Steadily, the remarkable fossil record in Cretaceous terrestrial sediments in China is revolutionising ideas about vertebrate evolution, particularly among small dinosaurs and early birds (see The Early Cretaceous lagerstätten of NE China in EPN March 2003). The long-held view that birds simply emerged fully fledged and flying from the dinosaurs has had to be thoroughly amplified. The sheer diversity, combined with intricate preservation in the Chinese sediments reveals feathering on a host of animals that are not birds, but earlier, bipedal dinosaurs. Some may have flown, but others had feathers for some other reason. Feathers are not a prerequisite for flying, and are so odd and complex in morphology and growth, that it has always been probable that they emerged and evolved over a long period preceding the appearance of true birds. Now it is possible to begin dissecting that strange evolutionary divergence, and Richard Prum and Alan Brush of the Universities of Kansas and Connecticut combine information about feathers and discussion of new fossils in a superbly illustrated review in the March issue of Scientific American (Prum, R.O. & Brush, A.H. 2003. Which came first, the feather or the bird. Scientific American, March 2003, p. 60-69).
Squirrels and tectonics
The squirrel family (Sciuridae) is one of the most widespread groups of mammals, only Australia, the Pacific islands and Antarctica being squirrel-free. The main reason is that squirrels are basically a primitive group among the rodents, themselves accounting for almost 50% of all living mammals species. The earliest fossil squirrel (Douglassciurus jeffersoni) was found in Late Eocene sediments in western North America, and the family seems to have originated there. The present wide distribution of squirrels bears witness to the many opportunities for migration in the Palaeogene, when continental masses were much less dispersed than they are today, together with changing environmental conditions that would have acted to drive migration. In the same way as human migrations have been charted and timed using genetic sequencing and molecular clock hypotheses, this unique group has been studied in detail (Mercer, J.M. & Roth, L. 2003. The effects of Cenozoic global change on squirrel phylogeny. Science, v. 299, p. 1568-1572). The general picture outlined by Mercer and Roth is that the Sciuridae migrated first across Beringea to reach Asia, then Europe and eventually Africa. In terms of migration rates, this was fast, the earliest European squirrel (Palaeosciurus) occurring in Early Oligocene sediments – this is also the earliest representative of squirrels that bear signs of the distinctive chewing muscles whose use today delights us all. Near identical musculature is found in the Red Squirrel and many other tree squirrels (Sciurus sp.), and their “living fossil” anatomy is borne out genetically.
As well as giving a fascinating insight into how modern genetic techniques help organise the cladistics of animals, the paper is full of information about the sheer diversity that this lowly group has achieved in about 50 Ma. Ground squirrels, rock squirrels, marmots, and tree squirrels abound, but none are so fascinating as the flying squirrels. Their teeth are similar to those in early Oligocene fossils, and genetic analysis suggests a common ancestry relatively early in squirrel evolution and migration. However, fossils of flying squirrels, in the areas where they are found today (North America and Asia) appear quite late in the stratigraphic column. The authors suggest that perhaps flying ability arose several times independently, based on a labile trait in the genes of their clade. There is also evidence for population “bottlenecks” that preceded adaptive radiation in several area. For instance, the entire radiation of South American squirrels seems to have stemmed from a single lineage that crossed the Isthmus of Panama shortly after it formed in Pliocene times. African squirrels can be accounted for by just two colonisations in the Miocene, and those of Indonesian archipelago east of the Wallace Line by migration during the Late Miocene, when sea-level was at its lowest before the Pleistocene lowstands. Most astonishing of all, is the Giant Squirrel of Borneo (Rheithosciurus), which is genetically closest to the squirrels of North America rather than its more diminutive cousins in the Sunda Shelf islands – did its ancestors move with astonishing speed, or did all related squirrels along its migration route become extinct quite rapidly?
A possible answer to the origin of the Giant Squirrel of Borneo lies in a collection made recently from a unique lagerstätten in a clay-filled pocket within laterites of northern Karnataka in India. The discoverer, Dr P.U. Siffli of Sringeri Institute of Palaeontology, has posted provisional results on his web site (http://geocities.yahoo.com/pusiffli/squirrels.html). The range of fossil rodents from near Sringeri is astonishing. Among them are bones of an undoubtedly primitive squirrel of enormous dimensions – approximately the size of a large child. Its masticatory musculature is similar to that of the North American Douglassciurus jeffersoni of Eocene age, i.e. unlike that of modern tree-squirrels. The biggest surprise lies in the dentition of the Sringeri giant squirrel. The typical rodent second incisors are serrated and arranged in a similar way to the shearing canines of mammalian carnivores. Its back teeth bear close resemblance to carnivore carnassials. As if this was not sufficient, the body cavity of the best preserved fossil contains pellets made up exclusively of bones from primitive hamsters, which abound in the lagerstätten. In a personal communication, Pandit Unmer makes a convincing case that he has discovered the only known predatory squirrel (provisionally named Titanosciurus sringeriensis), and will soon submit his finding for peer review. His only regret is that establishing a stratigraphic age for the laterite-bound pocket is proving to be very difficult. Sitting atop Archaean gneisses, the laterite can be correlated with similar palaeosols that cover the 64 Ma Deccan flood basalts some 130 km to the north, yet they defy dating by palaeontological or radiometric means. Dr Siffli would welcome offers to date the Sringeri lagerstätten (firstname.lastname@example.org).