Ordovician megamorinid demosponges from San Juan formation, Precordillera, western Argentina

Two new megamorinid sponges are described from Ordovician rocks of Western Argentina. Twenty three specimens of a new family were recovered from Arenigian beds of the San Juan Formation in La Silla and Buenaventura Luna sections. The new Family Nexospongiidae is erected to include a single genus Nexospongia composed by irregularly distributed heloclones and monaxons. The structural features of this new family are closely related to those of the Mesozoic megamorinids. However, spicule composition is similar to those of the Ordovician and Silurian megamorinids. A single specimen of the new genus Rugospongia was collected from Llanvirnian beds of the San Juan Formation in Cerro Viejo section. This genus is also composed of heloclones and monaxons as principal spicules, but they are grouped in tracts as are the other megamorinid genera known from the Paleozoic. Rugospongia nov. gen. is probably the ''stem-genus'' of the Early Paleozoic megamorinid lineage with tracts as a principal structure.