ARTHROPOD-DOMINANCE IN TRIASSIC POST-EXTINCTION RECOVERY FAUNAS: EVIDENCE FROM THE NORTHWESTERN MARGIN OF PANGAEA

Arthropod fossils & trace fossils clearly attributable to the activity of arthropods are disproportionately abundant in shallow marine Triassic post-extinction recovery faunas in on the northwestern coast of Pangaea. Lower Induan shallow marine successions (offshore transition to lower shoreface) contain abundant trace fossils inferred to have been constructed by arthropods including Cruziana, Diplichnites, Kouphichnium, Monomorphichnus, scratch-marked Rhizocorallium, Thalassinoides, Trichophycus & Spongeliomorpha. Many of these forms are atypical in post-Paleozoic strata indicating that either a group of Paleozoic arthropods survived into the Triassic or, alternatively, points to evolutionary convergence in the functional morphology of marine arthropods. Although trace fossils attributed to other invertebrates also occur, arthropod-constructed forms are disproportionately dominant. These trace fossil assemblages reflect both organismal adaptation to shallow marine hypoxia as well as local alleviation of hypoxic conditions. Arthropod dominance gradually diminished through the Upper Induan into the Olenekian. By the Middle Triassic (Anisian-Ladinian) the arthropod bias disappeared & shallow marine trace fossil successions possess balanced ichnofaunas similar in composition to later Mesozoic marine assemblages. Arthropod fossils (including penaeid & chimaerastacid decapods, possible stomatopods and phyllocarids) are locally common in these Lower and Middle Triassic successions. Factors that contributed to arthropod dominance in post-P-Tr extinction recovery faunas likely include the following: 1) high metabolism (thus high mobility) resulting in the ability to go where the food is but escape zones of reduced oxygenation; 2) high reproductive capacity (combining the invertebrate ‘shotgun' approach to reproduction with high metabolic rates); 3) evolving tolerance to hypoxia throughout ontogeny (i.e. larval tolerance to hypoxia); 4) adaptability in terms of food preferences (opportunistic generalists). Of these, the ability to reside in more optimum settings and travel to hypoxic settings in response to food availability likely provided earliest Triassic arthropods with a competitive edge over other, less mobile invertebrates.