Sponges

CARRERA, M.G.; RIGBY, J.K.

THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT

Columbia University Press

Lugar: New York; Año: 2004; p. 102 - 111

The phylum Porifera is one of the oldest and a very conservative group in the fossil record. Although, all major groups of sponges are represented in the Cambrian, Ordovician diversification shows an intensive radiation of some groups while others remain poorly developed or relegate. Relationships of the several major taxonomic groups to possible Cambrian origins are even obscure, largely because of significant gaps between Middle Cambrian and lower Ordovician sponge records. Records of several major sponge groups begin in the Ordovician, but their origins from Cambrian sponges are obscure. The lithistid suborder Orchocladina has Cambrian roots represented by 3 genera from which it rapidly diversified to over 45 genera in the Ordovician. Hexactinellid sponges are much less diverse than demosponges in the Ordovician record but they have more recognizable roots to Cambrian faunas, for at least three families continued from the Cambrian into the Ordovician. Cambrian hexactinellids are most common in soft-bottom facies and are primitive simple forms. Ordovician hexactinellids are found in similar rocks but have skeletons that are more complex and three dimensional.After a significant gap from Middle Cambrian to lower Ordovican (Late Tremadoc) the sudden worldwide appeareance of sponges are tied to the development of reef-mounds built by microbe-demosponge-calathiid consortium. However sponges do not show a significant diversification based upon scattered generic ocurrences, few genera of lithistid orchoclads including the common Archaeoscyphia. Abundant reef mound development coincided with a transgression that began in the Late Tremadoc. After their success in the Early Ordovician Lithistid microbe reef mounds virtually dissappeared at the end of the Arenig, with only scattered examples in the Llanvirn.The Middle Ordovician records the rapid diversification of the Lithistids the minor beggining of Orchoclads was replaced by an abruptly expansion in forms. This expansion was followed by other suborders like Sphaerocladina, Tricranocladina, Rhizomorina and Megamorina reaching a dominance comparable to Orchoclads by the Late Ordovician.Expansion of Calcareous Sphictozoan and heteractinid sponges in the Late Ordovician is remarkable, although they are restricted to a few areas.