Our Forgotten Cousins - The Multituberculate Mammals

Our Forgotten Cousins - The Multituberculate Mammals

When people think of prehistoric mammals, they usually think of the giant creatures of the Ice Age, such as the wooly mammoth, the wooly rhino and sabre-toothed cats - all animals which lived relatively recently. Generally speaking, however, most prehistoric mammal groups - like most groups of prehistoric animals that happen not to be dinosaurs - are little known to the general public. One of the most fascinating of these groups is one which lasted for 166 million years, the longest lived of all mammal groups. This group did not belong to any of the three mammal groups living today - placentals, marsupials and monotremes. They boasted over 200 species, the most diverse mammal group of all time, ranging in size from mouse-sized to beaver sized. They also demonstrated a wide variety of lifestyles - burrowing, tree dwelling, hopping. They are the multituberculates - also known as "multis" - so named from the several rows of small cusps, or tubercles, on their cheek teeth. Though their skulls and teeth were superficially similar to those of rodents (they have been dubbed "the rodents of the Mesozoic"), multituberculates are distinguished by their massive fourth lower premolar, a feature found in no modern mammals. Like rodents, they were able to gnaw, but with their lower jaws, not the upper jaws, as rodents do. These were probably used for crushing seeds and nuts, though it is likely that the multituberculates also fed on insects, worms and fruits: some may even have been scavengers, as indicated by tooth marks on fossils on Champosaurus, an aquatic crocodile-like reptile. Also, their teeth were replaced similarly to most living mammals, but unlike rodents, which have ever-growing teeth. They also boasted a palinal (front to back) jaw stroke, a feature possessed by very few living mammals: when eating, the food was held in place by the last upper premolar while sliced by the lower premolars; next, the lower jaw moved palinally, grinding the food between the cusp rows on the molars. And, lest these extinct species be thought of as "primitive" mammals, they also had complex brains, with some braincase regions unknown in mammals of today. They were also able to hyper-rotate their ankles, enabling them to move fast, a key reason for their success. In this post, I hope to bring to light these fascinating but little-known creatures.

Origins

The affinities of the multituberculates are disputed. Some scientists classify them as "crown mammals" - that is to say, descended from the most recent common ancestor of placentals, marsupials and monotremes, the three surviving mammal groups. Among those who hold this view, opinions diverge on whether multituberculates should be regarded as close relatives of the monotremes, the only surviving egg-laying mammal group, or whether they are the sister group to the therians (the live-bearing mammals, placentals and marsupials). Due to the narrow shape of the pelvis it is thought that, like marsupials, they gave birth to underdeveloped young: however, in evolutionary terms, the marsupials are the sister group to the placentals, and are thus not close relatives of the multituberculates.

The earliest known multituberculate fossil, Indobataar zofiae, is found in India and dates from 183 million years ago, in the Early Jurassic, a relatively early stage in mammalian evolution. Although recent research has shown that Jurassic mammals were more diverse than once assumed, they were nevertheless small and comparatively rare, with the dinosaurs the undisputed masters of the Earth. Life was dangerous for mammals, with most avoiding the dinosaurs by adopting a nocturnal lifestyle and eating insects. It was into this unpromising world that the multituberculates took their first tentative steps. But though they were destined never to grow to great sizes, unlike some much later mammal groups, the multituberculates were to prove that it was possible for mammals, against the odds, to survive and indeed thrive during the Age of Dinosaurs.

The earliest multituberculate known from good fossil remains is Rugosodon, which lived in China 160 million years ago, in the Late Jurassic. Known from a nearly complete skeleton, it was an omnivore, feeding on fruits, seeds and small animals. Its main significance is its flexible ankles: these were already known on much later multituberculates, but Rugosodon proved that this was an ancestral trait. Its dental features are similar to those of contemporary multituberculates found in western Europe, suggesting frequent faunal exchanges between the two continents. This should not be surprising: at this time, there were two major supercontinents, Laurasia in the north, consisting of Europe, Asia and North America, and Gondwana in the south, composed of Africa, South America, India, Australia, New Zealand and Antarctica. The multituberculates are found mainly in Laurasia, but they are also known from Gondwana.

The order Multituberculata is divided into two main suborders: Plagiaulacida and Cimolodonta. The plagiaulacids were the more primitive of the two, and the ones that predominated in the early stages of the multituberculates' history. They were largely confined to Laurasia and died out in the Early Cretaceous, to be replaced by their more derived cousins, the cimolodonts.

Heyday

In the Cretaceous Period, the multituberculates, in the form of the cimolodonts, began to diversify and spread, and were the most significant mammal group for 88 million years. In the Late Cretaceous, they comprised more than half of land mammal species in the northern hemisphere. As with all animal groups, most species were wiped out in the mass extinction 66 million years ago that also famously killed the dinosaurs, but the multituberculates were among the first to recover, For the next 20 million years, a time known as the Palaeocene Epoch, they reached the peak of their diversity, dominating life on land. The largest multituberculate group, the taeniolabidids, known from North America and Asia, originated in the latest Cretaceous, and flourished during the Palaeocene: on average they were about the size of a beaver, about as large as mammals got at this time. Indeed, they were the largest non-therian mammals. One member of this group, Taeniolaebis taonensis (below), was the largest multituberculate known, weighing about 100km, comparable to the (much later) largest ever beaver, Castoroides.

Taeniolabidids had gnawing teeth with a shelf-sharpening mechanism. Only the front side of the incisors was covered with hard enamel, with the rest consisting of softer material that wore away more easily, giving the tooth a sharp cutting edge at the front. The premolars were significantly reduced in size, while the molars were enlarged with an increased number of cusps, to form complex grinding surfaces. One taeniolabidid sub-group, the lambdopsalidae, grazing animals that lived in Asia in the late Palaeocene Epoch, also grew relatively large, and coexisted with the earliest rodents for millions of years. One species, Lambdopsalis, is notable for the earliest known example of fossilised hair, which is not usually preserved: they are found in the coprolites (fossilised dung) of carnivorous mammals which preyed on Lambdopsalis. It lived in burrows.

A highly diverse group was the ptilodontids, small, rodent-like animals, known only from sparse remains: one of their subgroups, the squirrel-like ptilodontids such as Ptilodus (below) were noted for their last lower premolars, which were larger and more elongated than those of other mammals, and formed a serrated slicing blade. As with squirrels, they lived in trees, and were able to reverse their back feet, enabling them to climb down trees with the head pointing downwards. However, unlike squirrels, the ptilodontids used their long tails as an extra limb when climbing.

Yet another significant group, the insectivorous kogaionids, originated near the end of the Cretaceous on Hateg Island, in what is now Romania (at the time Europe was a patchwork of islands, as sea levels were significantly higher than today), where they were the dominant mammal group, due to their isolation, and are the only multituberculate group known from Late Cretaceous Europe. One such species, Litovoi tholocephalos, experienced a significant reduction in its brain size, displaying one of the smallest brains in proportion to body size of any advanced mammal. Their insectivorous diet, an adaptation to their isolated environment, allowed them to survive the mass extinction that killed the dinosaurs 66 million years ago, and, as Hateg Island became connected to the rest of Europe, they spread, and their fossils are known from France, Spain and Belgium, becoming the most common mammals in Europe. But their dominance was to be short-lived, as the arrival of other multituberculate groups from North America brought about their extinction about 55 million years ago. In addition, some studies place the gondwanatheres, a herbivorous group of mammals that inhabited the southern hemisphere until as recently as 17.5 million years ago, as multituberculates.

Decline

From the late Palaeocene,  the multituberculates began to decline: their decline continued through the  next epoch, the Eocene, and they eventually died out in the early Oligocene. Traditionally, their demise was attributed to competition from rodents, which began to diversify rapidly during the Eocene: however, more recently this has been questioned. Firstly, this theory rests on the always questionable assumption that certain animal groups are "superior" to others. Secondly, multituberculates and rodents had coexisted for 15 million years. Though the decline in multituberculate numbers in North America and Europe does correlate with the arrival of rodents on these continents (though, of course, correlation does not indicate causation), in Asia, where rodents originated, the two groups long coexisted. Some research suggests that Asian multituberculates, unlike their European and North American cousins, never recovered from the end-Cretaceous extinction, which allowed rodents to evolve and spread in the first place. Also, the last multituberculate species appear to have been generalists, not the specialist animals that would be vulnerable to competitive exclusion. Some researchers link the decline of the multituberculates to a series of sharp extinction events. It seems most likely that they died out due to changes to the climate and vegetation: in which case, rodents would be the beneficiaries, rather than the cause, of the multituberculates' extinction. This is generally the case in evolution when one group is replaced by another: not because the superseding group is "superior", or because the declining group is unable to compete, but simply because the former takes advantage of the latter's decline, for factors outside the control of both. The best example of this is the replacement of the dinosaurs by the mammals: the mammals were not "superior" to the dinosaurs and did not competitively exclude them, but benefited from the dinosaurs' sudden extermination at the hands of an asteroid impact.

Conclusion

The multituberculates flourished for a remarkably long period of time, 166 million years - about the same length of time as the infinitely more famous dinosaurs. Though they never grew to great sizes, and were the dominant animals only briefly, their story is still a remarkable one, and it is a pity that they are little known to the general public. I hope that this blogpost has been able to make the point that an animal does not have to be huge to be interesting, and that life is so fascinating precisely because of the wide variety of shapes and sizes that it displays.