Jurassic evolution of southern hemisphere marine biochores based on benthic bivalves

DAMBORENEA, S.E.

Piacenza, Italia

Conferencia; International Conference of Paleobiogeography & Paleoecology; 2001

Biochores are dynamic units which change in range and rank through time, and also those based on benthic faunas have a different history to those based on pelagic taxa. These aspects are clearly exemplified by the history and evolution of southern hemisphere biochores during Jurassic times. The distribution of benthic Jurassic bivalve genera is globally analysed here, with emphasis on the southern hemisphere, where detailed patterns are studied. For this region, palaeobiogeographic units are quantitatively characterised according to their biological contents (mainly levels of endemism). Their evolution through time is followed on a stage by stage basis from the Rhaetian to the lowermost Cretaceous. The Tethyan Realm is undoubtedly the most mature and persistent through time. During Late Triassic times a Maorian Province has been long recognised, mostly on the basis of the distribution of genera of the superfamily Monotoidea. Along the Jurassic, the ecotone between southern (South Pacific, Maorian or Austral) and Tethyan biochores fluctuated in position with time. The approximate latitudinal location of the boundary area is recognised on the basis of faunal composition along the study region for each stage. During the Early Jurassic, in central Chile and Argentina the occurrence of typically Tethyan bivalves in the same area as high-latitude bivalve species is a consequence of a mid-latitude position of this region without significant barriers along the East Pacific margin, the probable pantropic nature of Tethyan faunas, and a shallow sea connection with the western Tethys from middle Early Jurassic times onwards. The palaeogeographical position of South America during the Jurassic and the availability of outcrops and fauna make this part of Gondwana a privileged place to study the evolution of biochores with time. In this region there is a southward shift of the northern boundary of the southern biochore between Hettangian and Toarcian times of about 700 to 900 km. At different times along the Jurassic, endemism within the southern biochore is high for some groups, such as pectinoids. Didemic (or bipolar) taxa are also common within this and other bivalve groups. During Middle Jurassic and early Late Jurassic times the southern hemisphere bivalve benthic fauna is apparently more cosmopolitan in composition, but still some bipolar genera are present. By the end of the Jurassic and beginning of the Cretaceous the situation changes and bipolar bivalve genera are again well established at least within inoceramoids and monotoids. Recognition of a relatively mature South Temperate (or Austral) Realm is clear, with well-defined Austral and East African provinces as subordinate biochores by the beginning of the Cretaceous.