The trace fossil record of the decapod crustacean radiations

CARMONA, N.B.; BUATOIS, L.A.; MÁNGANO, M.G.

Sidney, Australia

Congreso; First International Palaeontological Congress; 2002

The concept of evolutionary faunas represents a useful theoretical framework for describing and analyzing major changes in the composition of Earth´s biotas through time. Although trace fossils provide valuable paleoecologic and ethologic evidence, they have been relatively underutilized in evolutionary paleoecology. Based on the analysis of an extensive database constructed from the literature and from our own field studies, we evaluate the ichnologic record through the Phanerozoic of one member of the Modern Fauna, the malacostracan crustaceans. Although crustaceans originated early in the Paleozoic, decapods reach a maximum diversity in the Mesozoic and Cenozoic. More than 150 records of decapod-like burrow systems (mazes and boxworks) from all continents were included in the database; almost all of them belong to the ichnogenera Thalassinoides and Ophiomorpha. These data were plotted in a graphic that shows the abundance of these burrow systems through the Phanerozoic. Aspects considered in the database include burrow morphology, trophic type, burrowing depth, tiering position and potential tracemaker, among other features. Our analysis is restricted to softground shallow-marine ichnofaunas and therefore, examples of decapod burrows in deep-marine turbidite systems or in omission surfaces were not considered. Analysis of the database allows us to examine the fossil record of two- and three-dimensional burrow systems similar to recent decapod crustacean galleries, attempting to establish a link between the trace fossil record and the decapod radiations documented by body fossils. The number of early Paleozoic records is low and all the examples correspond to the ichnogenus Thalassinoides, generally preserved in carbonate deposits. The Middle to Late Ordovician advent of extensive three-dimensional burrow systems reveals the successful colonization of a moderately deep infaunal ecospace. However, identification of the potential tracemaker is uncertain. Despite the overall similarity in arquitectural morphology, unquestioned malacostracan fingerprints have not been identified. The oldest decapods come from Upper Devonian strata and the first thalassinidean body fossils are known from the Jurassic. Similarities between early Paleozoic burrow systems and modern examples point to behavioral convergence. Carboniferous-Permian records include both Ophiomorpha and Thalassinoides in clastic and carbonate shallow-marine successions, most likely reflecting the late Paleozoic radiation of malacostracan crustaceans evidenced from the body fossil record. However, decapod burrows commonly are accessory components of Carboniferous-Permian shallow-marine ichnofaunas, typically are small and generally display poorly developed pelletoidal walls. The low number of late Paleozoic decapod-like burrows is consistent with a macroevolutionary lag between the first appearance of decapods in the Late Devonian and their major radiation in the Jurassic, as suggested from the body fossil record. The Paleozoic picture changed in the Mesozoic, when there is a continuous increase in the number of records, reaching a pronounced peak in the Cretaceous. This pattern undoubtedly reflects the late Mesozoic decapod radiation. Crustacean burrows commonly are dominant in shallow-marine ichnofaunas and are morphologically similar to their recent counterparts. The biological affinities of these burrow systems with decapod crustaceans is unquestioned, based not only on burrow architecture and fine morphology, but also on the presence of crustacean claws within the structures. The post-Cretaceous records show a sharp decline in the number of reported specimens during the Paleogene. This sharp drop in abundance may be related to the Cretaceous-Tertiary mass extinction or may simply reflect a sampling bias. A maximum peak of abundance of decapod burrow systems is recorded during the Neogene, probably reflecting the decapod radiation that took place during the Eocene. Crustacean burrows are among the dominant elements of Neogene shallow-marine ichnofaunas, displaying an increase in burrowing depth and ecologic complexity, as revealed by multiple-tiered structure. Environmental conditions in shallow-marine settings were conducive to the development of evolutionary innovations. Infaunalization was one of the major innovations that favored the great diversification of the decapod crustaceans. Infaunalization allowed exploitation of an underutilized environment, the deep endobenthic realm, which provided new food resources, shelter during moulting and protection against predators. Our preliminary analysis suggests a tendency towards an increase in tiering complexity and burrowing depth through geologic time, although this general trend may have been disrupted by mass extinctions. The ichnologic record reflects decapod radiations documented from body fossil data and may be useful to refine available models.