Was Pelagiarctos a "killer" walrus? Part 3: new specimen from Orange County

In early spring 2011, just as I was finishing up my master’s degree at Montana State, I received an email from Tom Deméré, the paleontology curator of the San Diego Natural History Museum, inviting Morgan and I to study a new fossil of Pelagiarctos from the “Topanga” Formation. Fortunately, I would get a chance to examine it closely in person soon afterward – in June, I would be attending and presenting my master’s taphonomy research at the 6^th triennial conference on secondary adaptations of tetrapods to life in the water (usually abbreviated SATLW or simply referred to as the aquatic tetrapods conference), which was being hosted by Tom Deméré and Annalisa Berta at San Diego State University and the museum.

Although I had successfully delivered my master’s defense presentation and graduated without a hitch a month and a half prior, I was still nervous to give my presentation because it was in front of a totally different audience – technically, the conference was about secondary adaptations, and I was giving a talk on taphonomy. However, I tooled it towards what we can reasonably infer from the marine vertebrate fossil record, including about exactly how aquatic organisms were based on their preservation – which, I concluded at the time was not much. The talk was also fairly long; although I had 18 minutes to speak, which is fairly long, I had not had the time to shorten it. 36 hours before driving down I-5, I was on the beach at Bolinas prospecting with Dick Hilton when I got a funny phone message from Tom ‘asking’ me if it would be okay to move my talk to the first day; so I said my goodbyes to Dick and raced home down Highway 1 so I could spend a day and a half polishing the presentation off. And then worried half the drive down I-5 that I didn’t shorten it enough.

 

  

A comparison of the new Topanga Formation specimen (A) and the holotype (B) of Pelagiarctos.

 

The talk went without a hitch, and later in the conference Morgan and I were able to sequester a few hours in the SDNHM type room to examine the new specimen of Pelagiarctos. It consisted of a fragmentary pair of mandibles, with the left mandible being nearly complete and having much of its dentition (missing only a premolar, the two molars, and an incisor). Unlike the type specimen from Sharktooth Hill (which Morgan and I got a chance to examine in person at LACM in January 2012), these mandibles were not fused together at the symphysis (intermandibular joint). Symphyseal fusion is not common in modern pinnipeds, where it is restricted to the modern walrus. I’ve also seen pathologic (diseased) mandibles of modern otariids where, due to some bone disease, the symphysis has fused along with a large mass of bone at the chin, accompanied by incisor and canine loss.

 

  

 The left mandible of the Topanga Formation specimen of Pelagiarctos. From Boessenecker and Churchill (2013).

The teeth present in the new specimen confirm that the large teeth referred to Pelagiarctos thomasi by Barnes (1988) were correctly referred. It’s not so surprising, since you could predict the mandible shape from the teeth: they are like giant versions of Neotherium teeth, and the mandible is like a giant Neotherium jaw. I never really doubted Barnes’ identifications – but it was nice to confirm them. The mandible of this new specimen is damned huge – it’s wide, deep, with a short toothrow. The canines are robust, again with grooves on the lateral and medial surfaces, giving the canine a figure-eight shape in cross section. The premolars are large, primitively retaining what’s called the metaconid cusp; most modern pinnipeds have teeth that are unicuspid (single cusp, usually conical), with small anterior and posterior accessory cusps in otariids. The main cusp (on lower postcanine teeth) is called the protoconid. The anterior and posterior cusps are the remnants of the paraconid (anterior) and hypoconid (posterior) cusps. The metaconid cusp is a fourth cusp which is present in many primitive pinnipeds, such as the early enaliarctines, as well as Proneotherium, Neotherium, and Pelagiarctos. The metaconid is located just behind the principal cusp (protoconid). Many modern phocids primitively retain all four cusps – the harbor seal is an excellent example. The crabeater seal additionally bears a number of extra “neomorphic” (=new or novel structure) cusps on the posterior tooth crowns, which are used for filter feeding.

 

 

The dentition of the Topanga Formation specimen. From Boessenecker and Churchill (2013).

More details of the dentition of the Topanga Formation Pelagiarctos, from Boessenecker and Churchill (2013); I had a fun time drawing the medial view of those teeth.

In addition to having these primitive features, a couple of new features not seen in earlier walruses are present: a lingual cingulum with small little “crenulations” forming a sawtooth type pattern, and the presence of a labial cingulum. A cingulum is a thickened ridge of enamel at the base of a tooth crown. Pelagiarctos is the only walrus with a labial (cheek side of the tooth) cingulum, and only one other walrus has a crenulated lingual (tongue side of the tooth) cingulum – the late Miocene walrus Imagotaria downsi. At this point the uninitiated reader might ask ‘what exactly makes this thing a walrus?’ The truth is, for the earliest known walruses, the only synapomorphies allowing identification as a member of the Odobenidae (walrus family) are skull features. Many of the features of the known specimens of Pelagiarctos appear in some sea lions – such as a mandible that is deepest near the canines. Although the fossils don’t have any specific features that are diagnostic at the family level – several features of the dentition are only found in early diverging “imagotariine” walruses. The Imagotariinae was a subfamily named by Ed Mitchell and used extensively in various papers by Barnes, but as pointed out by several studies over the past two decades it is a paraphyletic assemblage of primitive walruses. Nevertheless, it is a useful vernacular term; imagotariines are sea lion-like with primitive dentitions, and ranged in size from harbor seal size (Proneotherium) to elephant seal size (Pontolis magnus).

 

 

 

Comparison of walrus mandibles, including the Topanga Fm. Pelagiarctos specimen, Imagotaria downsi from the late Miocene Santa Margarita Sandstone of Santa Cruz County, Proneotherium repenningi from the early middle Miocene Astoria Formation of Lincoln County, Oregon, and Pontolis magnus from the late Miocene Empire Formation of Coos County, Oregon. From Boessenecker and Churchill (2013).

 

Because our new specimen was more complete than the holotype, we were able to include Pelagiarctos within a phylogenetic analysis for the first time. Previous analyses did not use many mandibular characters, so at first we constructed a matrix which focused on mandibular and dental characters, and only used pinniped species known by lower jaws (i.e. we didn’t include some species for which jaws were unknown). This meant we didn’t initially include the early walruses Prototaria and Pseudotaria from Japan. We originally did this because we felt we’d get more accurate results than if we included Pelagiarctos in an analysis where it couldn’t be coded for any cranial characters – it was a reasonable hunch at first. One of our reviewers suggested we use a more comprehensive dataset, so we merged our data set with that of Naoki Kohno’s (2006) analysis for his Pseudotaria muramotoi paper. We ended up with fantastic results, and better support for some of the relationships.

 

 

Cladograms from Deméré (1994), Kohno (2006), and our new study showing the varying position of Pontolis (underlined in red).

Most of the relationships in our analysis are consistent with previous studies like Deméré (1994), Deméré and Berta (2001), and Kohno (2006), with one exception. In Deméré (1994), Pontolis magnus grouped as a dusignathine walrus, and closely related to Dusignathus itself. In Kohno (2006), Pontolis instead formed a sister taxon relationship with Imagotaria. The Imagotaria-Pontolis clade is only one node below the dusignathines, so admittedly it is not a far distance. In our analysis, however, Pelagiarctos formed a sister taxon relationship with Imagotaria instead, based on two features: grooved canines, and a crenulated cingulum. Neither of these features are present in Pontolis. Instead, Pontolis plotted out as the last diverging “imagotariine” and the sister taxon to the dusignathine + odobenine clade – in other words, intermediate between the phylogenetic hypothesis of Deméré (1994) and Kohno (2006). It’s sort of a compromise between the two, in a way. Obviously, there are more cranial characters that need to be explored and new walruses to describe, so there is clearly further scope for a more comprehensive study of walrus phylogenetics, which is in the early planning stages.

Next up: the dramatic conclusion to this series on the new publication, focusing on the feeding ecology of Pelagiarctos, and the life restoration.

References:

Barnes LG (1988) A new fossil pinniped (Mammalia: Otariidae) from the middle
Miocene Sharktooth Hill Bonebed, California. Contributions in Science,
Natural History Museum of Los Angeles County 396: 1–11.

Boessenecker, R.W. and M. Churchill. 2013. A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos. PLoS One 8(1) e54311. doi:10.1371/journal.pone.0054311.

Deméré TA (1994) The family Odobenidae: a phylogenetic analysis of living and
fossil forms. In: Berta A, Deméré TA, editors. Contributions in Marine Mammal
Paleontology honoring Frank C Whitmore, Jr: Proceedings of the San Diego
Society of Natural History. 99–123.

Kohno N (2006) A new Miocene odobenid (Mammalia: Carnivora) from
Hokkaido, Japan, and its implications for odobenid phylogeny. Journal of
Vertebrate Paleontology 26: 411–421.

Was Pelagiarctos a "killer" walrus? part 2: new publication in PLOS

Last thursday my new study which I collaborated with Morgan Churchill (University of Wyoming) on was published in PLOS One, regarding new fossil material of Pelagiarctos from the "Topanga" Formation of Orange County, California. There has been quite a bit of buzz about it, and it's gotten a surprising amount of media attention. To summarize it in one sentence - we describe the new material, reanalyze the paleoecological hypothesis of Barnes (1988), concluded it was not a specialized macrophagous predator, and conducted a phylogenetic analysis of the Odobenidae (walruses).

Brian Switek was kind enough to cover it on Laelaps, which you can see here. Also, there is an author spotlight on the PLOS EveryONE blog, viewable here.

Part of the new specimen of Pelagiarctos, which Tom Deméré (San Diego Natural History Museum) invited us to study.

Life restoration of Pelagiarctos, which I did last fall in my spare time. More on how I put this together at a later point.

This has been covered by a ton of news outlets, including the University of Otago news service, ScienceDaily, LiveScience, MSNBC, NBC, Huffington Post, Cosmos, Yahoo News, and even Fox News (perhaps a dubious honor...).

The full article can be viewed here at Plosone.org.

Boessenecker, R.W. and M. Churchill. 2013. A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos. PLoS One 8(1) e54311. doi:10.1371/journal.pone.0054311.

And, the morphobank account is available here.

Was Pelagiarctos a “killer” walrus? Part 1: Sharktooth Hill Pinnipeds

As a teaser for a forthcoming paper by Morgan Churchill and myself, I thought I’d introduce a (short) new series of posts (fewer than the last series, I promise). As the publication is not out quite yet, I thought I could at least give an introduction to the extinct “killer” walrus from the Sharktooth Hill Bonebed.

The Sharktooth Hill bonebed in Kern County, California is a widespread horizon within the Round Mountain Silt member of the Temblor Formation. It’s exposed near Bakersfield, California, and is middle Miocene in age. It’s approximately 10-50 cm thick, generally lacks calcareous invertebrate fossils, but is extraordinarily rich in teeth and bones of sharks, bony fish, birds, sea turtles, pinnipeds, dolphins, sperm whales, baleen whales, and occasionally sea cows, desmostylians, and terrestrial mammals. I visited Sharktooth Hill several times as a high school student, trying to find “local” vertebrate fossils – digging well through the night in the trenches with tiki torches and a headlamp. At many localities frequented by amateur fossil collectors, the bonebed is exposed on a hillside and a large linear scar follows the position of the bonebed, dug out by collectors removing overburden to get to the fossil layer. Amateur fossil collectors have done so much digging that a trench reminiscent of World War 1 battlefields encircles many hills in the region where the bonebed is exposed. Although some collectors will spend days at a time digging through overburden - admittedly backbreaking work - some decide to risk it and tunnel into the trench to get at the bonebed. Some collectors have paid for this tactic with their lives: on my first visit in 2002, a cross was placed at one of the localities where a collector had tunneled in about ten feet and was killed when the hillside slumped down onto him; it took the authorities several days to dig out his body. The rest of the Round Mountain Silt is mostly barren with respect to vertebrate fossils, not only explaining the attention given by collectors to the bonebed itself – but also suggesting a “unique” environment temporarily persisted in order to concentrate vertebrate remains. A number of strange biologic explanations have been offered, including red tides, extensive shark predation, and even a marine mammal calving ground. Several authors have quite rightly scrutinized these biologic explanations, and have suggested sedimentologic processes as a cause (Mitchell, 1966; Prothero et al., 2008; Pyenson et al., 2009). These studies have specifically suggested that a depositional hiatus (slowdown in the accumulation rate of sediment) permitted marine vertebrate remains to be concentrated on the seafloor. I have some minor taphonomic reservations, but those are best discussed another day.

 

One of the Sharktooth Hill localities, wife for scale.

According to Barnes (1976), the Sharktooth Hill bonebed is the most extensively studied and richest marine mammal locality in the eastern North Pacific; a faunal list compiled by amateur collectors can be viewed here, and it includes roughly 140 vertebrate taxa. Some of the species on the list are not yet described or published (“Neotherium ernsti”, for example) and other taxa are based on old identifications and may not be borne out in the long run (aff. Herpetocetus). Regardless of issues pertaining to the taxonomic identity of some fossil vertebrates, the ballpark number is probably accurate. It’s also fairly spectacular: I recently tallied up fossil vertebrates from the Purisima Formation, and there are roughly 70 taxa present – still impressive as hell, but not quite as gargantuan as Sharktooth Hill. Depending upon whose publication you look at, there are anywhere from seven (Barnes, 1972; Barnes and Hirota, 1995) to four pinnipeds present (Deméré et al., 2003). Papers by L.G. Barnes and colleagues list several desmatophocids, including Allodesmus gracilis, Allodesmus kelloggi, Allodesmus kernensis, Desmatophocine B, and Desmatophocine C in addition to the imagotariine walruses Neotherium mirum and Pelagiarctos thomasi. According to Deméré et al. (2003), only four taxa are present – Allodesmus kernensis (with A. kelloggi and A. gracilis subsumed as junior synonyms), an indeterminate desmatophocid (Desmatophocine B), and the two walruses. While it’s nowhere near as diverse as the cetacean assemblage from the same locality, it’s fairly comparable with other fossil pinniped assemblages from the eastern North Pacific.

 

The skeleton of Allodesmus kelloggi as exposed in the field. From Mitchell (1966).

            In 1980, future chief preparator of the Los Angeles County Museum of Natural History (LACM) discovered a curious chunk of bone with teeth at Sharktooth Hill. Several years later, he brought it in to LACM and showed it to Dr. L. G. Barnes (colloquially known as ‘Larry’ within the field), and insisted that it was the piece of a snout of some extinct mammal – it even had two small holes which look like nostrils to the uninitiated. Barnes kindly pointed out that those were mental foramina on the “chin” end of a very large jawbone of a pinniped. Larry and Howell enthusiastically recalled this whole story for Morgan Churchill and I when we sat at the very same table last January, thirty or so years later (Larry Barnes has an incredible, near photographic and certainly encyclopedic memory of marine mammal fossil specimens). Howell Thomas donated the fossil for study, and within a few years was hired as the Chief Preparator, and Barnes began to study the specimen. At the time, the marine mammal assemblage was already enormous, and the pinniped assemblage well documented by hundreds of specimens. Most of the fossils could be assigned to the large seal-like Allodesmus, although a single jaw described by Barnes (1972) as “Desmatophocine B” didn’t appear to be referable. “Desmatophocine B” was probably similar to Allodesmus, which has a long narrow skull, enormous eye sockets, single-rooted teeth, and a relatively large body. Furthermore, we know Allodesmus retained the ability to rotate its hindflippers forward for sea-lion like terrestrial locomotion, and it was probably a sea-lion like underwater “flyer”. Numerous small pinniped elements appeared to be similar to a handful of elements described by Remington Kellogg (1931) as Neotherium mirum. 

 

 

Skulls of Allodesmus (left) and Neotherium (right) roughly to scale. From 

Barnes and Hirota (1995) and Kohno et al. (1995).

Neotherium was an enigma for over 60 years, and it wasn’t until more complete remains of the early walrus Imagotaria downsi were recovered from the Santa Margarita Sandstone near Santa Cruz, California, that Neotherium began to make sense. Imagotaria was a sea lion-like walrus that lived about 9-12 million years ago – a bit younger than the 15-16 Ma Sharktooth Hill Bonebed – and by the close of the 1970’s was known by a number of well preserved skulls and partial skeletons from Santa Cruz County. Fossils of Neotherium, although never as common as Allodesmus, continued to trickle in from the bonebed and were referred to Neotherium piecemeal, one or two bones at a time by Mitchell (1961), Mitchell and Tedford (1972) and Repenning and Tedford (1977). By the 1980’s, Barnes had amassed a collection of nearly every skeletal element of Neotherium, identifiable as miniature and slightly more primitive versions of that found in Imagotaria – including partial skulls and several mandibles (eventually a complete skull was published by Kohno et al. 1995). Barnes has been for many years working on a monograph on Neotherium – I’m looking forward to seeing it published. 

The holotype of Pelagiarctos thomasi. From Barnes (1988).

Howell Thomas’ mystery jawbone appeared more similar to Neotherium relative to Allodesmus, with the exception of its comparably gigantic size as well as having a fused intermandibular joint (mandibular symphysis) and deep grooves on the sides of the canines. Eventually, several isolated teeth that were similar to Neotherium, but several times larger in size – were discovered from the bonebed. Some of these teeth even fit right in to the tooth sockets in the mandible fragment. Barnes published the fossils in 1988 and described them as Pelagiarctos thomasi, the species name honoring Howell Thomas. The genus name Pelagiarctos refers to the primitive dental anatomy, as ‘arctos’ refers to bears, the traditional sister taxon of pinnipeds (the root arctos is frequently used in pinniped genus names – Arctocephalus, Phocarctos, Hydrarctos, Pteronarctos, etc.), as well as the inferred pelagic ecology of the animal.

 

The isolated teeth referred to Pelagiarctos by Barnes (1988).

Several aspects of the anatomy of Pelagiarctos, although based on scant material, suggested a different approach to feeding in this fossil walrus relative to other Sharktooth Hill Pinnipeds. The teeth of Pelagiarctos were huge – very robust canines, and postcanine teeth with multiple large cusps and sharp crests. He likened the premolars and molars to those of modern hyenas and extinct borophagine dogs, two groups which (by observation or inference) crack and ingest bones, suggesting that Pelagiarctos had dental adaptations for large bite forces related to feeding on large prey items. Furthermore, the robust mandible and fused symphysis further suggested high bite forces. Barnes (1988) additionally noted that Pelagiarctos is very large and numerically rare in the Sharktooth Hill Bonebed – only known by five teeth and a mandible fragment at the time of his study, as opposed to the hundreds of specimens known of other pinnipeds such as Allodesmus and Neotherium. This suggested to Barnes that Pelagiarctos was rare in California waters during the middle Miocene, further supporting his hypothesis that it was an apex predator (apex predators at the top of the food chain can never be very abundant because they rely on a constant stock of abundant prey items). Barnes further postulated that the type specimen was a male, as it had proportionally large canines; modern and fossil pinnipeds are sexually dimorphic, including early walruses like Neotherium, Imagotaria, and Proneotherium. One of the canines in the holotype is broken and polished down, suggesting the tooth had been broken and worn down after continued use in life – damage which Barnes attributed to male combat, which occasionally results in such damage in modern pinnipeds. Furthermore, Barnes identified some of the fossil teeth as males because they fit right into tooth sockets on the type specimen, and those that didn't were of similar size.

As a result of these hypotheses, numerous fanciful reconstructions of Pelagiarctos have been produced by paleoartists (fanciful depictions can be seen here, here, and here). and Pelagiarctos has achieved the nickname "killer" walrus by some enthusiasts. But what do we really know about Pelagiarctos? Stay tuned...

References –

Barnes LG (1972) Miocene Desmatophocinae (Mammalia: Carnivora) from California.

University of California Publications in Geological Sciences 89: 1-69.

Barnes L.G., 1976, Outline of eastern Northeast Pacific fossil cetacean assemblages:

Systematic Zoology, v. 25, p. 321–343,

Barnes LG (1988) A new fossil pinniped (Mammalia: Otariidae) from the middle Miocene Sharktooth Hill Bonebed, California. Contributions in Science, Natural History Museum of Los Angeles County 396: 1-11.

Barnes LG, Hirota K (1994) Miocene pinnipeds of the otariid subfamily Allodesminae in the North Pacific Ocean: Systematics and Relationships. The Island Arc 3: 329-360.

Deméré TA, Berta A, Adams P (2003) Pinnipedimorph evolutionary biogeography. Bulletin of the American Museum of Natural History 13: 32-76.

R. Kellogg. 1931. Pelagic mammals of the Temblor Formation of the Kern River region, California. Proceedings of the California Academy of Science 19(12):217-397

Kohno N, Barnes LG, Hirota K (1995) Miocene fossil pinnipeds of the genera Prototaria and Neotherium (Carnivora; Otariidae; Imagotarinae) in the North Pacific Ocean: evolution, relationships, and distribution. The Island Arc 3: 285-308.

E. D. Mitchell. 1961. A new walrus from the imperial Pliocene of Southern California: with notes on odobenid and otariid humeri. Los Angeles County Museum Contributions in Science 44:1-28

Mitchell ED (1966) The Miocene pinniped Allodesmus. University of California Publications in Geological Sciences 61: 1-46.      

Mitchell ED, Tedford RH (1972) The Enaliarctinae: a new group of extinct aquatic Carnivora and a consideration of the origin of the Otariidae. Bulletin of the American Museum of Natural History 151: 203-284.

Repenning CA, Tedford RH (1977) Otarioid seals of the Neogene. Geological Survey

Professional Paper 992: 1-87.

D. R. Prothero, M. R. Liter, L. G. Barnes, X. Wang, E. Mitchell, S. McLeod, D. P. Whistler, R. H. Tedford, and C. Ray. 2008. Land mammals from the middle Miocene Sharktooth Hill Bonebed, Kern County, California. New Mexico Museum of Natural History and Science Bulletin 44:299-314

Pyenson ND, Irmis RB, Lipps JH, Barnes LG, Mitchell ED, et al. (2009) The origin of a

widespread marine bonebed deposited during the Middle Miocene Climatic Optimum.

Geology 37: 519-522.

Sinus anatomy of modern porpoises revealed by CT imaging

Congratulations are in order to my colleague and friend Rachel Racicot, a Ph.D. student at Yale (working under Jacques Gauthier), for getting her master's research published in the Journal of Morphology. Rachel did her Master's at San Diego State with Annalisa Berta, and was just finishing up when I visited the San Diego NHM for the first time in 2007. Aside from functional morphology and the endocranial anatomy of odontocetes (particularly cranial sinuses, brain endocasts, and the inner ear), Rachel is also interested in fossil porpoises and is currently researching the strange "half-beaked" porpoise from the San Diego Formation. Rachel's master's research concerns the pterygoid sinus morphology of modern porpoises.

Ms. Racicot had no idea that the journal had selected her image to be put on the cover 

of the January 13 issue, and she was quite surprised when I congratulated her on it. Pleasantly 

surprised, I should say.

Before I continue, I should briefly introduce phocoenids. The Phocoenidae, or true porpoises, are a small bodied group of delphinoid cetaceans that are not terribly diverse (6 species, 3 genera) in comparison to oceanic dolphins (Delphinidae; ~40 species, ~12 genera). They differ from delphinids in having short rostra, having symmetrical skulls, large bumps on the premaxillae just before the bony nares, and have inflated braincases without large bony crests. Phocoenids are considered to be paedomorphic -that is, retaining juvenile features into adulthood, thus explaining A) their inflated, juvenile-like braincases, B) lack of strong bony crests, C) cranial symmetry, D) short rostra, and E) small body size. It should be noted that the delphinid Cephalorhynchus is thought to parallel phocoenid paedomorphosis. Modern phocoenids also have strange, spatulate teeth which almost resemble the teeth of nodosaurs and ankylosaurs. Many fossil phocoenids, on the other hand, have longer rostra, conical teeth, cranial asymmetry, and better developed cranial crests.

 

Schematic view of a neonatal harbor porpoise (Phocoena phocoena) skull showing in blue the various parts of the pterygoid sinus. From Racicot and Berta (2013).

The pterygoid sinus is present in all Neoceti, and even within basilosaurids. It originates as an outpocket of the eustachian tube (an air filled cavity present in the middle ear of all mammals - hold your nose with your fingers and blow, and you'll feel crackling in your eustachian tubes; they are also what "pop" when changing altitude as the pressure changes). Parts of the sinus system can be seen externally, such as the hamular lobe of the pterygoid sinus, which is not completely encased in bone and is visible in a prepared skull as large cavities surrounded by thin flanges of bone. The pterygoid sinus system is elaborated in odontocetes relative to mysticetes. Although known to exist, the anatomy of the pterygoid sinus system in odontocetes - and true porpoises (Phocoenidae) in particular - is difficult to assess. Since they are cavities within a solid object, it's difficult to study them by any conventional means as they remain hidden in the skull. Certain aspects of the pterygoid sinuses have, for example, been used in phylogenetics - in multiple phylogenetic analyses which have included phocoenids, a cladistic character has been used - presence or absence of a dorsal extension of the preorbital lobe of the pterygoid sinus between the maxilla and frontal bone (po on the above diagram). This is a phocoenid feature, and the bottlenose dolphin lacks this. The preorbital lobe is well developed in the neonatal specimen (neonate = newborn individual, rather than a juvenile or subadult), although the dorsal extension is not as well developed as in the adults (an example of ontogeny recapitulating phylogeny).

Digital 'endocast' of the right pterygoid sinus (in medial view) from six skulls of Phocoena phocoena; anterior is to the left. Neonate specimen shown in F. The sinus shape looks pretty weird (but then again, so does the rest of a cetacean skull). From Racicot and Berta (2013).

 So, what's it for? Previous hypotheses for the function of the sinus includes A) an acoustic barrier to reflect sounds produced during echolocation forward through the melon, B) an acoustic barrier between sound producing and sound receiving structures (e.g. nasal region and petrotympanic complex, respectively), and C) to acoustically isolate the petrotympanic complex from sounds produced during echolocation (which, admittedly, sounds similar to B). A fourth hypothesis posits that the pterygoid sinus serves as a means to regulate pressure around the middle ear during diving.

 The paired sinuses (right and left) of Phocoena phocoena specimens in anterior view. Note that there is right-left asymmetry in each specimen. From Racicot and Berta (2013).

To test the sound reflecting ability of the sinus, Racicot and Berta calculated the minimum thickness necessary to reflect sound at the typical highest frequency sounds produced by Phocoena phocoena (~150 kHz). Many aspects of the cetacean skull make sense in the light of acoustic impedence - sound waves tend to bounce off of objects or features where there is a stark change in density. For example, echoes in air are sound waves bouncing off a solid surface. In water where the medium is much denser, sound not only travels faster, but flesh and bone are so similar in density that sounds travel through the vertebrate body rather than bouncing off of it - making things like external ears (which take advantage of sound waves bouncing due to acoustic impedence, and funnels sound in) useless. So, within a skull, a wall of air within a sinus is different enough in density to reflect sound, analogous to a solid object in air.

They calculated that the preorbital lobe would need to be 2.5mm thick at a minimum, which is less than what they observed in phocoenid sinuses - indicating they would function well at reflecting sounds. As for the asymmetry of the sinuses, they remarked that this could be explained by the fact that experiments have determined that porpoises produce sounds in an asymmetric fashion, preferring to use one nasal passage over the other, potentially explaining why the sinuses are asymmetrical.

Wild speculation time: it's also possible that aysmmetrical sinuses may be a vestige of cranial asymmetry. Fossils show that the earliest phocoenids had asymmetrical skulls in a similar fashion to delphinids; perhaps this is an example of phylogenetic inertia - the external skull changed at a faster pace than the sinuses, reaching symmetry first. However, paedomorphosis typically progresses by delaying adult morphology later and later during ontogeny, and retaining juvenile features longer and longer instead. In other words, paedomorphosis would suggest that asymmetry was once an adult feature which at some point was lost because juvenile symmetry prevailed - which doesn't totally jive with asymmetrical sinuses being retained, unless the two are decoupled somehow, progressing along different ontogenetic trajectories. Or, is asymmetry so ingrained within odontocetes that it's a juvenile feature in phocoenids, with symmetry really being secondarily gained via hypermorphosis, with asymmetry being pushed earlier on in ontogeny? Interesting questions, but they remain unanswered. We need more fossils and further studies of modern phocoenid cranial anatomy.

Another last thought - it's interesting to note that phocoenids are considered paedomorphic, but have relatively extensive pterygoid sinuses. The primitive condition among Neoceti, of course, is possessing less well developed sinuses (pterygoid sinuses in Neoceti and Basilosauridae are acquired stepwise in a piecemeal fashion). In other words - sinus development is not showing a paedomorphic trend - in fact, it's showing the opposite trend - it's a peramorphic feature, probably undergoing something like hypermorphosis (development is postponed and extended later into ontogeny) or acceleration (faster development of a feature during ontogeny). Perhaps hypermorphosis is not likely, given the short period it takes for phocoenids to mature.

References:

Racicot, R.A., A. Berta. 2013. Comparative Morphology of Porpoise (Cetacea: Phocoenidae) Pterygoid Sinuses: Phylogenetic and Functional Implications. Journal of Morphology 274:49-62.

Fordyce lab featured in local news

Perhaps it was a slow news day in Dunedin since there are no rascal scarfie students burning couches in the streets (it's summer break here, and relatively hot today), but we were interviewed and photographed for the Otago Daily Times yesterday. I don't have permission to include a copy of the photo here, but you can see it here on the Otago Daily Times webpage. Unfortunately fellow marine mammal students Cheng-Hsiu Tsai (currently conducting research in Japan) and Moyna Mueller weren't around.

Bone-eating zombie worms, part 4: more on bird bones, and Osedax colonizes whale teeth

As covered in a previous post, Kiel and colleagues (2011) recently reported on Osedax traces in Oligocene marine bird bones (Plotopteridae) from Washington, implying that by the late Paleogene Osedax was adapted towards using bones of many types of marine vertebrates (birds and cetaceans). This has direct implications for the divergence time of Osedax - two dates have been determined by molecular divergence dating: Eocene (coinciding with the radiation of pelagic cetaceans), or Cretaceous (a period with numerous large marine reptiles). Kiel et al. indicated that medium sized birds spanned the K/Pg interval and were some of the only marine tetrapods with available bony substrate during certain areas in this time interval. Fish bone, of course, is always going to be more plentiful in the marine record - but fish bones are generally small, and at the time of publication, Rouse et al. (2011) had not yet published their experimental colonization of fish bones by Osedax. Later that year, Higgs et al. (2011) indicated that the bimodal size of boreholes and lack of distinct individual cavities could suggest that Kiel et al. (2011) had not really reported Osedax colonizing bone, but that the traces on bird bones may actually be sponge borings - sponges form a trace called Entobia, which looks (on typical substrate like mollusk shells) like a bunch of tiny pinholes; the sponge inhabits the cavity, using the numerous tiny pinholes for inhalent papillae, and a few larger pinholes for the exhalent papillae.* Additionally, the borehole density reported by Kiel et al. (2011) was far denser than reported on experimentally colonized whale bones, which Higgs et al. (2011) further identified as evidence that the Oligocene bird bones were not in fact Osedax-bored, but sponge-bored.

Close up shot of the bone surface, showing the two sizes of pinholes in the Oligocene plotopterid. Are they the sponge trace Entobia, or the Osedax trace Osspecus? From Kiel et al. (2011).

Steffen Kiel and colleagues followed up their previous discoveries with a new paper in Paläontologische Zeitschrift, reporting in more Oligocene marine vertebrates bored by Osedax: fish bones and whale teeth. Altogether, it's not all too surprising: we have borings in a wide array of modern and fossil critters already. We already have modern fish bones - the really cool thing, in my opinion, is that Osedax will consume teeth. At the moment, I'm wrapping up a large manuscript which does include a little blurb and a figure showing possible Osedax borings in a dusignathine walrus tooth - which means I've got to add a new reference to my paper. But that's a story for another day.

Osedax-bored fish bone, from the Oligocene Makah Formation of Washington state. From Kiel et al. (2012).

The fossils hail from various Oligocene deep water rock units which are already known to produce whale fall faunas, cold seep assemblages, and wood falls. These include the Makah and Lincoln Creek Formations of the northern coast of the Olympic peninsula, as well as the (also) Pysht Formation just across the river from beautiful Astoria, Oregon. Coauthor Jim Goedert and his wife Gail have prospected the rugged coastline of the Pacific Northwest for decades, finding marine mammals, sharks, and marine birds. He's described Paleogene pelagornithids from the area, and named a plotopterid (Phocavis). The toothed mysticete Chonecetus goedertorum from Washington was named for them (they collected the holotype in 1984), and Gail also found what would later become the holotype of Pteronarctos goedertae in Oregon.

Archaic mysticete teeth from the Oligocene Pysht Formation of Washington with Osedax borings. From Kiel et al. (2012).

Arguably the most surprising finding of Kiel et al's new study is the pervasive boring of whale teeth. These teeth were all found in a partial, highly corroded mandible; the authors did not specify what kind of corrosion, but it was in all likelihood bioerosion (and Osedax related at that). They didn't identify what kind of mysticete the teeth belonged to - published aetiocetids from the Oligocene of Oregon and Washington have simpler teeth, but I have seen aetiocetid teeth in USNM collections similar to these. In several specimens, the crown was heavily bioeroded by Osedax, and the root just below the crown was as well. They argued that the loss of the crown in some cases was caused by scavenging invertebrates (possibly crustaceans) accidentally damaging the root while trying to eat the Osedax worms. Similar crustacean-mediated destruction of bone has been observed in modern whale falls. (As a total aside, it's mid summer here in New Zealand, and both my wife and I got pinched on the toes by shore crabs while in the water.)

CT images of the Osedax borings within the whale teeth, with individual borings shown in yellow in 3D below. From Kiel et al. (2012).

Kiel et al. (2012) also took the opportunity to respond to some of the comments by Higgs and colleagues about the bird bone traces. Kiel et al. pointed out that in numerous modern specimens, they observed comparatively dense Osedax borehole clustering, so extreme borehole density does not invalidate the their identification. Kiel et al. (2012) also point out that there were mollusk shells present along with the bird bones, and the shells were not bored; shells are the typical substrate for such boring sponges. As a taphonomist, I should note that this argument doesn't necessarily hold sway: most marine assemblages are time averaged, and because vertebrate and mollusk remains are of different chemistry, size, and have different soft tissue anatomy and production rates, they are subjected to different taphonomic pathways. In other words, it is certainly possible that the period of modification was different for the bones and shells. To make that more clear: the whale fossil could have been sitting on the seafloor exposed for a long period of time, allowing Osedax to colonize; towards the end of the pause in sedimentation, some mollusk shells are washed in and buried too quickly for the shells to have also been colonized. Or, the mollusks could have been burrowing taxa. Back on topic: Kiel et al. (2012) concluded that these fossils demonstrate that Osedax has been a generalist bone-consumer for over thirty million years, which strikes another blow to the "Osedax as a cetacean bone specialist" hypothesis.

*I'm no expert on Poriferan anatomy, but I can only assume that the papillae are homologous to the large openings through which water is pumped in and out.

Don't forget to see the other posts in this series:
 
Bone-eating zombie worms, part 3: Osedax consume more than cetacean bones

Bone-eating zombie worms, part 2: the discovery of fossil Osedax traces

Bone-eating zombie worms, part 1: whale falls and taphonomy

References cited:

Higgs, N.D., C.T.S. Little, A.G. Glover, T.G. Dahlgren, C. R. Smith, and S. Dominici. 2011. Evidence of Osedax worm borings in Pliocene (~3 Ma) whale bone from the Mediterranean. Historical Biology 24:269-277.

Kiel, S., Kahl, W. A. and Goedert, J. L. 2010 Osedax borings in fossil marine bird bones. Naturwissenschaften 55:51–55.

Kiel, S., Kahl, W. A. and Goedert, J. L. 2012. Traces of the bone-eating annelid Osedax in Oligocene whale teeth and fish bones. Paläontologische Zeitschrift DOI 10.1007/s12542-012-0158-9

Rouse, G.W., Goffredi, S.K., Johnson, S.B., and R.C. Vrijenhoek. 2011. Not whale-fall specialists, Osedax worms also consume fishbones. Biology Letters 7:736-739.

US research trip, part 11: more photos from Charleston

I realized that my photos from Charleston did not really include much in the way of candid photos, or photos of us (you know, people). My camera has a narrow field of view and is ill suited for getting 'action' photos. Fortunately, my colleague Tatsuro Ando has graciously allowed me to post some photos he took of the gang and exhibits while in Charleston. Thanks, Tatsuro!

Another view of King Street in Charleston.

Ewan and I enjoying some beer after a long drive from North Carolina. This was my first time eating real barbecue after starting my Ph.D. program, and damn was it good.

Ewan and I looking at the model of the Hunley outside the Charleston Museum.

The pier at Folly Beach, South Carolina - we ate dinner at a restaurant at the base of the pier.

 

The Sanders' treated us to dinner at a great seafood place in Folly Beach (south of downtown Charleston). I tried shrimp n' grits for the first time - it was delicious. From left to right, clockwise: Rhonda Sanders, Al Sanders, Ewan, Myself, and Eric Ekdale.

Al and Ewan on the Sanders' screened in porch, overlooking the South Carolina 'forest' (better termed jungle, in my opinion).

 The view out into the forest; a few minutes later, a raccoon came climbing by. It was a welcome sight, although it is pretty neat seeing brushy tailed possums here in Dunedin.

After a wonderful seafood dinner, discussions of cetacean evolution, key lime pie, and genuine southern hospitality - and we were ready to take a cab back into town. (From left: Eric, yours truly, Rhonda, Al, and Ewan).

 

A skull of Schizodelphis from the Calvert Cliffs, on display at the College of Charleston.

 A cacophony of mosasaurs on display at the College.

Ewan inspecting some Carcharocles megalodon teeth, unaware of how funny this ended up looking.

Basilosaurid teeth from South Carolina (College of Charleston).

 

 Desmostylian teeth from the middle Miocene Temblor Formation of California (College of Charleston).

 

To examine fine details of mysticete palates, sometimes you have to get in pretty close. 

Someone else (Ewan I suspect) snapped a pinup-esque picture of me doing this, which has hopefully 

been deleted forever.

A mounted cast of Pteranodon at the College of Charleston.

A Platecarpus (mosasaur) skull and cervical vertebrae mounted at the College of Charleston.

The gang's all here: Tatsuro, myself, Ewan, Al, and Eric in Charleston Museum collections. Thanks, Al, for a great time (and good food!).

Thanks again to Tatsuro Ando for letting me post these photos.

US Research trip, part 12: SVP, the hurricane, and the future

 We're finally at the conclusion of this long set of posts about my month-long trip back to the US. I've sort of told it in sequence, with the exception of SVP, which was in the middle of the trip, and right before the trip down to Charleston. The SVP meeting was great - my 8th meeting, and the 6th at which I've presented research. I gave my second SVP talk this year - this time, I presented results of my master's thesis research on marine vertebrate fossils from the Purisima Formation near Santa Cruz, California.

I carried out my master's research from 2008-2011 at Montana State, and collected taphonomic data from fossil vertebrates I had collected, as well as specimens from UCMP and the Santa Cruz Museum of Natural History. We have a very poor concept of the taphonomy of marine vertebrates - and sought to clarify some of these issues by studying changes in preservation among different shallow marine depositional settings.

Title page from my SVP talk. The image is taken near Halfmoon Bay, with my wife 

Sarah standing in for scale beside stacked beds of hummocky cross-stratified sandstone, 

characteristic of the "middle shelf".

Most previous taphonomic studies of marine vertebrates have focused on single skeletons or bonebeds - which admittedly doesn't tell us much about the big picture. I'll spare you the details for now (least of all because it's not published yet), but the research addresses some of these big picture questions and patterns. The talk went off without a hitch, and I was able to meet with quite a few colleagues. The night before the talk, I did benefit from finding a jacuzzi with a few friends in a vain attempt to relax. I'd given the talk before, three other times - at my defense, at the aquatic tetrapods meeting, and again at Fossil Coffee at UCMP - but getting up in front of a huge audience at SVP is something else altogether. Unfortunately, because the talk was on the last day of the conference, the meeting flew by way too quickly - one of the reasons I enjoy poster presentations much more.

Another slide from my talk, showing examples of Purisima Formation marine vertebrates.

SVP is also a fantastic time because it's one of the few times you ever get to see old friends. Many friends of mine I had not seen since my wedding last year; it was a bit difficult waking up the day after SVP and not being able to go find mobs of familiar faces milling about in the hotel lobby. Nevertheless, there is always next SVP.

I'm currently nearing submission of the manuscript version of my master's thesis, something I've been looking forward to for a long time. I try to keep projects moving, and would rather not get too caught up and sit on them; it's only been a year and a half since I graduated and completed my master's, so I suppose I'm doing well. The manuscript has taken a lot of time to modify - I've spent almost as much (or even more) time editing it over the past 6 months than I did writing it in the first place, and it is a far better piece of writing because of it. With this, and the September submission of another monstrous manuscript (also 100+ pages), I've cleared off a sizeable part of my research backlog.

After SVP, and Charleston, I had another week in Washington D.C. I was scheduled to fly out to California on Halloween. I kept hearing things on the news while in Charleston about a hurricane, but didn't really think too much of it - after all, hurricanes only happen on the east coast, right? As a Californian, I've always sort of filtered out news like tornadoes and hurricanes. And then I remembered, like some sort of bizarre moment of existentialist realization - that I was in Washington D.C. of all places, with a gigantic hurricane heading more or less right for us, with landfall in about four or five days. Fortunately, I was staying in D.C. with my childhood friend (and aide for the Senate Committee on Veteran's Affairs) Ben Merkel - who lives in a brownstone north of downtown, on a sloped street far from any drainage, and separated from the street by about 15 feet of steps. I felt pretty safe. Unfortunately, it also meant that I'd have to miss two days of museum work. Well, shit happens, and I was able to come in over the weekend before Sandy hit. We spent the 48 hour period while the city was shut down watching movies, eating and cooking, and drinking a healthy amount of beer. We only lost power for half a second when the lights flickered - once. We had internet the whole time, and I was able to work on some peer reviews and writing up a short manuscript on Herpetocetus. It was actually a pretty fun and productive time. Best of all, I was even able to make it out to the Garber facility for a few hours on my last day in DC to get some much needed photos. The storm had largely bypassed DC - we had some pretty heavy rain and high winds, and they had to close down the DC metro and buses and sandbag a bunch of federal buildings, but all the damage we saw included a few newspaper stands blown into the street. And my flight even left on time!

All in all, the trip was a total success, and I was able to arm myself with all the data and photographs necessary to complete my thesis here in New Zealand. I even had enough time while on the trip to start and finish an entire manuscript (a short one, anyway).