Coelacanth vestiges

Abstract

A vestigial second dorsal fin spine in a Carboniferous rhabdodermatid coelacanth (cf. Rhabdoderma sp.) provides new data bearing on the evolutionary origins of coelacanths. The fossil was recovered from a cannel coal deposit in the Allegheny Group of Ohio. A vestigial second dorsal fin spine occurs between the second dorsal fin endochondrial support plate and the second dorsal fin. The fin spine is preserved as a carbonized impression showing longitudinal ridges, comparable to the ridges seen on dorsal fin spine of the early osteichthyan Guiyu oneiros. A diagonal band of bends in the tracks of the spine ridges is seen in the medial spines of both Guiyu oneiros and cf. Rhabdoderma sp., as is compression of the ridges to form a reticulate pattern near the base of the spine. The vestigial spine in cf. Rhabdoderma sp. indicates the evolutionary derivation of coelacanths from the Guiyu-Psarolepis cluster, and furthermore suggests that the Guiyu-Psarolepis cluster is more closely allied to sarcopterygians than to basal actinopterygians. The vestigial coelacanth medial spine reported here represents a second type of osteichthyan medial spine and provides new data concerning the origin of coelacanths and possible affinities of the Guiyu-Psarolepis cluster. This conclusion is further supported by the reappearance of the Guiyu median dorsal plate as an otico-occipital shield in the bizarre Triassic latimeriid coelacanth Foreyia maxkuhni.