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Ichthyosaurs from the Early Cretaceous (Hauterivian - Barremian) from the western border of the Tyndall Glacier in the Torres del Paine National Park, Southernmost Chile

Pardo Pérez, Judith Mariana

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Abstract

Three field campaigns to the eastern lobe of the Tyndall Glacier in the Torres del Paine National Park, southern Chile, allowed us to record 46 specimens of ichthyosaurs, which are almost completely exposed and articulated, as well as several associated faunal elements such as teleost and ganoid fishes, land plants, belemnites and ammonites. The fossil-bearing unit is part of the Uppermost Jurassic to Lower Cretaceous Zapata Formation (Tithonian to Albian, 151-112 Myr.) and is Valanginian to Hauterivian in age (140-130 Myr.), as indicated by the fossil assemblage. The concentration of numerous ichthyosaurs in this area is explained by turbidity flow events, which must repeatedly have caught the ichthyosaurs hunting fishes inside a submarine canyon. The flows transported the ichthyosaurs downslope into the deep ocean, from about 200 to possibly > 2000 m. Ichthyosaurs lost their orientation and were unable to return to the surface for oxygen. Some died during the transport and were immediately covered by turbidity sediment, while others were buried still alive; agony of these latter ichthyosaurs is expressed in the vertebral column of their skeleton. Isolated ichthyosaur elements such as fins, portions of vertebral column, or ribs, are explained by a previous partial decomposition of carcasses and posterior transport by the turbidity flows, with random distribution of bones along the canyon. Presence of pyrite and calcite in ichthyosaur skeletons suggest an early diagenesis. This process, together with a rapid and almost instantaneous burial, the absence of scavengers and low oxygen environment, contributed to the excellent preservation of the specimens, which even favored the preservation of soft tissue. Abrasion of retreating glaciers polished the ichthyosaur bones and exposed the skeleton. The topography of the area shows some portions of the skeleton exposed, while other continue embedded. The excavated specimens have demonstrated that the portions not exposed 'in situ' are contained in the sediment. The ichthyosaurs have been assigned to five different taxa: Myobradypterygius hauthali von Huene, 1927, ?Myobradypterygius hauthali, Ophthalmosauridae indet., Platypterygiinae indet. Arkhangelsky, 2001 (sensu Fischer et al., 2012)and Thunnosauria indet. The new cranial and postcranial material obtained from the Tyndall fossil locality allowed to emend the original diagnosis of Myobradypterygius hauthali and to disassociate it from the genus Platypterygius von Huene, to which Myobradypterygius was synonymized in 1972. The amount of specimens assigned to M. hauthali has allowed to reconstruct the anatomy of the complete animal and to differentiate five ontogenetical stages: adult, subadult, juvenile, neonate and embryo. M. hauthali was an edentulous Platypterygiinae ichthyosaur that shared the same ecological niche with dentate platypterygiines. According to internal content and pellets, both edentulous and dentate ichthyosaurs fed on fishes, using similar hunting strategies but differing in their mode of ingestion:dentate platypterygiines used their teeth to pierce the food whereas edentulous one smashed prey with their strong jaws. The cranium is exposed in five specimens and articulated postcranial elements are exposed in four. An isolated and articulated forefin is also present. These specimens were assigned to three yet undetermined taxa of Ophthalmosauridae based on a remarkable differentiation in shape and organization of elements of the hind fin. Likely the three taxa represent new species of ophthalmosaurid ichthyosaurs. Ophthalmosauridae indet. documented here show similarities in the configuration of fore-and hind fin with Undorosaurus spp. and Cryopterygius kristiansenae from the Volgian (Tithonian) of Russia and Norway, but differ from these in the shape of the zeugopodium and the anterior and posterior extensions of the fins. The similarities with these northern hemisphere species suggest a convergence forming a yet undocumented Undorosaurus-like clade. Specimens asigned as Thunnosauria indet. are distinguised by possessing a particular morphology of neural spines, which favored the attachment surface of epaxial musculature and therefore contributed to the increase in thrust force during swimming and thus an enhances capacity to dive. The differentiation between specimens having this thoracic anatomy from others where it is absent, such as M. hauthali, may have dictated a different behavior, reducing competition. Specimens with an evolved thorax have been shown to possess a forefin architecture similar to those of Middle-Lower Triassic to Upper Jurassic ichthyosaurs, suggesting that the fins of ichthyosaurs evolved independently from the axis of the body. Morphological similarities between the Tyndall ophthalmosaurid ichthyosaurs and species from the northern hemisphere help us to understand the Jurassic to Cretaceous ichthyosaur distribution around the globe and pathways that these marine reptilians may have taken for dispersal. In this context the Tyndall fossil locality has a privileged geographical location that contributes to the comprehension of the geotectonic fragmentation of Gondwana.

Item Type: Dissertation
Supervisor: Stinnesbeck, Prof. Dr. Wolfgang
Date of thesis defense: 11 December 2015
Date Deposited: 11 Jan 2016 14:11
Date: 2015
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institut für Geowissenschaften
Subjects: 560 Paleontology Paleozoology
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