I spent the morning in a symposium on early vertebrates. The first talks were on the Cleveland Shale, a Devonian unit from this area that has produced large numbers of fossil fish; 66 species in fact, according to the first talk by Robert Carr and Gary Jackson. One of the most common Cleveland fossils is the giant placoderm Dunkleosteus (top and below):

Two talks, the first by Philip Anderson and the second by Eric Snively et al., discussed the variation in both shape and function in the placoderms, a group of jawed vertebrates from the Devonian. The second talk in particular looked at possible different feeding strategies in juvenile and adult Dunkleosteus, as well as in the even larger Gorgonichthys.

The next talk, by John Maisey, was about Cladoselache, a Devonian shark known from numerous well-preserved specimens from the Cleveland Shale, like the specimens below at the Cleveland Museum of Natural History:

Here is the American Museum of Natural History’s reconstruction of Cladoselache:

In spite of these beautiful specimens, Maisey showed that there is a great deal that isn’t known about this shark, including basic things like the number of gills and the structure of the dorsal fin.

Henning Blom and Mark Wilson presented papers on jawless vertebrates from the Silurian and Devonian (these groups are sometimes lumped together as “agnathans” or “ostracoderms”); this model of an anaspid (the topic of Blom’s paper) is from the Texas Memorial Museum:

Wilson’s talk was on another jawless group, the thelodonts. These had fantastic forked tails, and Wilson showed that their mouths were not just simple holes but may have in fact had a mobile lip-like structure.

John Long and Kate Trinajstic presented on new placoderm specimens from Australia. Incredibly, two of these specimens included embryos, including a mineralized umbilical cord in one specimen, showing that at least this group of placoderms used in vitro fertilization and gave live birth.

After lunch, I headed to a different room for some mammal talks. Julie Meachen-Samuels presented data on the forelimb modifications in saber--toothed cats and nimravids (like Barbourofelis, below, from the Florida Museum of Natural History) compared to other cats.

Saber-toothed forms have more powerful forelimbs than typical cats of the same size, but (surprisingly to me) the forelimb morphology of the two groups converges for very large body sizes.

Wendy Binder and Julie Meachan-Samuels presented a second paper on saber-tooths, specifically Smilodon, looking at sexual dimorphism. It turns out that Smilodon shows less sexual dimorphism (the differences between the sexes) than do lions.

Lauren Berg and Nick Pyenson gave an interesting talk on the relationship between diving depth and the size of the eye socket in pinnipeds (seals, sea lions, and walruses). It turns out that, for most pinniped groups, the deeper the species dives the larger the size of the eyes (most surprising in that no one had bothered to check before).

In the poster session Jason Schein et al. looked at the preservation of fossils across the Cretaceous-Tertiary boundary in New Jersey, to see if it was a lag or a condensed section. A lag deposit is formed when a rising sea erodes pre-existing sediment. The small particles are carried away, while the large particles (including the fossils) are redeposited as a bonebed in the newly formed sediment layer (these are typically referred to as “reworked”). In a condensed section, instead of the redeposition of fossils, the area is simply starved of new sediment for a long period of time. Any animals that die there lay on the surface for a long time without being buried, and over a long period of time a bonebed accumulates (sometimes called an “attritional deposit”). I was interested in this talk because these are the same issues we face in trying to interpret the origin of the Carmel Church bonebed.

Blaine Schubert present a study of changes in tooth shape and body size over time in the short-faced bear Arctodus (below, from the Oklahoma Museum of Natural History).

Arctodus got much larger over time, and the anterior grinding teeth getting relatively larger, suggesting a change in diet.

Kathleen Smith and Dan Fisher looked at the dentin thickness in the tusks of a female American mastodon (below, from the Cleveland Museum of Natural History).

By looking at the variation in thickness of the dentin layers, they were able to determine when that particular mastodont reached sexual maturity. In tusk year seven (about age nine), the tusk shows an abnormally low rate of growth in the spring for the first time. This is mating season, and apparently female elephants (and presumably mastodonts) are so harassed by males attempting to mate that they have difficulty getting enough to eat. The tusk then shows two years of low growth (while she was pregnant and then nursing), followed by three years of increasing growth (after the baby was weaned).

After the poster session I headed to a reception at the Cleveland Museum of Natural History, which has a great collection of Devonian fish from the Cleveland Shale, as well as many other fossils including the one-of-a-kind Haplocanthosaurus, a primitive sauropod dinosaur (this photo is from a trip to this museum from several years ago).