El registro fósil de equinodermos en el Eoceno-Mioceno de Patagonia Argentina y Antártida

Abstract

This thesis includes a review and a compendium of the record of the phylum Echinodermata in Argentina, including Antarctica and the South Atlantic Islands, from Eocene to late Miocene. Echinoderms are bilaterial organisms, with secondary radial symmetry, mutable connective tissue, aquiferous vascular system and multielemental calcareous endoskeleton, composed by calcite with high magnesium content. They constitute an important part of the benthic marine ecosystems, as part of the trophic chain. They have low tolerance to changes in water temperature and salinity, and to the sediment and organic matter input, being good environmental indicators. The environmental conditions that improve the echinoderm preservation are: little to null transport, rapid burial or in life entombment, absence of subsequent reworking, anoxia or dysoxia, and deposit of fine to very fine sediments. The first studies on echinoderms in Latin America date back to the 19th century and laid the foundations for the rest of the studies on the subject. In Argentina, rocks from the Eocene-Miocene interval yielding echinoderm remains are assigned to the Man Aike (Middle-Late Eocene), San Julián (Late Oligocene), El Chacay (Early Miocene), Río Foyel (Early Miocene), Monte León (Early Miocene), Estancia 25 de Mayo (early Miocene), Chenque (early Miocene), Gran Bajo del Gualicho (early-middle Miocene) and Puerto Madryn (late Miocene) formations. These formations represent several transgressive-regressive pulses within the Patagonian territory in the Golfo San Jorge, Peninsula Valdés and Austral-Magallanes basins. In the Antarctic Peninsula, the echinoderm records come from the La Meseta (Middle Eocene), Submeseta (Late Eocene-Early Oligocene), Polonez Cove (Middle-Late Oligocene) and Cape Melville (Early Miocene) formations. Using high resolution imaging (SEM and microtomography), the species Zoroaster marambioensis was described from La Meseta Formation. Its preservation allowed identifying death and burial positions, inferring their possible life habits. It was interpreted that starfishes colonized the distal part of an estuary during a period when salinity was normal, died from a freshwater discharge through a hyperpychnic flow, and were immediately buried in fine sand, without further rework. In the La Meseta and Submeseta formations (Seymour Island, Antarctica), the genera Abatus, Schizaster and Stigmatopygus were identified, using high contrast images and digital diagrams. These genera were previously recorded in Antarctica (Seymour and King George islands) and Patagonia (Santa Cruz and Chubut provinces, Argentina). These findings enabled making inferences about ancient marine connections between New Zealand, Australia, Antarctica, and Patagonia. In the Monte León Formation, a layer yelding spatangoid echinoids of the genus Brisaster preserved in situ, associated with their own traces and other ichnofossils that suggest a shallow marine environment. The depositional framework was interpreted as subtidal, affected by migration of large-scale bedforms, with important pyroclastic participation. The compendium of the Eocene-Miocene record of echinoderms from Patagonia Argentina and Antarctica gathered more than 1,200 specimens, housed in 26 institutions, 9 of them located in Argentina. It includes 37 genera grouped into 13 orders, from all current classes, except the Class Holothuroidea. The Class Echinoidea is the best represented, with 19 genera (9 of them endemic from Patagonia). Irregular echinoids are recorded in the Eocene of Antarctica, the Oligocene of southern Santa Cruz, the early Miocene of northern Santa Cruz and southern Chubut, and the middle and late Miocene of northern Chubut, Río Negro, and Entre Ríos. A possible distribution pattern was proposed, running from south to north over time. The Cenozoic echinoderm global diversity is marked by a peak in the Eocene and another in the Miocene. In Antarctica, generic greatest diversity occurs in the middle Eocene, in La Meseta Formation, with 17 genera (5 of crinoids, 6 of asteroids, 5 of echinoids and 1 of ophiuroids). The observed communities are similar to those of the Mesozoic, indicating a shallower and warmer environment than the current one in Antarctica, with a scarcity of durophagous predators and abundant organic matter content. In Patagonia, the echinoderm diversity peak was documented during the early Miocene as in the rest of the world, in coincidence with the maximum flood of the second cycle from the Patagonian transgression (~ 20 Ma).