Carr, R. K., 1995a. - Biological Sciences

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