This is the "Fossil Friday" post #218. Expect this to be a somewhat regular feature of the website. We will post any fossil pictures you send in to [email protected]. Please include a short description or story. Check the #FossilFriday Twitter hash tag for contributions from around the world!
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Rhabdoderma elegans was described by John Strong Newberry in 1856 as Coelacanthus elegans. Newberry (1822 - 1892) was an American physician, geologist, and paleontologist. He participated as a naturalist and a surgeon on three expeditions to explore and survey the western states during the 1850s. He served in the US Sanitary Commission during the Civil War. Among his many post war positions, he was professor of geology and paleontology at Columbia University, chief geologist of the Geologic Survey of Ohio, a member of the Geologic Survey of Illinois, and president of AAAS.
John Strong Newberry
Coelacanthus elegans was moved to Rhabdoderma elegans in 1937 by James Alan Moy-Thomas based on work by Otto Maria Reis. Reis proposed the Carboniferous coelacanths should be separated from the Triassic and Jurassic species due to morphological differences in the shape of the first dorsal fin support and the pattern of ornament on the scales. See The Coelacanth Rhabdoderma in the Carboniferous of the British Isles" by Peter Forey in 1981.
Up to 1937, most authors grouped species of Carboniferous coelacanths in the genus Coelacanthus Ag. , a genus founded on the Permian C. granulatus Ag. , and which also included Triassic and Jurassic species (Woodward 1891). Moy-Thomas, however, reinstated a suggestion by Reis (1888, pp. 71-72) that the Carboniferous species should be separated as the genus Rhabdoderma. Reis pointed out that the Carboniferous species are distinguishable by a well-developed ornamentation of closely spaced ridges and tubercles on the scales, lower jaw, andgular plates. To this Moy-Thomas was able to add further features by which the Carboniferous species differed from the type-species of Coelacanthus: these included the presence in Rhabdoderma of a triangular coronoid (versus rectangular), a basipterygoid process (absent in C. granulatus), and the absence in Rhabdoderma of ossified ribs and the presumed absence of an extracleithrum (both present in the Permian species). But these additional features, while enabling Rhabdoderma to be distinguished from C. granulatus , do not allow it to be distinguished from other coelacanth genera. For example, a triangular coronoid is present in Wimania, a basipterygoid process is present in Diplocercides, ossified ribs are absent from most coelacanths, and the extracleithrum is absent from Macropoma. In other words, these character states are not synapomorphies for the recognized species of Rhabdoderma. Furthermore, in two features (the presence of a basipterygoid process and the absence of an extracleithrum) previous statements and interpretation have to be modified (pp. 206, 211). But there do seem to be two features peculiar to Rhabdoderma ; scales bearing an ornament of ridges which converge to the midline of the scale (Reis 1888) and the kidney-shaped endochondral support of the first dorsal fin (Schaeffer 1941). A definition of Rhabdoderma incorporating these features would also reflect the primitive position of the genus amongst coelacanths (p. 224).
During a recent visit to the Field Museum, Arjan Mann, the new Assistant Curator of Fossil Fishes and Early Tetrapods, showed us a huge Rhabdoderma elegans from Mazon Creek. This stunning fossil fish was in a very large layered concretion, which looked similar to concretions found in the Mazon River near Morris, IL. Thanks for sharing, Arjan!
