Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
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clades of gnathostomes maintain stable levels. The Frasnian-Famennian boundary coincides with<br />
declines in all placoderm orders discussed here, but one (low diversity phyllolepids), and a continuing<br />
decline in acanthodians. At the Famennian-Tournaisian boundary, there was a major diversity<br />
increase for chondrichthyans and actinopterygians. Coincident with this increase was a<br />
decline among rhipidistians, dipnoans, and remaining placoderms. MCGHEE (1988) noted the<br />
importance of temporal resolution in evaluating the timing of events in the fossil record; however,<br />
resolution problems still exist and the relative timing of the osteichthyan radiation and placoderm<br />
decline is not clear. This is demonstrated by the epoch level resolution for sarcopterygian diversity.<br />
which limits any analysis of the Frasnian-Famennian extinction event for this clade.<br />
WILLIAMS (1990) provided direct evidence for the interaction among Late Famennian<br />
gnathostomes within the Cleveland Shale fauna. He documented evidence for predator-prey relationships<br />
among piscivorous members of the fauna concluding (p. 287) simply that "the big<br />
fish ate the little ones," suggesting a general absence of prey selectivity. Additionally, there was<br />
potenti\ll competition among durophages with independent evolution of durophagous feeding<br />
structures in placoderms (e.g. Ptyctodontida, Mylostomatidae), dipnoans, and chondrichthyans<br />
(e.g. Orodus). By the end of the Mississippian, a number of holocephalan and elasmobranch<br />
durophages had evolved (ZANGERL, 1981; CARROLL, 1988). AdditionaJly, VERMEIJ (1987) noted<br />
a doubling of marine durophagous families of eurypterid and crustacean arthropods, cephalopod<br />
molluscs, and vertebrates between the Middle and Upper Devonian.<br />
The above findings suggest that the Frasnian-Famennian extinction episode was critical in<br />
the initial evolution of actinopterygians and chondrichthyans providing a so-called "opportunity<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
A<br />
0 0<br />
30<br />
25<br />
20<br />
15<br />
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PRO GED SIG EMS ElF GIV FRS FAMTOU VIS NAM CJ) ::::!:=> LL ::::!:=> > ::::!:=> CJ) :E=> ::::!: ::::!:=> =><br />
::::!: W a a: « 0<br />
SIL I DEV I CARB w LL LL I<br />
-.J ...J<br />
-.J -.J -.J<br />
DEV CARB<br />
FIG. 10. - A, a comparison of stage-level generic diversity for major gnathostome clades: placoderms (squares), acanthodians<br />
(circles), osteichthyans (diamonds), and chondrichthyans (triangles). S, eubrachythoracid substage-level generic diversity: coccosteomorph<br />
arthrodires (squares) and pachyosleomorph arthrodires (circles). Note the decline of coccosteomorph arthrodires<br />
prior to the FRS-FAM extinction event.<br />
A, Courbe de comparaison de la diversile des groupes majeurs de Gnalhostomes en fonction des etages geologiques: Placodermes<br />
(carres), acanlhodiens (eercles), oSleiehthyens (Iosanges) el ehondriehthyens (Iriangles). E, meme eourbe pour les<br />
EubraehYlhoraei en fonelion des sous-hages: Arlhrodires coccosleomorphes (carres), Arthrodires paehyosreomorphes (eerdes).<br />
Remarquez Ie declin des coccosteomorphes a partir de l'evenement Frasnien-Famennien..<br />
B