FERNANDEZ Diana Elizabeth
congresos y reuniones científicas
Implications of recently found Early Cretaceous xiphosurid trackways from the Neuquén basin (Western Gondwana): evidence of paleocoastline continuity?
FERNÁNDEZ, DIANA E.
Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS The history of life: A view from the Southern Hemisphere; 2014
Recently reported xiphosurid trackways represent the first record of such trace fossils from the Early Cretaceous (worldwide) and the second Cretaceous fossil record of xiphosurids in the Southern Hemisphere. These trackways were found in the uppermost levels (interpreted as tidal flat facies) of the Agua de la Mula Member (late Hauterivian?early Barremian) of the Agrio Formation (Neuquén Basin). Based on the phylogeny and fossil record of xiphosurids and the age of these deposits, these trackways were interpreted as produced by members of the Subfamily Limulinae. The aim of the present work is to discuss the implications this finding might have in the reconstruction of marine seaways involving Western Gondwana. Recent xiphosurids (horseshoe crabs) are found in two regions of the world. Three species occupy coastal waters from India to Japan, and one is found along the North American Atlantic coastline from Maine to the Yucatan. The main large-scale limitations on the distribution of Recent horseshoe crabs are the extent of continental shelves (which define space availability), certain tidal regimes, and low temperatures. Although extant xiphosurids have planktonic larvae, their long-range dispersal is limited. Studies based on genetic analyses and the distribution of extant species, established that the divergence of the American species from the Indo-Pacific species took place approximately 135 million years ago, and the ancestor of the present species originated in the Mesozoic in Europe and migrated east and west. However, no palaeobiogeographic, migration or distribution studies on xiphosurids have been carried out using the fossil record. Only five Cretaceous limulid body fossils have been recorded, and two are of Early Cretaceous age: Victalimulus mcqueeni Riek and Gill from Victoria (Australia) and Crenatolimuluspaluxyensis Feldmann et al. from Texas (USA). During that time, the Neuquén Basin was connected to the Pacific Ocean through an island arc chain to the west. The arrival of nektonic (and planktonic in larval stage) North Pacific and Tethyan faunas along the western South American margin has already been documented. The global palaeogeographic reconstructions of the Early Cretaceous (Hauterivian-Barremian) and the marine connexions of the Neuquén Basin supported by body fossil information suggest a marine seaway between this locality and the previously mentioned North American and Australian sites. Nevertheless, given the large-scale limitations on the distribution of Recent horseshoe crabs, a direct passage between the three localities would most likely imply a continuous palaeocoastline with relatively shallow waters, which has not been accurately documented.