This is Mazon Monday post #214. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Rhabdoderma exiguum is a species of Mazon Creek coelacanth. Coelacanths are lobe-finned fish, thought extinct since the Cretaceous. The first living coelacanth was discovered in 1938 off the coast of South Africa. There are now two known living species.
Rhabdoderma exiguum was described by Charles R. Eastman (1868 - 1918) in 1902. Eastman was an American geologist and paleontologist, who specialized in fish. The paper "The Carboniferous Fish-Fauna of Mazon Creek, Illinois", which was published in The Journal of Geology. He named it Coelacanthus exiguus.
Description.-A small species, attaining a maximum length of about 4.5 cm. Trunk narrow and elongated, the head occupying about one-fourth of the total length. First dorsal consisting of relatively few stout rays, and situated slightly in advance of the pelvic pair; second dorsal midway between the first dorsal and principal caudal; the latter comprising nine stout rays
above and below. [Scale-structure and ornamentation of head-bones not: observed.]This species is represented by ten specimens in the Yale, and one in the Harvard Museum, most of them being only 3 cm long, and very deficient in preservation. They agree in having a narrow, gradually tapering body, which terminates in an equilobate caudal fin, with indications that the axis was prolonged into a supplementary caudal. The first dorsal and
caudal, owing to their stronger attachment, are present in nearly all specimens, but the remaining fins have in most cases become destroyed. The first dorsal has usually seven or eight stout rays, and is situated near the middle of the trunk. Ten long, hollow rays are to be counted in the single specimen displaying the posterior dorsal, and nine above and below in the symmetrical caudal. The neural and haemal spines are very long in the abdominal and caudal regions. The ossifications of the axial skeleton are continued nearly to the termination of the principal caudal. The squamation must have been exceedingly delicate, as no indications of scales are to be observed in any of the specimens, nor do any of them have the cranial elements satisfactorily preserved.
The genus Rhabdoderma was erected in 1937 by James Alan Moy-Thomas. He took a hint from Otto Maria Reis in 1888. Reis noticed morphological differences in the shape of the first dorsal fin support and the pattern of ornament on the scales between Carboniferous and Triassic and Jurassic species. 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).
R. exiguum appears on page 135 of "The Fossil Fauna of Mazon Creek" by Jack Wittry.
Rhabdoderma exiguum is a coelacanth frequently found in the small size typical of Mazon Creek fish. Specimens of juveniles are known which show attached yolk sacs in various stages of absorption. Even individual eggs, as the one shown in Figure 135.2 below, are found with the embryonic fish seen wrapped around the yolk.
The tail is a diphycercal type with the fleshy termination of the notochord extending straight out to the tip. The upper and lower caudal fin lobes are symmetrical around an elongated central lobe. Two dorsal fins are present and have a lobed appearance. The scales are diamond-shaped and densely covered with flexuous and generally parallel ridges. Because of the juvenile nature of the Mazon Creek specimens, scales are not always present and bones are not always ossified. Larger- sized specimens in the Mazon Creek Fauna are known only from their isolated scales.
R. exiguum is found in both the marine Essex and freshwater Braidwood Faunas. It is thought to be a predator and one of the more common fish. Genus Rhabdoderma is the common coelacanth of the Coal Measures of North America and Europe.
Drawings from Don Auler in the "ESCONI Keys to Mazon Creek Animals", published in 1989.
Specimens
From "The Fossil Fauna of Mazon Creek"
From "The Fossil Fauna of Mazon Creek" with an embryonic egg sac.
ESCONI Member David Duck (Fossil Friday #37)
ESCONI Member Dan Quasney (Fossil Friday #210)
