Page 283 - Introduction to Paleobiology and The Fossil Record

270 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a) (b)
Figure 11.10 Some archaeocyaths from the Lower Cambrian of Western Mongolia, in thin section:
(a) cryptic, solitary individual of Cambrocyathellus showing holdfast structures (×7.5), and (b)
branching Cambrocyathellus tuberculatus with skeletal thickening between individuals associated with
transverse sections of Rotundocyathus lavigatus (×5). (Courtesy of Rachel Wood.)

skeleton is usually buried in the sediment with material. These groupings have now been

a basal flange and roots or holdfasts adding shown to have little taxonomic value,
anchorage and stability. In some taxa, the refl ecting rather ecological preferences
intervallum is partitioned horizontally fi rstly (Debrenne 2007). Most archaeocyaths are
by porous shelves or tabulae or secondly with “Regulares”, including the orders Ajacicy-
aporous, convex dissepiments, often extend- athida and Coscinocyathida; however the
ing into the adjacent central cavity. apparent abundance of regular genera may be
Two main subdivisions have been defi ned due to excessive taxonomic splitting. There
within the group: the “Regulares” and the are fewer “Irregulares” but this ecogroup
“Irregulares”. The regular forms have an includes the orders Archaeocyathida and
initial aporous, single-walled stage lacking Kazachsetanicyathida.
dissepiments; soft tissue fi lled the entire body.
The inner and outer walls are punctuated by Synecology: archaeocyath reefs
septa and tabulae developed either singly or
together. The irregular forms have initial The archaeocyaths were exclusively marine,
aporous, single-walled stages with dissepi- probably living at depths of 20–30 m on car-
ments. The twin walls have irregular pore bonate substrates. The phylum developed an
structures, always dissepiments, and the skel- innovative style of growth based on modular
eton is asymmetric; soft tissue was restricted organization (Fig. 11.10). Such modularity
by the development of secondary skeletal permitted encrusting abilities and the possi-