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NEW INFORMATION ON THE PECTORAL FIN IN PLACODERMI FROM THE
LATE DEVONIAN CLEVELAND SHALE FAUNA (FRASNIAN OF NORTHERN
OHIO)
Gary L. Jackson, Cleveland Museum of Natural History
Robert K. Carr, Dept. Biological Sciences, Ohio University
Placoderms have been characterized as either obligate benthic organisms or that
they were limited to life just off the bottom. These interpretations are a
consequence of our lack of understanding of the locomotor structures in these
fishes. Preservation in most taxa is limited to perichondrally ossified pectoral
girdle and basal fin elements. An exception to this pattern is the ray-like rhenanid,
Gemuendina, with a pectoral fin consisting of a single proximal element with up to
three rows of radials. Other than this rare example, little is known of the fin distal
to the proximal basals.
Four placoderm specimens from the Cleveland Shale provide new information on
the pectoral fin. A large Dunkleosteus terrelli specimen documents the first
recognized occurrence of radials among the arthrodires. A second specimen of D.
terrelli possesses numerous basals with a carbon film representing the distal fin
(no ossified radials are seen in this specimen suggesting radial ossification may
represent an age-related phenomenon). Two specimens of unidentified selensoteid
arthrodires also demonstrate the presence of organic films. The importance of
these specimens is the information they provide on estimates for the aspect ratio of
the pectoral fin. In comparison to the chondrichthyan Cladoselache (a recognized
active swimmer in the pelagic realm), the pectoral fin in arthrodires appears to
have a comparable aspect ratio (standardized for size and thus suggesting
equivalent lift capabilities). In the arthrodire Heintzichthys, the presence of
potential muscle attachments on the basal elements suggests the possibility of
active fin retraction or alteration of fin camber further adding to controlled
maneuverability.
A proposed explanation for the extinction of placoderms is thought to be through
the process of competitive displacement by the contemporaneous chondrichthyans
and osteichthyans. This has been related, in part, to relative differences in effective
locomotion (based on incomplete evidence among placoderms). The recognition
of comparable locomotor structures in arthrodires and sharks of the Late
Devonian necessitates our reevaluation of placoderm extinction at the close of the
Devonian.

P.S: Поэтому есть надежда на новые находки окаменевших мягких тканей дунклеостея!