Today was the last day of the SVP meeting. I started off the day with a walk through the posters.
Yoshitsugu Kobayashi and others reported an amazing slab from the Cretaceous of China containing specimens of the aquatic choristodere Monjurosuchus. The specimens included one adult female surrounded by six young juveniles, providing the first evidence of parental care in this group. (Another choristodere from the same unit, Hyphalosaurus, is shown above.)
Andrew Beach examined two large specimens of Triceratops (example below) at the BYU Museum. There have been recent publications that proposed that Triceratops was the juvenile form of the large ceratopsian Torosaurus. One of the arguments against this hypothesis was the presence of exceptionally large Triceratops specimens such as those at BYU. Beach showed that once allowance was made for the reconstruction work done to one of the skulls, these specimens don’t refute, and may even support, the relationship between Triceratops and Torosaurus.
Ryan Littlewood and Laura Vietti developed a model of marine bone bed formation, to try and determine the likelihood of these units developing, by using certain starting assumptions concerning bone decay rate and sedimentation rate. Basically, they concluded that it’s almost impossible to get a whale bone bed to form, unless there either is a process that removes sediment or a process that concentrates the bones. This obviously has some bearing on Carmel Church.
SVP always seems to schedule the marine mammal talks for the very last technical session, and this year was no exception. Mark Uhen reported a new protocetid whale from the middle Eocene of Mississippi (the skull of the Pakistani protocetid Rodhocetus, on display at the University of Nebraska, is shown below).
Ewan Fordyce reported a new, apparently primitive squalodont from New Zealand. I’ve talked about the squalodonts several times on the blog, so I was obviously really anticipating this talk. The new squalodont really emphasizes how conservative this group was; they all look a lot alike, and it’s difficult even for experts to pin down what features can be used to define the group. The New Zealand specimen also has significant tooth wear, which we also see in the North American squalodonts like Squalodon whitmorei below (skull and tooth, both from the US National Museum). This tooth wear suggests that the squalodonts may have been biting into something hard, like bones.
Gabriel Aguirre-Fernandez and Ewan Fordyce also reported the occurrence of small kentriodontid dolphins from New Zealand at around the same time they appear in the Calvert Formation. This establishes that the group had spread worldwide fairly early in their history.
John Graf and others also reported some new whales, but these were Pliocene baleen whales from Angola. They interpret these as the first known fossils of the family Neobalaenidae, the pygmy right whales (modern example below from the US National Museum). The skulls represent two different taxa, and I have to admit I found one more convincing than the other, but it certainly appears that at least one of them is a pygmy right whale.
Jessica Martin, Annalisa Berta, and Tom Deméré looked at the effectiveness of using particular taxa for calibrating molecular clocks. There are two basic methods of determining when two taxa split and began evolving along separate paths from each other. One is to be lucky enough to have a detailed fossil record with reliable dates, so that the split can be directly observed. The other is to use molecular clocks, essentially determining the rate at which biomolecules diverge. However, molecular clocks can usually only be used in extant animals, and they require some type of calibration, which is often fossils. The commonly used fossil taxa tested in this study actually seem to perform quite well.
Jonathan Geisler and others presented a nice overview of the cetacean cochlea, which is contained inside the petrosal (such as the small odontocete example below, from Carmel Church). They found that characters that seemed in modern species to be related to high-frequency hearing appear to be more randomly distributed in fossil taxa.
Romana Govender and Anusuya Chinsamy-Turan reported on extensive bite marks on Pliocene whale bones from the west coast of South Africa. The bites seem consistent with the great white shark, Carcharodon carcharias, with still lives off the South African coast. (Bite marks below on a Pliocene whale jaw from Peru.)
Shoji Hayashi and others attempted to use internal bone structure to infer the swimming capabilities of desmostylids, an unusual group of probable-marine mammals from the Oligocene and Miocene of the Pacific (such as Palaeoparadoxia, below, from the American Museum of Natural History). They clearly showed that the desmostylan bone structure is more like marine mammals than terrestrial ones. They then attempted to tie rib density to swimming ability, suggesting that species with more dense ribs were poorer swimmers. I’m not sure I agree with this assessment. Rib density has been related to feeding style in sirenians, and possibly cetaceans, so I think it’s more likely that the differences they’re seeing in desmostylans are related to difference in feeding preferences.
Mark Clementz and Jason Sewall continued with marine herbivores, looking at stable carbon ratios in sirenians (sea cows). Carbon 12/13 ratios in sea grasses correlate with temperature, and those ratios will be reflected in the teeth of sea cows that feed on them. They found that in Eocene sea cows the ratios were about the same in all sea cows regardless of latitude, but that after the Eocene the ratios from different latitudes tended to differ. This shows that, during the Eocene, there was very little change in ocean temperatures across different latitudes, but that this changed beginning in the Oligocene.
In the final talk, Ryosuke Motani and Isabel Montañez studied the relationship between the evolution of marine tetrapods (animals that moved back into the ocean) and various other factors, including sea level and temperature. They found that these events tend to occur when sea level is high and temperatures are warm (although the warm correlation is more pronounced in reptiles than in mammals). This is interesting data, although I’d like to see more evidence that it’s not an artifact (that is, since sediments are more common when sea level is high, and high sea level tends to correlate with high temperature, that those are the times that the fossils are likely to be preserved). Even so, it is an interesting study of how global events can potentially affect evolutionary patterns.
So, that wraps up my four days at the SVP meeting. Tim and I begin driving home tomorrow.