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.

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    which are inserted on the occiput and pass backward to the dorsal parts of the vertebral column. In the case of the very flattened amphibia, such as Cyclotosaurus and Gerathorax, these muscles are inserted very in- advantageously, for in a large amphibian such as Metoposaurus, where the skull is 22.5 cm. wide, the upper border of the occiput lies only 3.75 of a centimeter above the centre of curvature of the condyle, and such a skull is 28.5 cm. in length. It is therefore obvious that the epiaxial muscula ture, when flattening is excessively developed, will be inadequate; hence the musculus depressor mandibulae is enlarged and its point of applica tion removed as far as possible from the fulcrum represented by the quadrate condyle. Hence as the skull becomes more and more flattened the retro-articular process tends to lengthen as a direct adaptation to enable the mouth to be opened by raising the head while the lower jaw remains in contact with the ground. In all the early labyrinthodonts the occipital condyles lie much in front of the quadrate condyles and hence, if the head be raised so as to open the mouth with the lower jaw in contact with the ground, not only must the head be raised but the anterior part of the vertebral column ( which is articulated with the occipital condyle) must also be pulled up, greatly increasing the weight which has to be lifted and the tension necessary in the muscles to bring the movement about. In the large late labyrin thodonts with exceedingly flattened skulls the shape is so changed that the occipital condyles come to lie in the same transverse plane as the quadrate condyles and are even lowered so that the two condyles are surfaces of rotation about the same axis. In these circumstances the head can be raised while the lower jaw remains in contact with the ground without any displacement of the anterior part of the body behind the skull. The process is indeed carried beyond this point in brachyopid forms, where the occipital condyles actually lie posterior to the quadrate condyles and considerably above them, and hence when the skull is raised the lower jaw is automatically pushed forward. It seems perfectly obvious that these changes in the relative positions of the quadrate and occipital condyles, and the development of the retro- articular process, serve a functional end and meet a need that has arisen through the flattening of the anterior part of the animal which places the fore limbs in such a position that they could certainly not have raised the head from the ground on land, and could have done so only very ineffectively under water. Furthermore the fore limbs of later labyrin thodonts are considerably reduced in comparison with Eryops and other Early Permian land living amphibia, and their feebleness may perhaps be related (at any rate in part) to the decrease of cartilage bone in such forms, not only in the skull but in the post-cranial skeleton. It is possible to give a functional explanation of many of the other