Sunday, March 25, 2012

5 Questions for Aaron Leblanc

It's been a bit quiet around here lately as I plug away at my thesis, but here's an interview with UALVP alumnus Aaron Leblanc, now a PhD student at the University of Toronto. Aaron's description of the new mosasaur Eremiasaurus was published in the January issue of the Journal of Vertebrate Paleontology.

1. What inspired you to conduct this study?

My inspiration for this study comes from my work on an undergrad honors thesis with Dr. Michael Caldwell back in 2007–2008. I remember being shown potential subjects for my project and being drawn to the skull of what is now Eremiasaurus right away. Back then only the left side of the skull was prepared and really didn’t look like much, because most of the elements of the skull had been badly crushed and displaced from their original positions (Al Lindoe did a great job preparing the other side of the University of Alberta specimen, but that didn’t happen until I had started my Master’s thesis on the same subject over a year later). Even at that point you could tell that the teeth of this mosasaur were really unusual. I think the teeth alone were what drew my attention and got this whole project rolling.

2. How does a mosasaur wind up in Morocco?

By the end of the Cretaceous, mosasaurs were everywhere, but this wasn’t the case around 90 million years ago when the first mosasaurs took to the seas. At that point in time, mosasaurs were rather small, occasionally reaching lengths of two meters or so. More importantly, these early mosasaurs were probably still capable of walking on land and may have been restricted to shallow marine habitats. Coincidentally, the early fossil record of mosasaurs is quite restricted geographically. Most of the early mosasaur fossils (a group traditionally called the aigialosaurs) have been recovered from Croatia, Slovenia, and possibly from Texas. If you fast-forward to the Maastrichtian, the time interval from which Eremiasaurus is found, mosasaurs had become incredibly large (some reaching body lengths of 15 meters) and their remains have been recovered from every continent, including Antarctica. The occurrence of mosasaurs in Morocco isn’t a new discovery, but it is certainly becoming an increasingly more valuable data point the more we learn about their diversity and paleoecology during that time and in that particular region of the globe. The Late Cretaceous seas of Morocco were home to some very unique mosasaur taxa that occupied nearly every aquatic predatory niche, from hard shell-crushers like Globidens phosphaticus to (what I believe, anyway) fish-eaters like Eremiasaurus heterodontus.

3. What is special about the tail of Eremiasaurus?

Nobody paid particular attention to the tails of mosasaurs until a recent study of Plotosaurus bennisoni from the Maastrichtian of California by Johan Lindgren and colleagues back in 2007. Their work was incredibly important, because it challenged a traditional view of mosasaurs as resembling giant sea serpents, slithering through the water using side-to-side motions of their whole bodies to move through the water. What Lindgren et al. (2007) were able to show is that there is anatomical evidence that the tails of more advanced mosasaurs were much more specialized for aquatic locomotion than this historical view. The vertebrae at the base of the tail (called pygals) did not allow much side-to-side movement and the bony supports for a tail fin were pushed further back along the body. What this means is that at some point in their evolutionary history, mosasaurs abandoned that slithering style of swimming for a more thunniform (tuna-like) body shape and swimming mode. This represents a more efficient swimming style for cruising the open waters and engaging in pursuits of food, instead of lurking and ambushing. Mosasaur researchers aren’t as lucky as those that study ichthyosaurs (an unrelated group of marine reptiles), because there are no recorded cases of soft tissue outlines of the tails of mosasaurs. While it is only speculative, the current view is that mosasaurs had a crescent-shaped tail fluke like an ichthyosaur or a Great White shark. The vertebral column in well-preserved mosasaur specimens, including Eremiasaurus, has a gentle downward bend that starts just behind the pelvic girdle, and the tall neural and haemal spines (upwards and downwads projections of the tail vertebrae respectively) form a broad sweeping fan at the back of the tail that would have supported an expanded dorsal tail fin made entirely of soft tissue.

Where Eremiasaurus differs from other mosasaurs is in how far back the bony supports of the tail fluke have been “pushed” compared to other species. While it isn’t the most ichthyosaur-like tail ever reported for a mosasaur (though it is a close second), it still suggests that it adopted a more thunniform style of swimming than most other related forms.

4. What is unusual about the teeth in Eremiasaurus?

The teeth are certainly the most conspicuous features of the skull of Eremiasaurus. I always imagined it having a fiendish grin, especially when looking at the right side of the skull of the University of Alberta specimen. The teeth are unusual in that they change shape so dramatically along the length of the jaws and along the roof of the mouth (mosasaurs had a second row of upper teeth that lined the pterygoid bones, just for good measure). The front teeth are straight and cone-shaped,

the middle ones are slender triangular blades,

and the back teeth are more bulbous and hooked.

Whatever Eremiasaurus was eating, it was very good at tearing it apart into smaller more palpable pieces. The front teeth interlock when the jaws are closed, making for an efficient trapping mechanism, while the middle and back teeth are serrated and shear past each other like a pair of scissors to cut and dismember prey. The pterygoid teeth are curved backwards and would have kept struggling prey in the mouth. From an evolutionary perspective, attributing these kinds of teeth, tail anatomy and other unusual features of the skull to anything but a new genus and species was all too difficult.

5. What does Eremiasaurus tell us about the relationships among mosasaurs?

Adding Eremiasaurus to an existing phylogeny of mosasaurs added a great deal more complexity to the story, particularly within the subfamily Mosasaurinae (the group to which Eremiasaurus belongs). In many ways Eremiasaurus is an intermediate between two well-established groups of mosasaurines, something that was clear from the first phylogenetic analysis we performed. Where these relationships became more complex was in a second analysis in which we added and updated information to the existing data set. We performed this second analysis, because many of the characters and species of mosasaurines that were originally used back in 1997 by Gordon Bell were in need of revision. New species had been described since then and had not incorporated into such an analysis, while others had since been re-examined and re-described. Using this updated data set, we were able to conclude that many of the features previously thought to be diagnostic for one of the major groups of mosasaurines were actually present in most of the members of the other group as well, or were secondarily lost. Because of this, many of the previously well-established relationships within the subfamily Mosasaurinae broke down in this analysis, suggesting that there is a need to reconsider some of our classifications, or re-evaluate the characters that we use to define them.

Thanks Aaron! You can read more about Eremiasaurus in:

Leblanc ARH, Caldwell MW, Bardet N. 2012. A new mosasaurine from the Maastrichtian (Upper Cretaceous) phosphates of Morocco and its implications for mosasaurine systematics. Journal of Vertebrate Paleontology 32:82-104.