Are Protospongiidae the Stem Group of Modern Hexactinellida?

Abstract

The family Protospongiidae Hinde, comprising the genera Protospongia Salter and Diagonella Rauff — both first found in rocks of Middle Cambrian age — has long been considered to be the stem group of all later Hexactinellida. They are small, conical to subcylindrical sponges, whose skeletal wall apparently consisted of a single layer of stauractins only (Fig. 1). These sponges have been considered as primitive among Hexactinellida (Rigby 1986) and their skeletons of nonfused stauractins were supposed to have supported a simple unfolded choanosome layer of ascon type (Finks 1960, 1970, 1983). On account of studies done by Okada (1928) on spicule-bearing parenchymella larvae of the recent hexactinellid Farrea sollasii Schulze, stauractins have been assumed to be the original hexactinellid spicule from which hexactins developed by addition of radial rays (this hypothesis was originally introduced by Minchin 1905). Mainly isolated stauractins have been found in the Lower Cambrian (the first whole hexactinellids are from the Middle Cambrian; only a questionable fragment of ? Protospongia hicksi Hinde is described by Rigby (1987) from the Early Cambrian Lower Parker Slate of North America), but some isolated hexactins have also been reported (Finks 1983; Rigby 1986; and others). Recently, rocks of the lowermost Cambrian of China (Sinian-Cambrian boundary strata) have been studied for fossil content (Wei-Ming and Qian 1988). Isolated triaxial spicules (hexactins and pentactins) were found in these strata, but no stauractins. A purpose of this chapter is to discuss the status of the Protospongiidae and the probability that they represent the stem group of modern hexactinellids. Also, Okada’s ontogenetic studies done on Farrea sollasii and their phylogenetic significance will be discussed.