Evolution of Dinosaurs - Fossil Record

Fossil Record

The first few lines of primitive dinosaurs diversified rapidly through the Triassic period; dinosaur species quickly evolved the specialised features and range of sizes needed to exploit nearly every terrestrial ecological niche. During the period of dinosaur predominance, which encompassed the ensuing Jurassic and Cretaceous periods, nearly every known land animal larger than 1 meter in length was a dinosaur.

One measure of the quality of the fossil record is obtained by comparing the date of first appearance with the order of branching of a cladogram based on the shape of fossil elements. Close correspondence exists for ornithiscians, saurischians and subgroups. The cladogram link between coelophysids and ceratosaurs is an exception, it would place the origin of coelophysids much too late. The simplest explanation is convergent evolution - ceratosaur bones evolved independently into a shape that resembles that of the earlier coelophysids. The other possibility is that ceratosaurs evolved much earlier than the fossil record suggests.

Most dinosaur fossils have been found in the Norian-Sinemurian, Kimmeridgian-Tithonian, and Campanian-Maastrichtian periods. Continuity of lineages across the intervening gaps shows that those gaps are artifacts of preservation rather than any reduction in diversity or abundance.

In many instances, cladistic analysis shows that ancestral lineages of varying durations fall in those gaps. The length of missing ancestral lineages in 1997 range from 25 Ma (Lesothosaurus, Genasauria, Hadrosauroidea, Sauropoda, Neoceratopsia, Coelurosauria) to 85 Ma (Carcharodontosauridae). Because the dinosaurian radiation began at small body size, the unrecorded early history may be due to less reliable fossilization of smaller species. However, some missing lineages, notably of Carcharodontosauridae and Abelisauridae, require alternative explanations because the missing range extends across stages rich in fossil materials.