A New Ice Age Toad From the La Brea Tar Pits, Found in a Drawer of 1950S Excavation Material

The find that matters here was not made in the field. It was made at a table, with a drawer of bones on it, in a collection nobody had worked through properly since the material came out of the asphalt in the 1950s.

Rancho La Brea has been giving up fossils since the beginning of the twentieth century, and the ones that built its reputation are large and photogenic. Dire wolf skulls line a wall of the museum by the hundreds. Sabertooth cats, mammoths, ground sloths and short-faced bears fill the halls. What comes out of the same deposits in far greater numbers, and gets far less attention, is small: rodents, lizards, birds, snakes, insects, seeds, and the occasional frog. Those remains were collected, catalogued and shelved, and there they stayed.

J. Alberto Cruz went through the amphibian and reptile shelves as a postdoctoral fellow at the La Brea Tar Pits, part of the Natural History Museum of Los Angeles County. He is now based at Mexico's National Institute of Anthropology and History and its Quinametzin paleontological research center, and remains a research associate at the tar pits. One bone stopped him: an incomplete sacro-urostyle, the fused element at the base of the spine where it meets the hips.

That fusion is itself informative. In most frogs and toads the sacrum and the urostyle meet at a joint; in spadefoot toads they are fused into one bone, so the element identifies the family on sight. This one was noticeably larger than the same bone in living spadefoots, and different enough, in Cruz's assessment, to belong to no known species. His first thought was more mundane. A bone this odd usually means a sick or injured animal of a species already described.

"I reviewed and reviewed a lot of specimens until I was convinced," Cruz said in the museum's announcement of the work.

The result is a paper published online Aug. 4 in the Journal of Vertebrate Paleontology, by Cruz and Emily L. Lindsey of the Samuel Oschin Global Center for Ice Age Research at the tar pits, describing the animal as a new extinct species and going on to what the site's frogs and toads collectively say about Ice Age Los Angeles. The new species is Spea labreae, its name taken from the place it came out of. The specimen is roughly 45,000 years old by the researchers' estimate, and it is likely to stay approximate: a direct radiocarbon date would consume the fossil.

Cruz and Lindsey report it as only the second extinct Pleistocene amphibian ever described from North America. That is their claim rather than an established count, and it is worth taking as scoped: the first, according to the museum, is a tree frog from Florida. Extinct Ice Age mammals number in the dozens on this continent. Extinct Ice Age amphibians, on this reckoning, number two.

The asymmetry is partly about bone. A mammoth femur survives almost anything; a frog's skeleton is small, thin and easily destroyed, and what remains of it is often unidentifiable past the family. It is also about attention. Nobody staffs a tar-pit excavation to find toads.

Which is a shame, because amphibians are unusually good witnesses to climate. They do not regulate their own body temperature, their skin is permeable, and they need standing or damp ground to breed, so their presence at a site is a statement about temperature and water rather than a statement about what they could tolerate while passing through. They also do not travel far. A herd animal can wander in from a different climate zone; a spadefoot found in Los Angeles was living in Los Angeles. The frog record at Rancho La Brea therefore points to a place with reliable water, cooler and wetter than the modern city.

One find in the same review makes that point more sharply than the new species does. Cruz identified remains of Rhinophrynus, the Mexican burrowing toad, which the paper reports as the first occurrence of the genus in the southwestern United States. Its nearest living populations today are roughly 2,500 kilometers, or about 1,550 miles, to the south, in southern Mexico. Whatever the animal was doing in Pleistocene California, it is not doing it anywhere near there now.

The work does establish that a collection can be excavated, boxed, shelved and still be unexcavated in every sense that matters. The bones that produced a new species were dug out of the asphalt around seventy years ago by people looking for something else. There are more drawers.