Development of the Cetacean Nasal Skull

Morphogeny of the nasal skull was investigated in 92 embryos of 13 species of Cetacea. Compared to the original nasal capsule of land mammals, the nasal structures of Cetacea show many weighty transformations. As a result, the nostrils are translocated from the tip of the snout to the vertex of the head. Several structures of the embryonic nasal skull remain preserved even in adult cetaceans. The translocation of the nostrils to the highest point of the surfacing body is among the most perfect adaptations of cetaceans to the aquatic life habits. The morphogeny of the nasal skull suggests that all cetaceans are of common origin and form a single monophyletic order. The hitherto usual division of this order into two suborders, Mysticeti and Odontoceti, appears to be unsubstantiated. Rather, at least three closely related superfamilies should be distinguished within the order Cetacea, viz., Balaenopteroidea, Physeteroidea, and Delphinoidea. The results are in a conspicuous accordance with the most recent molecular biological investigations on this topic.

Contenu

1 Introduction.- 2 Materials and Methods.- 3 Synopsis of Structures of the Embryonic Nasal Skull.....- 3.1 Cartilaginous Structures.- 3.2 Dermal Bones.- 4 Morphogeny of the Nasal Skull.- 4.1 Odontoceti.- 4.1.1 Spotted DolphinStenella attenuata.- 4.1.2 Common DolphinDelphinus delphis.- 4.1.3 White-beaked DolphinLagenorhynchus albirostris.- 4.1.4 Pilot WhaleGlobicephala melas.- 4.1.5 Common PorpoisePhocoena phocoena 28.- 4.1.6 NarwhalMonodon monoceros 29.- 4.1.7 Sperm WhalePhyseter catodon 31.- 4.2 Mysticeti.- 4.2.1 Fin WhaleBalaenoptera physalus.- 4.2.2 Minke Whale Balaenoptera acutorostrata.- 4.2.3 Humpback WhaleMegaptera novaeangliae.- 5 Metamorphosis and Translocation of the Nasal Structures.- 5.1 Stages Around 40 mm in Size.- 5.2 Stages Around 80 mm in Size.- 5.3 Stages Around 150 mm in Size.- 5.4 Course of the Most Important Transformations.- 6 Comparison of the Embryonic Nasal Structures.- 6.1 The Median Structures.- 6.1.1 Septum nasi.- 6.1.2 Spina mesethmoidalis.- 6.2 The Posterior Side Wall Structures.- 6.2.1 Tectum nasi.- 6.2.2 Lamina cribrosa.- 6.2.3 Paries nasi.- 6.2.4 Commissura orbitonasalis.- 6.2.5 Cupula nasi anterior.- 6.2.6 Maxilloturbinale.- 6.2.7 The Nasal Cavity.- 6.2.8 Processus paraseptalis posterior.- 6.3 The Anterior Side Wall Structures.- 6.3.1 Lamina transversalis anterior.- 6.3.2 Processus lateralis ventralis.- 6.3.3 Cartilago paraseptalis.- 6.3.4 Cartilago ductus nasopalatini.- 7 The Nasal Structures of Adult Cetacea.- 7.1 The Ethmoid Complex.- 7.2 Cartilaginous Rostrum.- 7.3 Cartilages of the Nostril.- 7.4 Nasal Roof Cartilage in the Sperm WhalePhyseter catodon.- 7.5 Pararostral Bones, or the so-called Meckelian Ossicles.- 8 Evolutional and Functional Relationships.- 8.1 Position of the Nostrils and the Mode of Surfacing in Cetaceans.- 8.2 Position of the Nostrils and the Mode of Surfacing in Ichthyosaurs.- 8.3 Position of the Nostrils and the Modell9 of Surfacing in Sirenians.- 9 The Systematics of Cetacea According to the Morphogeny of Their Nasal Skull.- 9.1 On the Phylogenetic Origin of Cetacea.- 9.2 The Present Results and Their Significance for the Systematics of Cetacea.- 9.3 Other Recent Criteria for the Systematics of Cetacea.- 10 Summary.- References.