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

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