Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
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(e.g. LELIEVRE, 1984a, b; 1988; LELIEVRE et al., 1981), Australia (e.g. MILES & DENNIS, 1979;<br />
YOUNG, 1988a; LONG, 1990b), Antarctica (e.g. YOUNG, 1988b) and South America (e.g. GOUJET<br />
et al., 1985) have added significantly to our understanding of diversity and biogeography among<br />
gnathostomes. Additionally, specific critical periods in gnathostome evolution (e.g. Famennian<br />
Tournaisian) require active exploration with known Devonian localities needing renewed interest.<br />
A number of Middle and Upper Devonian Michigan Basin invertebrate localities have been<br />
collected extensively (EHLERS & KESLING, 1970) with few or no records of fossil fishes. In a<br />
two week field period (Antrim Shale Formation and Traverse Group, Summer 1991), I collected<br />
remains belonging to 12 genera of gnathostomes, new to the basin, along with three genera<br />
which represent extensions of known ranges. The Middle to Late Paleozoic represents a key<br />
period in the evolutionary history of gnathostomes and a renewed interest in field work offers<br />
much potential.<br />
The early placoderms show a trend toward solidification of the head and thoracic shields<br />
associated with the origin and enhancement of the cranio-thoracic articulation and possible modification<br />
of anguilliform locomotion. Arthrodires reduced body scales, perhaps increasing maneuverability.<br />
Within aspinothoracid arthrodires, secondary locomotor trends included further<br />
development of pectoral fin maneuverability and lift along with mass reduction through lateral<br />
shortening of the thoracic shield and thinning of the dermal bone. If initial gains in mass were<br />
associated with inertial stabilization in a modification of anguilliform locomotion, secondary<br />
loss would suggest further modifications away from a purely anguilliform style of locomotion.<br />
However, there is no preservation of post-thoracic anatomy among aspinothoracid arthrodires to<br />
confirm this relationship. Several forms (brachydeirids) showed lateral compression which would<br />
minimize yaw associated with loss of anterior mass. Pachyosteomorph arthrodires developed the<br />
widest range of feeding modifications on the unique placoderm pattern of fixing the suspensorium<br />
to the dermal skeleton. This pattern may have been a limiting factor in their evolution and<br />
competition with other evolving gnathostomes, despite the evolution of a wide diversity of gnathal<br />
morphologies among pachyosteomorph arthrodires along with mechanical specializations. Ossification<br />
of the inferognathal blade (a brachythoracid character) provided attachment for enlarged<br />
adductor musculature. A number of taxa developed elongated inferognathals characterized by<br />
increased bite velocity and modification of the anterior cusp to impale prey. Large occlusal surfaces<br />
permitted a wide range of potential out-force for crushing or partitioning of food. Specialized<br />
durophages (e.g. Mylosfoma) reduced the occlusal portion of the inferognathaJ concentrating<br />
crushing surfaces posteriorly. Enlarged orbits were achieved independently in a number of arthrodire<br />
groups and are correlated with either increased acuity or specialization for low light<br />
intensity.<br />
In contrast to the generalized perception of placoderms as sluggish modified benthic fishes,<br />
the diversity of morphological specializations suggest that these fishes were ecologically diverse<br />
with some being active predators capable of effective locomotion (Fig. 17). Placoderm extinction<br />
cannot be attributed to any single cause. During the Upper Devonian their decline at the Frasnian<br />
Famennian boundary can be attributed to a global extinction event (SEPKOSKI, 1986; McMILLAN<br />
et aI., 1988); however, the event did not equally affect each of the gnathostome clades present<br />
at that time. After a reduction in diversity of over 57% (table 1) there is evidence for a partial<br />
recovery among arthrodires duling the Upper Famennian where they were competing with rapidly