The members of this, the last order of the Amphibia, are entirely extinct. They were Batrachians, probably most nearly allied to the Urodela, but all of large size, and some of gigantic dimensions, the skull of one species (Labyrinthodon Jaegeri) being upwards of three feet in length and two feet in breadth. The Labyrinthodonts were first known to science simply by their footprints, which were found in certain sandstones of the age of the Trias. These footprints consisted of a series of alternate pairs of hand-shaped impressions, the hinder print of each pair being much larger than the one in front (fig. 285). So like were these impressions to the shape of the human hand, that the unknown animal which produced them was at once christened Cheirotherium, or "Hand-beast." Further discoveries, however soon showed that the footprints of Cheirotherium had been produced by different species of Batrachians, to which the name of Labyrinthodonts was applied, in consequence of the complex microscopic structure of the teeth.
Fig. 285. - Footprints of a Labyrinthodont (Cheirotherium).
The Labyrinthodonts were "salamandriform, with relatively weak limbs and a long tail" (Huxley). The vertebral centra and arches were ossified, and the bodies of the dorsal vertebrae are biconcave (amphicoelous). "In the thoracic region three superficially sculptured exoskeletal plates, one median and two lateral, occupy the place of the interclavicle and clavicles, Between these and the pelvis is a peculiar armour, formed of rows of oval dermal plates, which lie on each side of the middle line of the abdomen, and are directed obliquely forwards and inwards to meet in that line " (Huxley).
The head was defended by an external covering or helmet of hard and polished osseous plates, sculptured on their external surface, and often exhibiting peculiar, smooth, symmetrical grooves - the so-called "mucous canals." The skull (fig. 287) was articulated to the vertebral column by two occipital condyles. The teeth (fig. 286) are rendered complex by numerous foldings of their parietes, giving rise to the "labyrinthine" pattern, from which the name of the order is derived.
There appear usually to have been both pairs of limbs developed, but some forms which have been referred here (such as Ophiderpeton) possessed a serpentiform body, and seem to have been apodal. Little is known, necessarily, of their development, but the singular genus Archegosaurus possessed permanent branchial arches, and was, therefore, apparently perennibranchiate (if not truly a larval form), whilst its notochord was persistent, and simply had rings of osseous matter deposited in it.
Fig. 286. - Section of the tooth of Labyrinthodon (Masto-donsaurus) faegeri, showing the microscopic structure. Greatly enlarged. Trias.
Fig. 287. - a Skull of Labyrinthodon faegeri, much reduced in size; b Tooth of the same. Trias. Wurtemberg.
As to their distribution in time, the Labyrinthodonts range from the Carboniferous to the Trias, inclusive, being most numerous in the former period, but attaining their maximum in point of size in the latter.