Pushing Back The Origin Of The Mammals

An artist's impression of the early mammal Morganucodon

Around 200 million years ago in a scrub forest, which one day would become Glamorgan in Wales, there lived a tiny and seemingly insignificant creature known as Morganucodon.

Warm blooded, covered in fur and almost certainly nocturnal it is considered to be one of the most primitive mammals on the planet. Yet tens of millions of years before Morganucodon there were no mammals, only a group of bizarre fur covered reptiles known as the cynodonts, their immediate ancestors.

Cynodont fossils are not overly rare, but the form which represents the transition from reptiles to mammals, namely Prozostrodon, was separated from Morganucodon and its kin by nearly 20 million years - a gap not easily dismissed. Yet this gap may be closed thanks to a recent discovery of three new species which are part of a group known as the haramiyids. Fossils of haramiyids crop up in the fossil record from 216 to 160 million years ago, but they are fragmentary, comprising mostly teeth and jaws. This makes it difficult to place them in the evolutionary tree. The general morphology suggests that they fall within the mammal group, but the lack of complete fossils has hindered a definitive classification. The recent discoveries, however, are complete enough to give a clear view of the haramiyids physiology and their placement within the tree of life.

Uncovered from 160 million year old rocks in China, the three new species - Shenshou lui, Xianshou linglong and Xianshou songae - weighed in at between 30 and 300 grams, with tails and feeding habits suggesting that they were arboreal. 'They were good climbers and probably spent more time than squirrels in trees,' said Jin Meng from the American Museum of Natural History's Division of Palaeontology. 'Their hands and feet were adapted for holding branches, but not good for running on the ground.' Overall the fossils confirmed the haramiyids' general form. The teeth, however, presented some interesting differences.

The 160 million year old fossil of
the haramiyid Shenshou lui

Mammals have cusps, or raised points, on the crowns of their teeth. Mammals are thought to have evolved from a common ancestor that had three cusps; human molars can have up to five. The newly discovered species, however, had two parallel rows of cusps on each molar, with up to seven cusps on each side. Despite this, their skeletal morphology confirmed that the haramiyids were a part of the mammal family rather than close relatives.

Mammals are unique in the fact that they have three bones in the inner ear, the malleus, incus and stapes, which are responsible for conducting sound waves to the sensory organs in the skull. These bones are so small that they are often lost in the fossil record, but the new species were so well preserved that these bones were present.

The earliest haramiyid fossils are 218 million years old, thus pre-dating Morganucodon by almost 20 million years, closing the gap on the most immediate mammal ancestor Prozostrodon. Not only is the fossil evidence a closer fit, it also corresponds with the molecular data. 'What we're showing here is very convincing that these animals are mammals, and that we need to turn back the clock for mammal divergence,' concluded Meng. 'But even more importantly, these new fossils present a new suite of characters that might help us tell many more stories about ancient mammals.'

Perhaps more intriguing still, this presents an entirely new paradigm within which the mammals evolved. If they had appeared 200 million years ago, they were entering a world filled with ever larger dinosaurs both predatory and herbivorous. 218 million years ago presents a scenario where they evolved alongside early dinosaurs. This suggests that their arboreal nature may have been a response to the threats presented by the fleet-footed, carnivorous early dinosaurs. More evidence is needed to confirm my hunch, but as time progresses, the origins of mammals will become more clear cut.