NEW CHANCELLORIIDS FROM THE EARLY CAMBRIAN SEKWI FORMATION WITH A COMMENT ON CHANCELLORIID AFFINITIES

Journal of Paleontology, Sep 2005 by Randell, Robert D, Lieberman, Bruce S, Hasiotis, Stephen T, Pope, Michael C

Exceptionally preserved chancelloriid integument reveals rows of microscopic, imbricated structures (Fig. 4). The material described here preserves regularly spaced oblong structures, 40-50 |xm wide, along the outer surface of the integument, with similar features possibly lining the sclerite walls (Fig. 4). Based on their size, position, and imbrication, they appear homologous with the platelets of Bengtson and Hou (2001) and the cellular structures of Janussen et al. (2002), though their position and morphology differs from the reticulated microstructure of Li (1999).

The spiny, saclike body of a chancelloriid is superficially very similar to that of a sponge (Conway Morris and Chapman, 1997), and comparisons of chancelloriids with sponges have focused on the supposed homology of chancelloriid sclerites and sponge spicules (Sdzuy, 1969) or spongin fibers (Butterfield, 1995; Butterfield and Nicholas, 1996). Both of these poriferan skeletal elements are built by secretory cells that externally surround them (Harrison and de Vos, 1991) within the body of the sponge. Bengtson and Missarzhevsky (1981), Bengtson et al. (1996), and Bengtson and Hou (2001) have argued that chancelloriid sclerites were secreted from the inside and cannot be homologous to sponge spicules, and material illustrated here supports their contention. Figure 4 suggests that the sclerites are external structures, linked to the body by short stalks of integument that only attach to the restricted undersurface of the sclerite. No evidence is seen in our material for the dermal covering of soft tissue postulated by Mehl (1996; Janussen et al., 2002). Thus, poriferan spicules and chancelloriid sclerites do not appear homologous. Moreover, sponge spicules are scaffolds whose primary function is to act as a skeleton, while chancelloriid sclerites are stiffened walls (Runnegar, 1990) that can only perform a defensive function (Bengtson and Hou, 2001). None of our specimens, however, expose the apex of the scleritome, and the presence of a terminal osculum or ostial sieve (characters of sponges) cannot be disproved. The presence of choanocytes or pinacoderm, which effectively define the Porifera (Harrison and de Vos, 1991), also cannot be disproved.

A comparison between chancelloriids and ascidians (Mehl, 1996) was based primarily on a supposed sclerite-spicule homology. Spicules of the ascidian tunic, however, are built outwards from concave basal plates into spinose projections by an external bundle of sclerocytes (Lambert, 1998), so ascidian spicules do not appear homologous to chancelloriid sclerites for the same reason sponge spicules and chancelloriid sclerites are not (see Bengtson and Hou, 1998, 2001). Also problematic to the ascidian model is the absence of siphons in chancelloriids, which are critical to the ascidian mode of life (Brusca and Brusca, 1990).

The concept of chancelloriids as cnidarians has not been investigated previously. Certain cnidarians, especially anthozoans in various families of the octocorals including the Primnoidae, Chrysgorgiidae, and Isididae, produce a dermal layer of imbricated, platelike sclerites (Bayer, 1990; Fabricius and Alderslade, 2001) that may resemble the plated integument of chancelloriids, though sponges also may shape spicules into an external layer of imbricating plates (de Vos et al., 1991; S. Bengtson, 2004, personal commun.). Further, sclerite width varies between 40 µm and 200 µm so the dimensions are quite compatible to those of chancelloriid platelets, and the sclerites of some octocorals (e.g., Echinomuricea Verill, 1869) bear pronounced spines that could be comparable in appearance to bristled chancelloriid platelets. This is not a powerful argument of homology, we merely note the potentially interesting similarity. Also, although many octocorals are colonial, there are extant examples of solitary and semisolitary octocorals discussed in detail by Ausich and Babcock (1998). Still, the oral extremity of most anthozoan polyps terminates with a slitlike mouth, and a similar opening could not be observed in a typical chancelloriid specimen, whose apex is laterally compressed and densely ornamented with sclerites. The rays of many chancelloriid sclerites are oriented towards the apex, as in Archiasterelia fletchergryllus n. sp., which could suggest that there was an apical opening to be protected. Also, the dense thicket of apical sclerites would have served locally to reduce water current velocities over the apex, causing food particles to fall out of suspension into an opening (see Lambert, 1998). Anthozoans typically possess tentacles, but none have been found in chancelloriids, although this problem is not insurmountable, as Conway Morris (1993b) listed several extant anthozoans that lack tentacles. Anthozoans are also characterized by mesenteries and an actinopharynx and none of these features can be proven for chancelloriids. Thus, the concept of chancelloriids as armored cnidarian polyps could be considered an interesting possibility, albeit speculative at this point.