Evolution of the mammal-like reptiles

Mammals, along with the biologically remarkably similar birds, are the vertebrates that are most completely adapted to the physiological rigours of the terrestrial environment. Whilst all the terrestrial dwelling tetrapods can operate in the absence of the buoyancy effect of water, and can use the gaseous oxygen available, mammals have in addition evolved a highly sophisticated ability to regulate precisely the internal temperature and chemical composition of their bodies in the face of the extremes of fluctuating temperature and the dehydrating conditions of dry land. From this perspective, the origin of mammalian biology may be said to have commenced with the emergence of primitive tetrapods onto land around 370 Ma, in the Upper Devonian. Until the 1990s, the only Devonian tetrapod at all well known was Ichthyostega from east Greenland, as described by Jarvik (e.g. Jarvik 1980, 1996). Famous for its combination of primitive fishlike characters such as the lateral line canals, bony rays supporting a tail fin, and remnants of the opercular bones, with fully tetrapod characters such as the loss of the gills and opercular cover, robust ribcage, and of course large feet with digits, Ichthyostega provided more or less all the fossil information there was relating to the transition from a hypothetical rhipidistian fish to a tetrapod. Subsequently, however, an ever-increasing number of other Upper Devonian tetrapods have been described, and the emerging picture of the origin of vertebrate terrestriality has become more complicated and surprising (Ahlberg and Milner 1994; Clack 2002). The earliest forms are Upper Frasnian in age, and include Elginerpeton from the Scottish locality of Scat Craig (Ahlberg 1995, 1998). So far known only from a few bones of the limbs and jaws, Elginerpeton adds little detail to the understanding of the evolution of tetrapods except to demonstrate that the process had commenced at least 10 million years prior to the existence of Ichthyostega. The next oldest tetrapods are Fammenian in age and include Ichthyostega, and a second east Greenland form, Acanthostega, which has been described in great detail (Coates and Clack 1990, 1991; Coates 1996).