By Mauricio Antón
08 May 2014
04 May 2014
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 :
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.
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.
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.
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).
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.
|Helicoprion by Ray Troll|
|Huehuecuetzpalli mixtecus from Reynoso 1998|
|Baja California in the Cretaceous, by William Stout|
|Mexican fossil horses, UNAM Museo de Geología|
|Mexican megafauna mural by Iker Larrauri|
Museo Nacional de Antropología
01 May 2014
|Smilodon Mosaic in the Nebraska State Capitol|
- 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|