New fossil provides clarity to the history of Alligatoridae

Discovery made at Dalquest Desert Research Site

New fossil provides clarity to the history of Alligatoridae

Families are complicated. For members of the alligatorid family, which includes living caimans and alligators - this is especially true. They are closely related, but because of their similarity, their identification can even stump paleontologists. 

But after the recent discovery of a partial skull, the caimans of years past may provide some clarity into the complex, and incomplete, history of its relatives and their movements across time and space. 

Michelle Stocker, an assistant professor of vertebrate paleontology in Virginia Tech’s Department of Geosciences in the College of Science, Chris Kirk of the University of Texas at Austin, and Christopher Brochu of the University of Iowa, have identified a 42 million year old partial skull that may have belonged to one of the last prehistoric caimans to roam the United States.

“Any fossil that we find has unique information that it contributes to understanding the history of life,” said Stocker, who is an affiliated faculty member of the Fralin Life Sciences Institute and the Global Change Center. “From what we have, we are able to understand a little bit more about the evolutionary history of caimans and the alligatorid group, which includes alligators and caimans.”

Their findings were published in PeerJ, an open access peer-reviewed scientific journal covering research in the biological and medical sciences.

The fossil was discovered in 2010 at Midwestern State University’s Dalquest Desert Research Site, which includes extensive exposures of the Devil’s Graveyard Formation, a geologic formation in the trans-Pecos volcanic field of West Texas. The Devil’s Graveyard Formation preserves fossils from the latter portion of the Eocene epoch, a period of time covering 15 million years of prehistory.

In 2011, Stocker and the team returned to the site to collect a key bone that remained in the hard sandstone block that once encapsulated the caiman skull.

“The Devil’s Graveyard Formation provides a unique window into the evolution of North American vertebrates during the middle and late Eocene,” said Kirk. “There are a host of extinct species that are only known from the Devil’s Graveyard, including several primates, rodents, lizards, and now this new fossil caiman.”

What they discovered was a partial skull. At the time of the discovery, paleontologists were convinced that the skull came from a closer relative to alligators than to caimans.

“When you are at the early diversification of groups, their features aren’t as differentiable,” said Stocker. “It was harder to tell if this is more closely related to caimans or to alligators because those two are really closely related already. And the differences between them are subtle, especially early in their evolutionary history.”

The skull’s braincase was a key component in the identification of the fossil. The braincase encases and protects the brain from injury. Since no two species have the same braincase, finding one can provide some much needed information for paleontologists.

After further investigation into this fossil’s braincase, Stocker and the team were able to determine that this was most likely a caiman.

The caiman was deemed to be 42 million years old using a combination of investigative techniques, including radiometric dating, biochronology, and biostratigraphy, where paleontologists use the relative order of the fossilized animals to find out how old the rocks are.

With the age of the fossil and its location in mind, paleontologists are able to add to an ever-growing story about a large biogeographic range contraction, or a climate-related extirpation, that occurred millions of years ago.

Michelle Stocker and Rachel Wallace seen excavating the caiman fossil in 2011.

Michelle Stocker, an assistant professor of vertebrate paleontology in Virginia Tech’s Department of Geosciences in the College of Science, and Rachel Wallace, a former graduate student at the University of Texas at Austin, are seen excavating the caiman fossil from sandstone in January of 2011. Photo courtesy of Chris Kirk.


Roughly 56 million years ago, the planet was experiencing temperatures so hot and methane levels so high that no polar ice caps could form. For large cool-blooded reptiles like alligators and caimans, it was their time to thrive and soak up the sun. In fact, the conditions were so favorable that these early reptiles roamed as far north as northern Canada.

“The presence of a fossil caiman in the Devil’s Graveyard, about 1200 kilometers north of where caimans are found today, really says something about how different the climate of West Texas was in the middle Eocene”, said Kirk.

But one epoch later, in the Oligocene, the entire world was experiencing cooler temperatures, forcing many species that require warm and humid conditions into more restricted geographic ranges. Caiman populations, in particular, are now only found in South and Central America. Although, a small number of caimans have been found outside of this range and are thought to be invasive species.

“This caiman seems out of place,” said Brochu. “Caimans today are a South American radiation, and data from modern forms, including DNA, would suggest a very simple single origin from a North American ancestor. This new form, along with some older North American fossil caimans, suggests a far more complex early history with multiple crossings of the seaway that separated North and South America until fairly recently.”

There is even more to know about caiman history. Since the specimen was an incomplete skull, and far from a complete skeleton, paleontologists still have some knowledge gaps to fill about their relationships.

“If we can find another individual, we will get a better sense of its relationships, and it might be able to say something about what variation could be present in this taxon, or how they grow, or where else they might be found,” said Stocker. “This is a one and done kind of fossil right now. Hopefully there are more out there.”

MSU Texas’ DDRS gives researchers views to the past

By Kathy Floyd

Midwestern State University’s Dalquest Desert Research Station near the Big Bend area of West Texas has been a rich resource of fossils and scientific data for researchers not only from MSU Texas but from other universities as well. Dr. Christopher Kirk, Professor of Anthropology at UT, has conducted paleontological fieldwork in the Devil’s Graveyard Formation on the Dalquest site since 2005.

Dr. Walter Dalquest was professor of biology at MSU Texas from 1952-1984, and continued to work part time until his death in 2000. He originally bought two sections of the land for hunting in 1968. He would also collect small mammals, geodes, and agates. He realized the scientific value of the land and in 1996, he and his wife, Rose, donated the land to MSU Texas with the restriction that it be used only for scientific research by biologists and geologists. In 2004, Rose provided funds to purchase adjacent sections from the State of Texas.

Dr. Norman Horner, former Director of Natural Laboratories at MSU Texas, said that the site has some of the best early primate fossils of anywhere in the world. “That area of Texas was tropical and a prime habitat for the early development of primates 45-50 million years ago,” he said.

In 2016, Kirk and researchers from The University of Texas at Austin’s Jackson School of Geosciences found a new species of a legless lizard fossil and named it in honor of Texas and an MSU botanist. The “Lone Star” lizard's official name is Solastella cookei— the species name in honor of Biology Chair Dr. William Cook. Solastella is a Latinized form of “lone star.”

The primate fossils Diablomomys dalquesti and Mescalerolemur horneri were named in honor of the Dalquests and Horner. Diablo refers to the Devil’s Graveyard Formation and Omomys, a closely-related fossil primate. Mescalerolemur is derived from the Mescalero Apache Indians that were in the area and a primitive lemur. The species name honors Horner because of his assistance in allowing Kirk to work at the DDRS.

In 2019, a new family of spiders found on the DDRS was recognized by taxonomists. Myrmecicultor chihuahuensis was originally found in 1999 by one of Horner’s graduate students, Greg Broussard.