In the wake of the dinosaurs’ demise, a bizarre beast that some researchers have nicknamed “ManBearPig” lived life in the fast lane. This sheep-sized mammal — which sported five-fingered hands, a bearlike face and the stocky build of a pig — gave birth to highly developed young. And those young grew up much faster than expected for an animal as massive as ManBearPig, new fossil analyses show.

That combination of long gestation and quick aging may have led to many rapid generations of bigger and bigger babies, researchers report online August 31 in Nature. Such an approach to life could help explain how some mammals took over the world after the dinosaur doomsday.

During the age of the dinosaurs, mammals “only got as large as a domestic cat, maybe, or a badger,” says Gregory Funston, a paleontologist at the Royal Ontario Museum in Toronto. But after an asteroid wiped out all nonbird dinosaurs about 66 million years ago, “we see this huge explosion in mammal diversity, where mammals start to get really big,” Funston says.

In particular, placental mammals got really big. Those are mammals whose babies develop mainly in the womb while fed by a placenta — unlike 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 world’s largest animals such as whales, elephants and giraffes.

Paleontologists have long wondered why placentals rose to dominance. Researchers suspected that the long gestation period of this mammal lineage was an important factor. But it was unclear how long ago such long gestation evolved.

For clues, Funston and colleagues turned to what they call ManBearPig, or Pantolambda bathmodon. 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 in the teeth sketched out a timeline of each animal’s life. On 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. Barium in the enamel spiked while an animal was nursing. Other details of the teeth and bones revealed how fast P. bathmodon grew throughout its life and each animal’s age at death.

P. bathmodon stayed in the womb for about seven months, nursed for just a month or two, reached adulthood within a year and lived at most about 11 years, the team found. A female’s pregnancy was much longer than the weeks-long gestation seen in modern marsupials and platypuses, but similar to the months-long pregnancies typical of modern placentals.

“It was reproducing like the most extreme placentals do today,” Funston says, such as giraffes and wildebeests — which are on their feet within minutes of birth. P. bathmodon gave birth to “probably just one baby in each litter, and that baby had a full set of teeth already in the mouth when it was born, and that means it was probably born with fur in place and with open eyes.”

The rest of P. bathmodon’s life trajectory, however, was markedly different from modern mammals. This species weaned and reached adulthood faster than expected for an animal of its size. Most died between two and five years old, with the oldest one studied dead at age 11 — only about half of the 20-year lifespan expected for an animal as big as ManBearPig.

That “live fast, die young” lifestyle may have helped placental mammals fill giant dinos’ empty shoes, says Graham Slater, a paleobiologist at the University of Chicago who was not involved in the study. “These things are going to be kicking out new generations every year and a half,” he says, “and because they’re having that rapid generation time … evolution can just act faster.”

Longer gestation could have led to bigger babies, which grew into bigger adults that had bigger babies themselves. With many such generations passing in quick succession, Slater says, “you’re going to get bigger and bigger animals very, very quickly.”

But no single species can tell the story of how mammals took over the world (SN: 6/7/22). Future studies should investigate whether other mammals that lived around this time had a similar life cycle, Slater says.