Fossil (Gondwanan) Vertebrate of the Month: Thylacosmilus atrox

Thylacosmilus atrox
By Mauricio Antón

Some animals are not as weird as we often think they are (zebras, for example, are really just horses with stripes, a not-uncommon color pattern among mammals).  Some animals are far stranger than people tend to realize (elephant trunks are like no other structure that has ever evolved).  Among my favorite animals, though, are those that people tend to recognize as being strange but that are, in fact, even more bizarre than they might appear at first glance.  The prime example among living animals is the platypus, which is aberrant in almost every single way possible.  This month's featured fossil vertebrate provides a nice paleontological example of an animal who's outer coating of weirdness just hides more profound weirdness underneath.  Thylacosmilus atrox lived in the savannas of South America in the Mio-Pliocene of South America.  A reasonably well-known fossil mammal, it is often referred to as the marsupial sabertooth, a name that highlights the two superficially strange things about it.  Thylacosmilus had among the largest sabers of any predator ever to have lived, and this trait has drawn comparisons to saber-toothed cats and to the cat-like Barbourofelis.  Unlike cats and barbourofelids, though, Thylacosmilus was not a carnivoran.  In fact, it wasn't even a placental mammal, but rather a metatherian more closely related to kangaroos and opossums than to cats.  So, was Thylacosmilus a marsupial that evolved convergently with cats on a continent without carnivorans?  This is certainly how it is often described and depicted (perhaps most notably by the great Czech paleoartist Zdenek Burian), but this view of Thylacosmilus does not do its weirdness justice.  To begin with, it wasn't actually a marsupial, but a member of the Order Sparassodonta a group of stem metatherians endemic to South America.  This means that, while Thylacosmilus was more closely related to marsupials than to placentals, it and its sparassodont kin were only distantly related to both groups.  Secondly, while Thylacosmilus certainly had its share of cat-like features, it was not a perfect felid analog.  As Julie Meachen has shown, there is more than one way to be a sabertooth: some carnivorans have sturdy sabers and gracile arms, while some have long, delicate sabers and robust arms, and some (such as Xenosmilus) don't fall neatly into either category.  With its long, thin sabers, it is tempting to think of Thylacosmilus as the South American version of Smilodon or Barbourofelis, but does the rest of the body follow suit?  As mentioned in a previous post, my student Laurel Perper has been looking into this question by visiting the type specimen of T. atrox at the Field Museum.  Her results show that some aspects of the forelimbs of Thylacosmilus are similar to those of saber-toothed cats, but the robustness of its humerus turned out to be literally off the charts, meaning that Thylacosmilus had arms so beefy that they compare better to those of bears than to even the burliest of cats.  So why is Thylacosmilus so similar to cats in some ways but so different in others?  Is it because it was descended from a very different group of mammals, from which it would have inherited a very different "evolutionary toolkit" than did cats?  Were differences in the environment and climate between North and South America responsible for the weirdness of Thylacosmilus?  Were its robust arms an adaptation to preying upon the unique megafauna of the Pampas, which would have included armored glyptodonts, enormous toxodonts, and sloths and rodents orders of magnitude larger than their living relatives?  Perhaps Thylacosmilus evolved as it did because of competition with other South American predators, most notably the giant "terror birds."  Between the measurements Laurel has made at the Field Museum and those that I intend to make in my upcoming trip to South America, my students and I are hoping to have sufficient data to be able to start answering these questions soon, so if they piqued your curiosity as much as they did mine, stay tuned!

An Appreciation of Mexican Fossils

It's May 5th, or, if you prefer to use Spanish, el Cinco de Mayo, which means it's time for perhaps the stupidest of all "holidays" (Mexico has a cultural and historical legacy that we in the US can only dream of and yet, generally speaking, the only time we celebrate our neighbor to the south is by getting drunk on cheap margaritas, bad tequila, or Corona on what is not even a major holiday in the country the day allegedly honors).  That said, there are many aspects of Mexico that deserve to be celebrated: the dazzling scientific achievements of the Maya, the monumental architecture and sculpture of the Aztecs, the stunning Baroque design of Spanish colonial buildings, and the powerful modernist art of Diego Rivera, Frida Kahlo, and their contemporaries all spring to mind.  However, Mexico has a heritage that predates its oldest archaeological artifacts.  I can hardly do justice to the entire Mexican fossil record in one off-the-cuff blog post, but here are some highlights of the country's 200+ million year history of vertebrates :

Helicoprion by Ray Troll

The oldest vertebrate fossil in Mexico (or at least the oldest that I could find in the Paleobiology Database) is a specimen of the whorl-toothed "shark" Helicoprion.  Not only is the age of the specimen noteworthy, but the location from which it was recovered, in the central Mexican state of Puebla, is as far south as this most bizarre of chondrichthyans has ever been found, making it geographically superlative as well.

Huehuecuetzpalli mixtecus from Reynoso 1998

Puebla is also home to the most spectacularly-preserved fossils in all of Mexico.  Tlayúa Quarry near Tepexi de Rodríguez represents a Cretaceous reef and preserves a primarily marine fauna, though a few terrestrial vertebrates have been found there as well.  Tlayúa has drawn comparisons to Solnhofen, which is very high praise.  While visiting the collections at the Universidad Nacional Autónoma de México a few years ago, I was fortunate enough to be able to be able to view some of the specimens from this site, and I can vouch for the fact that they are nothing short of spectacular.  The bulk of the vertebrate fossils from the site are fish, but several reptiles have been uncovered as well.  UNAM has a long-standing research program at the site and has established a museum there as well, making this one of the best-studied localities in the country.  The icing on the cake of this spectacular site is that many of the new genera and species from Tlayúa have been named in Nahuatl, giving them marvelous names to match their marvelous preservation; my personal favorite is the early iguana relative Huehuecuetzpalli mixtecus.

Baja California in the Cretaceous, by William Stout

The best place to find dinosaurs in Mexico is the north, particularly in Baja California and Coahuila.  This dinosaur fauna dates to the latest Cretaceous and is dominated by hadrosaurs, some of which grew extremely large.  Incidentally, if this post gets you really excited about Mexican fossils but you lack a passport or the money or time to fly south of the border, you can always check out the Fossil Mysteries exhibit at the San Diego Natural History Museum, the Cretaceous section of which focuses on fossils from the Two Californias (Alta and Baja) and features an excellent mural of Mexican dinosaurs by William Stout.

Mexican fossil horses, UNAM Museo de Geología

Volcaniclastic sediments throughout Mexico preserve a Miocene fauna of which the most impressive and abundant members are horses.  When I travelled to UNAM, it was these fossils that I was visiting, as they were part of my dissertation research on body size evolution in mammals.  Interestingly, and counter to any predictions that one would make based on modern mammals, horses in central Mexico and Oaxaca were much more similar in size to those from Oregon than to those from California or Nevada (an observation for which I have - as yet - no good explanation, though it is intriguing that both landscapes are highly volcanic).  However, my work is just a small, fairly inconsequential drop in the bucket of Miocene mammal studies in Mexico; the one person who has done more to fill the bucket than anyone else is Ismael Ferrusquía-Villafranca, whose lab is responsible for much, if not most, of what we know about the Mexican Neogene (who, besides being a hugely important figure in the field, also happens to be a really nice guy to whom I am still exceedingly grateful for allowing me to work with the collections he has helped amass).

Mexican megafauna mural by Iker Larrauri
Museo Nacional de Antropología

When the Isthmus of Panama closed, Mexico became part of the land bridge connecting North and South America.  Much of the wildlife that migrated north colonized Mexico before the rest of the continent, meaning that South American migrants such as sloths and glyptodonts are well-represented in the Mexican Pleistocene, as are native taxa such as mammoths and saber-toothed cats.  This Panamerican fauna forms the bulk of the displays in the UNAM's fantastic Museo de Geología, as magnificent an example of a classic academic museum as you will find anywhere (also the home of most of the important works of José María Velasco, who really should be one of your favorite old-school paleoartists).  Mexico's Pleistocene fossil record also overlaps with the country's incomparable archaeological record.  To cite just two examples, a mammoth skeleton from Santa Isabel Iztapan shows evidence of butchery by humans, while a camel sacrum from Tequixquiac carved to resemble the head of a wolf or coyote represents one of the earliest works of art in Mesoamerica.

Conference Review: North Central GSA

Smilodon Mosaic in the Nebraska State Capitol

Ask a random person on the street what state they most closely associate with fossils and, after having stared at you you just long enough to determine just what kind of lunatic you are for asking such a bizarre question, they will probably respond with one of the Northern Rockies states (I'd bet that the most common response would be Montana, though I have no actual data to back this up).  However, a case could be made that they all should be answering Nebraska, which is probably the best place on Earth to find Cenozoic mammal fossils (it's also very good for Cretaceous marine fossils, if that's the sort of thing that floats your boat).  This is best illustrated by proboscideans, which have been found in 91 of the state's 93 counties, but mammals of all kinds have been found throughout the state.  The University of Nebraska has had an active paleontology program since E.H. Barbour's arrival in 1891, and the quality of the collections and research at the university have made Lincoln a paleontological mecca ever since.  Needless to say, then, when the Geological Society of America holds a regional meeting in town, paleontology is going to headline the program.  I attended the North-Central GSA in Lincoln last week, and here are some of my thoughts on the paleontology currently being conducted in the middle part of the continent (the abstracts and presentations can be viewed here if you want more detailed information on any of the talks):

• Nebraska professor Jason Head spoke about a subject very near and dear to my heart: how the fossil record can inform models of future ecological change.  Ecological niche models take the climatic conditions in which an organism lives today and use these to predict where the range of that animal might shift as climate warms in the future.  However, many studies have shown that these models generally fail (sometimes spectacularly) to predict ranges of species during periods of past climate change.  Jason Head's study was interesting because he used fossil occurrence data and paleoclimatic reconstructions for boid snakes to augment ecological niche models, and found that the models that included paleoecological data predicted somewhat wider ranges for rubber and rosy boas in the future than did models based purely on modern data.
• Did you know sloths have an especially high rate of dental anomalies (i.e., they frequently have either fewer or extra teeth)?  I didn't until I saw Robert McAfee's talk last week.  What's more, not only do living tree sloths often have abnormal numbers of teeth, but so did extinct ground sloths.  In fact, McAfee suggested that rates of caniniform tooth loss in Paramylodon from Rancho La Brea were so high that the population may represent the beginning of a speciation event that was nipped in the bud by the Pleistocene megafaunal extinction.
• Joseph Peterson spoke about dinosaur paleopathology, and in particular on the use of 3D reconstructions of a hadrosaur vertebra with tooth marks on it (as well as theropod jaws) to determine the identity of the animal that bit the vertebra.  Turns out it was most likely a juvenile Tyrannosaurus.
• My old officemate Jonathan Calede spoke about his dissertation work in the Cabbage Patch Beds of Montana.  The Cabbage Patch Fauna (named after a bar, not the creepy dolls, to paraphrase Jonathan) has been uncovered from several sites, which Jonathan found to be broadly similar in terms of geology and quality of preservation, suggesting that any trends observed between these sites represent genuine ecological patterns rather than taphonomic bias.
• Nebraska's Shane Tucker discussed one of the coolest fossil sites of which I've ever heard: the Happy Jack Mine in the central part of the state.  The tunnels of the mine intersect infilled rodent burrows from the Miocene, allowing the 3D structures of these burrows to be observed.  On balance, the burrows most closely match those of ground squirrels, though gophers may have been responsible for some.  Shane and George Corner also presented talks about Miocene faunas from elsewhere in the state, underscoring the absurd richness of the Nebraska fossil record.
• Ross Secord and his student Tom Baldvins both spoke about isotopic records across two important intervals of climate change: the Paleocene-Eocene Thermal Maximum and the glacial-interglacial cycles of the Pleistocene, respectively.  I was particularly interested in Baldvins' talk, which showed that C4 grasses were more abundant in the northern Great Plains during interglacial periods.  More intriguingly (to me, at least) his analysis of horse body size from the same sites showed a positive correlation with temperature (i.e., horses were larger in warmer climates).  Always nice to see another study that shoots holes in Bergmann's Rule, though I'd be curious to see if the trend would persist with a larger sample size and using tooth-based estimates of body size.
• An entire session was devoted to the White River Group and its fossils, which was not only interesting in and of itself, but gave some great ideas for the design of the Sedimentology & Stratigraphy class I'll be teaching next year.  Particularly intriguing to me was Kurt Spearing's talk on the first evidence of inner ear bones in nimravids.  Ear morphology is hugely important in carnivore phylogeny, and I have high hopes that more nimravid ear ossicles will turn up in the coming years, which could potentially help determine where in the feliform tree these enigmatic animals belong.
• I also spoke at the conference (on the humerus morphology of felids and whether or not it can be used to identify the huge cat that was present in the Late Miocene of western North America), but perhaps more importantly, for the first time I had a student presenting.  Laurel Perper discussed some of the early findings of our project comparing the morphology of the bizarre South American marsupial sabertooth Thylacosmilus, the subject of this month's upcoming Fossil Vertebrate of the Month feature.

Carnivore reconstructions by Mark Marcuson

I would be remiss if I finished this post without mentioning that, as expected, the University of Nebraska was an excellent host.  Besides pulling off a very well-run conference, Lincoln remains an excellent place to spend a few days (if nothing else, it's one of the only places in the northern Plains where you can reliably find good food and drink after 8:00).  As I always do when I'm in town, I spent some time in the University of Nebraska State Museum, home of, among many other things, the largest collection of fossil proboscideans in the world.  They were also featuring an exhibit on the paleoart of Mark Marcuson, whose murals I'd seen in the museum before but with whom I had been largely unfamiliar.  Another paleoartistic effort of which I'd previously been ignorant but that I visited this time on the suggestion of Nick Famoso is the mosaic on the floor of the Nebraska State Capitol (see picture above), which depicts reconstructions of fossils animals from trilobites to mammoths.  Any state that puts prehistoric life front and center in the rotunda of its capitol is a good one in my book, and as far as I'm concerned cements Nebraska's status of most fossily of fossiliferous states.