SVP, Day 2 (updated)

This morning was the Romer Prize session, a competition for student research. While I didn’t stay for the entire session, I did make it to several talks. Taketeru Tomita attempted to look at shark feeding strategies based on tooth shape (essentially, “swallowers” that take their prey whole versus “cutters” that dismember their prey (such as the Cretoxyrhina above, from the Nebraska State Museum). He concludes that near the end of the Early Cretaceous that was a change in size and feeding habits of sharks, with a move toward much larger prey.

Nick Pyenson presented an interesting talk on comparing the records of modern whale strandings on the Pacific coast to the actual whale fauna living in the area, and found that, as long as the coastline is long enough, the records match pretty well. By implication, that suggests that attritional deposits like the Sharktooh Hill bonebed should also represent the fauna from that time pretty well.

Nick’s talk is an example of an old problem in paleontology, that gets discussed at length in college paleontology classes, of death assemblages versus life assemblages. The fossil record is composed of, well, fossils, specifically organisms that died and were preserved. The likelihood of a particular species being preserved as a fossil is not necessarily a function of how common that species was in the living ecosystem, because of various preservational biases. Strictly speaking, the fossil record is simply a indicator of which organisms became fossils. To begin reconstructing a past ecosystem these biases should be considered.

I went to the Caroline Rinaldi et al. talk on the giant beaver Castoroides because you can never hear too much about giant beavers (like the one below from Earlham College).

This talk specifically looked at the enamel ridges that run along the incisors of the two different species of Castoroides. These ridges always run perpendicular to the cutting edge of the tooth (even though the cutting edge is oriented differently in each species). When the enamel at the tip of the tooth wears, the enamal ridges give the tooth a serrated cutting edge (like a steak knife), making for a more efficient cutting action.

There were two talks on the desmostylians (such as Palaeoparadoxia, above, from the American Museum of Natural History), a strange extinct group from the margins of the Pacific Ocean. Brian Beatty looked at the changes in tooth shape with age in various desmostylians, which may have resulted in juveniles and adults of the same species receiving different names. Hikaru Uno et al. described a multi-faceted project attempting to get at the feeding preferences for two different types of desmostylians that are found together in the same deposits, Desmostylus and Palaeoparadoxia, using a combination of tooth wear, carbon isotope ratios, trace element ratios, and skull morphology. As an example, the amount of the trace elements strontium (Sr) and barium (Ba) are inversely related to trophic level. This means that plants have relatively large amounts of Sr and Ba, but in herbivores eating the plants the amounts of Sr and Ba is a little lower, and lower still in carnivores eating the herbivores. Desmostylus and Palaeoparadoxia have different concentrations of Sr and Ba, indicating that they were feeding at different levels in the food web. Uno et al. also suggested that the mouth structure in Desmostylusindicates that it was a suction feeder, sucking clams out of the mud in estuaries much like the modern walrus.

There were two fascinating presentations on the development of the vertebral column and the pelvis (sacrum). Sterling Nesbitt had a poster on how sacral vertebrae are added to the pelvis in different vertebrate groups, suggesting that they might add from the lower back (dorsosacral), add from the tail (caudosacral), or add new vertebrae in between the ancestral sacral vertebrae (insertion).

Emily Buchholtz had a presentation on essentially the same topic, specifically looking at tree sloths (like Choloepus, below, from the Omaha Zoo).

Sloths are relatively unique among mammals in that they have a highly variable vertebral column, and usually don’t have seven neck vertebrae (as almost all other mammals do). Buchholtz made a convincing case that the variation in sloths is because growth of the vertebrae, the upper parts of the ribs, and the lower parts of the ribs (along with the sternum and the pelvis) are controlled by different genes. The expression of these genes at different points in development causes the vertebral variation (and also means that sloths really do have seven neck vertebrae).

There a lot of posters that caught my attention tonight:

Phil Bell described a specimen of the theropod dinosaur Gorgosaurus (a type of tyrannosaur) with a whole suite of bone pathologies in the neck, upper right forelimb, and right foot. At least one injury was apparently caused by a rolled ankle that tore a ligament, while some of the others were infections spreading from bone to bone. The gorgosaur survived for some time in spite of the injuries.

Following the theme of both sloths and pathologies, Monalise Cruz and Deise Henriques reported on a specimen of the giant ground sloth Eremotherium from Brazil (like the one below from the North Carolina State Museum) with pathologies in the tail vertebrae. The nature of the injuries indicates that ground sloths did use their tails as a prop when rearing up on their hindlegs, as has long been suspected.

Brian Redmond and Greg McDonald showed a specimen of the ground sloth Megalonyx jeffersonii from Ohio (the model below is from VMNH) which showed evidence of butchering by humans. This is the frist evidence of humans butchering Megalonyyx in North America. A carbon-14 date on the femur was 11,740 years (± 35) is the oldest record of human activity in Ohio.

Tess Van Orden and Stephen Godfrey showed two examples of unusual bone preservation in the dugong Metaxytherium (below, from the Calvert Marine Museum) from the Calvert Formation in Maryland. The bones in this specimen had essentially been partially dissolved by an unknown mechanism, although they ruled out abrasion from current activity.


Update: following up on the comments on vertebral development, here is an image from my paper from a few years ago. The apparent extra head on each of these ribs is actually an extra, fused rib that results from having an extra thoracic vertebra (at the expense of a cervical vertebra):

This paper is available for sale at VMNH for $1.00 (email orders). The reference is:

Dooley, A. C. Jr. “Double-headed” ribs in a Miocene whale. Jeffersoniana 8, 8 p.

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5 Responses to SVP, Day 2 (updated)

  1. Decker says:

    Hi Dr. Dooley
    I didn’t know that tree sloths don’t have seven neck vertebrae. I have long suspected that the Boy Scouts’ Mammal Study merit badge pamphlet needed revision (there are a few things I noticed wrong with it) but this fact makes it really wrong. Steve Churchill and Lee Berger taught our Field School in South Africa that all mammals had 7 neck vertebrae as well (even giraffes). Did the presenters say that the tree sloth’s condition is derived, or does it mean that the ancestral condition of mammalian skeletal structure different than what we have previously thought?

  2. Alton Dooley says:

    Hey, Decker!

    The tree sloth condition is derived.

    The mammalian pattern of seven cervical vertebrae goes all the way back to the Jurassic; it may even predate the 3 middle ear bones, with I think of as being the most definitive mammal character.

    In the tree sloths, the genus Choloepus (2-toed sloth) typically has 6 neck vertebrae, while Bradypus (3-toed sloth) has 8-10. Fossil ground sloths, incidentally, have 7 like almost all other mammals.

    The really interesting thing about Buchholtz’s paper is that she’s proposing that the tree sloths really do have 7 cervical vertebrae like other mammals. In mammals, the cervical (neck) vertebrae are defined as the vertebrae immediately after the skull that do not articulate with a rib. Almost always, the 8th vertebra is the first one to have a rib. Buchholtz suggests that in tree sloths the development timing of the ribs and the vertebrae has decoupled, so that the vertebrae are developing as they should, but the ribs are developing too early (in Choloepus) or too late (in Bradypus), giving the appearance of a change in vertebral count.

    That’s not the end of the story, though. One other mammal, the manatee (Trichechus), only has 6 cervical vertebrae. In the question session after her talk, Buchholtz indicated that she didn’t think the mechanism is the same in manatees and sloths.

    I think in manatees the change in count might be due to a homeotic transformation. This is a mutation in a homeotic (or HOX) gene, which controls body segmentation (or at least a change in it’s activation during development).

    Homeotic transformations resulting in 6 cervical vertebrae also occur in a small percentage of humans, and in baleen whales. A few years ago I wrote a paper describing this sort of transformation in a fossil whale (although at the time I wasn’t familiar with HOX gene research). In whales and humans, this results in an extra thoracic vertebra (and rib); often the extra rib fuses to the first normal rib, giving the appearance of a double-headed first rib. If I can find the images from that paper, I’ll post them as an update.

  3. Doug says:

    Right you are! Never can get enough giant beavers.

    So desmostylians had different feeding habits, eh? I have heard that they were plant eaters and also that they were shellfish eaters. So I guess one was the latter and the other that former. Is that right?

    The sloth femur with butcher marks sounds cool.

  4. Alton Dooley says:

    With a name like Palaeoparadoxia you know it will be hard to pin down!

    In that talk, they actually didn’t speculate about Palaeoparadoxia’s feeding habits, except different from Desmostylus.

  5. Tess Van Orden says:

    Hi Dr. Dooley,

    This is Tess (from the Metaxytherium presentation), I was pleasantly surprised to see someone took a bit of an interest in Dr. Godfrey’s and my work. That certainly was an interesting project to work on and really opened my eyes to the world of Taphonomy!

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