Fossil Florida porcupine solves thorny 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 North America’s only porcupine belongs to a group that appeared 10 million years ago, but fossils appear to tell a different story. Some paleontologists believe they evolved only 2.5 million years ago, at the start of the Ice Age.

A new study published in the journal Current biology claims to have reconciled the dispute, thanks to an exceptionally rare and almost 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 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 study and fully process an entire skeleton,” said lead author Natasha Vitek. While studying for his doctorate at the Florida Museum of Natural History, Vitek teamed up with curator of vertebrate paleontology Jonathan Bloch to create a college course in which students gained hands-on research experience by studying the porcupine fossils.

Ancient radiation gave rise to the world’s largest rodents

Porcupines are a type of rodent and their ancestors likely originated in Africa more than 30 million years ago. Their descendants have since wandered overland through 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 to each other than today. They were the first rodents to set foot on the continent, where they evolved into well-known groups such as guinea pigs, chinchillas, capybaras and porcupines.

Some have taken on gigantic proportions. There were heavy, rat-like animals up to five feet long, equipped with small brains that weighed less than a plum. The capybara’s extinct relatives grew to the size of a cow.

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

Despite their excellent travel history, South America remained a dead end for millions of years. A vast, fast-moving sea route separated North and South America, and most animals were unable to cross it, 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 highway, with traffic in both directions.

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

The two groups experienced radically different destinies. Mammals migrating south fared quite well; many successfully established themselves in their new tropical environments and survived to the present day. But almost all lineages that ventured north into colder environments became extinct. Today, only three survivors remain: the nine-banded armadillo, the Virginia opossum, and the North American porcupine.

New fossils catch evolution in the act

Animals traveling north faced new environments that bore little resemblance to those they had left behind. Warm rainforests gave way to open grasslands, deserts, and cold deciduous forests. For porcupines, that meant facing harsh winters, fewer resources, and having to climb down from trees to walk on dry land. They haven’t quite figured out the latter yet; North American porcupines have a maximum speed of about 2 mph.

South American porcupines are equipped with a menacing layer of hollow, overlapping quills, which provide substantial 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 change their diet, which changed the shape of their jaws.

“In winter, when their favorite foods aren’t around, they bite the bark of trees to get to the softer tissues underneath. It’s not good food, but it’s better than nothing,” said Vitek. “We believe this type of feed is selected for a particular jaw structure that makes them better at grinding.”

They also lost their prehensile tail. Although North American porcupines still enjoy climbing, it is not their strong point. Museum specimens often show evidence of healed bone fractures, likely caused by falling trees.

Many of these traits can be seen in fossils. The problem is that there aren’t many fossils. Most are individual teeth or jaw fragments, according to Vitek, and researchers often associate them with South American porcupines. Those considered to belong to the North American group lack the essential characteristics that would provide paleontologists with clues to their evolution.

So when Florida Museum paleontologist Art Poyer discovered a perfectly preserved porcupine skeleton in a Florida limestone quarry, he was well aware of its importance.

“When they first introduced it, I was amazed,” said Bloch, lead author of the study. “It’s so rare to get fossil skeletons like this with not only a skull and jaws, but also many bones associated with the rest of the body. This allows for a much more complete picture of how this extinct mammal would have interacted with its environment. We immediately noticed that it was different from modern North American porcupines because of its specialized tail for grasping branches.

By comparing the fossil skeleton with modern porcupine bones, Bloch and Vitek were confident they could determine its identity. But the amount of work this would require was more than a single person could accomplish 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.

“This is the kind of thing 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 the details of the jaw, limbs, feet and tail. This required a series of very detailed comparisons that you might not even notice on first pass.”

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

With all data combined, the analyzes consistently provided 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 formation of the Isthmus of Panama. But questions remain about how many species once existed in this group or why they became extinct.

“One thing that is not resolved by our study is whether these extinct species are the direct ancestors of the North American porcupine that is alive today,” Vitek said. “It’s also possible that porcupines 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, Maria Vallejo-Pareja, Michael Ziegler, Michael Granatosky and Richard Hulbert of the Florida Museum of Natural History are also authors of the article. .