04 May 2014

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.

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