Fossil Hexactinellida: An Overview

Abstract

Hexactinellida (Porifera) are known from hexactin spicules from the Late Proterozoic of Mongolia (Brasier et al., 1997) and China (Steiner et al., 1993), possibly representing the oldest lineage of animals alive on earth today. Hexactinellida flourished and radiated rap- idly during the Middle Cambrian times, giving rise to many new taxa and new skeletal plans (Rigby, 1986c; Walcott, 1920). Our knowl- edge of Early Paleozoic Hexactinellida is largely derived from isolated spicules from sediments. Both Amphidiscophora and Hexasterophora are known since the Early Paleozoic but most Paleozoic families did not survive the Paleozoic-Mesozoic boundary. Lyssacinosida are the first Hexactinellida known since the Proterozoic. They are an ultra-conservative group and their body plan still exists in living species today. The first representatives of Hexactinosida are known since the Devonian, with maximum radiation and diversity occurring during the Mesozoic. The first lychniscosidan sponges were described from the Middle Jurassic of Europe and during the Cretaceous where they reached their maximum peak of diversity. After the Jurassic-Cretaceous there has been a gradual decline in diversity leading up to the present. The present overview discusses about 120 genera in 18 families and two orders.