On the origin of the so-called Meckelian ossicles in the nasal skull of odontocetes.

Although Cave (1987) accepts the theory that the Meckelian ossicles originate from the maxilloturbinals, evidence given in his study in fact supports the opinion of Klima and van Bree (1985) that the Meckelian ossicles arise from elements of the nasal floor, solum nasi, of the embryonic nasal capsule, in particular from the lamina transversalis anterior and the cartilago paraseptalis.

[Additional skeletal elements in the nasal skull structure of Phocoena phocoena and the development of the nasal region in toothed whales]. / Uberzählige Skeletelemente im Nasenschädel von Phocoena phocoena und die Entwicklung der Nasenregion bei den Zahnwalen.

Small nasal ossicles, occasionally occurring on the caudal border of the premaxillary bones in Phocoena and some other toothed whales (Odontoceti) up to now, were considered the remnants of maxilloturbinalia. The latter, however, originate from the cartilaginous lateral wall of the embryonal nasal capsule, whereas the ossicles mentioned are remnants of the nasal floor cartilages. These additional nasal structures are homologous with the lamina transversalis anterior, and also contain the material of the cartilago paraseptalis. In comparison with the quadrupedal terrestrial mammals, the skull of odontocetes shows some important differences in development. In the nasal region, the nasal tract is shifted backward from the top of the snout to the vertex of the head, while the bony rostrum projects far forward. These changes can already be observed during the early morphogenesis of skull. We have examined embryos of Phocoena, Lagenorhynchus, and Monodon, 16 different developmental stages altogether, representing the most complete material ever examined for this purpose. There is a marked developmental trend from a complicated nasal capsule, consisting of many isolated cartilages, to a simple reduced structure, being composed of some fused cartilages only. Whereas Monodon, Lagenorhynchus, and Globicephala still possess some of the original ancestral features, common to all mammals, Phocoena in this respect is the most advanced odontocete which has been investigated so far. In Phocoena, the tectum nasi and the lateral wall of the nasal capsule are widely reconstructed in embryonal life, turned upright and displaced caudally; thus it lies immediately in front of neurocranium. The floor of the original nasal capsule persists in 2 elements only, the lamina transversalis anterior and the paraseptal cartilage, both largely fusing with each other and losing their connection with the lateral nasal wall. During early morphogenesis they move to the top of the head, expanding along the upper border of the nasal septum. Together with the rostral part of the nasal septum, they join in the prolongation of the dolphin head and hold the space for the exoskeletal dermal bones which develop later. There is a direct connection between the lamina transversalis anterior and the premaxillary bone, as well as between the cartilago paraseptalis and the vomer. These topographical relationships persist until the adult stage of odontocetes.