Diversificación y Paleobiogeografía de los poríferos: de Ediacara al mundo globalizado del Ordovícico

CARRERA MARCELO G.

Gral Roca

Simposio; Seaways and landbridges: Southern Hemisphere biogeographic connections through time; 2011

Instituto de Investigación en Paleobiología y Geología de la Universidad Nacional de Río Negro.

A series of critical steps for the sponge evolutionary history occurred from the Proterozoic-Cambrian boundary to the Middle–Late Ordovician interval. It is fairly clear that sponges possess a long record back to the Proterozoic, represented in the Ediacara fauna. Such indirect methods as biomarkers and molecular clocks suggest that the origin of sponges may extend back to the late Paleoproterozoic. If true, it is not surprising that in the early Cambrian all the sponge classes had already diversified significantly.The lower-middle Cambrian faunas are considered as a Cambrian evolutionary sponge fauna, with archaeocyaths and diverse monaxonid demosponges as distinctive components. There was a transitional fauna in the upper Cambrian-lower Ordovician, with orchoclad lithistids dominating shallow environments. Hexactinellids began to colonize nearshore siliciclastic settings during this time. Middle-Upper Ordovician, corresponds to the Paleozoic evolutionary fauna, and is the interval during which lithistids diversified in several suborders and families, and the stromatoporoid and sphinctozoan calcified sponges experienced their first radiation. The earliest fossil demosponges and hexactinellids were adapted to quiet-water environments. In the Upper Cambrian and Early Ordovician, the anisotropic triangulated skeletons predominate, as in the orchoclad lithistids. These sponges invaded reef environments in the late Cambrian after the demise of archaeocyaths. The anthaspidellid orchoclads that appeared in upper Cambrian shallow-water environments were represented by only a few genera of simple cylindrical to obconical forms. They rapidly diversified at species, genus, and family level and occupied a variety of shallow to deep platform environments during the Lower and Middle Ordovician.             In the Middle and Upper Ordovician the main stage of sponge diversification took place. The main appearances occur among lithistids, with the first record of several orders and suborders. This is related with the appearance of the isotropic triangulated net (triangular in all directions) seen among the Hindiids and Sphaerocladins.Ordovician sphinctozoan sponges exhibit a distinctive biogeographic distribution restricted to Upper Ordovician fold-belt successions (island-arc terranes) of the Paleo-Pacific. Sphinctozoans are very rare in platform associations of the Lower Paleozoic, the positive record of sphinctozoans in the few preserved island arcs is overwhelming evidence that they must have diversified and dominated in such habitats, at least during the Ordovician. There are some possible factors and proposed explanations for the particular patterns exhibit by the Cambrian and Ordovician sponge distribution. The Upper Cambrian-Lower Ordovician increase in hard substrates is one of the most accepted. The important peak in diversification of sponges recorded by the Middle and Upper Ordovician is apparently tied to the changing paleogeographic configuration as a consequence of the increase of the tectonic activity.