A spiny pig fossil in Florida solves a spiny dilemma 10 million years in the making

There is a long-standing debate that has been simmering 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 North American wild pig belongs to a group that arose 10 million years ago, but fossils seem to tell a different story. Some paleontologists believe they may have evolved as recently as 2.5 million years ago, at the beginning of the ice ages.

A new study published in Current Biology claims to have settled the dispute thanks to an extremely rare, nearly complete skeleton of a porcupine discovered in Florida.

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

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

Ancient radiation spawned the world’s largest rodents

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

They crossed the Atlantic—probably by raft—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 acquired gigantic proportions. There were lumbering, rat-like animals up to five feet long, equipped with a small brain that weighed less than a plum. Extinct relatives of the capybara grew to the size of a cow.

Porcupines remained relatively small and developed adaptations for living in the treetops of the lush rainforests of South America. Today, they travel through the crown using long fingers covered in blunt, sickle-shaped claws, perfectly angled for grasping branches. Many also have long, prehensile tails capable of supporting their weight, which they use while climbing and reaching for fruit.

Despite their excellent track records, South America was a dead end for many millions of years. A vast seaway of swift currents separated North and South America, and most animals did not make it across—with a few notable exceptions.

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

Prehistoric elephants, saber-toothed cats, jaguars, llamas, peccaries, deer, skunks, and bears poured from North America to the South. The return trip was made by four different species of ground sloths, huge armadillos, terrifying birds, capybaras and even a marsupial.

The destinies of the two groups are radically different. These mammals migrating south fared quite well; many of them successfully established in their new tropical environments and survived to the present. But almost all the lineages that ventured north into colder environments have died out. Today, only three survive: the nine-banded armadillo, the Virginia opossum, and the North American porcupine.

New fossils capture evolution in action

The 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 cold deciduous forests. For porcupines, this meant dealing with harsh winters, fewer resources, and coming down from the trees to roam the land. They have not yet quite mastered the latter; North American porcupines have a top speed of about 2 miles per hour.

South American porcupines are equipped with a menacing coat of hollow, overlapping feathers 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 feathers 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 foods are gone, they will bite the bark of the tree to get to the softer tissue underneath. It’s not great food, but it’s better than nothing,” Vitek said. “We think that this type of feeding, selected for a certain jaw structure, makes them better at grinding.”

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

Many of these traits can be seen in fossils. The problem is, there aren’t many fossils to go around. According to Vitek, most are either individual teeth or jaw fragments, and researchers often associate them with South American porcupines. Those believed to belong to the North American group lack the critical characteristics that would provide paleontologists with clues to how they evolved.

So when Florida Museum paleontologist Art Poyer discovered a perfectly preserved spiny pig skeleton in a limestone quarry in Florida, they knew its significance.

“When they first brought it in, I was amazed,” said Block, the study’s senior author. “It’s so rare to get fossil skeletons like this with not only a skull and jaws, but also many associated bones from 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 differed from modern North American porcupines in that it had a specialized tail for grasping branches.”

By comparing the fossil skeleton with bones from modern porcupines, Block and Vitek were confident they could determine its identity. But the amount of work this would require is more than one person can 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 spiny pig bones.

“It’s something that can only be taught in a place like the Florida Museum where you have both the collections and enough students to study them,” Vitek said. “We focused on the details of the jaw, limbs, legs 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 possessed a prehensile tail, making it appear more closely related to South American porcupines. But, Vitek said, other traits bear a greater resemblance to North American porcupines, including the shape of the middle ear bone, as well as the shapes of the lower front and back teeth.

By combining all the data, the analyzes consistently give the same answer. The fossils belong to an extinct species of North American porcupine, meaning that this group has a long history that probably 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 research is whether these extinct species were direct ancestors of the North American spiny pig that is alive today,” Vitek said. “It is also possible that porcupines penetrated the temperate regions twice, once along the Gulf Coast and once in the west.” We’re not there yet.”

Jennifer Hoflich, Isaac Magallanes, Sean Moran, Rachel Narducci, Victor Perez, Jeanette Pirlo, Mitchell Rigler, Molly Selba, Maria Vallejo-Pareja, Michael Ziegler, Michael Granatoski and Richard Hulbert of the Florida Museum of Natural History also authored the report.