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Steuerungsfaktoren globaler Biodiversifizierungsmuster am Beispiel frühmesozoischer Muscheln


Hautmann, Michael (2007). Steuerungsfaktoren globaler Biodiversifizierungsmuster am Beispiel frühmesozoischer Muscheln. Beringeria : Würzburger geowissenschaftliche Mitteilungen, 37:61-74.

Abstract

The dominance of bivalves in today’s skeletonized benthic marine faunas reflects the acme of a long-term increase in the diversity and abundance of this class since its origination in the Early Cambrian. This great phylogenetic success has found three contrasting explanations, which are paradigmatic for an ongoing discussion on the controls of global biodiversification in general: It has been interpreted as a result (a) of a superior bauplan that enabled bivalves to outcompete inferior groups over geological time, (b) of the intrinsic diversification dynamic of bivalves as a substantial part of the „Modern evolutionary fauna“, or (c) of the end-Permian mass extinction event, which by chance affected bivalves less severely than most of their competitors. The present paper confronts these hypotheses with a taxonomically revised database of the generic diversity of marine bivalves in the critical time interval between the Late Permian and the Late Triassic. These data indicate that bivalves suffered less from the end-Permian mass extinction event than most other groups of organisms. This is particularly evident in comparison with brachiopods as the most important competitors of epifaunal bivalves: Whereas the Late Permian generic diversity of brachiopods was twice the diversity of epifaunal bivalves, the diversity ratio was opposite in the Early Triassic. This reversal was probably facilitated by the greater filterfeeding capacity of bivalves, which made them less vulnerable to the reduction of primary production in the wake of the extinction event. Additionally, epifaunal bivalves have a longer larval stage than brachiopods, which enabled quicker interregional re-dispersal after the extinction event and a corresponding lead in the occupation of vacant niches. The steady increase of competition since the ecological vacuum of the Early Triassic provides a unique frame for testing general concepts of global biodiversification, which differ chiefly in attributing either positive (expansion models) or negative (equilibrium models) effects of biotic interaction on diversification rates. The boundary-crossing standing diversity of bivalves shows an initial exponential increase, which persisted until the Anisian-Ladinian boundary. Since the Ladinian, the increase progressively slowed down, and diversity approached a plateau of approximately 100 genera. The resulting logistic shape of the diversification curve matches nearly perfectly the prediction of equilibrium models, which postulate damping of diversification when a global carrying capacity is approached, i.e. when the rate of origination equalled the rate of extinction as a consequence of ecospace filling. The theoretical concept of equilibrium models is also confirmed by the quick filling of vacant niches that is observed in several orders and suborders of the Bivalvia. The Late Triassic diversity plateau indicates that the re-diversification of bivalves was more or less completed at this time, but diversity suffered a temporary setback by the mass extinction event at the end of the Triassic. Bivalves with massive aragonitic shells were particularly affected by this event, which is in accordance with the general extinction pattern of marine animals, especially with the preferred extinction of reef-building organisms with aragonitic skeletons. This selectivity against taxa that secreted large amounts of skeletal aragonite was probably the consequence of a strongly reduced CaCO3-saturation of seawater, which resulted from massive volcanogenic CO2 and SO2 release of the Central Atlantic Magmatic Province. Evolutionary responses to this non-actualistic chemical condition of sea water were size reduction (e.g., Megalodontoidea) and replacement of skeletal aragonite by calcite (e.g., Ostreidae, Gryphaeidae, Plicatulidae und Pectinidae).

Abstract

The dominance of bivalves in today’s skeletonized benthic marine faunas reflects the acme of a long-term increase in the diversity and abundance of this class since its origination in the Early Cambrian. This great phylogenetic success has found three contrasting explanations, which are paradigmatic for an ongoing discussion on the controls of global biodiversification in general: It has been interpreted as a result (a) of a superior bauplan that enabled bivalves to outcompete inferior groups over geological time, (b) of the intrinsic diversification dynamic of bivalves as a substantial part of the „Modern evolutionary fauna“, or (c) of the end-Permian mass extinction event, which by chance affected bivalves less severely than most of their competitors. The present paper confronts these hypotheses with a taxonomically revised database of the generic diversity of marine bivalves in the critical time interval between the Late Permian and the Late Triassic. These data indicate that bivalves suffered less from the end-Permian mass extinction event than most other groups of organisms. This is particularly evident in comparison with brachiopods as the most important competitors of epifaunal bivalves: Whereas the Late Permian generic diversity of brachiopods was twice the diversity of epifaunal bivalves, the diversity ratio was opposite in the Early Triassic. This reversal was probably facilitated by the greater filterfeeding capacity of bivalves, which made them less vulnerable to the reduction of primary production in the wake of the extinction event. Additionally, epifaunal bivalves have a longer larval stage than brachiopods, which enabled quicker interregional re-dispersal after the extinction event and a corresponding lead in the occupation of vacant niches. The steady increase of competition since the ecological vacuum of the Early Triassic provides a unique frame for testing general concepts of global biodiversification, which differ chiefly in attributing either positive (expansion models) or negative (equilibrium models) effects of biotic interaction on diversification rates. The boundary-crossing standing diversity of bivalves shows an initial exponential increase, which persisted until the Anisian-Ladinian boundary. Since the Ladinian, the increase progressively slowed down, and diversity approached a plateau of approximately 100 genera. The resulting logistic shape of the diversification curve matches nearly perfectly the prediction of equilibrium models, which postulate damping of diversification when a global carrying capacity is approached, i.e. when the rate of origination equalled the rate of extinction as a consequence of ecospace filling. The theoretical concept of equilibrium models is also confirmed by the quick filling of vacant niches that is observed in several orders and suborders of the Bivalvia. The Late Triassic diversity plateau indicates that the re-diversification of bivalves was more or less completed at this time, but diversity suffered a temporary setback by the mass extinction event at the end of the Triassic. Bivalves with massive aragonitic shells were particularly affected by this event, which is in accordance with the general extinction pattern of marine animals, especially with the preferred extinction of reef-building organisms with aragonitic skeletons. This selectivity against taxa that secreted large amounts of skeletal aragonite was probably the consequence of a strongly reduced CaCO3-saturation of seawater, which resulted from massive volcanogenic CO2 and SO2 release of the Central Atlantic Magmatic Province. Evolutionary responses to this non-actualistic chemical condition of sea water were size reduction (e.g., Megalodontoidea) and replacement of skeletal aragonite by calcite (e.g., Ostreidae, Gryphaeidae, Plicatulidae und Pectinidae).

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Paleontological Institute and Museum
Dewey Decimal Classification:560 Fossils & prehistoric life
Language:German
Date:2007
Deposited On:31 Mar 2014 09:15
Last Modified:24 Sep 2019 20:16
Publisher:Institut für Paläontologie der Universität Würzburg
ISSN:0937-0242
OA Status:Green
Free access at:Publisher DOI. An embargo period may apply.
Related URLs:https://www.gzn.nat.fau.de/

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