How the earliest mammals thrived alongside dinosaurs
An explosion of fossil finds reveals that ancient
mammals evolved a wide variety of adaptations allowing them to exploit
the skies, rivers and underground lairs.
Early mammals like this rat-sized species Liaoconodon hui coexisted with feathered dinosaurs like Sinotyrannus in the temperate ecosystems of the Cretaceous in what is now Liaoning in northern China. Illustration by Davide Bonadonna
Night is falling in the early Jurassic 185 million years ago, and the Kayentatherium
is tending to her newly hatched brood. Heavy rains pummel the bank
above her den as she looks over her dozens of tiny young. She is about
the size of a large cat and could easily pass for a mammal, but her
large jawbone, characteristic teeth and lack of external ears give her
away: she is a cynodont, a member of the group from which mammals
evolved. At some point without warning, the sodden bank collapses,
entombing the hatchlings and their mother in mud.
There they
remained until the summer of 2000, when a fossil-hunting crew led by
Timothy Rowe at the University of Texas at Austin chanced upon their
scattered bones among rocks of the Kayenta Formation in northern
Arizona.
That initial encounter with the fossils did little to
impress the palaeontologists. They dug up the block and shipped it back
to the laboratory for safekeeping. It wasn’t until nine years later that
a specialist preparing the fossil for study noticed something
startling: embedded in the block were tiny teeth, and jawbones just 1
centimetre in length. “Immediately they stopped the preparation and
thought about ways of non-destructively examining the babies,” says Eva
Hoffman, at Texas with Rowe at the time and now a palaeontologist at the
American Museum of Natural History in New York City. Instead of
breaking into the rock, Hoffman and Rowe digitally extracted the bones
with a microcomputed tomography (microCT) scanner, which uses X-rays to
create fine-grained 3D images.
What they found inside the rock
were the first known babies of mammals or their relatives from the
Jurassic — and not just one, but 38 of them, placing this among the most
significant discoveries related to mammal origins made in the past
decade1. Kayentatherium
is at the cusp of mammalhood — and researchers say that it provides
crucial insights into which traits define mammals and which were present
in their earlier relatives.
Kayentatherium’s skeleton is
mammal-like in many ways, but the fossil suggested that it still
reproduced very much like a reptile, giving birth to large litters of
small-brained offspring. By contrast, “mammal moms invest a lot in a
smaller number of babies, each of which has a better chance of
surviving”, says Hoffman. Mammal babies spend longer under their
parents’ care, developing relatively large brains, whereas these fossil
hatchlings had well-developed bones and teeth, hinting that they could
fend for themselves and were not nourished by milk, as all mammals are
today.
The find is among a mass of discoveries in the past 10–20
years that are illuminating milestones in mammalian evolution. Although
major finds are emerging all over the world, the largest number are
coming out of China; together, they have overturned the now dated belief
that dinosaur-era mammals were small, unremarkable insectivores, eking
out a life in the shadows of the giant reptiles.
This rat-sized Liaoconodon hui is one of many fossils from northern China that are sharpening the picture of how mammal traits evolved.Credit: J. Meng et al. Nature472, 181–185 (2011)
The fossils have revealed that early mammals were
ecologically diverse and experimenting in gliding, swimming, burrowing
and climbing. The discoveries are also starting to reveal the
evolutionary origins of many of the key traits of mammals — such as
lactation, large brains and superbly keen senses.
“The explosion
of early-mammal discoveries, particularly from China, over the last two
decades has been eye-opening, mind-numbing and absolutely dazzling,”
says David Krause, a vertebrate palaeontologist at the Denver Museum of
Nature and Science in Colorado.
This avalanche of discovery is
also stirring up debate: some researchers disagree over which fossil
groups are true mammals and the shape of the mammal family tree. “We
want to understand our early history in the language of evolutionary
biology, and that’s what fires me up,” says Zhe-Xi Luo, a
palaeontologist at the University of Chicago in Illinois. “That’s why
this entire field is so interesting, because the fossil record is
getting better and better, and we are starting to really tackle some of
these questions.”
Out of the shadows
In 1824, at the
Geological Society of London, naturalist William Buckland presented
bones from one of the first known dinosaurs, Megalosaurus. At the
same talk, he revealed tiny mammalian jaws that had been found in the
same fossil deposit. Their presence suggested that mammals had a very
deep history, but as would happen repeatedly, the dinosaur discoveries
completely overshadowed the mammal ones.
The slow trickle of
mammal finds from around the world continued for 150 years. Then in
1997, researchers described the first ancient mammal from the
fossil-rich rocks of Liaoning in northeastern China2,
and the floodgates opened. Since then, 50 or more near-complete and
“beautiful specimens” have been found there, according to Jin Meng, a
palaeontologist at the American Museum of Natural History. Like the
dinosaur fossils, they are dug up by local farmers and sold on to
museums.
But the dinosaurs continued to get the vast majority of
the attention, says palaeontologist Steve Brusatte at the University of
Edinburgh, UK. “It’s only that very recently, through the work of Luo,
Meng and others, that the mammals are getting their due.”
Most of
China’s mammal fossils were formed when volcanoes buried the animals in
ash — and they are exquisitely detailed. Typical mammal fossils from the
Mesozoic era (252 million to 66 million years ago) are little more than
teeth and jaw fragments, but Chinese specimens often have entire
skeletons, with fur, skin and internal organs. “We have a lot of detail
to answer scientific questions,” says Meng, He is interested in
understanding the evolution of the mammalian ear, for instance.
The finds overturned previous dogma. “We used to say that
during the time of dinosaurs, mammals were totally unspectacular. That
they were just these little mousey things scampering around in the
shadows,” says Brusatte. But these animals “were undergoing their own
evolutionary explosion”, he says.
Mammals first appeared at least
178 million years ago, and scampered amid the dinosaurs until the
majority of those beasts, with the exception of the birds, were wiped
out 66 million years ago. But mammals didn’t have to wait for that
extinction to diversify into many forms and species. “These new
discoveries document a huge, hitherto-undreamed-of ecological
diversity,” says Richard Cifelli, a palaeontologist at the University of
Oklahoma in Norman.
Among the first innovations that researchers
began to find in fossil form were those to do with locomotion. In 2006,
Meng’s team reported the first gliding mammal3, 164-million-year-old Volaticotherium, which had wing membranes made of furry skin, like today’s flying squirrels. In 2017, Luo’s team added Vilevolodon and Maiopatagaium4,5,
which lived at around the same time and belonged to a group called the
haramiyids. These animals swooped between the trees alongside some of
the first flying dinosaurs, taking advantage of previously unexploitable
food resources.
Researchers found other specializations that they assumed had evolved only later: Agilodocodon could climb trees and gnawed into bark to feast on sap6; the platypus-sized river-dweller Castorocauda had webbed feet and a beaver-like tail for swimming7; and Docofossor had paws and claws for digging, and looked like a modern mole8.
These
mammals had also adapted to a multitude of diets, much more diverse
than previously assumed. In 2014, Krause described the groundhog-like Vintana from Madagascar9, a herbivore that perhaps fed on roots and seeds. And the wolverine-sized carnivore Repenomamus, which Meng’s team reported in 2005, had baby dinosaur bones in its stomach10.
Many of these new-found fossil mammals belong to long-extinct
subgroups, says Meng. In contrast to the panoply that existed in the
Mesozoic, mammals today come in just three varieties: placentals, which
make up the majority of species and include humans; marsupials, such as
kangaroos and koalas, in which gestation in the womb is brief and
development continues in a pouch; and the egg-laying monotremes,
represented only by the platypus and several echidnas. “But in
geological history, there were many other groups such as
multituberculates, triconodonts and haramiyids,” says Meng. “Mammals
were actually very diverse in the Jurassic.”
Some, such as the shrew-like Juramaia
— described by Luo’s team in 2011 and dated to 160 million years ago —
are among the earliest placental mammals and therefore have the
potential to be our ancestors11.
And a few dinosaur-era mammals were much bigger than suspected, too. Repenomamus was 12–14 kilograms, and the racoon-sized Vintana
weighed in at 9 kg. “It’s exciting that we kind of busted the old myths
that early mammals came from a very humble generalized ancestor,” says
Luo.
The finds are not solely from China. Important fossils are
also coming from the United States, Spain, Brazil, Argentina, Madagascar
and Mongolia. Some of the most intriguing and oldest fossils — as well
as the biggest gaps in our knowledge — relate to the southern
continents, where only five genera of Mesozoic mammals and their
relatives are known, compared with more than 70 genera from northern
latitudes. In the past two decades, Brazil has yielded several Triassic
fossils that are more than 200 million years old. Guillermo Rougier, a
palaeontologist at the University of Louisville in Kentucky, describes
them as “incredible discoveries” that are right on the cusp between
mammals and their cynodont ancestors. “These forms really show a very
transitional progression from things that are typically non-mammalian,
to things that pretty much have all the features of early mammals.”
Mammal must-haves
The
latest finds are also offering clues to the evolution of key mammal
features. For instance, the keen hearing of mammals is partly down to
tiny bones in the middle ear — the malleus, incus and ectotympanic. But
in the reptilian ancestors of mammals, these bones were part of the jaw,
and were used for chewing instead of hearing. Mammal forerunners, such
as shrew-like Morganucodon from 205 million years ago, sported a prototype of the mammal arrangement that allowed for both functions12.
In 2011, Meng reported an intermediary13: a 120-million-year-old specimen from China belonging to a group of mammals called eutriconodonts and named Liaoconodon hui
(see ‘Mammal hallmarks’). The rat-sized fossil revealed three
middle-ear bones, but they were still attached to the jaw by cartilage.
“The hearing function and the chewing function were still not completely
separated,” he explains. This was hard evidence of the evolutionary
transition from jaw to ear.
This exquisitely preserved 160-million-year-old specimen of Maiopatagium furculiferum shows how early gliding evolved.Credit: Zhe-Xi Luo/UChicago
Another unique trait of mammals is the sophisticated way
they chew and ingest food in small parcels, rather than swallowing
things whole as snakes and alligators do. To make that possible, mammals
evolved a wide variety of complex teeth for biting and grinding food.
But
as babies, mammals are nourished another way — by suckling from their
mother’s mammary glands. “Our whole group is named after this incredible
biological innovation,” says Luo. Drinking milk is made possible by the
ability to suck and swallow, aided by the hyoid bones in the throat and
muscles that support them. This apparatus also forms the voice box.
In
July, Luo published a paper revealing a 165-million-year-old vole-sized
docodont — a close relative of true mammals — that had the hyoid bones
of its throat preserved14. Microdocodon gracilis is the earliest animal known to have been able to suckle like a modern mammal.
This level of detail is rare, and — similar to the study of the Kayentatherium
hatchlings — the work on both the ear and throat bones has been made
possible only through advances in microCT scanning techniques, says
Krause. The technique has also revealed details about the olfactory
abilities and brains of early mammals. These revelations are “breathing
life into these early mammals in ways that were previously impossible
and almost inconceivable”, he says.
Much of the constellation of
features we think of as defining mammals — complex teeth, excellent
senses, lactation, small litter size — might actually have evolved
before true mammals, and quite quickly. “More and more it looks like it
all came out in a very short burst of evolutionary experimentation,” Luo
says. By the time mammal-like creatures were roaming around in the
Mesozoic, he says, “the lineage has already acquired its modern look and
modern biological adaptations”.
Family drama
Although the
experts concur on many points, there is still much debate about how
early mammal groups are related, and which groups are true mammals. That
leads to uncertainty about how key traits evolved, says Hoffman.
One
sticking point between Meng and Luo, who have each developed their own
evolutionary trees, is the haramiyids. Meng thinks this early group
belongs with true mammals, whereas Luo is convinced it’s a side branch.
The oldest known haramiyids are from 208 million years ago in the
Triassic. If they are true mammals, then mammal origins date back at
least that far — if not, then the oldest known mammal is 178 million
years old, well into the Jurassic.
More fossils will help to
resolve such questions, and bring more surprises. Krause and Meng say
they are both studying exciting fossils, but are yet to publish their
findings on them, and tens of unstudied specimens lie piled in the
offices of their Chinese colleagues.
Palaeontologists have many
items on their wish lists. One characteristic that Luo wants to
understand is growth rates. Reptiles grow slowly throughout their lives,
whereas mammals grow in bursts in youth and then plateau. He’d love to
find a series of fossils from babies to adults to watch this happening.
“That is one of the most critical features in mammals that help to
define our biology,” he says.
Both Hoffman and Meng agree that embryos and more babies would be significant finds — and, like the Kayentatherium
discovery with its dozens of hatchlings, they would help us to pinpoint
the date that mammal-style small litter sizes appeared. Meng’s dream is
to find a pregnant mammal. “This is always in my mind that I will find a
mammal that inside the skeleton you can see some very delicate
skeleton, which is either an egg that hasn’t hatched, or it’s a more
interesting fetus,” he says.
If the flurry of discoveries has
taught researchers anything, it’s that every fossil find has the
potential to add a chapter to evolutionary history or even flip the
prevailing narrative on its head. “We’re really in this exciting, almost
manic phase of lots of new evidence coming in, and it’s going to take
time to synthesize,” says Brusatte.
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