A Florida fossil porcupine solves a prickly dilemma 10 million years in the making

There is a long-standing debate among biologists who study porcupines. There are 16 species of porcupine in Central and South America, but only one in the United States and Canada. DNA evidence suggests that the only porcupine in North America belongs to a group that originated 10 million years ago, but fossils seem to tell a different story. Some paleontologists believe they may have evolved as little as 2.5 million years ago, at the beginning of the Ice Age.

A new study published in the journal Contemporary Biology claims to have settled the dispute thanks to an exceptionally rare, nearly complete porcupine skeleton discovered in Florida.

The authors reached their conclusion by studying key differences in bone structure between North and South American porcupines, but it wasn’t easy to get there. It took an entire class of graduate and undergraduate students and several years of careful preparation and study.

“Even for a seasoned curator with all the necessary expertise, it takes an incredible amount of time to fully study and process an entire skeleton,” said lead author Natasha Vitek. While a doctoral student at the Florida Museum of Natural History, Vitek teamed up with curator of vertebrate paleontology Jonathan Bloch to create an undergraduate course in which students gained hands-on research experience studying porcupine fossils.

Ancient radiation gave rise to the world’s largest rodents

Porcupines are a type of rodent and their ancestors probably originated in Africa more than 30 million years ago. Their descendants have since traveled overland to Asia and parts of Europe, but their journey to South America is a particularly defining event in mammalian history.

They crossed the Atlantic Ocean – probably by rafting – at a time when Africa and South America were much closer together than they are today. They were the first rodents to ever set foot on the continent, where they evolved into familiar groups such as guinea pigs, chinchillas, capybaras, and porcupines.

Some have grown to gigantic proportions. There lived crude rat-like animals up to five feet long, equipped with tiny brains that weighed less than a plum. Extinct relatives of the capybara grew to the size of cows.

Porcupines remained relatively small and evolved adaptations for living in the treetops of the lush rainforests of South America. Today, they travel through the canopy using long fingers tipped with blunt, sickle-shaped claws perfectly angled for grasping branches. Many also have long, prehensile tails capable of supporting their weight, which they use to climb and reach fruit.

Despite their excellent travel experience, South America was a dead end for many millions of years. North and South America were separated by a vast seaway with fast currents, and most animals could not swim across—with a few notable exceptions.

About 5 million years ago, the Isthmus of Panama rose above sea level, cutting off the Pacific from the Atlantic. This land bridge, a few million years later, became the ancient equivalent of a congested freeway with traffic flowing in both directions.

Proto-elephants, saber-toothed cats, jaguars, llamas, peccaries, deer, skunks, and bears flowed south from North America. The reverse trek was done by four different species of ground sloths, oversized armadillos, terror birds, capybaras and even a marsupial.

Both groups met a radically different fate. These southward migrating mammals did quite well; many have successfully established themselves in their new tropical environment and survived to the present day. But almost all lineages that ventured north into colder environments became extinct. Only three survive today: the nine-banded armadillo, the Virginia opossum and the North American porcupine.

New fossils capture evolution in action

Animals that traveled north had to contend with new environments that bore little resemblance to the ones they left behind. Warm tropical forests gave way to open grasslands, deserts, and cool deciduous forests. For porcupines, this meant coping with harsh winters, fewer resources, and coming down from the trees to roam the land. The latter they have not yet fully understood; North American porcupines have a maximum ground speed of about 2 mph.

South American porcupines are equipped with a formidable coat of hollow, overlapping quills that offer considerable protection but do little to regulate body temperature. North American porcupines have replaced them with a mixture of insulating fur and long needle-like quills that can be raised when they feel threatened. They also had to adjust their diet, which changed the shape of their jaw.

“In the winter, when their favorite foods aren’t there, they’ll bite into the bark of trees to get to the softer tissue underneath. It’s not great food, but it’s better than nothing,” Vítek said. “We think this type of feed has been selected for a particular jaw structure that makes them grind better.”

They also lost their prehensile tails. Although North American porcupines still like to climb, it is not their forte. Museum specimens often show signs of healed bone fractures, likely caused by falls from trees.

Many of these features can be seen in fossils. The problem is that there aren’t many fossils around. According to Vítek, these are mostly individual teeth or fragments of jaws, and researchers often associate them with South American porcupines. Those considered members of the North American group lack critical features that would give paleontologists clues to how they evolved.

So when Florida Museum paleontologist Art Poyer found a perfectly preserved porcupine skeleton in a Florida limestone quarry, they were well aware of its significance.

“When they first brought it up, I was amazed,” said Bloch, the study’s lead author. “It’s so rare to get a fossil skeleton like this, not just with the skull and jaws, but with 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. We immediately noticed, that it differs from modern North American porcupines in having a specialized tail for grasping branches.”

By comparing the fossil skeleton with the bones of modern porcupines, Bloch and Vítek were convinced that they could determine its identity. But the amount of work it would require was more than one person could do alone in a short time. So they co-created a college paleontology course in which the only assignment for the entire semester was the study of porcupine bones.

“It’s something that can only be taught at a place like the Florida Museum where you have both the collections and enough students to study them,” Vítek said. “We focused on the details of the jaws, limbs, feet and tails. It required a very detailed series of comparisons that you might not even notice on the first pass.”

The results were surprising. The fossil lacked reinforced bark-gnawing jaws and had a prehensile tail, making it appear closer to South American porcupines. But, Vítek said, other features more strongly resembled the North American porcupine, including the shape of the middle ear ossicle as well as the shape of the lower front and back teeth.

When all data were combined, the analyzes consistently provided the same answer. The fossils belonged to an extinct species of North American porcupines, meaning the group has a long history that likely began before the Isthmus of Panama formed. But the question remains as to how many species once existed in this group or why they became extinct.

“One thing that our study did not resolve is whether these extinct species are the direct ancestors of the North American porcupine living today,” Vítek said. “It’s also possible that porcupines have entered temperate regions twice, once along the Gulf Coast and once to the west. We’re not there yet.”

Jennifer Hoeflich, Isaac Magallanes, Sean Moran, Rachel Narducci, Victor Perez, Jeanette Pirlo, Mitchell Riegler, Molly Selba, María Vallejo-Pareja, Michael Ziegler, Michael Granatosky and Richard Hulbert of the Florida Museum of Natural History also contributed to the paper. .