TRACE FOSSIL PALEOGEOGRAPHY. LATE CRETACEOUS PROGRESSIVE REPLACEMENT OF COSMOPOLITAN ANTARCTIC ICHNOFOSSILS (CRUZIANA ICHNOFACIES): PALEOENVIRONMENTAL IMPLICATIONS

OLIVERO, E. B.; LÓPEZ CABRERA, M.I.

Punta Arenas

Congreso; IX Congreso Latinoamericano de Ciencia Antartica; 2017

Instituto Antártico Chileno

Ichnofossils record animal behavior preserved in sediments, but as phylogenetically unrelated organisms may present similar behaviors, quite commonly evolutionary convergence in widely separate geographic regions precludes trace fossil provincialism. This is exemplified in the Late Cretaceous by worldwide distributed ichnogenera of the Cruziana ichnofacies. Nonetheless, in the Santonian-Danian Marambio Group, James Ross Basin,Antarctica, cosmopolitan ichnogenera of the Cruziana ichnofacies are progressively replacedby a distinctive group of mostly endemic, complex spreite ichnotaxa (Fig. 1). This replacement of cosmopolitan ichnofossils by endemic ones in the Late Cretaceous of Antarctica is of considerable paleogeographic interest and in this presentation we explore possible causal relationships.The 3 km-thick Marambio Group records a variety of deltaic, estuarine, and shelfal storminfluenced deposits bearing ichnological suites of the proximal and distal Cruziana ichnofacies. Typical ichnogenera of worldwide distribution include Asterosoma, Chondrites,Nereites, Ophiomorpha, Phycodes, Phycosiphon, Planolites, Rhizocorallium, Rosselia,Schaubcylindrichnus, Scolicia, Taenidium, Teichichnus and Thalassinoides. Among these, a distinctive group of complex spreite burrows, which are not recorded in the Coniacian, startsto appear in the Santonian with increasing diversity and dominance, includingParadictyodora antarctica (Santonian); P. antarctica and Tasselia ordamensis (Fig. 2; late early Campanian); P. antarctica, T. ordamensis, and Euflabella singularis (mid Campanian);P. antarctica, T. ordamensis, E. singularis and Euflabella multiplex (Fig. 3; late Campanian);and P. antarctica, T. ordamensis, E. singularis; E. multiplex, Euflabella radiata, Nereitesbiserialis; Patagonichnus stratiformis and Patagonichnus thalassiformis (Maastrichtian) (Fig. 1). These complex spreite burrows are shallow-tier fodinichnia or repichnia. Many of their producers were also trophic generalists, living on detritus from the burrowed sediment,fresh detritus from the surface, and bacterial gardening (Fig. 2). Parts of these burrows arepoorly known outside Antarctica-Tierra del Fuego and most are restricted to southern high paleolatitudes (Olivero and López Cabrera, 2010; 2013; 2016).Progressive endemism of Late Cretaceous ichnofossils occurred during a significant climatic and biotic reorganization in Antarctica. A cooling trend of seawater temperatures from ~18C°(Santonian) to ~8C° (Maastrichtian) was accompanied by remarkable shifts in ammonite biogeography, with replacement of Santonian-early Campanian cosmopolitan-Indo Pacific faunas by endemic mid Campanian-Maastrichtian ones. The latter are characterized byradiation and dominance of stenothermal kossmaticeratid ammonites and its transition from the former by early extinction of warm-temperate scaphitid, nostoceratid, and baculitid ammonites and of inoceramids and trigoniid bivalves (Olivero, 2012). We hypothesize that the striking replacement, both in trace fossils and mollusks during the Late Cretaceous of Antarctica reflects the appearance of evolutionary novelties adapted to changing environments characterized by the combination of severe cooling, high-latitude and strongseasonality in food production. As in modern polar marine ecosystems, the new ethological strategies of the Antarctic trophic generalists trace makers were advantageous to cope with seasonally fluctuating food resources, resulting in a markedly endemic Late Cretaceous trace fossils assemblage