05 July 2012

Fossil Vertebrate of the Month: Rutiodon carolinensis

At the end of last month, I spent a week out East, visiting the collections at the American Museum of Natural History and in Chapel Hill at a workshop that I would recommend to any geologists out there that are in the late stages of a PhD or the early stages of a postdoc.  While the two states have followed very different historical trajectories, to vertebrate paleontologists New York and North Carolina have one big thing in common.  The Newark Supergroup is a series of Late Triassic and Early Jurassic formations that run from Nova Scotia to the Carolinas; representing a wet, seasonal rift valley environment, these rocks are among the best places in the world to find fossils from early in the "Age of Dinosaurs" (and certainly the best in North America outside of the red beds of the Southwest).  Pictured above is perhaps the most iconic fossil from the Newark Supergroup, the phytosaur Rutiodon carolinensis.  Any student that's ever taken a class from me will be familiar with phytosaurs, which are one of my favorite examples of convergent evolution.  Superficially, members of this now-extinct group were very similar to modern crocodiles and doubtless filled a similar ecological niche (piscivores/ambush predators).  However, several features of the skeleton show that they are not the direct ancestors of crocodiles (which, in the Triassic, were primarily small, agile, and land-living); the most notable of these is the placement of the nostrils above the eyes rather than at the end of the snout as is the case in alligators and crocodiles.  The specimen pictured above is a historically significant one.  As the species name suggests, the first Rutiodon specimens to be uncovered were found in North Carolina.  One of these specimens, from a coal mine near the town of Egypt, was acquired by W.D. Matthew in 1895.  He brought it back to New York, where it became the first vertebrate fossil in the collection of the American Museum of Natural History (it bears the specimen number AMNH 1), which in the subsequent decades would grow to become the largest collection of fossil vertebrates in the world.

12 March 2012

Fossil Vertebrate of the Month: Panthera atrox

P. atrox in the Natural History Museum of L.A. County
John Orcutt, 2010
At least in Oregon, this March has certainly come in like a lion, which inspired this month's FVOTM: Panthera atrox, the American lion.  There has recently been some debate over whether or not this common name is correct: while very lion-like, Per Christiansen and John Harris have suggested that it was actually more closely related to jaguars.  I come down on the other side of the debate: the morphological data are, at best, ambiguous (and I would argue that they tend to favor the interpretation of P. atrox as a true lion) and molecular data from subfossil specimens indicate that P. atrox was very closely related to both modern lions and to European cave lions (in fact, some have argued that all three species really should be lumped into one).  Panthera atrox is best known from the "tar" seeps of Rancho La Brea, where it is the most common conical-toothed cat (though not the most common cat; that honor goes to the saber-toothed Smilodon).  Because it is less common than its machairodontine "cousin," it has been suggested that, unlike modern lions, P. atrox might have been solitary; this should come as no surprise as modern lions are very unusual among cats in living in large social groups.  Panthera atrox certainly would have been large enough to tackle prey on its own.  In fact, with the largest individuals weighing in at somewhere around 350-400 kg, it was probably the largest conical-toothed cat ever, and would have been in the same size range as the largest saber-tooths (as well as a few species of bear).

01 February 2012

Fossil Vertebrate of the Month: Nothrotheriops

Nothrotheriops in the Nevada State Museum, Las Vegas
John Orcutt, 2011
Sloths have figured prominently on this blog lately (see my posts on Megatherium and Megalonyx from the past year).  That theme continues this month with Nothrotheriops, a genus of ground sloth that has been found throughout the southwest US and northern Mexico (and at one site in Florida), including within the Phoenix metro area, site of this month's Western Association of Vertebrate Paleontologists annual meeting.  There are two known species: N. texanus and its apparent descendant N. shastensis, both of which lived during the Pleistocene.  While my previous sloth posts have focused primarily on the taxon's place in the history of science, Nothrotheriops is remarkable as an example of how paleontological data can inform our knowledge of both paleo- and modern ecology.  This is in large part because it lived primarily in arid climates and apparently frequented caves, possibly even using them as sites for dens, meaning that Nothrotheriops has a fantastically high preservation potential and that preserved soft tissue and dung are not uncommon.  Analysis of the dung has been particularly informative, providing direct evidence of the plants eaten by the sloth (primarily desert globemallow) as well as of interactions with other organisms (fungus gnat larvae fed on vegetable matter in the dung, which also contains traces of parasitic nematodes).  Perhaps the most remarkable aspect of Nothrotheriops ecology, though, is its connection to Joshua trees.  Despite being icons of the Mojave Desert, Joshua trees may be doomed to extinction (or at least widespread local extirpation) due to their extremely slow dispersal rate keeping them from shifting their range in response to climate change.  Based on relatively large amounts of Joshua tree material having been found in Nothrotheriops dung, it is not unreasonable to assume that the sloths, in eating and excreting seeds, served as important dispersal agents for the trees.  A recent paper has suggested that the extinction of sloths and other desert megafauna made the Joshua tree an effectively immobile species.  While this hypothesis awaits a rigorous paleontological test, if it proves to be correct (as seems very likely) it will have important implications for the conservation of Joshua trees, underscoring the importance of fossil data in predicting and mitigating the effects of future climatic change.

09 January 2012

Fossil Vertebrate of the Month - Lystrosaurus

One hundred years ago this month - on January 6th, 1912 - Alfred Wegener presented his idea that the continents had once all been joined (in a supercontinent he termed Pangaea) and were slowly floating apart as part of a process he called continental drift.  His hypothesis was largely derided at the time, but would eventually evolve into plate tectonics, which is now the unifying theory of geology.  Numerous lines of evidence were brought together to establish the validity of plate tectonics, but among the most convincing was the presence of remarkably similar fossils on distant continents.  Perhaps the most famous of these organisms was the Permo-Triassic dicynodont Lystrosaurus.  First described from South Africa, the discovery of Lystrosaurus fossils in Antarctica in the 1960s showed beyond a reasonable doubt that the two continents must have been joined (it has subsequently been uncovered in India, East Asia, and Europe, driving home the reality of plate tectonics even further).  Besides its utility as a biogeographic marker, Lystrosaurus is remarkable for being one of the few survivors of the Permian-Triassic Extinction, which, by some estimates, wiped out over 90% of life on Earth.  Why it was able to survive this cataclysm and to prosper in its aftermath is something of a mystery, as in many ways Lystrosaurus is a very unimpressive animal (they are often referred to as the pigs of the Triassic).  In fact, my original dissertation project (before I was romanced by its distant mammalian relatives) was to be a test of the hypothesis that Lystrosaurus, as a burrowing animal, was adapted to the low-oxygen conditions that may have characterized the Early Triassic.  In another Oregon-related note, the picture above is of a model from Prehistoric Gardens, south of Coos Bay, one of the more atmospheric and well-preserved "dinosaur parks" in the world.