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