They are the most successful mammals that you probably haven’t heard of. Multituberculates were squirrel-like mammals that first evolved in the Jurassic, about 170 million years ago, and survived until about 35 million years ago, an enviable length of time for any group of beasts. Multis, as they are affectionately known, were around for more than twice as long as primates have been around. And now, paleontologists have learned, multis were among the first to look after their offspring the way that many modern mammals do, keeping close to their young until they were weaned and ready to leave their mothers. The secret is in their bones.
How quickly young mammals grow and when they stop weaning from their mothers leaves telltale clues in the microscopic structure of their skeletons. By zooming in close to these details, paleontologists were able to discern that multituberculates raised their young more like most modern mammals than marsupials or egg-layers like the platypus, according to a recent American Naturalist study. That means mammal mothers were looking after their offspring back in the heyday of the dinosaurs.
Studying reproduction and parental care in the fossil record is a challenging task. Research often relies on rare discoveries such as nests or pregnant adults that were carrying their offspring inside them when they died. The sparseness of the fossil record has confounded attempts to understand the ways that prehistoric mammals gave birth and brought up their babies, leaving significant gaps in our understanding of our broader bestial family’s history. But paleontologists have found alternate paths to gain insights that were previously impossible, and new discoveries are changing our perception of when mammals started to care for their little ones.
First, let’s go back to the beginning. The earliest mammals, much like reptiles, laid eggs just as the duckbilled platypus and the echidna do today. The very ancient mode of reproduction goes all the way back to the last common ancestor of mammals and reptiles that lived over 300 million years ago. Sometime after the origin of the first true mammals over 210 million years ago, however, mammals started to reproduce in new ways. Some were like today’s marsupials, giving birth to very tiny, helpless babies that had to nurse for a very long time and required an extended period of growth before they could venture off on their own. Others were more like our own family, the placental mammals, where offspring gestated inside their mothers until they were more developed and then spent a comparatively shorter time nursing and growing before becoming independent.
Paleontologists used to think that these ways or reproducing developed one after another, from laying eggs to caring for underdeveloped offspring to our placental mode of reproduction. Given that multituberculates were a very, very old mammal group, some scientists thought that they reproduced much like opossums rather than like squirrels. But the new study by University of Washington paleontologist Lucas Weaver and colleagues has changed the story and affirmed that multituberculates were much more like placental mammals—perhaps moving the date for this particular way of raising young to a time millions of years before Stegosaurus and Allosaurus roamed the Earth.
More than one way can be used to learn about the life histories of creatures long extinct. And in this case, dinosaurs provided some inspiration. Back in 2016, Weaver recalls, he was watching University of Washington paleontologist Megan Whitney give a talk about understanding the lives of the long-necked sauropod dinosaurs through microscopic details in bone tissues. The tiny clues documented how fast the dinosaurs grew and outlined their developmental stages through hatchling, yearling, juvenile, subadult and mature animals, offering greater insight into the lives of these reptiles than experts previously knew.
If it worked for dinosaurs, Weaver wondered, why not mammals? The two began a collaboration to look at how multituberculates were growing.
“We tend to think of bones as being lifeless structures that simply provide a scaffold for our flesh, but our bones are living tissue,” says Oxford University Museum of Natural History paleontologist Elsa Panciroli, who was not involved in the new study. Because bones are made of living, dynamic tissue, they can record important events from the life of an animal. For mammals, which often grow relatively rapidly to their adult sizes, their bones can act as time capsules of their youth.
Minute details of the bone tissue in the thighbones, or femora, of multituberculates are affected by lactation. Egg-layers, marsupials and placental mammals all carry different signatures. While baby placental and marsupial mammals start with bones made of a fast-growing, messy-looking tissue called woven bone, over time different bone types grow around that scaffold. In marsupials, a dense bone tissue builds up around the outside of that early bone scaffold. In placentals, more bone tissue grows around the inside of the scaffold, a pattern that multis match. “Multis developed a litter of offspring, those offspring were born, nursed for a short time, and then the offspring were off to do their own thing,” Weaver says, noting the similarity to small rodents today.
The findings run contrary to the classic idea of a continuum of ever-improving evolution from egg-laying mammals to marsupial mammals to placental mammals. In other words, the way kangaroos reproduce, with very tiny offspring that require prolonged care, is a specialization and not a middle step. At the same time, that means that the placental method of birthing more developed offspring that stay close to their mothers for a shorter time appeared much earlier than anyone thought, with multis pioneering the strategy. That leaves paleontologists with one of two possibilities—either multis evolved parental care on their own, and placental mammals evolved it anew, or their form of parental care goes back to their last common ancestor. “It is possible that multituberculates evolved the same caregiving independently,” Panciroli says, “but this research suggests that this life history is shared by their common ancestors.”
Understanding that multis were ahead of their time changes what paleontologists have thought about their history, why they became extinct, and the rise of placental mammals. For a long time, perhaps because of our bias as placental mammals ourselves, paleontologists thought that giving birth to well-developed offspring that mothers doted on for a short time gave placental mammals an evolutionary edge over other groups. But now we know that multis shared this trait, meaning that placentals didn’t have the unique advantage we thought they did. As a result, understanding why ancient forms of mammals like multis disappeared while placental mammals became the most numerous on the planet has become a mystery once more. “There are probably multiple reasons for the patterns we see today,” Panciroli says, “but much of it is serendipity rather than reflecting any inherent superiority in either group.”
The new details make the legacy of the multis even more impressive. “They basically invented the small-herbivore niche, were likely important seed dispersers and flourished in a world otherwise dominated by large dinosaurs,” Weaver says, with the group’s eventual extinction doing nothing to tarnish its reputation. “They were the queens of kings of the mammal world for at least 130 million years,” Weaver says, and we are only now just beginning to understand what made them bestial royalty for so long.