A new Ice Age species was hiding in la brea tar pits fossils for decades

The find is exceptionally rare. Only one other extinct Pleistocene amphibian has been discovered in North America (a tree frog from Florida). Although S. labreae was tiny compared with the mammoths and saber toothed cats associated with La Brea Tar Pits, its fossils could provide important clues about the climate and environmental changes that occurred near the end of the last ice age.

The species is described in theJournal of Vertebrate Paleontology. The discovery also highlights the new findings continuing to emerge from the Samuel Oschin Global Center for Ice Age Research at La Brea Tar Pits as the museum undergoes a historic transformation.

An Unexpected Discovery in an Old Fossil Collection

Lead author Dr. Alberto Cruz did not set out to discover a new species. He began studying the Tar Pits' relatively understudied reptile and amphibian collections while working there as a Postdoctoral Fellow. Cruz remains a Research Associate at La Brea Tar Pits and works full time as Researcher for Mexico's SECIHTI Centro de Investigación Paleontológica Quinametzin (CIPAQ), Instituto Nacional de Antropología e Historia (INAH).

"I'm really more of a paleoecologist, paleogeographer, not a taxonomist," says Dr. Cruz, who noticed unusual features in the Tar Pits specimens that distinguished S. labreae from known spadefoot toads.

At first, Cruz wondered whether the unusual bones belonged to an animal that had been sick or injured during its lifetime, which would make them an example of paleopathology. Comparisons with modern spadefoot toads eventually convinced him that the differences represented something else entirely.

"I reviewed and reviewed a lot of specimens until I was convinced. I thought, 'Oh my gosh, this is a new species.' This material is originally from the 1950s, but when you study it again, you can find gold in these specimens. It's very cool."

The fact that the fossils had been collected decades earlier shows how much scientific information can remain hidden in museum collections.

"It's a great example of how we're still finding new things at La Brea Tar Pits, even after a century of ongoing excavation and research," says Dr. Emily Lindsey, Curator and Excavation Site Director at the Samuel Oschin Global Center for Ice Age Research at La Brea Tar Pits. "There's definitely more to come."

Tiny Amphibians Can Reveal Ancient Climate

Large Ice Age mammals tend to attract the most attention at La Brea Tar Pits, but the much smaller reptiles and amphibians studied by Cruz can sometimes provide scientists with more detailed information about past climate and environmental conditions.

Amphibians generally do not travel long distances during their lives, and they are highly sensitive to changes in their surroundings. Those traits make them useful climate proxies. By determining where particular amphibians were present or absent, researchers can reconstruct environmental changes at a more local scale.

"If you change the environment, you change the vegetation, the climate is warmer or colder, it affects these animals directly," says Cruz.

Their usefulness comes with a major obstacle. Frogs, toads, and salamanders have delicate skeletons that do not fossilize easily, so their remains are uncommon in the fossil record outside asphaltic sites such as La Brea Tar Pits. That makes the discovery of a previously unknown small amphibian such as S. labreae especially remarkable.

The same fragility may also mean that many extinct reptiles and amphibians have disappeared from the fossil record entirely. Cruz and other researchers suspect that additional species once existed but left remains too delicate to survive long enough to be discovered.

Comparing Fossils With Modern Toads

To establish that the fossils represented a new species, Cruz closely examined amphibian bones from the La Brea Tar Pits collection, which had not received a detailed assessment in nearly 30 years. He documented differences in the shapes and sizes of the bones and compared the fossils with 80 specimens housed in the Herpetology Collections at the Natural History Museum of Los Angeles County and the Museum of Vertebrate Zoology at the University of California, Berkeley.

Those comparisons helped demonstrate that the La Brea fossils belonged to a distinct species of spadefoot toad rather than to an unusual individual of a known species.

Another Toad Reveals a Dramatically Different Los Angeles

The research produced another surprising discovery. Scientists identified the first known record of the Mexican burrowing toad, Rhinophrynus, in the southwestern United States.

These toads no longer live anywhere near Los Angeles. Their closest living populations are now nearly 2,500 km away in southern Mexico, indicating that the species once occupied a dramatically different geographic range.

That major shift in distribution, combined with the extinction of S. labreae, adds another piece to the complex history of environmental change as the last Ice Age came to an end. By examining how amphibians responded to those ancient changes, researchers may also gain insight into how similar animals could respond to human caused climate change today.