Page 406 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 393

Box 15.2 Origin of the echinoderms and the status of the helicoplacoids

During the major Early Cambrian radiation of echinoderms, many rather bizarre forms appeared
suddenly with very different morphologies. At least nine genera were present, of which about half
had pentameral symmetry, but the others were not pentameral at all. One such non-pentameral
group, the helicoplacoids (Fig. 15.2), is unique in having only three ambulacral areas with tube feet
wrapped around their spindle-shaped bodies. Moreover, the group lacked appendages and individu-
als probably lived with their shorter ends anchored to the sediment. However, helicoplacoids have
many plates with the distinctive stereom structure, ambulacra and a mouth sited laterally together
with an apical anus. The helicoplacoids have thus been interpreted as primitive echinoderms, surviv-
ing by suspension feeding in the sessile benthos. Helicoplacus may be very close to the stem group
of all subsequent Echinodermata, and something like this animal might have given rise to the pel-
matozoan and eleutherozoan body plans. Other groups of echinoderms were already diverse and
widespread during the Early Cambrian, but the helicoplacoids were apparently restricted to western
North America, where they were very abundant during only the Early Cambrian. Their extinction
may have been a very important ecological signal. Such groups of unattached “sediment stickers”
were well adapted to the algal mat substrates of the Neoproterozoic. Perhaps they could not cope
with the increased bioerosion and bioturbation of soft substrates that were part of the move away

from seafloors covered by microbial mats that prompted the Cambrian substrate revolution (Bottjer
et al. 2000).

Figure 15.2 Helicoplacus from the Lower Cambrian (×10). (Based on Treatise on Invertebrate
Paleontology, Part S. Geol. Soc. Am. and Univ. Kansas Press.)