reconstruction of Euthenopteron Reconstruction of Strepsodus, a Carboniferous relative of Sauripterus ©

Juvenile Rhizodont
(c.f. Sauripterus, lobe-fin fish)

fossil of juvenile rhizodontFossil of the larger of the two juvenile rhizodonts found at Red Hill. Note the large pectoral fins. (Photo courtesy of Ted Daeschler, ANS.)

Two well preserved specimens of juvenile rhizosdont lobe-fins were collected in the shallow channel margin of Red Hill. The larger of the two was approximately 25 cm long; the less complete smaller specimen was probably about 75% as large. Similarities in the pectoral girdle and fins indicate that these fishes compare favorably (c.f.) with Sauripterus.

Rhizodonts are large (up to 3m), freshwater lobe-fin fishes that lived during the Devonian and Carboniferous. Although their remains have been recognized since the 1830s they remain poorly known. Most specimens consist of scales, isolated teeth and vertebrae, but more complete material is known from Scotland, Australia and Antartica. Sauripterus is known primarily from a pectoral fin discovered in the Catskill Formation by J. Hall in 1840 and a second pectoral fin discovered by Ted Daeschler and Neil Shubin in 1995.

Sauripterus, Fin Ontogeny and Fingers

One conspicious feature of these two Red Hill rhizodonts is the relatively large size of their pectoral fins, which equal to about 1/4 of the body length. In contrast, the fins of adult rhizodonts and many other lobe-fin fishes are about 15% of the body length. More significant, however, is the extent and type of bone that occur in these fins. Both specimens exhibit well-developed, unjointed lepidotrichia (fin rays) which are a type of dermal bone. The larger of the two fishes has some weakly ossified, but identifiable endochondrial bone (internal bones that form from cartilage) near the body; the smaller individual exhibits no endochondrial elements. The small size of these two specimens and the absence or incomplete ossification of endochondrial bone indicate that these two are juveniles.

Comparisons of these two specimens with a recently discovered adult specimen of Sauripterus from another Catskill Formation deposit of similar age allows us to view three stages of fin development. Lepidotrichia are well developed but endochondrial ossifications are absent in the earliest stage. Endochondrial ossification is evident in the larger juvenile, but only for those bones near the trunk. By adulthood, the fins contain both an expanded set lepidotrichia and ossified and articulating endochondrial elements. These endochondrial bones include the distal (finger-like) preaxial radials well as the proximal elements (e.g., humerus, radius and ulna) seen in the older juvenile.

Current theories on fin (or limb) ontogeny generally view the developmental pathways for lobe-fin and ray-fin fishes very different. Ray-fin fishes are believed to experience an early shift in favor of the development of dermal elements (lepidotrichia). In contrast, lobe-fins are believed to have either a late developmental shift from endochondrial growth to dermal elements or no shift at all. Consequently their fins consist primarily or exclusively of endochondrial elements. Sauripterus, on the other hand, exhibits a mosaic of dermal and endochondrial elements; lepidotrichia are prominent early and continue to expand throughout life, while robust endochondrial elements develop later.

Fish with Fingers?

One confounding aspect of Sauripterus is the presence of eight finger-like radials in the adult fin. As with fingers, these radials are jointed and six of them articulate with the ulnare and the intermedium; both homologues of the carpus; the other two radials are conspiciously more robust and articulate directly with the radius. In some respects, the fin of Sauripterus looks like a "better" intermediate between lobe-fin and early tetrapod limbs than those of the osteolepiform lobe-fins, Eusthenopteron and Panderichthys.

However, osteolepiforms and early tetrapods share a number of derived characters that indicate they belong to a single clade. The relationships of rhizodonts to osteolepiforms is still unresolved, but many authorities consider the two groups to be closely related. In any case, the radials of Sauripterus indicate that "fingers" are not unique to tetrapods.

Other lobefins, including an unidentified lungfish, Holoptychius sp., Red Hill rhizodont, Red Hill megalichthyidid, and Hyneria lindae were also found at Red Hill. You can also learn more about lobe-fin fishes.

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Web:
Academy of Natural Sciences press release on Sauripterus:
www.acnatsci.org/news/tedsfish/fish08.html
Uiversity of Pennsylvania's web page on Sauripterus:
www.upenn.edu/gazette/0398/0398gaz6.html
Jonathan Jeffery's web site on rhizodont fishes:
www.donnasaxby.com/rhizodonts/
Books:
Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
Scientific Papers:
Andrews, S.M. 1985. Rhizodont crossopterygian fish from the Dinantian of Foulden, Berwickshite, Scotland, with a re-evaluation of this group. Trans. Royal Soc. Edinburgh 68:207-239.
Davis, M. C., N. H. Shubin and E. B. Daeschler. 2001. Immature rhizodontids from the Devonian of North America. Bull. Mus. Comp. Zool. 156(1): 171-187.
Daeschler, E. B. and N. Shubin. 1998. Fish with fingers? Nature 391:133.
Image Credits:
The reconstruction of Strepsodus is copyrighted © 2002, Dennis C. Murphy. (See Terms of Use.) It's based on Andrews (1985).

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