Skin and no bones

By M.K. | LOS ANGELES

DINOSAURS are best known for their bones. But skeletons are not all that remains of the ancient beasts. Once considered rare, skin impressions are increasingly being recognised during excavations and brought back to museums for preservation and analysis. As Katherine Clayton, a student from the University of Utah doing research at the state's Natural History Museum, told the annual meeting of the Society of Vertebrate Palaeontology in Los Angeles, the cutaneous reliefs contain plenty of useful information.

Skin impressions are most often found next to river beds and lakes. The scales of animals which died in such places were pressed against soft sediment before they rotted away. The markings left behind subsequently hardened as the sediment dried (see picture). Such impressions had already enabled palaeontologists to determine that some dinosaurs had feathery structures on their bodies. Ms Clayton was keen to find out if the imprints could reveal other titbits.

She studied the impressions made by various hadrosaurs, a family of crested and duck-billed beasts that were abundant during the late Cretaceous period around 70m years ago. Together with colleagues, she surveyed 56 impressions that had been excavated in southern Utah next to a troves of bones. As a result, the researchers were even sometimes able to identify which bit of the body the impressions represented.

Ms Clayton found that bodies of adult hadrosaurs were dotted with round or polygon-shaped nodules. More important, many had unique features. Several hadrosaur genera, for instance, had corrugated nodules ornamented with ridges; many did not. A number had distinct nodule patterns all over their bodies or had nodules that were consistently larger in some parts of their bodies, like their arms, and smaller in other parts, like their backs. Corythosaurus had parallel rows of nodules across its abdomen.

The features are not enough on their own to allow individual dinosaurs to be identified. But differences in skin patterns were often distinctive enough to provide useful additional information. The lucky few paleontologists working at sites where bones are plentiful and in relatively good shape may not find Ms Clayton's findings all that useful. But for the many more who are forced to work with barely identifiable scraps, they may prove to be invaluable.