Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Sponges

Sponge lineages had been present for much more than 200 My by the time of the explosion, first appearing as chemical markers of probable demosponges in the fossil record that have been dated to 635 Ma. There were 2 possible sponges in the macrofauna from the Ediacaran, both of which were found in the White Sea location, Russia (Kouchinsky et al., 2012) Serezhnikova & Ivantsov, 2007). During the explosion is the first time sponge spicules became common, though there have been reports of siliceous spicules from many localities that date from the Neoproterozoic, most of which can be discounted.

3 of the 4 extant sponge clades are represented in the explosion faunas. Calcareous spicules are secreted by 1 clade, the Calcarea, the others that secrete spicules, secrete siliceous spicules. The formation of spicules in hexactinellids and demosponges is considered to be homologous as it is so similar. The LCA of these clades is indicated by molecular clock estimates to have originated about 700 Ma. There are many reports of spicules from the Cryogenian and Ediacaran, though most of them have been reinterpreted as pseudofossils. In the Late Ediacaran sections of Namibia (Reinter & Worheide, 2002) reports of spicules are considered to be more reliable. The result is a “spicule gap” of more than 200 My, though it is believed that spicules were probably secreted but the ocean conditions were not conducive to their preservation, the authors¹ suggesting that at those times the ocean water or the geochemistry of the substrate led to the dissolution of the spicules (Sperling et al., 2010). During the explosion fossil homoscleromorphs are not known of during the explosion.

During the explosion spicules appeared in Stage 1 deposits of China and Siberia (Kouchinsky, et al., 2012), becoming relatively common, and Hexactinellids and demosponges have been found as body fossils in Cambrian Stages 2 and 3 (Xiao, Hu et al., 2005), though known from relatively few localities. These body fossils of sponges are of simple construction and thin-walled, which suggests they were adapted to quite waters (Carrera & Botting, 2008). Extant Hexactinellids and demosponges have siliceous spicules in which there is a central organic filament, but have no organic sheath. The spicules of most Hexactinellids are 6-rayed (hexactines) with 3 axes (triaxons), of which 1 axis is perpendicular to the plane of the others. The Early Cambrian is also the time when sponges with calcareous spicules first appear, having usually been assigned to the crown class Calcarea, though it has been shown by recent studies that the picture is more complicated. The spicules of extant calcareans have no internal filament but do have an organic layer as a sheath, most being 3-rayed (triaxons) or 4 axes (tetraxons). In extant sponges sheathed spicules are excreted extracellularly, though spicules with internal filaments are largely secreted intracellularly. Spicules recovered from Cambrian Stage 4 in Newfoundland display an extinct combination of these characters (T. P. H. Harvey, 2010): 6-rayed triaxines that resemble Hexactinellid spicules were originally sheathed, as are calcareans spicules, and are preserved as carbonaceous films, lacking any obvious internal filaments. According to the authors¹ these Stage 4 spicules recall the results of a detailed study of spicules the Stage 5 Burgess Shale sponge Eiffelia, the best known of the Cambrian sponges (Botting & Butterfield, 2005), which has a scleritome containing a unique variety of calcareous spicule types. The largest of the spicules are hexactines, though they lack a perpendicular axis, while in size classes that are successively smaller, there is a progressive increase in the number of tetraxons; these spicules were originally sheathed. Also here is an extinct combination of spicule types. Therefore combinations of characters, such as spicules, are present in some of the earliest sponges, which in extant forms are either restricted to different crown classes or are unique. The authors¹ suggest that either extinct clades of sponges that combined characters are ancestral to extant clades that have lost characters differentially or some of these characters evolved independently in different clades. Whichever is the case, it appears there was an early Cambrian radiation of sponge clades, some of which have become extinct and may not belong to any crown classes.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado