Palaeospondylus—a larval lungfish, and not a jawless fish. Thomson et al. (2003) clarified the puzzle in paleontology as to the identity of Palaeospondylus gunni, a tiny (5-60 mm) and abundant vertebrate fossil from the Middle Devonian (approximately 385,000,000 years ago) from primarily Achanarras Quarry, Caithness, Scotland, first discovered in 1890. They showed that it is the larval stage of a lungfish, most probably Dipterus valenciennesi.
Branchiostegals and gulars absent; caudal fin diphycercal, confluent with dorsal and anal fins; premaxilla and maxilla absent; lungs functional.
Extensive fossil record since the Lower Triassic and three extant genera and six species.
Although the three families with living species are restricted to widely separated continents, this was not always the case. Ceratodus and Lepidosiren tooth plates occur together in the lower Paleocene in Bolivia, and Ceratodus is also known from Africa and Madagascar. This and the fact that some fossil South American ceratodontids and lepidosirenids have their closest relatives in Africa and Australia may suggest that the species involved dispersed in freshwater on a united supercontinent; but, apparently, as also noted in 1991 by H.-P. Schultze, Cretaceous ceratodontids show marine tolerance and might, therefore, have been able to disperse without land connections.
Order CERATODONTIFORMES (62)—living lungfishes. All three extant families are placed in this order, in two suborders (each suborder was recognized at the ordinal level in Nelson, 1994). This change was made to better accommodate the changed classification based on Cloutier and Ahlberg (1996). Three families, three genera, and six species.
Suborder Ceratodontoidei. Pectoral and pelvic fins flipperlike; scales large; air bladder (lung) unpaired; larvae without external gills; adults do not estivate.
Family CERATODONTIDAE (513)—Australian lungfishes. Freshwater; Southeast Queensland, Australia.
R. S. Miles in 1977 and Cloutier and Ahlberg (1996) recognized the extant Neoceratodus in the family Neoceratodontidae. Cloutier and Ahlberg (1996) acknowledged that their fossil family Ceratodontidae may be paraphyletic, and until relationships are resolved I prefer to continue to recognize the Triassic Ceratodus and the extant Neoceratodus in the same family.
One species, Neoceratodus forsteri.
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