Development of the Axial Skeleton and Median Fin in the Australian Lungfish, Neoceratodus forsteri

ABSTRACT

New observations on the axial skeleton of the extant lungfish Neoceratodus forsteri (Dipnoi; Sarcopterygii) indicate that neural and haemal arch elements develop more independently than previously believed. For example, while the cartilaginous neural arches/spines begin development anteriorly, just behind the skull, the distal supraneurals first form separately in the posterior region of the axial skeleton. Proximal supraneurals develop subsequently, but initially lack clear association with either neural arches/spines or distal supraneurals. This contradicts previous studies of Neoceratodus and other extant lungfish suggesting that the supraneurals and more distal radials develop from a single cartilage. This was suggested as a unique sarcopterygian pattern, but our new observations suggest a closer resemblance to the actinopterygian condition. With respect to the caudal fin skeleton, the heterocercal tail of Devonian lungfish is replaced in Carboniferous and younger taxa by a diphycercal tail, comprising elongate and equal dorsal and ventral lobes. Whether these lobes resulted from fusion of dorsal/anal and caudal fins or expansion of dorsal and anal fin rays at the expense of the caudal fin is uncertain. However, comparing ontogenetic development of Neoceratodus with Devonian taxa indicates that the elements of the ventral diphycercal lobe in Neoceratodus are homologous to haemal elements present in the caudal fin of Devonian forms, but loss of supporting basal plates in the dorsal fins of Devonian taxa allowed supraneural elements of the dorsal fin to expand to form the dorsal lobe of the Neoceratodus tail.

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