Page 261 - Introduction to Paleobiology and The Fossil Record
P. 261
248 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Hertzina Lapworthella Pelagiella Aldanella Fomitchella
Camenella Tommotia Latouchella Anabarella
Figure 10.12 Elements of the Tommotian-type or small shelly fauna. Magnifi cation approximately ×20
for all, except Fomitchella which is about ×40. (Based on various sources.)
interest in the definition of the base of the brian. The small shelly fauna probably domi-
Cambrian System. Nevertheless, the biologi- nated the earliest Cambrian ecosystems when
cal affinities of many members of the Tom- many metazoan phyla developed their own
motian fauna have yet to be established. The distinctive characteristics, initially at a very
assemblage, although dominated by minute small scale. Nevertheless, some of this small
species, together with small sclerites of larger size may be a preservational artifact, since
species, represents the fi rst major appearance phosphatization only works at a millimeter
of hard skeletal material in the fossil record, scale.
some 10 myr before the fi rst trilobites evolved
(see p. 363). Composition and morphology
This type of fauna is not restricted to the
Tommotian Stage; small shelly fossils are also Many of the Tommotian skeletons (Fig. 10.12)
common in the overlying Adtabanian Stage were retrieved from residues after the acid
(see below) and similar assemblages of mainly etching of limestones; thus there is a bias
phosphatic minute shells have been reported towards acid-resistant skeletal material in any
from younger condensed sequences in the census of the group as a whole. Moreover,
Paleozoic. The shell substance of the carbon- there is currently discussion concerning
ate skeletons within the fauna seems to have whether the acid-resistant skeletons of the
been controlled by the ambient seawater Tommotian-type animals were primary con-
chemistry; Nemakit-Daldynian assemblages structions or secondary replacement fabrics.
were mainly aragonite, whereas younger Or perhaps these shells survived in the sedi-
shells were mainly calcitic (Porter 2007). ments because of particular chemical condi-
Tommotian-type faunas probably fi nally dis- tions in the oceans at the time that allowed
appeared with the escalation of predation phosphatic fossils to survive (Porter 2004).
during the Mesozoic. The Tommotian animals had skeletons com-
Some scientists such as Stephen Jay Gould posed of a variety of materials. For example,
suggested the less time-specifi c term, small Cloudina and the anabaritids were tube-build-
shelly fossils to describe these assemblages. ers that secreted carbonate material, whereas
The fauna is now known to include a variety Mobergella and Lapworthella consisted of
of groups united by their minute size and sclerites comprising organisms that secreted
sudden appearance near the base of Cam- phosphatic material; Sabellidites is an organic-
