19 April 2011

Fun With Body Mass Estimates

The latest issue of the Journal of Vertebrate Paleontology has been a minor media sensation because it includes a description of the largest known rabbit, Nuralagus rex from the Pliocene of Menorca (as neat an illustration of the Island Rule as ever there was).  For those of us interested in carnivores, though, the issue also included the description of, as the authors put it, 'a gigantic otter.'  The otter is from the Pliocene Awash region of Ethiopia - site of many a famous hominid discovery - and is a new species in the genus Enhydriodon: E. dikikae.  Giant animals are always fun to read about, and they're even more so to someone writing a dissertation on mammal body size evolution.  The paper includes dental measurements for the new species, including the dimensions of the first lower molar (the carnassial, for those that know your carnivore teeth), which is tightly correlated with body size.  I couldn't resist plugging the E. dikikae measurements into the body mass regression for mustelids developed by Blaire Van Valkenburgh to get some kind of idea of the size of the animal.  There are two specimens included in the paper: the smaller, more complete tooth suggests a 77 kg animal, while the larger, incomplete tooth may represent an animal of 126 kg.  The former mass would put the individual in the same size range as modern giant otter (Pteroneura brasiliensis), but the larger individual would be in a class by itself; a 126 kg otter would be roughly the same size as a large jaguar or a small lion.  Of course, as with any body mass estimate for exceptionally large (or small) animals, these numbers should be taken with a grain of salt: the very fact that E. dikikae is so big means that it falls well outside the range of masses in living mustelids (with the not insignificant exception of the giant otter) and that any estimate of its weight is therefore extrapolating beyond the available data (the same issue has been raised for mass estimates of giant South American rodents).  Still, it's fossils like this that remind me why I consider myself lucky to be a paleobiologist; after all, what other field would publish journals describing giant rabbits and otters in the same issue?  Life, and in particular prehistoric life, is just so cool.

02 April 2011

Fossil Vertebrate of the Month: Baryonyx walkeri

Last month's fossil vertebrate was an Irish icon, so in honor of April's St. George's Day, it only seems fair to put the spotlight on an English animal.  The earliest dinosaurs to ever be described were English, but for the most part the end of the Victorian Era was also the end of new dinosaur discoveries in Great Britain.  A few new taxa have come to light in the previous few decades, though, perhaps the most impressive of which is the bizarre Baryonyx walkeri.  Uncovered in Surrey in 1983 and named in 1986, the genus name translates as 'heavy claw,' a reference to the large, recurved claws on the animal's hands.  The other noteworthy feature of Baryonyx is its elongated skull with a kinked jaw, remarkably similar to that of a crocodile.  This skull, in conjunction with fish scales found within the specimen's body cavity, led to the conclusion (since challenged by some, but still largely accepted) that Baryonyx was piscivorous.  It's unusual morphology meant that the relationship of Baryonyx to other dinosaurs remained a mystery for some years, though the discovery of similar dinosaurs have shown that it was a member of the Spinosauridae, a group of large (in at least one case very large), sometimes sail-backed, likely fish-eating group of theropods that lived mainly on the southern continents during the Cretaceous.  Baryonyx was very closely related to the much-publicized African spinosaur Suchomimus tenerensis; in fact, the two were likely members of the same genus.