Sensory lines and rostral skull bones in lungfish of the family Neoceratodontidae (Osteichthyes: Dipnoi)

Abstract

Skull bones in early lungfish contain permanent insignia of the sensory lines of the head, but osteological evidence of sensory lines in derived lungfish is reduced to foramina for nerves to neuromasts, superficial grooves, or elevated ridges on some bones. This is particularly evident in anterior bones, making definition of these bones difficult, and creating problems for phylogenetic analyses. Despite a close association of the sensory lines with the bones of the developing skull in the Australian lungfish, Neoceratodus forsteri, few traces of the lines remain in the bones of the adult animal. Among derived dipnoans, Mioceratodus, a genus of neoceratodont fossil lungfish from Tertiary deposits in central and northern Australia, is unusual because traces of the supraorbital sensory line are retained in the anterior skull roofing bones of large specimens. Equivalent traces are absent from the rostral bones of N. forsteri, and from small specimens of Mioceratodus. The supraorbital sensory line grooves in Mioceratodus bones pass over the posterior surface of the rostral bone, and not the anterior process as in Neoceratodus. The rostral bones of Mioceratodus may be formed from the fusion of different bones in the primitive dipnoan skull compared with those that formed the rostral (EQ) bone of N. forsteri. Alternatively, the association between sensory lines and skull bones may not be constant.