An exceptionally rare porcupine fossil skeleton found in Florida has helped to resolve a long-standing evolutionary mystery.
The research, published in the journal Current Biology, sheds new light on the evolutionary history of North American porcupines.
The origin of North American porcupines has long been the subject of debate among biologists who study these animals.
Central and South America are home to 16 porcupine species, all belonging to the genus (group of species) Coendou. By contrast, only one porcupine species lives today in the United States and Canada—the North American porcupine, known scientifically as Erethizon dorsatum.
DNA evidence suggested that the genus E. dorsatum belongs to (simply called Erethizon) originated around 10 million years ago when the North and South American continents were still separated by a vast seaway. On the other hand, evidence from the fossil record hints that North American porcupines may have evolved around 2.5 million years ago after a land bridge emerged to connect the two continents.
The latest findings, based on an analysis of the Florida fossil, appear to reconcile these differing timelines, according to the authors.
The fossil, which was found in a limestone quarry, is the oldest nearly complete porcupine skeleton documented from North America, the researchers say. They determined that it belongs to an extinct species known as Erethizon poyeri, which belongs to the Erethizon genus.
Analysis of the fossil indicated that instead of originating in North America, the Erethizon genus likely first appeared and existed for much of its evolutionary history in South America, with a body form and ecological role that broadly resembled those of its sister group, Coendou.
Both Coendou and Erethizon belong to a larger group known as the New World porcupines. These animals originated in South America before dispersing into North America as part of the Great American Interchange.
This interchange occurred around 3-4 million years ago after the Isthmus of Panama rose above sea level, joining the two continents by a land bridge. This stretch of land enabled animals to migrate in both directions, with porcupines among those that headed north.
Today, Coendou porcupines live in the tropical forests of Central and South America. They feature several characteristics that have enabled this group to adapt to life in the treetops. These traits include a relatively small body size, long fingers capped with sickle-shaped claws for gripping branches, and long, prehensile tails capable of grasping, which can be used for climbing and reaching for fruit.
North American porcupines, meanwhile, are adapted to temperate forests. They have larger bodies, shorter tails and a different jaw shape—the latter adaptation enabling them to eat tree bark when food is scarce during cold winters.
A scarcity of fossils has hindered biologists' understanding of the timing of these ecological adaptations in relation to the dispersal of porcupines from South to North America. But the Florida fossil sheds new light on this issue, indicating that only after Erethizon expanded into temperate North American habitats did it acquire the characteristic features that the group is known for today.
"When they first brought [the fossil] in, I was amazed," Jonathan Bloch, a paleontologist and senior author of the study with the Florida Museum of Natural History, said in a press release.
"It is so rare to get fossil skeletons like this with not only a skull and jaws, but many associated bones from the rest of the body. It allows for a much more complete picture of how this extinct mammal would have interacted with its environment. Right away we noticed that it was different from modern North American porcupines."
Unlike the modern North American porcupine, Erethizon poyeri had traits similar to those of the tropical Coendou genus, including a long, prehensile tail specialized for holding on to branches and grasping feet. It also lacked a specialized jaw and teeth for gnawing bark.
However, other Erethizon poyeri traits, such as the shape of the middle ear bone and the shape of the lower front and back teeth, appear to more closely resemble those of North American porcupines.
While the study suggests that the Erethizon genus originated before the Isthmus of Panama had formed, questions remain regarding how many species once existed in this group or why they went extinct. E. dorsatum is the only living member of the genus although at least four extinct relatives are known.
"One thing that isn't resolved by our study is whether these extinct species are direct ancestors of the North American porcupine that is alive today," study lead author Natasha Vitek, a former doctoral student at the Florida Museum, who is now at Stony Brook University in New York, said in the release. "It's also possible porcupines got into temperate regions twice, once along the Gulf Coast and once out west. We're not there yet."
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