Pectoral girdle and fin anatomy ofGogonasus andrewsaelong, 1985: Implications for tetrapodomorph limb evolution

Abstract

Recently discovered material has yielded new information on the pectoral girdle and fin endoskeleton of Gogonasusandrewsae (Frasnian Gogo Formation, Kimberley Region, Western Australia). These elements permit the first comprehensive description of the anocleithrum, cleithrum, scapulocoracoid, and lepidotrichia. New autapomorphies of Gogonasus include a square exposed region on the supracleithrum, an unusual knob-like process on the scapulocoracoid, a relatively small entepicondyle, and lepidotrichia with I-beam-shaped cross sections. Several poorly ossified regions on the scapulocoracoid and humerus indicate an early ontogenetic state, as with other immature tetrapodomorph fish specimens. A phylogenetic analysis indicates a more stemward position for Gogonasus in a weakly supported clade with other "osteolepidid" taxa, compared to other recent studies placing Gogonasus crownward of osteolepidid fishes and the Tristichopteridae, as the sister taxon to the "Elpistosteglia" + Tetrapoda. A phylogenetic position among megalichthyid fishes is suggested for Sterropterygion, while radiographs of the megalichthyid Cladarosymblema show a scythe-like radius terminating distally with that of the intermedium. New data on the scapulocoracoid of the rhizodontid Barameda reveals a coroacoid crest and small supraglenoid foramen.

Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins‐to‐limbs transition

“…8C) (Parker et al, 2005) and ventromesially in Barameda ( fig. 5 of Holland, 2013). The scapulocoracoid has a dorsal process in Strepsodus (dp, Fig.…”

“…In Gogonasus, the dorsal process of the scapula bears faint grooves, possibly for muscle attachment (Holland, 2013). The scapulocoracoid of Gogonasus has a crest on the ventromedial aspect of the anterior buttress (corresponding to the coracoid crest in rhizodontids) with a probable muscle attachment scar and a knob-like process on the ventral side between the anterior and posterior buttresses (Holland, 2013). In Eusthenopteron, two small pits located on a similar part of the scapulocoracoid were interpreted as the origin of the coracobrachialis (cb?, Fig.…”

“…Like Sauripterus, Gogonasus has a 'ventral cleithral ridge', and Eusthenopteron has a similar ridge (vcr, Fig. 8E) (Holland, 2013) which was interpreted as the anterior extent of the pectoralis origin in the latter taxon (Andrews & Westoll, 1970a). The cleithrum in Eusthenopteron also has a thickened posterior margin just above the glenoid (del?, Fig.…”

“…4A) and stated that the deltoid and supinator processes in Gogonasus are part of a continuous flange (p. 303). Indeed, the 'supinator process' of Holland (2013) resembles the ectepicondyle in other tetrapodomorph fish in that it is located at the extreme distal end of the bone at the end of a thick, longitudinal ridge. Furthermore, Andrews & Westoll (1970a) identified 'incompletely separated' supinator and deltoid processes in Eusthenopteron (p. 240) (arrows, Fig.…”

“…In Rhizodopsis, the dorsal surface is smooth with a small pit at the distal end; the ulna lacks a postaxial process, and the ventral surface is unknown (Andrews & Westoll, 1970b). In Gogonasus, a concave, circular region of unfinished bone with a distal groove is described on the proximal dorsal surface (Holland, 2013). In Eusthenopteron, a strongly developed ventral postaxial process (vpu, Fig.…”