The little mammal that emerged after the dinosaurs

A tree-dwelling rat-like creature that lived in China provides information about the earliest mammals. Elizabeth Finkel reports.

Reconstruction of a new mammal species, Xianshou songae. This mouse-sized animal was a tree dweller in the Jurassic forests and belonged to an extinct group of Mesozoic mammals called Euharamiyida. – Zhao Chuang

Dinosaurs left a hefty fossil record. Not so the small rat-like creatures who emerged in their shadows, such as the haramiyids. Until recently most of what we knew of them came from the occasional tooth, which was quite different to that of any living mammal. A report this week in Nature from Jin Meng at the American Museum of Natural History and colleagues describes what haramiyids were like. Six exquisitely preserved skeletons, found in 160-million-year-old Jurassic deposits in China, reveal them to be fully-fledged mammals – an extinct early branch on the mammalian family tree. The findings support the idea that the age for the origin of mammals was in the late Triassic, some 200 million years ago.

“It’s the first time we have convincing evidence these creatures were actually mammals,” says Meng. Tom Rich, a palaeontologist at Museum Victoria who studies 100-million-year-old Australian mammals and reptiles, agrees. “They’ve made a convincing case,” he says.

Teeth are a mainstay in the fossil record. They are the most durable part of an animal because of their hard enamel coating. Reptilian teeth end in a single point – they’re ideal for tearing and gulping food. But mammals have molars for chewing. Their shape, number and the arrangement of their cusps and ridges can distinguish individual species.

Haramiyid teeth, first identified in 1847 in late Triassic rocks in England, were neither reptilian nor quite mammalian. They had two roughly parallel rows of cusps. This likely meant the animal ground its food by moving its lower jaw backward and forward rather than the sideways action of modern mammals. Two distinct groups possessed these teeth – a common variety dubbed multituberculata and older ones, the haramiyida. The discoveries raised many questions. Were they mammals? Were haramiyids the ancestors of the multituberculata?

These questions couldn’t be answered only on the basis of teeth. “We had a feeling they were mammals but they were so different from mammals we know,” says Meng. Last year he and his colleagues published a report on the first haramiyid skeleton, unearthed from the bountiful fossil beds in Liaoning Province in northeast China. The remains suggested these animals were unlikely to be the closest kin of multituberculates.

Now some 50 km away in the Tiaojishan formation – rocky outcrops rising from corn fields – six more haramiyid skeletons preserved in slabs of fine sandstone have been discovered. The fine detail of the skeletons helps shed light on the unresolved questions.

They are clearly mammals, not mammal-like reptiles. The hearing bones of the middle ear provides evidence. Reptiles have a single hearing bone, the stapes, to amplify sound to their ear drums. Mammals have an additional two, the malleus and incus, which are derived from two small bones in the joint of the reptilian jaw. The new arrangement increases the sensitivity and extends the hearing range to higher frequencies – it’s a signature feature of mammals. Creatures in between reptiles and mammals – like Yanoconodon, a Chinese fossil reported in Nature in 2007 – have an intermediate structure with the two middle ear bones still connected to the jaw.

The new skeletons could be resolved into three different species,
based on features such as their size and tooth morphologies

But the new Chinese specimens have fully developed middle ear bones that are completely split from the jaws. “That’s what I find exciting. They’ve already made the transition,” says Rich.

The skeletons also had long grasping fingers, slim builds and long prehensile tails, that leads Meng to propose they were tree dwellers. It helps explain the paradox that while this general group (haramiyids) existed for a long period – their teeth are found through rocks from at least 200 million years ago to 140 million years ago – other types of remains are rare. “Tree living is not conducive to forming fossils”, explains Meng. The rare Tiaojishan formation where these skeletal specimens were found appears to have resulted from volcanic action blasting the mammals out of the trees and into nearby ponds. It has been the source of much Jurassic flora and fauna, including many feathered dinosaurs.

Finally, based on the six new skeletons, the team could show haramiyids were not a single species. Though somewhat surprising to find this degree of diversity in a single site, Meng is confident the new skeletons represent three different species based on features such as their size and tooth morphologies. “This is probably the first Nature paper to report three new species at once”, says Meng. The haramiyids varied in size from 40 to 300 grams but were all mature animals as ascertained by their teeth. Meng and colleagues suggest the new species (four including the one published last year) merit being grouped under the new name Euharamiyidia (meaning true haramiyids).

That these Euharamiyidia were quite diverse suggests that their ancestors, the first mammals, evolved much earlier on in the late Triassic. No doubt their remains lie waiting to be discovered in the siltstones of Northeastern China. “I hope the new discovery will inspire more discussion and research on the early evolution of mammals,” says Meng.

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