The Island Effect in Dinosaurs

Everyone these days knows about the "island effect" where certain animals evolve to a diminutive size because they live on islands. You know this because of the Flores hominid. Now, it has been shown to have operated in a dinosaur.

Thecodontosaurus Thecodontosaurus is also known as the Bristol Dinosaur. It is one of the first named dinosaurs, having been found in 1834, even before dinosaurs were recognized as a phenomenon. It is a diminutive dinosaur that was originally thought to have lived in an arid area of the mainland. Research just published in Geological Magazine shows that this dinosaur probably evolved on an island.

Thecodontosaurus (below) compared with PlateosaurusThis very intensive and extensive study of this rich set of fossil deposits allows the development of a number of interesting conclusions. The fauna represented here contains no amphibians. Other aspects of the deposit suggest an island in a saline sea, which would explain the lack of amphibians since they would not have survived long in the surrounding salt water.

The diminutive nature of some of the remains in this deposit as well as the relatively low species diversity also conform to expectation of an island habitat.

According to John Marshall, one of the study's authors:

The cave deposits with dinosaurs have been known for over 150 years and are world famous. You would think there would be nothing new to find. But by looking at new deposits with a fresh mind we have been able to radically change the environmental interpretation. The big surprise was discovering that these reptiles did not live on arid uplands but rather on small well-vegetated tropical islands around Bristol about 200 million years ago. It is only the microfossil pollen and algae that can tell us this. The outlines of the islands can still be seen today in the shape of the land.

This changes the context in which we should view Thecodontosaurus. It has many similarities to the giant Plateosaurus that lived at the same time and other researchers have not taken into account the rapid changes that take place when large animals are isolated on islands of decreasing size. We believe that the Bristol dinosaur is probably a dwarfed species that derived from the giant Plateosaurus or a very similar animal.

Another interesting conclusion from this research concerns extinctions. Previous researchers had concluded that tetrapods of the period went extinct during a series of distinct events in the Late Triassic, rather than during a single end-Triassic event. This has been the subject of much debate. The research reported here suggests that some of the dinosaurs that were thought to have gone extinct before the end--Triassic were not quite extinct yet, as they are found in this well dated deposit later than they should have been alive.

At Cromhall, where we believe that the evidence strongly favours a [later] age assignment, there are representatives of the Kuehneosauridae (Kuehneosaurus) and Stagonolepididae (... Aetosaurus ...). If Tricuspisaurus, known from Tytherington and Cromhall, is classified as a trilophosaur (although we believe it is not), then this extends that group also into the [later period].

Therefore at least four of the reptile families considered ... to be extinct at the end of the Norian are certain or likely to be present [later in time]. ... then this gives further support to a significant large extinction event at the very top of the Triassic.

Illustrations and quotes from a Bristol Press Release

Original Paper:

WHITESIDE, D. I. and J. E. A. MARSHALL. (2008) The age, fauna and palaeoenvironment of the Late Triassic fissure deposits of Tytherington, South Gloucestershire, UK. Geological Magazine (2008), 145: 105-147 Cambridge University Press. doi:10.1017/S0016756807003925. Published online by Cambridge University Press 17 Sep 2007