Chihuahua-sized fossil 'beardogs' shed new light on evolution of dogs and their relatives

Fossil discoveries don't always happen out in the field, with scientists armed with pick-axes realizing they've found something special. Sometimes, fossils lie in wait in museum collections until the right researcher comes along and realizes there's something unusual about them. That's what happened this time, and the fossils in question are prehistoric dog relatives called "beardogs."

In a new paper published in Royal Society Open Science, paleontologists revisited two species of fossil carnivores whose true identities had remained obscure for years, finding them to be some of the earliest and most primitive members of the beardog family, also known as amphicyonids. The beardogs in the study aren't new species--they were discovered and officially described decades ago--but until now, scientists didn't know where they belonged on the carnivore family tree.

"We've known about these curious little critters for 30 years, but couldn't tell exactly what kind of carnivores they were," explains lead author Susumu Tomiya, a postdoctoral scholar at The Field Museum who reidentified the specimens in the Museum's collections. The two newly classified animals--now renamed Gustafsonia and Angelarctocyon--lived 38 to 37 million years ago in what's now Texas and were surprisingly small for creatures called "beardogs"--some of their descendents were large and bear-like, but these ones were the size of Chihuahuas. But it's their family membership (and their scientific name) that was in need of revision, not the moniker "beardog."

In the scientific naming process, organisms are broken down into more and more specific groups. For instance, humans are Homo sapiens--Homo is our genus, which is a bigger group that includes other human relatives like Neanderthals (Homo neanderthalensis). Meanwhile, sapiens is our species, a more precise group that only describes us. In the case of the beardogs in this study, they're still the same species, but they'd been grouped into a wrong genus and family. The fossil formerly known as Miacis cognitus is now renamed Gustafsonia cognita (in honor of Eric Gustafson, who first described them), and Miacis australis is Angelacrtocyon australis, meaning "messenger beardog" since it heralds the other beardogs that evolved down the line.

Tomiya's discovery was made possible by The Field Museum's world-class fossil mammal collection. "I had just started at the Field, and I was getting the lay of the land, exploring our collections. In one room of type specimens, the fossils used as a standard to describe their species, I stumbled across something that looked unusual," he says. "There were beautiful jaws of a small carnivore, but the genus the specimen had been assigned to didn't seem to fit some of the features on the teeth. It made me suspect that it belonged to a very different group of carnivores."

The upper teeth on the specimen, which had been called Miacis australis, have expanded shelves for increased crushing surfaces. "The fact that these teeth appear to have been well adapted for crushing suggests that these animals were eating more than just meat--they may well have branched out into other foods like berries and insects, like small foxes do today," says Tomiya. "Subtle dental features like these gave me the first hint that the animal was actually related to well-known beardogs from later time periods." As he delved into the study, he noticed that another fossil carnivore from the same rock formation in Texas, then called Miacis cognitus, had similar traits. Unlike the Field Museum specimen, this second specimen (housed at the University of Texas at Austin) was missing lower jaws but had well preserved back portion of the skull, which is useful for identification and classification of carnivores.

To further test the idea that these specimens represented early beardogs, Tomiya and coauthor Jack Tseng of the University at Buffalo generated a 3D digital reconstruction of the largely intact skull of Miacis cognitus using high-resolution X-ray CT scans available through the Digimorph database, funded by the National Science Foundation and run by the University of Texas at Austin. This digital reconstruction allowed the team to "see" important internal features of the skull that are otherwise invisible, and provided additional evidence for the beardog identity of the mystery carnivores.

"We used CT scans of the only cranium specimen of Gustafsonia from Digimorph to reconstruct the internal spaces surrounding the auditory apparatus, a critical anatomical region with very useful characteristics for reconstructing carnivore species inter-relationships," explains Tseng.

The study sheds light upon the dawn of beardog evolution, and more broadly on the earliest stage of diversification of the dog branch. For example, based on their findings as well as other recent fossil discoveries, Tomiya and Tseng think that southern regions of North America may have played a more important role in the history of mammals than has been appreciated.

"These are some of the earliest beardogs--they lived 38 to 37 million years ago," says Tomiya. "By about 15 million years ago, the beardog family had given rise to huge predators bigger than modern lions, but the early members reported in this study were tiny, around the size of a Chihuahua." The evolution of beardogs from tiny to huge is familiar to vertebrate paleontologists, who frequently see this trend. "There are advantages to being bigger--you can take down bigger prey, you have fewer predators and fewer competing carnivores that can steal your food--but there are disadvantages too. Bigger animals require more food and space, and they reproduce more slowly. Larger animals go extinct at higher rates than smaller animals--recent studies suggest that getting bigger may be a dead-end strategy in mammalian carnivores," notes Tomiya. "Studying how the diversity of beardogs waxed and waned over time could tell us about larger patterns in carnivore evolution."

Members of the dog branch today include dogs, wolves, foxes, bears, sea lions, and weasels. Beardogs are from an earlier point in that group's history, before they branched out into the dog relatives we see today. "They're equally related to all of the dog relatives alive today--they're not the direct ancestors of modern wolves and bears, but more like their cousins," explains Tomiya.

The study has broader implications relating to evolutionary response of mammals to climate change, too. "Gustafsonia and Angelarctocyon lived at a time when North America was transitioning from a subtropical climate to a cooler, drier climate," says Tomiya. "There were big changes happening in the landscape, forests were probably opening up, and the fossil assemblages including these beardogs can tell us about what kinds of animals did well in that environmental context."

Tomiya also notes the importance of museum collections for discoveries like his. "There's so much history embedded in these collections," he says. "All biodiversity research is contingent on good understanding of what species lived where and when in the past, and how they are related to one another. Museum collections are vital to building and continually refining such fundamental data sets."

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Susumu Tomiya is a postdoctoral scholar at The Field Museum in Chicago; he did his graduate work at the University of California Museum of Paleontology at Berkeley. Zhijie Jack Tseng is an assistant professor at the University at Buffalo and was previously at the American Museum of Natural History.

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