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

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PRDlGED SIG EMS ElF GIV FRSFAMrOU VIS NAM PRD1GEDSIG EMS ElF GIV FRSFAM,TOU VIS NAM
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FIG. 7. - Chondrichthyan and acanthodian stage-level generic diversity patterns. Note the end Devonian diversity increase among
chondrichthyans and the Middle and Late Devonian decline of acanlhodians. A, chondrichthyan diversity: Chondrichthyes
(squares), Elasmobranchii (triangles), and Subterbranchialia (circles). B, acanthodian diversity: Acanthodii (filled squares),
order Acanthodida (circles), order Climatiidae (triangles), order Ischnacanlhida (diamonds), and incerlae sedis (open squares).
Courbes de modetes de diversile des genres d'Acal1lhodiens et de Chondrichlhyens en fonclion des etages geologiques. Remarquez
la croissance de la diversile des Chondrichlhyens a La Jin du Devonien el Ie declin des Acanthodiens pendalll Ie
Devonien moyen et terminal. A,.diversile des Chondrichrhyens: Chondrichthyes (carres), Elasmobranchii (Iriangles), Subterbranchialia
(cercles). B, diversile des Acanthodiens: Acanlhodii (carres pleins), ordre des Acanthodida (cercles), ordre des
Climatiidae (triangles), ordre des Ischnacanthida (Iosanges) et incertae sedis (carres).
(Fig. 6B), the decline of phlyctaeniids begins prior to the origin of coccosteomorph arthrodires;
however, a comparison of phlyctaeniids and brachythoracid arthrodires is suggestive of a pattern
of competitive displacement in analyses carried out at both stage- and substage-level resolution.
Generic patterns for pachyosteomorph and coccosteomorph arthrodires are roughly parallel
(Fig. 6) and suggest a single pattern differing only in levels of di versity with pachyosteomorphs
reaching a higher maximum diversity. In contrast, a substage-level analysis (Fig. lOB) shows
the peak diversities of the two clades to be offset which accounts for the bimodal maxima seen
in the species- and genus-level Frasnian diversities for placoderms (Fig. IE). Coccosteomorph
arthrodires reached maximum diversity in the Lower Frasnian (which includes the well documented
Gogo Formation fauna) with their greatest decline in the Middle Frasnian prior to the
Frasnian-Famennian extinction event. This decline does not coincide with known extinctions and
suggests a possible biotic cause, although, it is not clear as to which taxa are interacting during
this short interval. Pachyosteomorph arthrodires reached peak diversity in the Upper Frasnian
prior to the Frasnian-Famennian extinction episode. The conclusions of MILES (1969) and
GARDINER (1990), concerning biological interactions among arthrodires, provide a basis for a
number of hypotheses that still need regional evaluation and analysis at a finer time scale.
Most placoderms were extinct by the end of the Devonian with antiarchs possibly surviving
until the Lower Carboniferous and some arthrodires surviving into the Tournaisian; however,
patterns for placoderms and other gnathostomes do not support an ubiquitous effect for the numerous
extinction events reported from the Middle and Upper Devonian (HOUSE, 1985; SEPKOSKI,
1986). At the Givetian-Frasnian boundary (Figs. 5-8, 10) few of the major gnathostome clades
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