Page 269 - Introduction to Paleobiology and The Fossil Record

256 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Tardigrada
Machaeridia Onychophora Pantopoda
Polychaeta

Xenusia

Chelicerata

Cambrian Trilobita
Vendian
Crustacea
parapodium
antennulae
? compound
eye
biramous
arthropodium
lobopodium

Figure 10.19 Significance of the diverse worm-like animals at the Precambrian–Cambrian boundary
and the postulated origins of some major clades. (Based on Dzik, J. & Krumbiegel, G. 1989. Lethaia
22.)

Burgess Shale and other such exceptionally
SOFT-BODIED INVERTEBRATES
preserved faunas suggests that many of these
Of the 25 or so commonly recognized animal soft-bodied groups dominated certain marine
phyla, fewer than nine (35%) have an ade- paleocommunities in terms of both numbers
quate fossil record. Many are small phyla rep- and biomass and additionally contributed to
resented by relatively few species. However, associated trace fossil assemblages.
there are a number of larger phyla whose The platyhelminths or the fl atworms are

poor fossil record reflects the lack of a pre- bilateral animals with organs composed of
servable skeleton, although a number of these tissues arranged into systems. Most are para-
soft-bodied forms are preserved in fossil sites, but the turbellarians are free-living
Lagerstätten. Most are worms or worm-like carnivores and scavengers. The Ediacaran
organisms (Fig. 10.19). But in spite of unspec- animals Dickinsonia and Palaeoplatoda
tacular fossil records, there is considerable have been assigned to the turbellarian fl at-
interest in these poorly represented inverte- worms by some authors; similarly Platyden-
brates. The origins of many higher taxa must dron from the Middle Cambrian Burgess
be sought within the plexus of worm-like Shale has been ascribed to the
organisms. Moreover, the evidence from the platyhelminthes.