In the wake of the demise of the dinosaurs, a strange beast that some researchers have nicknamed “Man-Bear-Pig” lived in the fast lane. Sporting five-fingered hands, a bear-like face, and the stocky build of a pig, this sheep-sized mammal gave birth to well-developed young. And those young grew much faster than expected for an animal as massive as ManBearPig, new analyzes of fossils show.
That combination of long gestation and rapid aging may have given rise to many rapid generations of ever-larger babies, researchers report online August 31 at Nature. This approach to life could help explain how some mammals took over the world after the end of the dinosaur world.
During the age of the dinosaurs, mammals “were only as big as a house cat, maybe, or a badger,” says Gregory Funston, a paleontologist at the Royal Ontario Museum in Toronto. But after an asteroid wiped out all non-bird dinosaurs about 66 million years ago, “we see this huge explosion in mammalian diversity, where mammals start to get really big,” says Funston.
In particular, placental mammals got really big. Those are mammals whose babies develop primarily in the womb while feeding on a placenta, as opposed to egg-laying platypuses or marsupials, whose tiny newborns do much of their development in their mother’s pouch. Today, placentals are the most diverse group of mammals and include some of the largest animals in the world, such as whales, elephants, and giraffes.
Paleontologists have long wondered why placentals rose to dominance. The researchers suspected that the long gestation period of this mammalian lineage was an important factor. But it was unclear how long ago such a long gestation evolved.
Looking for clues, Funston and his colleagues turned to what they call ManBearPig, or Pantolambda batmodon. This ancient herbivore, which lived about 62 million years ago, was one of the first large mammals to appear after the dinosaur apocalypse. The team examined fossils from the San Juan Basin in New Mexico, including two partial skeletons and scattered teeth from several other individuals.
Daily and annual growth lines on the teeth outline a timeline of each animal’s life. In that timeline, chemical signatures recorded when the creature underwent major life changes. The physical stress of being born left a deposit of zinc on the tooth enamel. The barium in the enamel spiked while an animal was suckling. Other details of the teeth and bones revealed how quickly P. bathmodon grew throughout its life and the age of each animal at death.
P. bathmodon it remained in the womb for about seven months, nursed for a month or two, reached adulthood in a year and lived to a maximum of about 11 years, the team found. A female’s pregnancy was much longer than the weeks’ gestation seen in modern marsupials and platypuses, but similar to the months’ gestation typical of modern placentals.
“It was reproducing as the most extreme placentals do today,” Funston says, like giraffes and wildebeests, which stand up within minutes of birth. P. bathmodon she gave birth to “probably only one baby in each litter, and that baby already had all her teeth in her mouth when she was born, and that means she was probably born with hair in place and her eyes open.”
The rest of P. bathmodonThe life path of , however, was markedly different from that of modern mammals. This species weaned and reached adulthood faster than expected for an animal of its size. Most died between two and five years of age, with the oldest studied dying at age 11, only around half the expected 20 years to live for an animal as large as ManBearPig.
That “live fast, die young” lifestyle may have helped placental mammals fill the empty shoes of giant dinosaurs, says Graham Slater, a paleobiologist at the University of Chicago who was not involved in the study. “These things are going to spawn new generations every year and a half,” he says, “and because they’re having that fast generation time … evolution can act faster.”
A longer gestation could have led to larger babies, who grew into larger adults who also had larger babies. With many of those generations passing in quick succession, says Slater, “you’re going to have bigger and bigger animals very, very quickly.”
But no single species can tell the story of how mammals conquered the world (Serial number: 7/6/22). Future studies should investigate whether other mammals that lived around this time had a similar life cycle, Slater says.