“Giant mammals.” With these words Mark Norell, curator of palaeontology at the American Museum of Natural History in New York, completely stunned me. I had asked him what was new in the world of dinosaurs. Why was he talking about mammals?
Norell wasn’t talking about ice-age behemoths such as woolly mammoths or cave bears: those didn’t evolve until long, long after the dinosaurs had gone extinct. His giant mammals lived alongside the dinosaurs, at a time when textbooks say they should have been tiny, primitive and shrew-like.
A few months later I got the chance to see one of these “giant mammals” for myself. The skeleton of Repenomamus robustus was in a glass case in the foyer of the American Museum. Bones in its belly showed it had been dining on dinosaurs. But at the size of an opossum it was hardly a giant, and the dinosaurs it had eaten were babies.
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For a moment I was disappointed. But then I thought about it. Though Repenomamus weighed only 5 or 6 kilograms, that was still around a hundred times heavier than early mammals were supposed to be – comparable to the size difference between humans and the largest dinosaurs. From that perspective, Repenomamus was indeed a giant. And it wasn’t alone. After decades believing that the mammals of the dinosaur era were tiny and insignificant, palaeontologists are suddenly discovering a wealth of size and diversity that nobody suspected even five years ago.
Studying the mammals of the Mesozoic era – between 251 and 65 million years ago – has always been something of a minority pursuit. Fossils from that time are in very short supply. Nature is superbly efficient at recycling dead animals, and the smaller the animal, the more efficient the recycling. Other than large dinosaurs, most Mesozoic land animals left no fossils except teeth. For palaeontologists and their paymasters, dinosaurs have always been a bigger draw. “Traditionally, Mesozoic mammals were not the path to glory,” says Hans Sues of the National Museum of Natural History in Washington DC.
What was known about Mesozoic mammals came about through painstaking work. Palaeontologists sifted through piles of sediment looking for fragments of jawbone or, more likely, teeth. Their finds were unimpressive to the untrained eye; even specialists often needed a microscope to identify them. And while the fossils revealed relationships between groups and species of mammals, they said little of the animals themselves other than that most were generalised insectivores ranging in size from shrew to chipmunk and weighing no more than 50 grams.
All this meant that the prevailing view of Mesozoic mammals was that they were small, primitive and rather dull – an assortment of rodent-like creatures living in the background, eking out a jack-of-all-trades existence on insects and worms. Dinosaurs took centre stage, and predation and competition from them kept mammals on the sidelines for millions of years. It was only after the dinosaurs went extinct 65 million years ago that mammals were free to evolve into larger and more specialised forms.
A few careful palaeontologists, though, thought this might not be the whole story. They saw hints of a richer cast of Mesozoic mammals: teeth and skeletal remains that looked as though they came from larger and more interesting creatures. In the late 1980s Farish Jenkins of Harvard University discovered the bones of an opossum-sized mammal in Montana, in rocks from the early Cretaceous – around the middle of the Mesozoic era. The creature, which he named Gobiconodon, was a whopping half-metre from nose to tail and weighed about 5 kilograms. It was powerfully built and had the sharp teeth of a predator or scavenger. American geologists working in Portugal later turned up an incomplete Jurassic skeleton that was clearly leading a more interesting lifestyle than the standard-issue insect eaters. Haldanodon appeared to be adapted to a semi-aquatic, digging lifestyle similar to that of modern water moles.
Interesting as these finds were, they weren’t enough to overturn the orthodoxy. That didn’t start to happen until the discovery of the world-famous “feathered dinosaur” fossil deposits in China. Just as these are transforming our understanding of the dinosaurs themselves (Â鶹´«Ă˝, 21 May 2005, p 40) so they are changing our view of the animals they shared the world with. The first hints that the Chinese mammals were something special came from the Yixian formation in Liaoning province, a series of stunning deposits dating from the early Cretaceous, about 125 million years ago. When western palaeontologists first visited the site in 1997, they were stunned to see near complete skeletons of mammals alongside those of small dinosaurs and birds, preserved in the sediments of an ancient lake bed. Though small and shrew-like, the mammals had something never seen before: fringes of fur around their skeletons. This was confirmation of something that palaeontologists had long suspected – that fur goes way back in the mammal family tree.
Even more revealing was a second type of Yixian deposit: beds of volcanic ash that buried animals and preserved their skeletons in three dimensions. These gave palaeontologists a chance to see Mesozoic mammals in a way they had never done before, giving a unique window on what they looked like, the niches they occupied and how they behaved.
The first mammals to emerge from Yixian were the sort of finds that excite specialists more than the public. Eomaia, described in 2002, is the earliest animal in the line leading to placental mammals; it was the size of a large mouse. A similar find was 15-centimetre Sinodelphys, the oldest known marsupial by about 15 million years. Neither could be described as a giant, but both fossils showed unmistakeable signs of a lifestyle that had never been seen before in a Mesozoic mammal: they were adapted to climb trees.
It wasn’t long before spectacular finds from the ash beds made an appearance. The first to gain worldwide attention was Repenomamus, which hit the headlines two years ago after a joint team from the American Museum and the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, China, described it in Nature (vol 433, p 149).
Repenomamus giganticus instantly became by far the biggest and baddest mammal of the dinosaur age. About the size of a badger or Tasmanian devil, it was a metre long from head to tail and weighed at least 12 kilograms. With a low-slung posture, large head and sharp front teeth, the researchers reckoned that it had to be a predator.
The team also found a closely related species that confirmed their hunch. Repenomamus robustus was about half the weight of Repenomamus giganticus, but the specimen had a feature which more than made up for that: shortly before its death, it had gobbled down a young dinosaur about 14 centimetres long. This was the fossil I saw sitting in the foyer of the American Museum.
The two species of Repenomamus belonged to a group of mammals called the Eutriconodonts (see Diagram). These were the first mammals to hunt prey bigger than bugs, according to Rich Cifelli of the University of Oklahoma. They evolved massive skulls and heavy jaw muscles that made them look like “pit bulls on steroids”,he says. “These things were built for biting.” They probably couldn’t move quickly, but they were tough. Not for them a life of cowering in the shadows.
“These things were built for biting”
Then other sites started to throw up mammals with a difference too. Early last year a surprising creature emerged from the Daohogou deposits of Inner Mongolia, thought to have been laid down in the mid-Jurassic about 164 million years ago (the dates are disputed by some palaeontologists).
The animal’s proper name is Castorocauda but it is known as the “Jurassic beaver” because of its broad, flat 20-centimetre tail. At half a metre long and weighing 500 grams, Castorocauda is the largest Jurassic mammal found so far (Science, vol 311, p 1123).
Like Repenomamus, the Jurassic beaver was anything but a shrew-like generalist. With a body that was part otter, part platypus, limbs and tail built for swimming and seal-like teeth for catching fish, it was clearly adapted for a semi-aquatic lifestyle – pre-dating other semi-aquatic mammals by 100 million years. After it died, its body sank to the bottom of a lake, where it was fossilised complete with imprints of its webbed feet and fur – the oldest fur known from any mammal. Zhe-Xi Luo, curator of vertebrate palaeontology at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, says its lifestyle was probably much like a platypus, “digging a tunnel to nest and lay eggs, and going from the tunnel into the water to feed”. Yet despite its similarities to modern mammals, it is only distantly related. It was part of a group called docodonts, which split from the other mammals early in the Jurassic and died out about 100 million years ago.
Jurassic mammals had even taken to the air, as a jaw-dropping discovery from Daohogou revealed at the end of last year (Nature, vol 444, p 889). The fossil first caught the eye of researchers from the IVPP in Beijing because of its teeth, which were unlike those of any other Mesozoic mammal. Next they noticed its unusually elongated limbs and tail. “Then looking more carefully, we found preservation of soft tissues around the skeleton and outside,” says Jin Meng of the American Museum of Natural History, who was visiting IVPP at the time. Further study revealed the impression of a furry flap of skin alongside the skeleton, which the researchers concluded was a gliding membrane.
The animal, named Volaticotherium antiquus, “ancient gliding beast”, presumably used the flap to glide through the air, like a modern flying squirrel. The discovery pushed the first evidence of airborne mammals back as much as 100 million years, before the oldest bats 55 million years ago. Even if the disputed age of the Daohogou fossil beds is at the lower limit, 125 million years, that’s 70 million years before bats. If the 164 million year date is correct, as both Meng and Luo firmly believe, mammals may have been in the air before birds. Yet Volaticotherium left no living descendants. It’s the first of a new order of mammals, Meng says, yet another sign that large chunks of the mammals’ evolutionary history remain undiscovered.
Not all the surprises are coming from China. Two years ago, Luo announced the discovery of a 150 million-year-old chipmunk-sized skeleton near Fruita, Colorado. Its forelimbs were so powerfully built it quickly acquired the nickname “Popeye”. Formally called Fruitafossor, it had the peg-like teeth of an anteater or armadillo and probably made a living by digging into nests of colonial insects – the oldest example of this adaptation in mammals (Science, vol 308, p 103).
Mesozoic weirdness
In the light of such discoveries, researchers are taking a fresh look at some previously enigmatic Mesozoic mammal remains. One such creature is Bubodens, a late Cretaceous mammal known from a single tooth found in South Dakota in 1987. The tooth is a couple of centimetres long and “looks like a little elephant tooth”, with ridges for chewing leaves, says Anne Weil of Oklahoma State University in Tulsa. If Bubodens ate leaves it would have had to have been a fairly big animal, as leaf-eaters need a large and complex gut to extract enough nutrients. “Leaves are a poor source of nutrition,” Weil says. “There’s a size threshold below which leaf-eating herbivores cannot exist.” She thinks Bubodens weighed up to 10 kilograms.
Palaeontologists also are puzzling over other fossils from the late Cretaceous. A group called Gondwanatheres, which lived on the southern continents, are known only from an assortment of teeth and a partial jaw. “They’re pretty weird,” says David Krause of Stony Brook University in New York, and it’s not yet clear where they fit into the mammalian family tree. The teeth suggest they might have been grazers, and some believe they may have been the largest mammals to survive the mass extinction that wiped out the dinosaurs, but by 50 million years ago they too were extinct.
Overall, it’s hard not to conclude that discoveries made over the past few years paint a very different picture of Mesozoic mammals. Dinosaurs may have been the dominant creatures, but mammals were very much a part of their world, from the water to the air. “Their scope of diversity would be hard to imagine just five years ago,” says Luo. “They invaded many more ecological niches and developed many more lifestyles than was previously thought possible before the extinction of the great dinosaurs.”
That’s not to say that shrew-like generalists didn’t have an important part to play in evolution. After all, being small and adaptable was probably what allowed some mammals to survive the mass extinction that wiped out the dinosaurs. Among those survivors, of course, was our own direct ancestor.
For now though, large and unusual Mesozoic mammals are where the action is, and there are surely more surprises to come. “How much we missed is really hard to tell,” says Meng. “If an animal in the early stage can evolve to have this aerial lifestyle, it’s logical to think that other kinds of niche could be occupied by mammals as well. So Mesozoic mammals could be very diverse – we have to look for more fossils.”
Sues agrees. “I’m sure there’s so much more to be learned about Mesozoic mammal evolution,” he says. “The last word is never said. There are always new, weird things to discover.”
The origin of mammals
Mammals evolved from a group of reptiles called Dicynodonts that prospered during the Triassic period, 250 to 200 million years ago. The larger members of this group went extinct at the end of the Triassic and were displaced by dinosaurs, but some smaller ones survived into the Jurassic.
The line that gave rise to mammals is called Mammaliformes. In the early to middle Jurassic these evolved into the first true mammals (see Diagram). The monotremes – a group containing just five living species, the platypus and four echidnas – split from the main lineage first. Placental mammals and marsupials split in the early Cretaceous.FIG-mg25891601.jpg
Many extinct branches also split from the main mammalian lineage. The best known, the Multituberculates, split in the Jurassic and survived until about 30 million years ago.
Several groups of mammal survived the extinction that marked the transition from Cretaceous to Tertiary, and with the dinosaurs out of the way they had plenty of room to expand and diversify. Within 750,000 years a Multituberculate called Taeniolabis approached the size of the modern capybara. Placental mammals, which as far as is known remained tiny throughout the dinosaur age, soon caught up, some reaching elephant size by about 50 million years ago
