Genetics, paleontology, and evolution / edited by Glenn L. Jepsen, Ernst Mayr, George Gaylord Simpson, for the Committee on Common Problems of Genetics, Paleontology, and Systematics, of the National Research Council.

Date:
1949
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics, paleontology, and evolution / edited by Glenn L. Jepsen, Ernst Mayr, George Gaylord Simpson, for the Committee on Common Problems of Genetics, Paleontology, and Systematics, of the National Research Council. Source: Wellcome Collection.

    76/502 (page 56)
    until in the Late Triassic labyrinthodonts the whole skull is very shallow, even in the occipital region. This reduction in the height of the skull seems to have proceeded at very different rates in different stocks of labyrin thodonts, but taking the group as a whole the change from first to last is obvious. The dorso-ventral depression is recognizable in a highly de veloped condition throughout the skull even in such Early Permian creatures as Eryops, which were certainly inhabitants of the land. It is thus difficult to believe that its occurrence was induced by environmental influences, although its effect, when carried to an extreme, is to render the animal incapable of progress on land and thus to compel it to live through out its life on the floor of a pond or river. But this dorso-ventral depression has many mechanical sequels and is associated with a series of changes in the basi-cranial region which are made necessary in part also by the deficient cartilage ossification. One of the most easily discussed of all the changes which take place in very many of the labyrinthodont stocks is the development and steady exaggeration of the retro-articular process. In all the labyrinthodonts of the British Carboniferous, whether they be anthracosaurs or loxommids, there is a complete absence of any trace whatsoever of any protuberance behind the articular facet on the articular bone; the crest which marks the hinder edge of the articulation of the quadrate descends smoothly and un interruptedly to curve round into the lower border of the jaw; even in later labryinthodonts from the Texan Permian the retro-articular process is still little developed. In Eryops it may perhaps be said to exist, in the much flattened Trimerorachis it is relatively rather larger, in Cacops (as shown by a photograph published by Case ) it is relatively well developed. When we go to the later forms we find that in Capitosaurus, for example, it is a considerable projection which is further exaggerated in Cyclotosaurus. In the brachiopids it is present as a small projection in the Late Permian Dwinasaurus. It is relatively somewhat larger in Bothriceps australis, in the Middle Triassic Plagiosuchus it is a remarkably large and powerful process, while the Bhaetic Gerathorax has an enormous greatly elongated retro-articular process. The function of this process is very evident. It gives attachment to the depressor mandibulae muscle which arises from the posterior surface of the upper border of the skull, from tabulars and post- parietals, dorsal and internal to the otic notch. This muscle passes down ward and outward to be inserted on the posterior extremity of the retro- articular process; its function is to open the mouth by depressing the lower jaw. In animals whose legs are so short that the body habitually lies in contact with the ground, and the lower jaw is thus in contact with the soil, the mouth can only be opened by raising the head, leaving the lower jaw where it was; this condition is very familiar in crocodiles. The raising of the head is normally brought about by the contraction of those muscles