FERNANDEZ Diana Elizabeth
congresos y reuniones científicas
Asteroid trace fossils from a Lower Cretaceous shallow marine paleoenvironment in Patagonia: the first record of starfishes for the Agrio Formation
Santa Rosa
Congreso; IIº Latin-American Symposium on Ichnology.; 2013
Institución organizadora:
Universidad Nacional de La Pampa
Body fossils of any group of starfish are seldom preserved, since the discrete skeletal elements (ossicles) most commonly dissarticulate after death. Many fossil asteroid examples are known either from one single sample or from a single bed containing more than one specimen. Several reports worldwide have attributed trace fossils to ophiuroids. In contrast, asteroid trace-fossil occurrences are scarcer and mostly restricted to a few Triassic and Jurassic examples.    Fossil asteroids in general are infrequent in South America. In the Neuquén Basin, asterozoan trace fossils are known from the Mulichinco Formation (early Valanginian, Mendoza Group) where Asteriacites lumbricalis specimens have been described and attributed to ophiuroid producers. The only fossil asteroid known in the entire basin comes from that unit and was described as an astropectinid (Asteroidea, Neoasteroidea, Paxillosida, Astropectinidae).     Given the scarcity of the body fossil record of asterozoans in the Neuquén Basin (Patagonia), the finding of asteroid trace fossils is important in the reconstruction of its palaeontological framework. The material presented in this work comes from the type locality of the Agrio Formation (late Valanginian?early Barremian, Mendoza Group). These specimens represent the first record of starfishes for this unit.     The aim of this work is threefold: i) to report the existence of these specimens, taking into account that the ichnospecies they were assigned to has not been found before in deposits of Cretaceous age; ii) to describe the trace fossils, discuss their producers along with some palaeoecological and taphonomic inferences; and iii) to provide a more accurate paleoenvironmental context than previously reported.    The Agua de la Mula Member or Upper Member of the Agrio Formation is a Late Hauterivian?Early Barremian mixed carbonate?siliciclastic marine and marginal-marine succession. Previous works have interpreted the upper part of the Agrio Formation as shallow subtidal to proximal offshore deposits influenced by fair weather and storm waves. In the study site, the uppermost part (63 m) of the Agua de la Mula Member represents a paleoenvironment evolving from marine to marginal-marine, including tidal flat deposits with high and/or fluctuating salinity. The interval of interest in this work begins at 339.5 m above the base of the unit and ends with an oolitic-skeletal bar. Two main facies are recognized. The asteroid trace fossils were found within one of the appearances of facies 2. The logged section included in this work represents upper shoreface to upper foreshore deposits. The inferred paleoenvironment is therefore shallower than previously reported.     The trace fossils are shallow, star-shaped burrows preserved as negative epirelief. They are approximately 10.5 cm in total diameter. Five axes radiate from a central area of 3 cm in diameter. These are straight, with a broad base and distally tapering. Their width varies, and some of the tips are diffuse. These depressions display irregular ornamentation in the shape of discontinuous, small bulges, without any transverse striae. Given their diagnostic features, these trace fossils are assigned to Asteriacites cf. quinquefolius.     The most plausible producers are, in this case, asteroids belonging to the family Astropectinidae. These starfishes are semi-infaunal, capable of self-burial in soft substrates, sometimes using this as a feeding strategy. These ichnofossils could be associated with: i) a non-sustained resting behavior (cubichnia) with shallow burial; ii) a more temporally prolonged shallow burial, typical of a semi-infaunal life style; iii) an active search for prey within the uppermost centimeters of the substrate; or iv) a combination of behaviors.