The earliest known fossil stages in the evolution of the oral cavity and jaws

Abstract

1. I. In accordance with much evidence we may assume that at some extremely remote time (Fig. 22), long antecedent to the Basal Cambrian (550,-000,000 years ago), single-celled organisms subdivided into many-celled forms, some of which later gave rise to two-layered basal coelenterates (of which hydroids and jellyfishes are modern representatives). At this stage the pouchlike gastraea appeared; its single opening or prostoma, which led to the primitive food bag, was homologous not with the mouth but with the anus of vertebrates. True homologues of the vertebrate mouth are not to be found in any of the annulated, articulated, or arthropod invertebrates. 2. II. In the remote common ancestor of the echinoderms and chordates the beginnings of the vertebrate mouth were represented by a stomodaeal inpushing in a Tornaria-like, floating sacklike organism feeding by ciliary ingestion (Fig. 1 D). 3. III. The oldest known fossil chordates, which are the ostracoderms (Fig. 1, I; Fig. 7) of the Silurian and Devonian periods, had already settled down on or near the sea bottom and had acquired an oralo-branchial chamber with a stomodaeal mouth pouch and a series of large paired gill pouches. The endoskeletal tissue of the head was diffuse, not subdivided either by sutures or articulations. The floor of the throat, probably moved by slips of the locomotor muscles, pumped in small food. The more advanced ostracoderms (Anaspida) may have pursued small prey, seizing them by means of a bony knob at the end of the lower lip, but complex jaws with inner and outer skeletal parts had not yet been evolved. 4. IV. The acanthodians (Fig. 10) were predatory, swift swimmers, with complex jaws, the inner jaws being derived from. gill bars (Fig. 1, II). The endoskeletal tissue around the oral pouch was already segregated into articulated pieces, the primary upper and lower jaws, while the tissue between the hyoid and branchial sacks had already given rise to articulated bars of the hyoid and branchial arches. The hyoid arch was a complete gill-bearer and no part of it had been modified for the support of the jaws. At this and the next stage (Fig. 1, III) the future derm bones of the face were still on the surface and covered with enamel or a ganoine-like layer. The teeth were already differentiated from dermal denticles. 5. V. In the oldest known ganoid fishes the primary jawbones (Fig. 1, III) were covered by superficial dermal plates, surfaced externally with ganoine and bearing an incrustation of teeth. These surface plates were closely appressed to the old primary jaws (palato-quadrate, Meckel's cartilage). Thus arose the complex jaws of the higher vertebrates. When certain air-breathing fishes began to come up on land, their teeth and jaws were still essentially like those of the oldest ganoids. 6. VI. In the line of the mammal-like reptiles (Fig. 19) one pair of the surface bones of the mandible, the dentaries, became dominant, the ascending rami of the opposite dentaries grew upward toward the squamosal bones bordering the primary upper jaw until they came near to the squamosal portion of the temporal complex. 7. VII. As the mammalian grade was reached, the temporomandibular joint came into being, the jawbones behind the dentary became reduced and were taken over into the middle ear (Fig. 20), while the pterygoid part of the upper jaw dwindled into slender slips, forming the medial pterygoid lamina of the sphenoid (Fig. 16, VII to X). Meanwhile part of the sphincter colli profundus spread forward over the face, lips, cheeks and throat, forming a mobile mask (Fig. 13) which completely covered the old “fish-trap” bones (premaxillae, maxillae, dentaries), while transverse flanges from the maxillae and palatines formed a secondary palate to lead the inspired air back to the trachea (Fig. 16). The tongue (Fig. 21), inherited from the floor of the hyobranchial skeleton of the fish, became overlaid with thick, actively moving muscle layers. The teeth, now masked and covered by surrounding structures, represent long persistent holdovers from the ancient days when they, in common with the long-lost ganoid scales, were exposed on the lateral surfaces of the head. Thus the complexities of the human oral cavity have resulted from its long and intricate history, which is a part of the vast picture of the Tree of Life (Fig. 22), the understanding of which is now slowly emerging as the resultant of thousands of contributory studies in systematic zoology, embryology, comparative anatomy and paleontology.