Saturday, April 28, 2012

Glauconite, mosasaurs, and asteroids: does the taphonomy of a bonebed in New Jersey really indicate preservation of a catastrophic assemblage?

During the course of my master's thesis research, I attempted to find and read every paper ever written on the taphonomy of marine vertebrates so that I could not only get a good idea of what ideas had stood the test of time and which ones hadn't (or for that matter, were profoundly embarassing even to read), but also so that I could identify which areas in marine taphonomy that we had no answers or even hypotheses for yet. For example: we know extremely little about the everyday processes on the continental shelf that affect fossil preservation, from an actualistic perspective. I found that we have two areas where we do in fact have quite a bit of actualistic data: the beach (where we can walk around and poke dead stuff) and the offshore environment (i.e. outer shelf through abyssal plain - where we can send down a submersible to go and poke dead stuff). Everything in between - from ~10 through ~100 meters water depth - we have very little data, except for a couple of failed whale fall experiments. There is now quite a lot of data generated through whale fall studies for the deep sea, and plenty of actualistic taphonomists have walked around and watched dead vertebrates decay, disarticulate, and have their skeletons dispersed across the seashore. The likely reason we don't know much about the shelf is because in the intervening environments, currents and sediment transport processes likely cover/uncover/transport carcasses or bones too quickly to make a submersible visit really feasible. Using submersibles is damned expensive - and even scuba diving is expensive as well (let alone dangerous), and quite frankly, there probably aren't enough people in marine vertebrate taphonomy who care enough to try and get those sorts of funds. It is quite a bit easier to look at the fossil record in continental shelf settings and go from there and what we know about shelf sedimentation - which is exactly what my master's thesis focused on.

Even though I finished my thesis about a year ago now, I'm still quite keen to learn more and read more articles - and there have certainly been a flurry of articles recently, which I'll have to cover later on. Not only because I am quite obsessed with the subject of taphonomy, but also because I still haven't published my thesis yet, and will need to update it with citations to some of these new studies. If all goes well, I'll be presenting at SVP this year on my thesis, and hopefully will be putting a shorter version of it together for submission to "Geology". That all being said - I was quite interested to see this paper come up in a search on Georef: "Obasi et al. 2011. Glauconite composition and morphology, shocked quartz, and the origin of the Cretaceous? Main Fossiliferous Layer (MFL) in southern New Jersey, U.S.A. Journal of Sedimentary Research 81:479-494. Reading the abstract, the authors had apparently used a combination of glauconitic mineralogy, mineralogy of shocked quartz, and taphonomic evidence to identify a marine vertebrate bonebed in New Jersey as representing a catastrophic assemblage - and not just any catastrophic assemblage, but at the K/Pg boundary (for the unitiated - formerly K/T, meaning Cretaceous-Tertiary boundary) and representing a mass death assemblage from the end Cretaceous extinction.

Map of K/Pg strata in New Jersey. From Obasi et al. (2011).

Now, keep this in mind as you read: I do not study catastrophic extinctions, and I generally do not read literature on the K/Pg extinction unless (as in this case) it deals with taphonomy. Otherwise, given my background- the entirety of my reading focuses on the Cenozoic; but with taphonomy, there have been marine vertebrates dying and being moved around by sediment and other phenomena for nearly half a billion
years. Taphonomy has, in the past, been used as a means to an end to try and substantiate catastrophic hypotheses, and has even been used in weird ways by young earth creationists. So when I read a taphonomy article that does not seem sober, I immediately become skeptical. My last point - I have read a lot of other articles about catastrophic assemblages and their taphonomy, and very few ever really make a great case (especially in marine rocks). So it's not that I don't believe in catastrophic assemblages per se - it's that I haven't read very many good articles on them. They certainly do exist, but there is a substantial volume of evidence you need to provide in order to fully eliminate alternative hypotheses. In the past, people have identified many criteria for identifying catastrophic assemblages - are the fossils all preserved on the same bedding plane? Is there no evidence of post-mortem destruction or transport of bones? Can the fossil assemblage reasonably attributed to one event of short duration? Are the fossils well-preserved and reasonably complete (i.e. articulated or associated)? Is the assemblage biased, and can it reasonably be interpreted as a death assemblage? These are all questions that must be asked. I've probably forgotten a few, and there are certainly others that other researchers have demanded as requirements.

Stratigraphic column of the late Cretaceous and early Paleogene of New Jersey. From Obasi et al. (2011).

Now, to get to the paper. The authors studied the Main Fossiliferous Layer (MFL), which is a bonebed in the lowermost Hornerstown Formation, which is a 10cm thick concentration of invertebrate and vertebrate skeletal elements, including fish, sharks, mosasaurs, and marine birds. These fossils co-occur with Paleocene microfossils. Previous studies (cited within Obasi et al. 2011) have identified the MFL as a major sedimentary hiatus or unconformity (and even as a sequence boundary), and the fossils from the MFL as being reworked from Cretaceous strata into the Paleocene. There has not been a consensus with the placement of the K/PG boundary within the local section - either at the base of the Hornerstown, within, or above the MFL (referenes within Obasi et al. 2011). The Hornerstown Formation and underlying New Egypt and Navesink Formations are very rich in a mineral called glauconite, and the sediment can probably be characterized as greensand (more on this later). Interestingly, within the MFL, are bivalves infilled with sediment from underlying strata, gray rip-up mud clasts, shocked quartz grains (within a single burrow below the MFL). The MFL and associated underlying and overlying strata are also intensely bioturbated. Lastly - vertebrate fossils from the MFL occur as isolated bones and teeth which are occasionally abraded, as well as partially articulated and associated specimens.

Seeing as the author's primary contribution is in the form of mineralogical analysis of glauconite, I'll discuss that first. Glauconite is an authigenic mineral that forms as small sand-grain size pellets, often around fecal pellets or foraminifera, and other tiny organic elements. I am no mineralogist, but if there is one thing I have consistently read in nearly every publication regarding glauconite - it forms during periods of very low sedimentation or during sedimentary hiatuses, often in concert with phosphogenesis (formation of phosphate nodules), and it generally forms on the continental shelf in upwelling regimes. Glauconite often occurs in marine bonebeds, hiatal surfaces, transgressive surfaces of erosion, and sequence boundaries. Using geochemistry and exhamining the morphology of glauconite grains, Obasi et al. (2011) determined that the "maturity" of glauconite grains (i.e. features indicating how well-formed and long the glauconite was able to form for) increased up section through these strata that contain the MFL. Maturity - FYI - is a term applied to mineral grains in sedimentology (and probably other fields in geology); for example (the most widely known use of 'maturity'), the relative maturity of a sandstone is identified based on what proportion of the rock is composed by quartz (the most durable and stable type of grain in sandstone) or not, as opposed to less-stable rock fragments and feldspar grains. Less mature sandstone includes more unstable fragments and grains and less quartz, indicating it is 'fresher' and has not been subjected to as much weathering. The same concept applies to glauconite - mature glauconite has had a long residence time on the seafloor, and vice versa. Lastly, the authors make the case that the sedimentation rate - while already low to form glauconite at all - must have decreased even further through time during "Hornerstown" time.

The trenches, stratigraphic sections, glauconite (middle two rows) and shocked quartz (lower right) from Obasi et al. (2011). Note the green color of the sediment in A and B.

The authors then discuss the formation of the MFL and what processes could have led to its genesis. They begin by trying to evaluate whether or not the MFL represents a transgressive lag deposit. A transgressive lag, for those of you not versed in the arcane ways of sequence stratigraphy - forms during a period of sea level rise; during this time sediment becomes trapped within estuaries (which are backed-up rivers common during periods of sea level rise, whereas deltas are more common during periods of sea level fall), and sedimentation slows down on the continental shelf. The fairweather and storm currents that normally happen on the shelf then erode and rework older sediment instead of transporting around and depositing new sediment that would normally be shed off the continental margin. This erosion can rework fossils from multiple stratigraphic levels into a single bonebed/shellbed (but does not always happen). It is worth noting that transgressive lags are typically identified based on their position within a stratigraphic sequence, in addition to features indicating erosion or lack of deposition. Obasi et al. (2011) argue that because vertebrate fossils do not appear to be reworked based upon analysis of rare earth element concentrations in bones (published in Staron et al. 2001), the presence of partially articulated marine reptiles, the lack of primary sedimentary structures related to sedimentary reworking, and the fact that the MFL does not co-occur with the slowest sedimentation rate (as determined from glauconite maturity). They do acknowledge, however, that there are reworked invertebrate fossils within the MFL, supporting the identification of it as a transgressive lag. They also argue that the middle-outer shelf setting (inferred from glauconite presence) is too deep for a transgressive lag to form and state "Transgressive lags form as the surf
zone and wave base migrate inland during transgression".

The authors then discuss the possibility of the MFL representing a condensed deposit. A condensed deposit is formed by low to zero net sedimentation, as opposed to erosion, and fossil assemblages may appear fairly similar, and may not be associated with a sharp erosional surface. They admit that the intense bioturbation supports this interpretation, as does the preservation of fossil vertebrates, but the steady increase in glauconite maturity does not-  in theory, it should peak at the time of lowest sedimentation rate, which should be the MFL under this hypothesis. Finally, they discuss the reasons why they interpret the MFL as being a thanatocenosis formed by the K/Pg bolid impact. Although the authors have not found any evidence for the famed iridium anomaly in their study area (Obasi et al. 2011:491), they argue that the presence of shocked quartz grains in a single burrow below the MFL. Although not found elsewhere in the section, the shocked quartz appears to have been reworked form the Hornerstown Fm. down into the burrow, which extends down into the Navesink Fm. (Shocked quartz is often used as evidence of a bolide impact, as it is a high-pressure low-temperature type of deformation that happens to quartz). They argue that the gray rip up clasts are mud rip-ups from the tsunami wave from the bolide impact, and the shocked quartz is evidence of the bolide impact as well coinciding with bonebed formation. In total, they argue that the MFL formed as marine vertebrate carcasses littered their remains across the seafloor after being killed by the event, leaving some isolated bones and some associated specimens.

There are a lot of serious problems with this study, and I'll try to deal with them quickly so that this post doesn't get any longer. First and foremost, transgressive lags can occur in quite deep environments, and certainly glauconite-producing middle and some outer shelf environments, and glauconite has been identified at sequence boundaries and other large-scale sequence stratigraphic surfaces before. Transgressive lags are not confined to the 'surf zone', and storm waves can rework sediment at the depths which glauconite forms at. Secondly - you cannot make the argument that the lack of primary sedimentary structures (erosional surfaces, cross-bedding, etc.) indicates anything if you also note that the strata are massive and bioturbated (except perhaps that the strata are structureless because they are bioturbated!). Bioturbation is not an alternative mode of sedimentation - it is a process which erases certain physical information and overprints it with biogenic information.

What about the glauconite and taphonomy? We already know that A) glauconite forms during periods of slow sedimentation, B) there are clearly reworked invertebrate fossils, C) some of the vertebrate fossils are abraded, D) the vertebrates are probably not reworked, E) the vertebrates are not concentrated onto a single resolvable bedding plane as a number of clearly complete skeletons. Taken in full, the evidence does not really look that good for a catastrophic assemblage. Relatively little of the negative evidence cited by Obasi et al. (2011) for physically-controlled modes of formation are negative. Instead, nearly all of the evidence cited would positively support a physical control on bonebed formation. My alternative and less hyperbolic interpretation would go something like this: 1) slowdown in sedimentation results in glauconite formation; 2) vertebrates concentrated into 10cm interval due to current reworking of sediment at sea floor surface and transport and abrasion of some bones and disarticulation of skeletons (other elements shed from carcasses over protracted period of time); 3) slow sedimentation allows pervasive bioturbation, deleting the physical sedimentary structures from the strata; 4) K/Pg impact happened sometime before or after, or possibly during. We just don't know when given the evidence, and I don't think that Obasi et al. (2011) really have sufficient evidence to point at any one part of the strat column and say "here!".

Glauconite in the Santa Margarita Sandstone at Limantour Beach, Point Reyes. This is glauconite from a transgressive surface of erosion in California, which apparently should not exist.

Interestingly, Liebig et al. (2007) reported on the taphonomy of a mass assemblage of false killer whales, and found that not too long after the mass death, the skeletal remains were extremely variable in their taphonomic mode and would not be identifiable as a mass death assemblage if it were in the fossil record. Does this mean that an assemblage with variable taphonomy like that can be identified as catastrophic? No - it means in cases like that, we cannot tell, one way or the other. Clearly, there has been some residence time of the fossils on the seafloor - not all are articulated, and some are even abraded, meaning they have been transported around on the seafloor. We also know that there are other cases of many articulated skeletons concentrated onto what appears to be a single bedding plane (Peters et al. 2009) - in the case I'm referring to, a series of basilosaurid skeletons in Eocene strata at Wadi Al Hitan, Egypt-  but sequence stratigraphy identified it as a maximum marine flooding surface. An MMFS is a period of low sedimentation, and in this case it was muddy deposition, and essentially a condensed deposit with a concentration of well-preserved whales on what otherwise would look like a mass death assemblage. So - even in a case where all of the questions I asked above might be addressed positively - you still may not be able to tell the difference. The taphonomic support for the catastrophic interpretation of Obasi et al. (2011) just doesn't really hold up very well.

Edit: I realize that I ran out of steam a bit and neglected to mention some of the most serious reasons why the results of Obasi et al. (2011) do not hold up, taphonomically speaking. For one - the premise that a fossil assemblage deposited in glauconite can be interpreted in any way except that it is extremely time averaged to some degree or another (and thus preserving a physical depositional signal - and any purely biological/catastrophic signal being deleted permanently from the rock record) - is absurd. Glauconite is de facto geologic evidence (i.e. independent of the taphonomic evidence, which in the case of the MFL is just as strong) of time averaging and slow sedimentation. Secondly - what taphonomic characteristics need to be positively addressed in order to accurately demonstrate a catastrophic death assemblage in the marine realm? I guess I will start by asking (and answering) a third question - how many previously published marine tetrapod assemblages can realistically be interpreted to represent a catastrophic death assemblage? I can really only think of one well-published example: the death assemblage of Shonisaurus published by Hogler (1992). There may be a couple of others preserved under similar circumstances - but in this case, all of the Shonisaurus skeletons are articulated, in a small geographic area on a single bedding plane, not preserved in a concentration that appears to be caused hydraulically or by changes in sedimentary budget, etc. Virtually none of these variables are present in the assemblage from the MFL in New Jersey (Obasi et al., 2011), and even many of these have been argued to be of variable utility by taphonomists. 

Lastly, I get the impression that the identification of this exact horizon as the MFL rests solely on the shocked quartz grains in a single burrow. This would correlate the death assemblage with the K/Pg boundary, and voila! There's your cause of death. Once again, here's a perfect example of where correlation and causation may not necessarily be linked, and in this case - there isn't sufficient evidence to link them.


Hogler, J. A. 1992. Taphonomy and Paleoecology of Shonisaurus popularis (Reptilia:
Ichthyosauria). Palaios 7:108-117.

Liebig, P. M., K. W. Flessa, and T. A. Taylor. 2007. Taphonomic variation desite
catastrophic mortality: analysis of a mass stranding of false killer whales (Pseudorca
crassidens), Gulf of California, Mexico. Palaios 22:384-391.

Obasi, C.C., Terry, D.O., Myer, G.H., and D.E. Grandstaff. 2011. Glauconite composition and morphology, shocked quartz, and the origin of the Cretaceous(?) Main Fossiliferous Layer (MFL) in southern New Jersey, U.S.A. Journal of Sedimetnary Research 81:479-494.

Peters, S. E., M. S. M. Antar, I. S. Zalmout, and P. D. Gingerich. 2009. Sequence
stratigraphic control on preservation of late Eocene whales and other vertebrates at Wadi
Al-Hitan, Egypt. Palaios 24:290-302.

Thursday, April 26, 2012

The "King Lizard" and the origin of cranial asymmetry in whales

 Whales and dolphins are among the most bizarre and derived of all modern mammals, to the point where it took a while for a consensus to develop that they were even mammals. To skip all of the obvious soft tissue features that the average joe can point out, and get to the part most paleontologically minded folks are interested in - the skull of cetaceans is extremely weird in a number of ways. From cranial telescoping and the posterior location of the blowhole, the strangely indistinct orbit, homodont and polyodont dentition (in odontocetes), a series of complex basicranial sinuses, and the straplike jugal bone - the skulls of cetaceans bear little similarity to other mammals. Toothed whales - the suborder Odontoceti - are further perplexing in the asymmetry of their skulls. Baleen whales - mysticetes - are at least more normal in having symmetrical skulls. The asymmetry of toothed whale skulls occurs in a number of ways. Firstly - asymmetry is confined to the facial bones of the skull, and primarily the premaxilla and maxilla (the bones that form the middle and sides of the snout or rostrum, respectively). Secondly, the foramina or holes in the facial region (for nerves and arteries) are differently positioned on each side. Thirdly, bones of the right side of the face are wider than their counterparts of the left side, and the very top of the skull (called the vertex) is accordingly displaced to the left side. Fourthly - in some modern toothed whales, the whole posterior facial region has undergone a clockwise rotation. The skulls of sperm whales and ziphiids in particular are among the most asymmetrical of odontocetes, while porpoises, some delphinids, and the Franciscana (Pontoporia) have what appear to be symmetrical skulls (but still exhibit slight asymmetry).
A new reconstruction of Basilosaurus isis by the folks at University of Michigan, with small and edible child for scale. From

Asymmetry of the odontocete skull has been conclusively tied to their unique mode of sound production. The left nasal passage is unmodified and retained for breathing, while the right nasal passage is hypertrophied and has a host of soft tissue structures and muscles which produce the sounds (i.e. the well-known whistles and clicks of bottlenose dolphins) used during echolocation.  These facial muscles attach to the skull, and because they are larger on the right side - these bones are enlarged on the right side, resulting in asymmetry. And thus, odontocetes exhibit soft-tissue facial asymmetry as well. Cranial asymmetry shows up in different manners in different groups of odontocetes - the basal odontocete Simocetus shows slight asymmetry in the shape and proportions of some of the skull bones, as do certain other Oligocene odontocetes. It has been argued before that cranial asymmetry has evolved multiple times within odontocetes, while facial asymmetry is probably a shared derived feature of all odontocetes. The obvious lack of asymmetry in mysticetes and apparent symmetry of archaeocete skulls suggested for the longest time that cranial and facial asymmetry was unique to odontocetes.
Much to my (and everyone else's) surprise, I saw an abstract at last year's conference on Secondary Adaptations of Tetrapods to Life in the Water (SATLW) by friend and colleague Julia Fahkle on the discovery of cranial asymmetry in archaeocetes. I was skeptical at first - suggestions of looking for asymmetrical archaeocetes and mysticetes were murmured in the mid 1990's regarding the provocative and absurd hypothesis that sperm whales were more closely related to baleen whales (rendering Odontoceti paraphyletic), and I've read elsewhere about people speculating that ichthyosaurs and plesiosaurs may have echolocated and cranial asymmetry should be looked for in these groups (taking the ecological analogy a bit far, I think). When I saw Julia's talk, though, I was surprised, impressed, and dumbfounded- and no longer skeptical. I was happy to see the paper (Fahlke et al. 2011) come out only a couple of months later (although it's taken me about 6 or 7 months to get around to posting about it).
The skull of Basilosaurus isis, from

After taking CT scans of a skull and lower jaws of Basilosaurus isis from the Eocene of Egypt, Julia thought that the twisting of the snout was due to post-burial deformation, and attempted to correct for deformation and modeled the skull to be symmetrical. However, the digital model of the jaws would not close properly - she discovered when the non-modified skull scan was used, the jaws would close properly - suggesting that it was natural. In Basilosaurus, she noticed that the snout was curved a little to the left (insert inappropriate joke here), and that the midline of the skull was deviated to the right side behind the orbits; Fahlke et al. (2011) characterized cranial asymmetry in Basilosaurus as 'curvature and axial torsion of the cranium'. Furthermore - there are thin parts of the lateral wall of the lower jaw called the 'pan bone', which in modern odontocetes are symmetrical in their thickness; In Basilosaurus, the thinnest parts of the pan bone are placed differently - in the left jaw, the thinnest part is placed further forward than on the right. 

Fig. 4 from Fahlke et al. (2011), showing the different placement of the thinnest part of the pan bone in the lower jaws of Basilosaurus isis.

The 'pan bone' is an extremely thin wall of relatively dense bone in the lower jaw, which is placed alongside the very enlarged 'mandibular foramen' (a hole in the jaw that transmits arteries and nerves), which in modern toothed whales is filled with a large lens-shaped body of fat, termed the mandibular fat pad. The enlargement of the bony opening has resulted in the loss of bone on the inside of the jaw - so that looking at the medial surface, there is a boneless window exposing the mandibular fat pad. This structure has been implicated as a key innovation in cetacean evolution as an adaptation for directional hearing in water. Land mammals - including humans - have earbones that are firmly sutured to the skull, and airborne sounds travel slowly and bounce off of soft tissue and are funneled into the ear canal. The 'bouncing' of sound waves is called acoustic impedance, and only occurs when there is a strong contrast in the density of a given material. In water, sounds travel much faster, and because soft tissue is nearly the same density as water - waterborne sounds travel extremely quickly and because there is little to no acoustic impedance between water, flesh, and bone - sounds travel through (rather than bouncing off) soft tissue and bone, arriving at each ear too quickly for the direction of the sound to be determined by the brain. This is called bone-conducted hearing, and most (if not all) non-cetacean mammals hear through this manner when underwater. Next time you're in a pool, try an experiment in functional morphology and close your eyes and have a friend make noises (talking or yelling works), and try to correctly tell the direction of the sound - I challenge you!  The expansion of the mandibular foramen and the mandibular fat pad forms an acoustic pathway to each ear. In addition to this, modern cetaceans exhibit a series of extremely complex air-filled sinuses which surround each set of earbones, and the earbones have lost their bony connections to the skull, which are ways that the ears of cetaceans are acoustically isolated (these sinuses and loss of bony connections appear in baleen whales as well). The pan bone, air sinuses, and earbones separated from the skull are all features that have been previously identified as adaptations for directional hearing underwater.
In her talk last June, Julia showed a slide that further solidified the identification of asymmetry as natural rather than just deformation in archaeocetes - they identified, in a number of protocetid and basilosaurid skulls (roughly a dozen well-preserved specimens from different ages, localities, formations, and countries) - that the direction of torsion and curvature of the snout was the same in each specimen. Fahlke et al. (2011) argued that the appearance of cranial asymmetry in archaeocetes followed closely after the ability to hear directionally underwater, and that it must somehow be related. Pan bones and enlarged mandibular foramina (a bony correlate of a mandibular fat pad) are found in nearly all cetaceans with the exception of Pakicetids; if I recall correctly, the earliest appearance of a pan bone is in the "crocodile-otter" remingtonocetids, and pan bones are known in protocetid and basilosaurid archaeocetes, in addition to archaic mysticetes (read more about that here). Fahlke et al. (2011) argue that the asymmetry in archaeocetes is possibly analogous to that in owls - which enhances the hearing ability of owls in the dark.
Figure 1 from Fahlke et al. (2011).

While Fahlke et al. (2011) convincingly establish archaeocetes as having cranial asymmetry - an extraordinary and provocative discovery - they didn't spend much time in the article contrasting cranial asymmetry of archaeocetes and odontocetes. They do explain that cranial asymmetry in archaeocetes is NOT related to echolocation, as archaeocetes clearly lack many of the facial features of odontocetes. A major point worth stating is that asymmetry in archaeocetes is related to sound reception, while in odontocetes, these types of asymmetry are lacking, and the facial asymmetry is instead related to sound production. While this point was not elaborated on much by Fahlke et al. (2011) - which is certainly no problem anyone should complain about, given the page limits of the journal PNAS in which it was published - it certainly complicates the picture, and if anything - certainly makes early whale evolution much more interesting. It implies that cranial asymmetry might even be decoupled from echolocation altogether, but also that there are now two known modes of cranial asymmetry in cetaceans - longitudinal curvature and torsion, versus facial asymmetry. It suggests that cranial asymmetry predated echolocation and facial asymmetry in odontocetes; perhaps cranial asymmetry as documented by Fahlke et al. (2011) laid the structural blueprints for the derived form of asymmetry in odontocetes. Secondly – Fahlke's discovery implies that mysticetes are secondarily symmetrical, because they evolved from ancestors (basilosaurids) with asymmetrical crania. So then, what is going on in toothed mysticetes? Do they have symmetrical skulls, or not? If not, then why did they lose the asymmetry seen in archaeocetes? Were they already hearing at low frequencies like modern cetaceans, rendering directional symmetry less useful? If not in toothed mysticetes – then when was asymmetry lost in baleen whales? Julia Fahlke's exciting discovery really throws a giant wrench in what previously appeared to be a simpler view of cetacean hearing evolution, and leaves us asking more questions - and most excitingly, a series of weird questions we did not expect.

Edit: There is another hypothesis for cranial asymmetry that has been proposed, which this discovery (among other fossils) demolishes. But, that's a topic for another time.
Fahlke, J.M., P.D. Gingerich, R.C. Welsh, and A.R. Wood. 2011. Cranial asymmetry in Eocene archaeocete whales and the  evolution of directional hearing in water. Proceedings of the National Academy of Science 108:35:13545-13548.

Tuesday, April 24, 2012

Fieldwork in North Otago

 A couple weeks ago Ewan invited fellow student Cheng-Hsiu Tsai and myself along on some fieldwork in North Otago. We drove up the coast on highway 1 (sounds eerily familiar to my usual routine in California!), while Ewan gave us a geological narration of the drive. For the uninitiated - New Zealand has historically been a source of scattered Oligocene and Neogene fossil cetaceans (and other marine vertebrates), and many early discoveries include the baleen whales Mauicetus parki, "Mauicetus" lophocephalus, "Mauicetus" waitakiensis, and "Mauicetus" brevicollis, the early odontocete Notocetus marplesi, the possible archaeocete Kekenodon onomata, and fossil penguins, sharks, etc. However, it was not until the 1980's when Ewan and his dedicated preparator Andrew Grebneff received a National Geographic grant to excavate more fossils - and since, there has been an explosion in the volume of Oligocene marine vertebrates (particularly cetaceans) from the South Island of New Zealand. Many of these localities are in North Otago and South Canterbury, but I won't give out any more detailed information due to the sensitivity of the localities.
We arrived at this first locality - a lime quarry - to meet up with 3rd year student (and part time fossil preparator) Nichole Moerhouse, who was collecting data for a research project on the depositional environment of the Kokoamu Greensand and the Otekaike Limestone. The Kokoamu Greensand is early Late Oligocene in age (~30-26 Ma), and is a relatively thin richly glauconitic sandstone overlying a temporally significant unconformity, which is angular in places. It gradually transitions to the latest Oligocene-earliest Miocene Otekaike Limestone (~25 Ma), which can be 'sandy' in places, and basally contains glauconite. The Kokoamu-Otekaike section represents an overall shallowing - glauconite can only form in relatively deep environments (at least deeper than 'middle' shelf) during very slow sedimentation), and a gradual transition from slow offshore glauconite-rich deposition to inner shelf/shoreface calcareous deposition with abundantly preserved invertebrates. For those less familiar with stratigraphy and sedimentology - this is a fairly straightforward and commonly encountered type of depositional 'sequence' in marine strata, and generally represents an initial deepening of the shelf (possibly due to the continental shelf being down-dropped due to tectonic subsidence, or an increase in sea level) and subsequent filling of the basin and sediment marching out onto the shelf from the shoreline. Understanding concepts like this is crucial to paleontology, as these processes are going to affect the preservation, abundance, and three dimensional distribution of fossils within a body of rock.

Here, Tsai climbs up an exposure of the Otekaike Limestone to look for cetaceans. This is the only photo my camera took that day - it's particularly finicky and doesn't like cheap batteries, unfortunately for my wallet.

Nicole and I looking at a fossil dolphin in the Otekaike Limestone - it's below Tsai's feet, who took the picture. Ewan had spotted this years before, and sent Tsai and I up to locate it. It was exposed when the original road bed to the quarry was dug out (what we're standing in), and in the time since, a lower road was dug (the trench can be seen to the upper right).

 A fossil baleen whale mandible! Unfortunately it was not in great shape, but there was a thin flat bone above it - which could be a maxilla, the large flat bone in the snout of a baleen whale. It may be worth excavating in the future. We found nothing else of note at this locality. We visited two other spots - at one of which I spotted what most likely is a partial jawbone from a small dolphin. At the third locality for the day, we walked around and found a few cetacean bone fragments, part of a penguin vertebra, a penguin coracoid, a swordfish vertebra, and part of a large shark vertebra. Ewan also found a partial cetacean earbone, part of which had been scraped away by heavy machinery. He went and fetched his special chainsaw - and after 45 seconds of work, had a block of sandstone with the earbone inside. I am extremely interested in bringing this excavation method back to the states, as it reduces the time spent excavating by 75% or so.

Next up: pictures from a hike into native New Zealand forest.

Sunday, April 8, 2012

An Eocene pinniped? A critique of Diedrich (2011)

Several months ago I was kindly asked by Dr. Cajus Diedrich to remove this post. I have edited certain parts for content. Following the mantra that "extraordinary claims require extraordinary evidence", I've decided to leave the post up to inform those interested in fossil pinnipeds with a series of critical comments and observations regarding the "Eocene" seal. -R.W. Boessenecker, 11/28/2012

While I was in Montana enjoying my first christmas vacation with my in-laws, I got an email with an attached pdf of a new paper that had just been published. I had expected quite a bit of time to get some work done -after all, my wife and I were up there for a week and a half and her parents would be at work much of that time - and the weather was too poor to go and do anything outside. I had already expected to get some reading and writing done, so I was pleased to hear of the new publication. Once I saw the title page, though, I was immediately skeptical, and indeed - my skepticism did not go away once I was finished with the article. The title of the article was "The world’s oldest fossil seal record", and the abstract indicated that an Eocene seal - not just any pinniped, but a phocid seal - had been discovered in Germany. I was not familiar with the author, Cajus Diedrich, whose previous work has focused on other groups (Pleistocene carnivores, Triassic marine reptiles) - but I did remember reading an article by him revising placodont (you remember, those funky sauropterygian marine reptiles with big crushing teeth) paleoecology, suggesting that most placodonts were Triassic analogs of sea cows.

The skeleton of the dawn seal, Enaliarctos - the oldest known
bona fide pinniped. From Berta et al. (1989).

Before I go any further, I should summarize why exactly one should be skeptical of an Eocene pinniped. For starters, the majority (either by taxa or number of specimens, ~99%) of fossil pinnipeds are from the Miocene or younger strata. There are a handful of bona fide pinnipeds from the late Oligocene, though, which are represented by skulls. These include Enaliarctos tedfordi from the Yaquina Formation of Oregon (~28-25 Ma), and Enaliarctos barnesi from the uppermost Yaquina Formation or lowermost Nye Mudstone (~26-23 Ma, also Oregon). There are a bunch of other species of Enaliarctos known from the early Miocene and roughly 20-25 Ma in age, including Enaliarctos emlongi from near the Nye Mudstone-Astoria Formation contact in Oregon, Enaliarctos mitchelli (early Miocene Jewett Sand of California and Nye Mudstone of Oregon), and Enaliarctos mealsi from the Jewett Sand. Slightly younger fossil assemblages from the Astoria Formation, further upsection in the Newport Basin of Lincoln County, Oregon, show a mix of "enaliarctine" pinnipeds (Pteronarctos, Pacificotaria), two species of Desmatophoca, an early 'allodesmine', and the dawn walrus Proneotherium (Barnes, 1989; 1990; 1992; Barnes and Hirota, 1995; Kohno et al., 1995). Note that during the early Miocene in the North Pacific, yes - pinnipeds first start to diversify and different groups (e.g. non-"enaliarctines"), but most of these are either "enaliarctines", members of a wholly extinct clade (Desmatophocidae), or extremely archaic and "enaliarctine"-like members of extant clades (Proneotherium).

The femur of the dawn seal, Enaliarctos. This is Enaliarctos emlongi
from Oregon. From Berta (1991).

In other words, there aren't any crown-clade pinnipeds anywhere close to the Oligo-Miocene boundary. In the North Atlantic, the oldest known pinniped with diagnostic remains is Leptophoca from the middle Miocene Calvert Formation of Maryland, crania of which were described by Koretsky (2001). In fact, this is the oldest bona-fide and widely accepted record of fossil phocids. Irina Koretsky and Al Sanders published a paper in 2002 about partial fossil femora reputedly from the late Oligocene of South Carolina (I have discussed these specimens elsewhere). To summarize - these fossils are roughly 10 Ma older than Leptophoca and were presented as being 1) the oldest fossil phocids, and 2) evidence for pinniped diphyly. For the uninitiated, there have been several morphological hypotheses for pinniped evolution, and the diphyletic view states that true seals (Phocidae) are related to mustelids/musteloids, and sea lions (Otariidae) and walruses (Odobenidae) form a monophyletic clade (Otarioidea) and share a common ursid-like ancestor, having adapted to water separately from phocids. I won't go into it now, as the Eocene seal is totally separate from diphyly/monophyly. There are several problems with Koretsky and Sanders (2002) - 1) they did not examine femora of other Oligocene terrestrial mammals, and 2) the stratigraphic provenance of those specimens are questionable (see here for more on this). To summarize: late Oligocene pinnipeds consist of only a few diagnostic fossils from the North Pacific, and modern "family" level clades do not appear until a ways into the Miocene.

The femora of the alleged Oligocene seal. From Koretsky and Sanders (2002).

Phew, now that the introduction is done, I can talk about the paper. Diedrich (2011) published a partial, proximal femur fragment (just like Koretsky and Sanders 2002), from the Fürstenau Formation of Northern Germany, which is a Lutetian age shallow marine unit deposited on the southern margin of the pre-North Sea basin, roughly 45-49 Ma in age. So, we're not even talking latest Eocene and bordering on Oligocene - this is early middle Eocene, about 5 Ma before basilosaurids show up, just before protocetids evolve. The fossil itself is phosphatized and exhibits several borings, is missing the distal end, and has clearly been reworked (for the uninitiated: phosphatization can only occur on bones or sediment below the sediment-water interface, so a bone that is both phosphatized and abraded or polished by default has been reworked). It does look remarkably phocid-like: it lacks a fovea capitis for the teres ligament - a little pit on the femoral head. It also appears to genuinely lack a lesser trochanter (as opposted to being abraded or broken off) and is very anteroposteriorly flattened. All of these features are phocid or pinniped characteristics; the lack of a fovea capitis is a probable pinniped synapomorphy (Berta and Wyss, 1994). A lesser trochanter is absent in all modern and fossil phocids and the modern walrus, but present in all fossil walruses and modern and fossil otariids. Interestingly, these are all the same features listed by Koretsky and Sanders (2002) to identify their femora from the Oligocene of South Carolina. I'm not sure that erecting the name Praephoca bellunensis for this fragment of an element with dubious diagnostic utility was prudent.

The holotype femoral fragment of Praephoca bellunensis, the alleged
Eocene seal. From Diedrich (2011).

So far so good. When oddball fossils like this crop up - ones that just smell fishy - the best thing to do is to see if something could have gotten seriously screwed up between it leaving the ground and entering the annals of a journal. Reading the Materials and Methods, it goes through a long description of the large excavation conducted at Dalum, Germany, where the Fürstenau Formation is exposed. Buried toward the end of the Materials and Methods section, I found this:

"The femur illustrated in Figure 2 was actually found was actually found in these gravels during the 1980s, and has prompted a major program seeking to understand the biodiversity of marine vertebrates in Europe during the Eocene, in relation to that of the terrestrial vertebrates. This femur, together with all material from the 2011 excavations, is housed in the Shark Center at Bippen (SCB) in northwest Ger-many, a public visitor center and museum in the UNESCO- supported “Geo and Naturpark TERRA. Vita”."

So, it sounds like this started with the discovery of the fossil femur 25-30 years ago, and then the excavation was undertaken in May 2011. Who collected the fossil originally? I just don't know. Whoever it was - especially if was an amateur collector unfamiliar with local stratigraphy - do we know that they were able to positively remember the exact locality and horizon at which the fossil was collected? 25-30 years is a long time - and people often have unreliable memories, which is why most scientists can't afford to not write important things down. I have also met collectors who have admitted to intentionally making misleading statements to researchers about the locality and provenance of certain specimens, and I've met collectors who can't remember what they collected last week. To be clear, I know many collectors who know local stratigraphy very well and remember the exact location, time, date, etc. of a fossil collection. The same variable quality of memory exists within paleontologists - which is why we really must write everything down. To summarize, the stratigraphic provenance is poor, and it is not clear if the fossil really came from that locality or not. Secondly: the fossil is reposited at the "Shark Center at Bippen". Look it up - the only results are the pdfs of Diedrich's articles. I'm not sure where this place is, or who runs it.

Modern pinniped femora (from right to left - walrus, California sea lion, and harbor seal), arrow showing the position of the lesser trochanter. While it is reduced and absent in the modern walrus, it is present in nearly all fossil walruses for which femora are known (e.g. Imagotaria, Gomphotaria, Valenictus, Proneotherium).

An additional bit of interesting contextual data is a paper published on the results of the May 2011 excavation (Diedrich 2012), which yielded 13,690 shark teeth (!!!!), 206 ray teeth, a handful of other marine vertebrates, and two indeterminate mammal bones. Not even a single isolated seal tooth; my own field collecting suggests that you should find a tooth for every 5 pinniped bones or so, and perhaps a pinniped bone for every 10 shark teeth (Purisima Fm. data from my still unpublished Master's Thesis). So, where are they? There should have been hundreds of phocid fossils, and there aren't even any cetacean bones (probably because it's too old for archaeocetes; only a couple of protocetid and remingtonocetid specimens are known from Europe, and it's too early for basilosaurids). Cetaceans are almost always more common than pinnipeds in any given marine assemblage. It just doesn't add up.

Cladogram with fossil-calibrated molecular divergence dates, modified from Fulton and Strobeck (2010). This study isn't perfect by any means (and warrants further discussion on this blog), but is a hell of a lot closer to the mark than what Praephoca would do to this cladogram.

Another line of evidence are molecular divergence dates for pinnipeds, and the "fissiped" carnivoran fossil record. The most recent molecular divergence dates published by Fulton and Strobeck (2010) suggest an Oligo-Miocene divergence of basal pinnipeds (this is, however, based on Enaliarctos as a fossil calibration). The pinniped + mustelid divergence is in the latest Eocene, and the caniform divergence occurs just earlier in the early-middle Eocene. The earliest true carnivorans don't even appear until the Eocene, and the earliest possible Caniformia appear about 42 Ma - about 3 Ma after this alleged seal fossil. Purported pinniped sister taxa like Amphicticeps, Amphicynodon, Pachycynodon, and Allocyon don't show up until the Oligocene; apparently more pinniped-like taxa like Kolponomos and Puijila aren't even in the picture until the earliest Miocene. Just on grounds of parsimony, given the ranges of these other taxa - this record should be considered suspect. Accepting Praephoca at face value, and putting it into a phylogeny would 1) telescope nearly all cladogenesis within the Caniformia ~30 million years earlier than previously thought, and 2) add dozens of ghost lineages for nearly every caniform clade across the entirety of the Oligocene and halfway across the Eocene, at that. Where are all the fragmentary scraps of the dozens of other crown-clade carnivorans in the early Eocene? They just don't exist, although they would be a natural consequence of having phocids in the Eocene.

On the other hand, I am glad that the study got published, because it gives us something interesting and controversial to talk about it - just as long as molecular systematists don't take it too seriously. This is a nagging worry, as I've seen in happen before (e.g. the Milinkovitch 1993 hypothesis, which I will talk about another time). In all seriousness - there are a number of problems with the work of Diedrich (2011), and should not be taken at face value. Extraordinary claims require extraordinary evidence - a busted up femur that may or may not be from a phocid seal and may or may not have been collected at the same site which later produced nearly 14,000 vertebrate fossils and not a single other pinniped element is not extraordinary evidence. 

References/further reading:

L. G. Barnes. 1989. A new enaliarctine pinniped from the Astoria Formation, Oregon, and a classification of the Otariidae (Mammalia: Carnivora). Contributions in Science 403:1-26

L. G. Barnes. 1990. A new Miocene enaliarctine pinniped of the genus Pteronarctos (Mammalia: Otariidae) from the Astoria Formation, Oregon. Contributions in Science 422:1-20

L. G. Barnes. 1992. A new genus and species of middle Miocene enaliarctine pinniped (Mammalia, Carnivora, Otariidae) from the Astoria Formation in Coastal Oregon. Contributions in Science 431:1-27


C. Diedrich. 2011. The world's oldest fossil seal record. Natural Science 3(11):914-920.

C. Diedrich. 2012. Eocene (Lutetian) Shark-Rich Coastal Paleoenvironments of the Southern North Sea Basin in Europe: Biodiversity of the Marine Furstenau Formation Including
Early White andMegatooth Sharks. International Journal of Oceanography doi:10.1155/2012/565326

T. L. Fulton and C. Strobeck. 2010. Multiple fossil calibrations, nuclear loci and mitochondrial genomes provide new insight into biogeography and divergence timing for true seals (Phocidae, Pinnipedia). Journal of Biogeography 37:814-829

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

I. Koretsky. 2001. Morphology and systematics of Miocene Phocinae (Mammalia: Carnivora) from Paratethys and the North Atlantic region. Geologica Hungarica Series Palaeontologica 54:1-109

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.


X. Wang, M. C. McKenna, and D. Dashzeveg. 2005. Amphicticeps and Amphicynodon (Arctoidea, Carnivora) from Hsanda Gol Formation, Central Mongolia and phylogeny of basal arctoids with comments on zoogeography. American Museum Novitates 3483:1-57

Saturday, April 7, 2012

New Zealand Eomysticetidae - first look

Starting in the late 1970's and early 1980's, Dr. R. Ewan Fordyce received a grant from National Geographic to start conducting extensive fieldwork on the South Island of New Zealand in search of Oligocene cetaceans and other marine vertebrates. Incidentally, non-cetaceans such as abundant penguins, sharks, and bony fish were collected as well. This fieldwork was not limited to the Oligocene, but also included forays into Paleocene, Eocene, and Miocene localities. Over the past 30 years, Ewan has established a massive collection with an astonishing number of beautiful cetaceans. Not only are these fossils beautiful in terms of their preservation, but many of them are extremely bizarre, and the assemblage as a whole includes squalodontid, squalodelphinid, ?dalpiazinid, kentriodontid, and waipatiid odontocetes, as well as several types of baleen whales (toothed mysticetes, Mauicetus and similar "cetotheres", eomysticetids, and others), and even late surviving archaeocetes. Some of these cetaceans have been described, including Waipatia and an eocene archaeocete (Zygorhiza sp.), and several other taxa are on their way to being described.

I first met Ewan in 2005 at the SVP meeting in Arizona, and I vividly remember watching him discuss how to excavate fossil whales with a chainsaw, of all things; I immediately thought it was too extreme of an excavation method for me, but after I saw it in action last monday in the field (ironically, at the same quarry the photographs from the 2005 presentation), I immediately decided I would bring this method back to the United States. In fact, it was quite funny after watching him rev up the chainsaw - an adrenaline-inducing activity in and of itself - and afterwards stating in a polite Kiwi accent "that should clean up quite nicely".

But I digress - prior to graduation from Montana State University last spring, I contacted Ewan about a Ph.D. project, and he suggested studying the large collection of eomysticetid baleen whales from the Kokoamu Greensand and Otekaike Limestone that he had established. I remembered his talk from the 2006 SVP meeting in Ottawa, part of which included a slideshow of beautiful new fossil eomysticetids. I was pretty shocked to have been offered such a beautiful (and large!) collection of fossils to study. The family Eomysticetidae was named by Al Sanders and Larry Barnes in 2002 to accommodate the new taxon Eomysticetus, which is the most primitive described toothless baleen whale (i.e. baleen-bearing baleen whale, if that makes any sense, as opposed to a toothed baleen whale). Previously, the most primitive toothless mysticetes were some of the "cetothere" whales described by Remington Kellogg from the Chesapeake Group on the east coast, AKA "Kelloggitheres"; these however were much younger than any toothed mysticete (such as aetiocetids), and there was an apparently substantial morphological gap between toothed mysticetes and Kelloggitheres. My job is to fill a bit more of this gap in with more eomysticetids from the southern hemisphere- and so far, none of them seem to be identifiable as Eomysticetus, and there are probably several new genera and species represented.

Ewan Fordyce also took on another student recently, which was a total surprise for me. Even when I first got here, it sound like it would be several months away; instead, the new student arrived only two weeks after I did, and even stayed in the same temporary apartment my wife and I stayed in the first week we were here. Cheng-Hsiu Tsai, who goes by just 'Tsai', will be studying the other big group of fossil mysticetes from the Oligocene of New Zealand: Mauicetus and Mauicetus-like mysticetes, which may be the earliest Kelloggitheres. Tsai can be seen inspecting the ventral side of one of the eomysticetid skulls in the above photo.

This specimen, for example, is one of my dissertation specimens: a new taxon, with an extremely narrow rostrum, elongate dentaries, enormous temporal fossae with a long intertemporal region, and really weird squamosals.
Yours truly, examining the extraordinarily freaky squamosals of the specimen.

Tsai, demonstrating the proper way to photograph a mysticete skull.

Yours truly, demonstrating how to use yourself as a scale bar. I am 5'8" tall.
The beautiful skull in oblique view.

Tsai examining the skull. The brass seam on the floor is actually a joint where the floor opens for a small elevator used to bring large fossils up from the basement. On thursday, I spent most of the afternoon lifting a really really heavy plaster jacket a total of about eight feet - this ordeal took about an hour and a half, three other students, Ewan, and our preparator, Sophie. Fortunately, when the jacket is prepared, it will hopefully be a lot lighter when it goes back downstairs.

Thursday, April 5, 2012

Visit to the Otago Museum

I know there are some readers of this blog who have patiently waited and waited for pictures of beautiful Oligocene marine mammal fossils - to you I say, sorry for the delay. I'm going to try and get several blog posts written this weekend so I can post them incrementally. This one will mostly be in 'slideshow' format.

I've been fairly busy since I got here, and I've bordered on stress trying to figure out 1) where all the eomysticetid specimens are in collections, 2) which earbones belong to which skull or skeleton (just taking a while to become familiarized with the specimen numbers), 3) trying to make some sense out of the earbones and trying to group them based on consistently seen characteristics (and I have made a bit of headway), and 4) just generally trying to figure out how many taxa I am dealing with and thus 5) how many manuscripts/dissertation chapters this will end up making. Since I've finally made some headway and started describing the first material (a partial skull with earbones and a very partial postcranial skeleton), I've relaxed a bit and can allocate time to other activities. That being said, I'm also locked out of the building for four days due to construction/maintenance activities in the building. Fortunately, this will give me an opportunity to divert some time to my Pelagiarctos study with Morgan Churchill. Also, in other news - I finally finished up my massive manuscript describing an entire marine mammal assemblage from a locality in the Purisima Formation, which resulted in being just over 200 double spaced pages long with 45 figures; Felix Marx graciously offered to take a look, as did Ewan Fordyce. I have a bit of work left cleaning up some figures, but it should be submittable soon.
A spectacularly beautiful dalpiazinid dolphin! Look at those damn teeth! There's another specimen with even crazier incisors, and a full dentition, and jaw.

An archaic edentulous mysticete which may fall somewhere on the cetacean family tree near eomysticetids. This specimen will be part of my dissertation.

The holotype skeleton of the giant moonfish Megalampris keyesi. This set of slabs is seriously about 15 feet long and about 8 feet wide. Described by Gottfried et al. 2006.

A disarticulated skeleton of a squalodelphinid dolphin. My labmate and office mate Yoshi Tanaka is studying squalodelphinids for his dissertation (although their skulls are in better shape than in this specimen).
A partial skeleton of the giant shark Carcharocles angustidens, described by Gottfried and Fordyce (2001). Believe it or not, this specimen was found above the dolphin and moonfish skeletons in the same quarry; the shark was found first, and underneath they ran into dolphin bones; below that, they started seeing fish bones (from what turned out to be a truly monstrous fish). They called the shark Carcharodon angustidens instead, as Mike Gottfried is in the Carcharodon camp; that's fine, we all get along pretty well. Mike will be visiting University of Otago for paleo research in May, which will be a great opportunity to catch up.
Detail of the big, beautiful teeth of Carcharocles angustidens.

Beautiful jaw fragment of the undescribed squalodelphinid from the block photographed above.

The skull of the "Shag Point Plesiosaur", now known as Kaiwhekea. That's pronounced "Ky-feh-key-uh"; one Maori pronunciation is "wh" as an 'f'.

The holotype skeleton of Kaiwhekea; yes folks, that's all one gigantic concretion that is ~20 feet long. It took a crew of 3-6 to collect those blocks over the course of a month (each day).

More photos will be coming soon!