Development of the Snout of the Australian Lungfish Neoceratodus forsteri (Krefft, 1870), with Special Reference to Cranial Nerves

Abstract

The anatomy and embryology of the dipnoan snout and olfactory organ play a major role in the discussion about the phylogenetic position of Dipnoi and the question of ancestry of Tetrapoda. This is primarily due to the fact that an internal nostril is regarded as an important preadaptive organ of the ancestors of tetrapods. Two conflicting scenarios of phylogenetic change were proposed in favour of different hypotheses of relationship. One emphasizes the similarities between Dipnoi and Chondrichthyes concerning this complex of characters. The other supports the idea of a close relationship of Dipnoi and Tetrapoda and a common origin of a ‘choana’ from the posterior external nostril of fishes. Accordingly, there is need for a detailed embryological and anatomical study which could help to clarify homologies and the basis of character evaluation. This, in the first place, concentrates on Neoceratodus forsteri. A plate reconstruction of the larval head provides many new insights which are important for comparison with the lepidosirenid lungfishes Protopterus and Lepidosiren and with other vertebrate taxa. The value of the peripheral nervous system as a topographical reference is critically reviewed. The results support the hypothesis that lungfish form a ‘primitive’ teleostomate group, closely adjoining Chondrichthyes in many characteristics of the snout formation. The relations between cranial nerves in the snout and the developing nasal sac, however, do not support the conclusion that the recent selachian condition is the exceptional ancestral character state of the lungfish snout. Dipnoi lack a ‘choana’, but nevertheless a closer relationship to tetrapods is not excluded. A vestigial and transitory naso‐buccal connection in larvae of Neoceratodus might indicate such common ancestry, but has a completely different functional significance among fishes. It is shared with several plagiostomes and is, therefore, probably not useful as a synapomorphy of Dipnoi and Tetrapoda.

Early development of the olfactory organ in sturgeons of the genus Acipenser: a comparative and electron microscopic study

“…The formation of the nasal cavity and the anterior (incurrent) and posterior (excurrent) nostrils from an original single pit as described in this study, is considered the phylogenetically primitive (plesiomorphic) type among actinopterygians (Bertmar 1969;Zeiske et al1992). It closely resembles the formation of the naso-buccal pits and ventilatory openings of the olfactory organ in Dipnoi (Bertmar 1965;Bartsch 1993), and the incomplete separation of incurrent and excurrent opening in sharks (Berliner 1902). This type of formation has also been described for sturgeons (Devitsina and Kazhlayev 1993a), paddle fish Polyodon spathula (Ballard and Needham 1964;Bemis and Grande 1992) and the bowfin Amia calva (Dean 1897) as well as for many teleosts (e.g.…”

Olfactory metamorphosis in the Coastal Giant Salamander (Dicamptodon tenebrosus)

“…Finally, it is worth noting that, although no other amphibians have a larval olfactory epithelium with ridges of respiratory epithelium separating troughs of sensory epithelium like that of larval salamanders, similar structures have been described in early larval stages of lungfishes (Bartsch, 1993). In later stages, this arrangement persists in the "receptor grooves" on the surface of the primary gill lamellae (Derivot, 1984).…”

Cranial neural crest cell migration in the Australian lungfish, Neoceratodus forsteri

“…The development of the head, early events in particular, in the Australian lungfish are very poorly known. Only the later stages of development and the larval anatomy have been the subject of recent, thorough study ( Bartsch 1993, 1994; Kemp 1999). In the most recent study of neural crest development in the Australian lungfish, Kemp (1995) suggests that “In embryos of … the dipnoi, the cells of the neural crest may not form a part of the mesenchyme from which the dental and skeletal structures develop.” If true, this is very different from what would be expected and it would force us to develop reasonable explanations for why the Australian lungfish deviates so dramatically from other vertebrates in the development of its head.…”