Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
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information, one can begin to evaluate specific hypotheses of extinction effects, biological interactions,<br />
and morphological evolution.<br />
During the Devonian there was a major radiation within gnathostomes. The water column,<br />
in which these fishes lived, was not devoid of other predators; however, the history of early<br />
vertebrates shows the origin and evolution of organisms with specializations in locomotion, feeding<br />
structures, and sensory organs, as well as central processing and coordination of sensory<br />
and motor systems (NORTHCUTT & GANS, 1983). The appearance of these new morphologies<br />
are suggestive of an adaptive radiation.<br />
Devonian sediments potentially offer an unique and important view of early gnathostome<br />
history since they represent the largest estimated volume and geological map area for Paleozoic<br />
systems (DINELEY, 1984). The globally distributed Old Red Sandstone developed during this<br />
period and was associated with a number of tectonic events. A major facies shift occurred within<br />
the series of North American carbonate basins at the beginning of the Upper Devonian, characterized<br />
by the widespread deposition of anoxic black shales. The Upper Devonian further represents<br />
a time of complex biotic and abiotic events which include numerous orogenies putatively<br />
associated with the suturing of Pangaea (McMILLAN et at., 1988). JOHNSON (1970) noted shifts<br />
among brachiopods from earlier provincialism to cosmopolitanism associated with Upper Givetian<br />
onlap, effectively lowering the North American continental arch. HOUSE (1985) described eight<br />
separate extinction events among Devonian ammonoids of which six range from Upper Givetian<br />
to Lower Tournaisian (Taghanic, Frasnes, Kellwasser, Enkeberg, Annulata, and Hangenberg<br />
Events). Of these events, SEPKOSKI (1986) considered three to be significant (Frasnes or Givetian<br />
Frasnian, Kellwasser or Frasnian-Famennian, Hangenberg or Famennian-Tournaisian; Fig. 9).<br />
MCGHEE (1982, 1990) reported 65% extinction among marine placoderm species and 23% among<br />
putative freshwater forms during the Frasnian-Famennian extinction episode, though current data<br />
suggest an overall placoderm species extinction of 48-51 % and a generic extinction of 52-53%<br />
(table 1); five out of six placoderm orders survived (Antiarcha, Arthrodira, Petalichthyida, Phyllolepida,<br />
Ptyctodontida). MCGHEE commented upon the potential significance of differences in<br />
freshwater and marine extinction to the evaluation of causal mechanisms; however, freshwater<br />
interpretations for many Old Red Sandstone style sediments have been called into question (e.g.<br />
Spitsbergen, GOUlET, ] 984a; Escuminac Formation, CHIDIAC, 1989 and VEZINA, 1991; East Baltic<br />
and Podolia, MARK-KuRIK, 1991). The Frasnian-Famennian decline was possibly associated with<br />
a global event that affecled both invertebrate and vertebrate benthic and pelagic communities<br />
(McLAREN, ]988). Causes and timing of this event are currently under debate (for a discussion<br />
see McMILLAN et at., 1988; HOUSE, 1985; SEPKOSKI, ]986). The final decline of placoderms<br />
occurred in another association with a major event (Fig. 9). Despite the possibility of a physical<br />
cause, it is worthwhile also to evaluate potential biological factors for this final decline. To this<br />
end, gnathostome diversity patterns are evaluated.<br />
In the absence of an established phylogeny, MILES (1969) characterized placoderm evolution<br />
in terms of specializations related to their life on or just off the bottom. Within arthrodires, he<br />
described a succession of competitively superior grades related to improvements in feeding and<br />
locomotion. GARDINER (1990) further noted a number of morphological innovations associated<br />
with feeding and locomotion. LONG (l990a: 255) evaluated aspects of placoderm evolution in<br />
terms of "guiding factors" related to evolutionary trends, although, it should be noted that his