THE MESOZOIC FISSURES OF THE BRISTOL CHANNEL AREA AND THEIR VERTEBRATE FAUNAS.

Abstract

SUMMARY. Terrestrial vertebrates have been discovered in Mesozoic deposits occurring as fissure fillings in the Carboniferous Limestone of some of the counties round the Bristol Channel (Somerset, South Gloucestershire, Glamorgan). These terrestrial vertebrates fall naturally into two groups: one group consists mainly of mammals and mammal-like reptiles, of Rhaetic or Lower Liassic age, which are found in fissures of submarine origin. Most of these fossils have already been described, and a summary is given of Kuhne's studies of the fissures in which they are found. The other group consists of sauropsid reptiles only, found in a different type of fissure, of continental origin. The paper is mainly concerned with the second group of reptiles and the fissures in which they are found. These fissures are shown to be small underground watercourses, formed during Permo-Triassic times when the area was part of a continent. The age of the sediments and reptile bones found in these underground watercourses is considered, and shown to be late Triassic, both on geological grounds and on the character of the fossil reptiles. The reptiles from different localities probably form part of a fauna, the Bristol Channel fauna, of late Triassic age. In considering the age of the sauropsid reptiles a reconstruction of their physical background is outlined, and this is completed by a tentative sketch of climatic conditions. From this reconstruction emerges an important feature which characterizes the whole Bristol Channel fauna, namely, that the members of this fauna lived on the higher ground of the continent of the period. This is the only known example of a mesozoic fauna of this type, and when its members have been studied it will be interesting to oompare them with those from the lowlands of the Upper Triassic continents. The members of the Bristol Channel fauna identified so far are all diapsids and protorosaurs. During the Triassic the diapsids radiated into the orders and suborders whose members form the common elements of later Mesozoic terrestrial faunas. These late Triassic diapsids are therefore particularly interesting, especially as those of the Bristol Channel fauna are all small (less than six inches to over six feet in length) and therefore free from the specializations which are the concomitant of large size.