mysticetes, but is also characteristic of pelagic archaeocetes as well (I am not referring to the clade Pelagiceti, by the way). So, it is certainly possible that this is an adaptation for benthic feeding. However, it is also possible that this is a case of phylogenetic inertia, similar to the retention of an enlarged mandibular foramen in mysticetes.

The nature of the likely cause of the mandibular injury may also suggest benthic feeding as well (unless this was a freak accident; i.e. a lunge feeding whale impacting the seafloor). While the repetitive feeding behavior that kept the fracture from healing may have been caused by benthic feeding, *if* Diorocetus had been a lunge feeder, the incredible stresses experienced by mysticete dentaries during this action would certainly keep the fracture from healing. In any event, taken as a whole, the benthic feeding idea is very intriguing, and raises some very interesting questions regarding the primitive mode of feeding by baleen-bearing mysticetes. I understand that the authors have received criticism for some of the more speculative ideas in the paper, you'll find none from me; this study brings up some very interesting ideas, and I'll be covering more on the topic of benthic feeding on my next post, regarding the enigmatic toothed mysticete Mammalodon.

Also see:
Alton Dooley's blog post about the article, and Brian Beatty's post as well.


Beatty, B.L. and A.C. Dooley. 2009. Injuries in a mysticete skeleton from the Miocene of Virginia, with a discussion of bouyancy and the primitive feeding mode in the Chaeomysticeti. Jeffersoniana 20:1-28.

Bioturbation and ash beds in the Purisima Formation

This is really just a collection of annotated photos I took over winter break, and are sedimentological and ichnological in nature. These are all photos taken of Purisima Formation exposures in San Mateo county. During the winter, more intense storm activity cleans off the coastal cliffs and makes examining trace fossils, sedimentary structures, and bed geometry an easier task. In some cases, the most beautiful trace fossils and sedimentary structures are associated with ash beds. Above is part of a very thin ash bed (8-30cm thick - the ligher colored sediment) that has almost been destroyed by bioturbators (by the way, bioturbation is the disruption of primary sedimentary fabric by burrowing organisms/infauna). In fact, for most of its exposure it has been pierced by burrows (where the ash has been displaced into the burrow fill) that it looks like a dotted line. Above shows a U-shaped burrow; I can't remember what taxon this is - because some of them (i.e. Diplocraterion?) are defined based on spreiten (laminae within the burrow) inside the 'U'.

Here yu can see how little of the original bed is left; much of the ash has been introduced as burrow fill, and mixed and diluted with regular sediment (fine-very fine silty sandstone in this case), hence the blotches looking slightly less 'pure' than the primary ash.

A couple hundred feet away this bed thickens and isn't completely chewed up by bioturbation, enough so that you can see original sedimentary fabric within. In this case, it is a mix of swaley cross stratification (a small-scale version of hummocky cross stratification) and climbing ripples. Both of these sedimentary structures indicate uni- or bi-directional flow with a relatively high rate of sedimentation, i.e. sediment is just dumping out of suspension. This can happen during hyperpycnal flow - often occuring as a dense plume of sediment rich water introduced into the ocean from a river mouth, say after a big storm.

This ash bed, on the other hand, is huge. There's a reason I don't have a scale bar; this is about a 5-10 meter thick ash bed. This has a completely different set of weird features - some pretty incredible soft sediment deformation. These look like giant scale ball and pillow or loading structures.
This cliff right here is about 200 feet high, and we're looking at about 1/3 of it or so. These S.S.D. features continue for the entire outcrop length of this bed. The thinner ash bed featured above is visible in the very bottom of this photograph. The intense loading features here could, of course, be caused by a relatively rapid influx of a LOT of ash to the seafloor, which could lead to liquefaction of more typical sediment at the former (i.e. pre eruption) sediment-water interface (i.e. sea bottom), and leading to the big pillow-like lobes of ash, and the long upward pointing 'fingers' of sand.

Uranocetus and hearing in mysticetes

Hey Folks, Sorry about the delay; I realize its been over a month since I last posted anything. Winter break was not exactly relaxing, and the parts that neared relaxation were spent doing fieldwork (which definitely yielded some interesting material). In other news, my first technical paper has been tentatively accepted for publication by the UCMP-published journal PaleoBios; I'm approximately 99% done with revisions at this point, so you'll hear more about it after it's in press.

Recently two mysticete related papers have been published - Erich Fitzgerald's monograph on the truly bizarre Mammalodon colliveri, which I'll cover later, and M.E. Steeman's (2009) thought provoking paper naming the new "cetothere" Uranocetus from the Miocene of Denmark and its implications for mysticete hearing.

The cranium of Uranocetus, from Steeman (2009).

First off, "cetotheres" are a wastebasket group of generalized archaic baleen whales that don't fit nicely in modern families, although Bouetel and Muizon (2006) have redefined the Cetotheriidae sensu stricto as a small group with some very strange cranial features, including my personal favorite, Herpetocetus. Most other cetotheres (Cetotheriidae sensu lato) were placed into newly named families (Pelocetidae, Aglaocetidae, and Diorocetidae) which were sister taxa to the Balaenopteridae all included in her concept of the Balaenopteroidea (but not in the concept of the Balaenopteroidea advocated by Demere et al. 2005, which is Eschrictiidae + Balaenopteridae). Bottom line is Uranocetus is some kind of stem baleen-bearing mysticete, no matter whose phylogeny you use. The dentary of Uranocetus (from Steeman, 2009).

Interestingly, while it is placed rather close to Balaenopteridae, it still retains a large mandibular foramen, a plesiomorphic feature for mysticetes. The mandibular foramen is very small in extant mysticetes, but extremely large in odontocetes, so much that the posterior portion of the dentary is a thin bony shell (the "pan bone") that houses the mandibular fat pad. The lateral margin of the dentary is extremely thin, so that high frequency sounds can pass through without significant volume loss (Nummela et al. 2007, Steeman 2009). High and mid frequency sounds pass through this, and are then channeled up through the mandibular fat pad and up to the tympanic plate; in odontocetes, this is more or less a functional analog of the external ear pinna. And, by the way, all these strange auditory features are adaptations for allowing directional hearing underwater; otherwise terrestrial mammals hear via bone conduction hearing (sound travels faster in water, and the mammalian body is roughly as dense as the surrounding aqueous medium), and sounds more or less arrive at each ear too quickly to discern the direction. Cetaceans have separated their ear bones (petrosal, tympanic, and ossicles) from the temporal bone and surrounded them by sinuses to isolate these complexes from the skull to hear directionally. While this was initially thought to be an adaptation for hearing high frequency sounds and thus an adaptation for echolocation (a capability restricted to the odontoceti, and associated with high frequency sounds), recent research has identified the pan bone/enlarged mandibular foramen (i.e. bony correlates of the mandibular fat pad) in many archaeocetes, including Ambulocetus, remingtonocetids, protocetids, and basilosaurids (Nummela et al. 2007) as well as many archaic toothed- and toothless mysticetes, such as Aetiocetus weltoni, Mammalodon, Eomysticetus, and even Herpetocetus. This led Nummela et al. (2007) to reason that, since neither archaeocetes or mysticetes have any anatomical features associated with echolocation, that this feature is probably instead related to underwater hearing in general, and not just echolocation.

Dentaries of various archaic mysticetes and an archaeocete, from Fitzgerald (2009).

The fact that most basal mysticetes have an enlarged mandibular foramen suggests that this is a feature inherited from basilosaurid ancestors. Interestingly, modern mysticetes are adapted for hearing low frequency sounds, which pass through dense bone without significant volume loss. While Uranocetus has a large mandibular foramen, the lateral wall is too thick to be useful for hearing anything aside from low frequency sounds (which Uranocetus is adapted to hear based on its cochlear structure; Steeman 2009). The exact same thing is seen in Herpetocetus, which is also adapted for low frequency hearing, but has a large foramen with a thick lateral wall. This suggests that at least in these later diverging taxa, that the large mandibular foramen was a vestigial feature perpetuated by phylogenetic inertia.

Lateral aspect of a (not so typical) mysticete (Eshrichtius robustus, the Gray Whale) skull and dentary in articulation, from Johnston et al. (2009).

Steeman (2009) reasoned that the mandibular foramen decreased in size to strengthen the dentary due to the intense forces involved during feeding. Above shows a gray whale skull and mandible in articulation, just to give you an idea of how strange the mysticete feeding apparatus is (exclusive of baleen). In any event, I've been thinking about this quite a bit recently, and got to add a (very short) synopsis of this in my manuscript revisions, but you'll hear about that soon enough.

References-
Deméré, T.A. and A. Berta (2008). Cranial anatomy of the toothed mysticete Aetiocetus weltoni and its implications for aetiocetid phylogeny. Zoological Journal of Linnean Society, 154(2): 308-352. PDF

Deméré, T.A., A. Berta, and M.R. McGowen. 2005. The taxonomic and evolutionary history of fossil and modern balaenopteroid mysticetes. Journal of Mammalian Evolution 12:99-143.

Fitzgerald, E.M.G. 2009. The morphology and systematics of Mammalodon colliveri (Cetacea:Mysticeti), a toothed mysticete from the Oligocene of Australia. Zoological Journal of the Linnean Society 110p.

Johnston, C., T. Deméré, A. Berta, J. St. Leger and J. Yonas. 2009. Observations on the musculoskeletal anatomy of the head of a neonate gray whale (Eschrichtius robustus). Marine Mammal Science PDF

Nummela, S., J.G.M. Thewissen, S. Bajpai, T. Hussain, and K. Kumar. 2007. Sound transmision in archaic and modern whales: anatomical adaptations for underwater hearing. Anatomical Record 290:716-733.

Steeman, M.E. 2009. A new baleen whale from the late Miocene of Denmark and early mysticete hearing. Palaeontology 52 :1169-1190.