Florida’s fossil porcupine solves a difficult dilemma ten million years in the making

There has long been a debate among biologists who study porcupines. There are 16 species of porcupines in Central and South America, but only one in the United States and Canada. DNA evidence suggests that North America’s only porcupine belongs to a group that originated 10 million years ago, but fossils seem to tell a different story. Some paleontologists think they only evolved 2.5 million years ago, at the beginning of the ice ages.

A new study published in the journal Current biology claims to have reconciled the dispute, thanks to an exceptionally rare, nearly complete porcupine skeleton discovered in Florida.

The authors reached their conclusion by studying the key differences in bone structure between North and South American porcupines, but getting there wasn’t easy. 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,” says lead author Natasha Vitek. While studying as a doctoral student at the Florida Museum of Natural History, Vitek worked with vertebrate paleontology curator Jonathan Bloch to create a college course in which students gained hands-on research experience studying porcupine fossils.

Ancient radiation created 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 migrated overland to Asia and parts of Europe, but their journey to South America is a particularly defining event in mammal history.

They crossed the Atlantic Ocean – probably by rafting – 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 well-known groups such as guinea pigs, chinchillas, capybaras and porcupines.

Some reached gigantic proportions. There were lumbering, rat-like animals up to five feet tall, equipped with small brains that weighed less than a plum. Extinct relatives of the capybara grew to the size of cows.

Porcupines remained relatively small and developed adaptations for living in the treetops of South America’s lush rainforests. Today, they travel through the canopy using long fingers, capped with blunt, sickle-shaped claws that are perfectly angled to grip branches. Many also have long, prehensile tails that can support their weight, which they use while climbing and reaching for fruit.

Despite their excellent record of travel, South America was a dead end for millions of years. A vast seaway with swift currents separated North and South America, and most animals could not cross the border – with a few notable exceptions.

About five million years ago, the Isthmus of Panama began to rise above sea level, cutting off the Pacific Ocean from the Atlantic Ocean. This land bridge became the ancient equivalent of a busy highway a few million years later, with traffic flowing in both directions.

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

The two groups faced radically different fates. The mammals that migrated south did quite well; many became successfully established in their new tropical environment and survived to the present day. But almost all genera that ventured north into colder environments have become extinct. Today there are only three survivors: the nine-banded armadillo, the Virginia opossum and the North American porcupine.

New fossils document evolution in the act

Animals traveling north encountered new environments that bore little resemblance to the one they left behind. Warm, tropical forests gave way to open grasslands, deserts and cold deciduous forests. For porcupines, this meant coping with harsh winters, fewer resources, and getting out of the trees to walk on land. They still haven’t completely mastered the latter; North American porcupines have a maximum ground speed of about 2 mph.

South American porcupines are equipped with a menacing layer of hollow, overlapping quills, which provide a significant amount of protection but do little to regulate body temperature. North American porcupines have replaced these with a mix of insulating fur and long, needle-like quills that can be raised when they feel threatened. They also had to change their diet, which changed the shape of their jaw.

“In the winter, when their favorite food isn’t there, they bite into tree bark to get to the softer tissue underneath. It’s not great food, but it’s better than nothing,” Vitek said. “We think this type of food was selected because of a certain jaw structure, which allows them to sharpen better.”

They also lost their prehensile tails. Although North American porcupines still enjoy climbing, this is not their forte. Museum specimens often show signs of healed bone fractures, probably caused by a fall from trees.

Many of these properties can be found in fossils. The problem is that there aren’t many fossils available. According to Vitek, most are individual teeth or jaw fragments, and researchers often add them to South American porcupines. Those thought to belong to the North American group lack the critical features that would give paleontologists clues to how they evolved.

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

“When they first brought it in, I was amazed,” said Bloch, senior author of the study. “It’s so rare to get fossil skeletons like this, with not just a skull and jaws, but lots of associated bones from the rest of the body. It gives a much more complete picture of how this extinct mammal would have interacted with its environment. We immediately noticed that it differed from modern North American porcupines in that it had a specialized tail for grasping branches.”

By comparing the fossil skeleton with bones of modern porcupines, Bloch and Vitek were confident they could determine its identity. But the amount of work this would require was more than one person could do alone in a short time. So together they created a paleontology course where the only assignment for the entire semester was to study porcupine bones.

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

The results were surprising. The fossil lacked the reinforced bark-gnawing jaws and possessed a prehensile tail, making it appear more closely related to the South American porcupines. But, Vitek said, other traits showed a stronger similarity to North American porcupines, including the shape of the middle ear bone and the shape of the lower front and back teeth.

With all data combined, analyzes consistently produced the same answer. The fossils belonged to an extinct species of North American porcupine, meaning this group has a long history that likely began before the Isthmus of Panama formed. But questions remain about how many species ever existed in this group or why they became extinct.

“One thing our study doesn’t resolve is whether these extinct species are direct ancestors of the North American porcupine that is alive today,” Vitek said. “It’s also possible that porcupines have entered temperate regions twice, once along the Gulf Coast and once in 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 are also authors on the paper .