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

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-100 clades of gnathostomes maintain stable levels. The Frasnian-Famennian boundary coincides with declines in all placoderm orders discussed here, but one (low diversity phyllolepids), and a continuing decline in acanthodians. At the Famennian-Tournaisian boundary, there was a major diversity increase for chondrichthyans and actinopterygians. Coincident with this increase was a decline among rhipidistians, dipnoans, and remaining placoderms. MCGHEE (1988) noted the importance of temporal resolution in evaluating the timing of events in the fossil record; however, resolution problems still exist and the relative timing of the osteichthyan radiation and placoderm decline is not clear. This is demonstrated by the epoch level resolution for sarcopterygian diversity. which limits any analysis of the Frasnian-Famennian extinction event for this clade. WILLIAMS (1990) provided direct evidence for the interaction among Late Famennian gnathostomes within the Cleveland Shale fauna. He documented evidence for predator-prey relationships among piscivorous members of the fauna concluding (p. 287) simply that "the big fish ate the little ones," suggesting a general absence of prey selectivity. Additionally, there was potenti\ll competition among durophages with independent evolution of durophagous feeding structures in placoderms (e.g. Ptyctodontida, Mylostomatidae), dipnoans, and chondrichthyans (e.g. Orodus). By the end of the Mississippian, a number of holocephalan and elasmobranch durophages had evolved (ZANGERL, 1981; CARROLL, 1988). AdditionaJly, VERMEIJ (1987) noted a doubling of marine durophagous families of eurypterid and crustacean arthropods, cephalopod molluscs, and vertebrates between the Middle and Upper Devonian. The above findings suggest that the Frasnian-Famennian extinction episode was critical in the initial evolution of actinopterygians and chondrichthyans providing a so-called "opportunity 120 100 80 60 40 20 A 0 0 30 25 20 15 10 5 PRO GED SIG EMS ElF GIV FRS FAMTOU VIS NAM CJ) ::::!:=> LL ::::!:=> > ::::!:=> CJ) :E=> ::::!: ::::!:=> => ::::!: W a a: « 0 SIL I DEV I CARB w LL LL I -.J ...J -.J -.J -.J DEV CARB FIG. 10. - A, a comparison of stage-level generic diversity for major gnathostome clades: placoderms (squares), acanthodians (circles), osteichthyans (diamonds), and chondrichthyans (triangles). S, eubrachythoracid substage-level generic diversity: coccosteomorph arthrodires (squares) and pachyosleomorph arthrodires (circles). Note the decline of coccosteomorph arthrodires prior to the FRS-FAM extinction event. A, Courbe de comparaison de la diversile des groupes majeurs de Gnalhostomes en fonction des etages geologiques: Placodermes (carres), acanlhodiens (eercles), oSleiehthyens (Iosanges) el ehondriehthyens (Iriangles). E, meme eourbe pour les EubraehYlhoraei en fonelion des sous-hages: Arlhrodires coccosleomorphes (carres), Arthrodires paehyosreomorphes (eerdes). Remarquez Ie declin des coccosteomorphes a partir de l'evenement Frasnien-Famennien.. B
-101 or open window" for their early radiation. Within the Famennian there is clear evidence of direct predator-prey interactions and apparent competition for other resources. Much of placoderm evolution revolved around specializations on a basic plan with retention of a relatively primitive placoderm suspensorium and vertebrate locomotor pattern (see discussion below). In contrast, chondrichthyans and actinopterygians evolved a number of innovations associated with feeding and locomotion which have been well documented (e.g. SCHAEFFER, 1975; ZANGERL, 198 I; LAUDER, 1982; WEBB, 1982; LUND et ai., 1984). With the rapid increase in diversity among actinopterygians and chondrichthyans after the Frasnian-Famennian extinc;;tion event, it is proposed that these forms may have competitively displaced contemporaneous placoderms; however, the suggested presence of a major Famennian-Tournaisian extinction event is consistent with a model of opportunistic replacement by surviving actinopterygians and chondrichthyans. Tests of this hypothesis must await detailed basinal and regional faunal analyses. Current field work and review of the open basin faunas associated with the Catskill Delta and Michigan Basin may shed light on the history and extinction of placoderms in the Upper Devonian. PACHYOSTEOMORPH DIVERSITY PATTERNS Among pachyosteomorph arthrodires, the aspinothoracid subclade accounts for 50% of all described pachyosteomorph species. Remaining pachyosteomorphs comprise the Dunkleosteidae (and possibly Panxiosteidae). Aspinothoracid arthrodires first appeared in the Upper Givetian with an apparent increase in species diversity until the Frasnian-Famennian extinction episode. The Laurasian record for aspinothoracids includes Lagerstatten on both sides of the extinction episode suggesting the Frasnian peak does not represent a sampling bias. Each Lagerstatten (Upper Frasnian KelJwasserkalk of the Manticoceras beds of Bad Wildungen, Germany, and Late Famennian Cleveland Shale, northern Ohio, USA) represents similar deep water sedimentary environments which suggest potentially similar taphonomic processes. There is no data available for Devonian stage-level sediment volumes and surface exposures. RONOV (1980) provides series-level global data which indicates equivalent sediment volumes and areas for the Middle and Upper Devonian. However, differences in estimated duration (HARLAND ef ai., 1989) suggest a potential sampling bias in favor of Frasnian sediments, but SEPKOSKI (/991) has pointed out that ages for the Devonian stage boundaries are poorly constrained and time averaging may be omitted until better estimates are available (HARLAND et ai., 1989, note a high level of uncertainty for estimating the lower boundary date for each Upper Devonian stage. They note an error of plus or minus an amount equal to or greater than the duration of the stage). During the Famennian, there was little if any numerical recovery of diversity at the species level following the Frasnian Famennian extinction event. However, among arthrodires there was a secondary radiation associated with habitat utilization, feeding structures, food acquisition, and locomotor patterns. Aspects of this radiation are seen clearly in the Late Famennian Cleveland Shale (auna of North America with its morphologically diverse assemblage of aspinothoracid arthrodires. The question then arises: what might account for this apparent increase in pachyosteomorph diversity? Two major adaptive aspects of the phenotype are associated with feeding and locomotion. It is difficult to determine the prey of most placoderms, but the biological role, in mechanical terms, of the structures associated with feeding and locomotion can be evaluated. An analysis of potential functional consequences of evolutionary changes in feeding morphologies must consider both
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Page 32 and 33: -114 CARR, R.K., 1991. - Reanalys
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Carr, R. K., 1995a. - Biological Sciences

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