An unusually clear picture of the early stages of mammal evolution comes from a fossil skeleton of a 145-m.y.-old rat-sized creature from China. Zhangheotherium is described in the 13 November 1997 Nature by Zhexi Luo of the Carnegie Museum of Natural History in Pittsburgh and colleagues in Beijing. It is a symmetrodont from the Jurassic-Cretaceous boundary. Until now, symmetrodonts have been known only from jaws and teeth, but Zhangheotherium is preserved as an entire skeleton.

Many of the lineages that gave rise to modern mammals were already distinct well back into the Cretaceous, even if all the mammals looked rather similar: small, rodent-like animals, possibly nocturnal and insectivorous. Marsupials and placentals were already distinct in the Cretaceous.

In Jurassic rocks, we have a host of poorly known extinct groups, such as multituberculates and symmetrodonts. Multituberculates are particularly interesting. They have distinctive, comb-like teeth of a kind seen nowhere else, and were also the longest-lived mammalian group of all time, living for around 150 m.y. They handily outlived the dinosaurs and died out only around 40 Ma.

The precise relationships between monotremes and archaic, extinct mammals such as symmetrodonts have been a matter of some debate. Some have suggested that monotremes are modern-day multituberculates. Comparative anatomical studies based on Zhangheotherium clarify these issues. The first flush of mammalian evolution in the Triassic produced a series of archaic forms, some of which survived to the present-day to become monotremes. The monotreme lineage, however, had become distinct from that leading to multituberculates and other mammals, so that monotremes have no particular relationship with multituberculates. Instead, multituberculates form the basis of a second radiation of mammals, separating from a lineage which produced symmetrodonts such as Zhangheotherium and eventually, marsupials and placentals.

Zhangheotherium is primitive in many ways. For example, it has a spur on its hind foot, reminiscent of the poison spurs on the feet of a male platypus. Superficially rat-like, Zhangheotherium would have walked differently from modern mammals: it had advanced from the reptilian sprawl of monotremes, but had not quite acquired a posture we would consider modern.

Zhangheotherium was found in the province of Liaoning, which is currently yielding the most sensational fossils from anywhere in the world. The same deposits have yielded other mammals, some preserved with their fur, together with more than 200 specimens of Confuciusornis, complete with feathers and beaks. The most controversial finds have been three specimens of Sinosauropteryx, a small bipedal dinosaur with a skin covered in what look like hairs or possibly simple feather-like structures. Of perhaps even greater interest is Protarchaeopteryx, a dinosaur or possibly a primitive bird which, like Confuciusornis, seems to have had true feathers.

The relevance of Sinosauropteryx and Protarchaeopteryx to the evolution of birds is much in question at present. Few outside China have seen the original specimens, and even fewer have read the original research on these creatures, hitherto only in Chinese. One thing is certain: the lives of these creatures was as intertwined as those of any creature alive today. The last meal of one Sinosauropteryx appears to have been a small mammal, possibly even Zhangheotherium.

© Macmillan Magazines Ltd 1997 - NATURE NEWS SERVICE

By special permission of Nature, this item from the Nature News Service is mounted on this Web page.

[Return to Chapter 15]

[Return to UC Davis Geology Department Home Page]