XII.—The Postcranial Skeleton of Rhipidistian Fishes ExcludingEusthenopteron

Abstract

SynopsisIn a previous paper (Andrews and Westoll 1970) the postcranial skeleton of the best known rhipidistian, Eusthenopteron, was described, and its bearing on the origin of the tetrapod postcranial skeleton discussed. The postcranial remains of other Rhipidistia are now described as far as they are known, and comparisons are made with Eusthenopteron and other forms where relevant. Possible modes of function are considered in relation to the habitats in which these fishes may have lived. These studies have made it necessary to revise rhipidistian classification; the Family Rhizodontidae is re-defined and placed alone in a third Order of Rhipidistia (the Rhizodontida, alongside the better known Osteolepidida and Holoptychiida). Fresh insight has been gained into the following morphological problems: the composition of the osteolepid ring-like centrum, the origin of the tetrapod scapular blade and the diphyletic origin of the tetrapods.

New information on the enigmatic tetrapodomorph fishMarsdenichthys longioccipitus(Long, 1985)

“…The only other members previously assigned to the Rhizodopsidae include Callistiopterus clappi Romer, 1942, a taxon described from a fragmentary juvenile specimen "whose adult form is hypothetical" (Andrews and Westoll, 1970) and Taeniolepis trautseholdi Chabakov, 1927, which is known from isolated teeth and scales (Lebedev, 1996). There has never been a detailed modern systematic description and diagnosis published for Rhiz odopsis, although partially complete skull material (e.g., Traquair, 1881;Säve-Söderbergh, 1936) and complete postcranial skeletons, including pectoral fin elements (e.g., Andrews and Westoll, 1970), are known. Several species have been described based on juvenile specimens (Schultze andHeidtke, 1986, 1993) and isolated scales (e.g., Woodward, 1891;Obruchev, 1955).…”

“…However, several possible megalichthyid synapomorphies proposed by Young et al (1992), such as the enclosure of the nares anteriorly by the lateral rostral and posteriorly by the anterior tectal, are unknown from Rhiz odopsis, because the bones surrounding the external nostrils have not been described (e.g., Traquair, 1881; Thomson and Hahn, 1968). Other shared features between Rhiz odopsis and megalichthyids, such as the presence of fused ring centra, are also present in some specimens of Thursius (Andrews and Westoll, 1970) (although absent from Marsdenichthys). We recommend further study to establish synapomorphies pertaining to Rhiz odopsis that could clarify any possible relationship between the Rhizodopsidae and the Megalichthyidae.…”

“…It is worth noting that cosmine-deficient specimens of Megalichthys (Fig. 6G) have been misidentified as Rhiz odopsis, with Andrews and Westoll (1970) noting the two taxa are distinguishable by the presence of the central median boss on the inner surface of the scale in Rhiz odopsis. Examples of Megalichthys specimens showing cosminedeficient scales with concentric rings on the external surface include NHM P386 and NHM P6607.…”

“…Long justified this resolution on the assumption that only tristichopterids evolved this character within the 'Osteolepiformes', but cautioned that if other 'osteolepidids' displayed similar scale morphology, Marsdenichthys could be a specialized 'osteolepidid'. Since that time, both the 'Osteolepiformes' and 'Osteolepididae' have been shown to be paraphyletic (Ahlberg and Johanson, 1998), with several 'nontristichopterid' former constituents of the 'Osteolepiformes' exhibiting rounded scales with a median boss, including Medoevia Lebedev, 1995 from the Upper Famennian of Russia, the canowindrid Canowindra Thomson, 1973, from the Famennian of NSW, Australia, and Rhiz odopsis from the Carboniferous of Illinois, USA (Schultze, 1974), Ireland (Andrews and Westoll, 1970;Wyse Jackson and Monaghan, 1995), England (Williamson, 1837), Spain (Iwaniw, 1984), Silesia (Roemer, 1865), France (Leriche, 1908), Belgium (Cloutier and Candilier, 1995), Russia (Vorobyeva and Obruchev, 1964) and the Lower Permian SW-Germany (Schultze and Heidtke, 1986). Long (1999) tentatively placed Marsdenichthys within the ill-defined group, the Rhizodopsidae (Berg, 1940), based on similar scale ornamentation.…”

Rhipidistians (Sarcopterygii) from the Hunter Siltstone (Late Famennian) near Grenfell, NSW, Australia

“…For example, the tripodal scapulocoracoid supported by three distinct buttresses present in Grenfellia is characteristic of all known basal Tetrapodomorpha (e.g.. Andrews & Westoll 1970a, Long 1985a: fig. 11, Jarvik 1980, Lebedev 1995, Fox et al 1995.…”