IV.—The Origin of Continental Vertebrate Faunas

Abstract

upon-Tyne, The Univer sity of Durham.It is now well-established that the land surfaces of the world have not always carried an extensive vegetation or a substantial animal population, and there are innumerable papers in the scientific literature of Geology, Zoology and Botany that have dealt with various aspects of the evolution of present conditions. It is very understandable that man should be particularly con cerned with the problems of his own ancestry, and we can now be reasonably sure of the main outlines of the history of truly terrestrial vertebrates from at least early Permian times onwards. The mammal-like reptiles (the Pelycosauria) of the early Permian differentiated into several stocks, from one of which arose the diversified Therapsida of the later Permian and Triassic periods. These include quite certainly the ancestral group or groups from which mammals evolved, probably in late Triassic times. Mammal remains of Mesozoic age are very scarce, and a dispassionate survey of the geological evidence indicates that this is probably due very largely to the geological accident that extensive sequences of essentially continental and fossiliferous deposits like those of the Lower Permian of north-central Texas (Romer, 1958) or the Karroo of South Africa are not known in the Jurassic and Cretaceous of the well-explored parts of the world. But through these "Dark Ages" of mammalian history scattered and fragmentary remains do provide a patchy record, however inadequate they may be to permit a detailed story to be de ciphered. The main theme of mammalian, and especially eutherian, expansive evolution is much more abundantly docu mented from Palaeocene times onwards.During the whole of this last 200 million years or so of earth history one is in no doubt of the existence of a substantial fauna and flora on the continents, though these land-masses of course have undergone many changes in extent and outline, and probably also in relative position. The Reptilia (Fig. IV, 5), and even some of the Amphibia, of early Permian age show the on July 3, 2015 at West Virginia University http://trngl.lyellcollection.org/ Downloaded from

Palaeoecology and sedimentology of the Achanarras fish bed of the Middle Old Red Sandstone, Scotland

“…The generally variable features of the environment with cyclic fluctuating lake levels and periods of desiccation obviously provided severe pressures on the fish population as discussed by Westoll (1958) and Donovan (1980). Factors such as water temperaure and water chemistry were clearly of paramount importance.…”

“…No palaeozoological work has been attempted for this study, use being made of existing literature. Some fish are illustrated here but reconstructions and illustrations of most forms can be found by reference to Westoll (1958), Moy-Thomas and Miles (1971) and Saxon (1975).…”