Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
Carr, R. K., 1995a. - Biological Sciences
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From these data, the diversity patterns for a number of monophyletic placoderm groups are<br />
described and compared. Additionally, the generic diversity for all remaining Devonian and Early<br />
Mississippian gnathostomes is evaluated to document patterns of change during this critical time<br />
in vertebrate history. Data for chondrichthyans (ZANGERL, 1981; CARROLL, 1988), and acanthodians<br />
(DENISON, 1979) are recorded at stage level; however, osteichthyan data (taxonomy<br />
and range data from CARROLL, 1988) are recorded using series. MILES' (1969; see also GARDINER,<br />
1990) characterization of arthrodiran evolution as a succession of competitively superior grades<br />
is specifically c;onsidered by subjectively comparing his and current estimates of arthrodiran<br />
clades against predicted patterns for competitive and opportunistic replacements. The radiation<br />
of pachyosteomorph, or more specifically aspinothoracid, arthrodires in the Late Devonian is<br />
evaluated. Morphological changes within this clade are compared through analogy with extant<br />
fishes and mechanical aspects of these changes are evaluated in terms of biological roles and<br />
mechanical effectiveness.<br />
Tbe specimen number prefix CMNH denotes the Cleveland Museum of Natural History.<br />
The suffix "id" when used to form taxonomic adjectives does not refer to family-level Linnean<br />
classification and is used as a convenience for discussing informal taxonomic units. Abbreviations<br />
for stage names used in figures and Appendix I follow that of HARLAND et at. (1989).<br />
RESULTS<br />
PATTERNS OF DIVERSITY FOR PLACODERMS<br />
Placoderm global diversity rose at a nearly steady rate from the Silurian to the Frasnian<br />
Famennian boundary reaching both generic and specific peaks within the Frasnian (Fig. I; see<br />
also GARDINER, 1990). At the Frasnian-Famennian boundary current data suggest an overall placoderm<br />
species extinction of 48-51 % and a generic extinction of 52-53% (table 1).<br />
The genus- and species-level diversity patterns for placoderms are equivalent for both substage<br />
and stage-level analyses of all available data, only minor fluctuations are noted at substage<br />
levels (Fig. 1). Figure 2 demonstrates similar generic patterns for:<br />
1) stage-level analysis of all available data;<br />
2) data with a temporal resolution to stage level or finer;<br />
3) taxonomically resolved data (indeterminate forms and doubtful generic assignments are<br />
excluded);<br />
4) and data resolved both temporally and taxonomically.<br />
The orders Rhenanida (sensu stricto, i.e., excluding palaeacanthaspids) and Peta1ichthyida<br />
(Figs. 3A, B, 4-5) showed low specific diversity from their first appearance in the Gedinnian<br />
to their final appearance in the Upper Frasnian (Upper Devonian records are represented by<br />
single species). Both orders were marine and have been characterized as being dorsoventrally<br />
compressed (DENISON, 1978). This characterization is seen in Gemuendina stuertzi (a rhenanid,<br />
Fig. 3B) with its dorsally placed orbits and enlarged ray-like pectoral fins; however, little is<br />
known concerning the body form among petalichthyids. Lunaspis broilii (Fig. 3A), one of the<br />
better known petalichthyids from the Hunsrtickschiefer of Germany, has been secondarily com