Page 263 - Introduction to Paleobiology and The Fossil Record

250 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 10.5 Coelosclerites, mineralization and early animal evolution

The coeloscleritophorans are an odd group of animals based on the unique structure of their sclerites

that appeared first in the Tommotian (Fig. 10.13). The sclerites are made of thin mineralized walls
surrounding a cavity with a small basal opening. Once formed, the sclerites did not grow and were
secreted by the mineralization of organic material occupying the cavity. The sclerites have longitu-

dinal fibers and overlapping platelets within the mineralized wall. These animals may be extremely
important in understanding the origin of biomineralization and the fuse for the Cambrian explosion,
as argued by Stefan Bengtson (2005). Coelosclerites may be structures that are not known in any
living animal but that were shared by both the bilaterians and non-bilaterians and probably charac-
terized both ecdysozoans and spiralians. Coelosclerites may then have been lost, possibly by pro-
genesis (see p. 145) from the larval to juvenile stages. If these features were developed in larger
bilaterians then it is possible that within the Ediacara fauna giant forms – tens of centimeters in
length – lurked, adorned by spiny and scaly sclerites. This is a controversial but nonetheless stimu-
lating view that adds even more variety to our interpretations of early metazoan evolution.

100 μm

8
1

7 6
2

3

4 5

Figure 10.13 Coelosclerites. Chancelloriids: 1 and 2, Chancelloria; 3, Archiasterella; 4,
Eremactis. Sachitid: 5, Hippopharangites. Siphonoguchitids: 6, Drepanochites; 7, Siphogonuchites;
8, Maikhanella. Scale bars, 100 μm. (Courtesy of Stefan Bengtson.)