The ecostratigraphic transition and selective extinction of bivalves across the Triassic-Jurassic boundary in the Lombardian Alps, Italy
Date of Award
Doctor of Philosophy (PhD)
mass extinctions, Triassic, Jurassic
Geology | Paleontology
A major biotic crisis affecting virtually all major marine invertebrate clades occurred at the close of Triassic time. Ecostratigraphic and bivalve diversity analyses of carbonate strata from the Lombardian Alps of Italy document the patterns of extinction and suggest a possible causal mechanism. Three paleoenvironmental phases and a severe decline in bivalve diversity characterize the Late Triassic to Early Jurassic history of the Lombardian Platforms in the southern Alps of Italy. The first paleoenvironmental phase, of Late Triassic time (?Choristocera Zone), consists of 1-5 m thick shallowing-upward subtidal cycles of molluscan, coralline, and echinoderm wackestone and packstone of the Zu Limestone. Taxonomic loss by the end of Zu deposition was severe, where 71% of the bivalve species were eliminated, including all infaunal and 50% of the epifaunal species. The decline in diversity correlates stratigraphically with changes in sedimentary facies related to a fall in relative sea level. The second ecostratigraphic phase, of latest Triassic time (?upper Choristoceras Zone), consists of shallow Surface provides description only. Full text is available to ProQuest subscribers. Ask your Librarian for assistance. marine or peritidal carbonates of the Conchodon Formation dominated by barren lime mudstone and dolostone, algal laminites, and oolitic grainstone. Few megalodontid bivalves were able to tolerate the harsh environments of shifting substrates and perhaps hypersaline conditions. Where observed, the upper and lower contacts of the Conchodon Formation are conformable and do not constitute sequence boundaries as suggested by some workers. The Lower Jurassic (?Psiloceras Zone) Sedrina Limestone marks the beginning of the third phase, with the onset of transgression and return of normal marine conditions to much of Lombardia. Typical microfacies include molluscan, echinoderm, and sponge wackestone and packstone with abundant anomuran microcoprolites. Bivalve diversity analyses indicate a selective survivorship of epifaunal bivalve taxa, whereas infaunal species were selectively eliminated before onset of Sedrina deposition. Physiologic differences and selective resistance to physical stress between infaunal and epifaunal bivalves are consistent with the pattern of selective extinction. The selection against infaunal bivalves may be caused by their decreased capacity to filter feed relative to their metabolic demands. A decrease in primary productivity may have been responsible for selectively eliminating the infauna. Oceanographic processes or atmospheric darkening, perhaps caused by an extraterrestrial impact, could drastically limit food resources (primary productivity) and is consistent with the selective extinction at the end of the Triassic.
Surface provides description only. Full text is available to ProQuest subscribers. Ask your Librarian for assistance.
McRoberts, Christopher A., "The ecostratigraphic transition and selective extinction of bivalves across the Triassic-Jurassic boundary in the Lombardian Alps, Italy" (1994). Earth Sciences - Dissertations. Paper 20.