Findings: BBC missing one beast

With an average weight of 60g, it is small wonder that the BBC's Walking With Beasts overlooked the Paleocene's micromammals in favour of hulking mammoths, smilodons and giant sloths.

Yet the 50 million-year-old fauna unearthed by vertebrate palaeontologist Jonathan Bloch and colleagues at the University of Michigan in the United States is possibly the most complete to date from the age following the extinction of the dinosaurs. It is certainly the best-preserved.

Bloch himself has found between 20 and 30 new species of mammal.

They include Batodonoides vanhouteni , a 1.3g shrew-like insectivore that is believed to be the lightest mammal yet found; the primitive squirrel-like primates Plesiadapiformes, which may be ancestors of humankind; and the weasel-like Viverravus rosei , one of the smallest known mammal predators.

The fossils come from isolated limestone blocks in the Clarks Fork Basin, in Wyoming.

The deposits are thought to have formed from the hollows of rotting trees or shallow ponds. The peculiar circumstances of the fossilisation in the tropical rainforest in which these creatures lived enabled very delicate bones to survive.

Bloch and his colleagues isolated the remains by dissolving the limestone with formic acid. One block yielded 414 identifiable mammal bones from 13 individual skeletons. Birds and amphibians have also been found.

The quantity and quality of the fossils are allowing the scientists to attempt to reconstruct significant elements of whole extinct ecosystems. The high proportion of tree-dwelling adaptations among the animals is an early observation.

Much of the research has been published this year in the American Journal of Physical Anthropology and the Journal of Vertebrate Paleontology .