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

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or open window" for their early radiation. Within the Famennian there is clear evidence of direct
predator-prey interactions and apparent competition for other resources. Much of placoderm evolution
revolved around specializations on a basic plan with retention of a relatively primitive
placoderm suspensorium and vertebrate locomotor pattern (see discussion below). In contrast,
chondrichthyans and actinopterygians evolved a number of innovations associated with feeding
and locomotion which have been well documented (e.g. SCHAEFFER, 1975; ZANGERL, 198 I;
LAUDER, 1982; WEBB, 1982; LUND et ai., 1984). With the rapid increase in diversity among
actinopterygians and chondrichthyans after the Frasnian-Famennian extinc;;tion event, it is proposed
that these forms may have competitively displaced contemporaneous placoderms; however,
the suggested presence of a major Famennian-Tournaisian extinction event is consistent with a
model of opportunistic replacement by surviving actinopterygians and chondrichthyans. Tests of
this hypothesis must await detailed basinal and regional faunal analyses. Current field work and
review of the open basin faunas associated with the Catskill Delta and Michigan Basin may
shed light on the history and extinction of placoderms in the Upper Devonian.
PACHYOSTEOMORPH DIVERSITY PATTERNS
Among pachyosteomorph arthrodires, the aspinothoracid subclade accounts for 50% of all
described pachyosteomorph species. Remaining pachyosteomorphs comprise the Dunkleosteidae
(and possibly Panxiosteidae). Aspinothoracid arthrodires first appeared in the Upper Givetian
with an apparent increase in species diversity until the Frasnian-Famennian extinction episode.
The Laurasian record for aspinothoracids includes Lagerstatten on both sides of the extinction
episode suggesting the Frasnian peak does not represent a sampling bias. Each Lagerstatten
(Upper Frasnian KelJwasserkalk of the Manticoceras beds of Bad Wildungen, Germany, and
Late Famennian Cleveland Shale, northern Ohio, USA) represents similar deep water sedimentary
environments which suggest potentially similar taphonomic processes. There is no data available
for Devonian stage-level sediment volumes and surface exposures. RONOV (1980) provides series-level
global data which indicates equivalent sediment volumes and areas for the Middle and
Upper Devonian. However, differences in estimated duration (HARLAND ef ai., 1989) suggest a
potential sampling bias in favor of Frasnian sediments, but SEPKOSKI (/991) has pointed out
that ages for the Devonian stage boundaries are poorly constrained and time averaging may be
omitted until better estimates are available (HARLAND et ai., 1989, note a high level of uncertainty
for estimating the lower boundary date for each Upper Devonian stage. They note an error of
plus or minus an amount equal to or greater than the duration of the stage). During the Famennian,
there was little if any numerical recovery of diversity at the species level following the Frasnian­
Famennian extinction event. However, among arthrodires there was a secondary radiation associated
with habitat utilization, feeding structures, food acquisition, and locomotor patterns.
Aspects of this radiation are seen clearly in the Late Famennian Cleveland Shale (auna of North
America with its morphologically diverse assemblage of aspinothoracid arthrodires. The question
then arises: what might account for this apparent increase in pachyosteomorph diversity? Two
major adaptive aspects of the phenotype are associated with feeding and locomotion. It is difficult
to determine the prey of most placoderms, but the biological role, in mechanical terms, of the
structures associated with feeding and locomotion can be evaluated. An analysis of potential
functional consequences of evolutionary changes in feeding morphologies must consider both