Page 95 - Introduction to Paleobiology and The Fossil Record

82 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
logical evidence, however, remains the main
test of these comparisons and models. In this
chapter we focus on the community aspects of
paleoecology (synecology), reviewing the tools
available to reconstruct past ecosystems and
see how their organisms socialized.
Taphonomic constraints:
sifting through the debris
As noted above, most fossil assemblages have
been really messed about before being buried
and preserved in sediment. The decay and
degradation of animal and plant communities
after death results in the loss of soft-bodied
organisms, while decay removes soft tissue
with the disintegration of multiplated and
multishelled skeletal taxa (see Chapter 3). If
that were not enough, transport and compac-
tion add to the overall loss of information
during fossilization. On the other hand, areas
occupied by dead communities may be recolo-
nized and animal and plant debris may be
supplemented by material washed in from
elsewhere. This process of time averaging can
thus artificially enhance the diversity of an
assemblage over hundreds of years. But can
we rely on fossil assemblages to recreate
ancient communities with any confi dence and
accuracy? Paleoecologists know we can, with
varying degrees of precision.
The similarity of a death assemblage to its
living counterpart, its fi delity, can be assessed
in different ways. In a series of detailed studies
of the living and dead faunas of Copana Bay
and the Laguna Madre along the Texas coast,
George Staff and his colleagues (e.g. Staff
et al. 1986) discussed the paleoecological sig- zoans, gastropods, bivalves, cephalopods,
nificance of the taphonomy of a variety of
nearshore communities, sampled over a
number of years. Most animals in living com-
munities are not usually preserved, neverthe-
less the majority of animals with preservation
potential (mainly shelled organisms) are in
fact fossilized. More were actually found in
death assemblages than in living assemblages,
where the effects of time averaging were
clearly significant. Suspension feeders and
infaunal organisms were the most likely to be
preserved (Fig. 4.2). Measurements of biomass
and taxonomic composition rather than those
of numerical abundance and diversity are the
best estimates of the structures of communi-
ties, and counts of the more stable adult pop- methods. A variety of methods involving the
suspension feeders
Laguna Madre
Copana Bay
living
assemblage
potential death
assemblage
death
assemblage
detritivores/herbivores predators/parasites
Figure 4.2 The transition from a living
assemblage to a death assemblage. Relative
proportions of different types of organism
change in two living marine assemblages off the
Texan coast. Living assemblages are dominated
numerically by detritivores and herbivores, death
assemblages by suspension feeders. (Based on
Staff et al. 1986.)
ulations are the most realistic monitors of
community structure.
Another method to estimate taphonomic
loss involves a census of an extraordinarily
preserved Lagerstätte deposit. Whittington
(1980) and his colleagues’ detailed reinvesti-
gation of the mid-Cambrian Burgess Shale
fauna revealed a community dominated by
soft-bodied animals with very few of the more
familiar skeletal components of post-
Cambrian faunas such as brachiopods, bryo-
corals and echinoderms. More importantly,
the deep-water Burgess fauna is quite differ-
ent from more typical Cambrian assemblages
with phosphatic brachiopods, simple echi-
noids and mollusks together with trilobites.
Although the Burgess fauna has many other
peculiarities (see Chapter 10), the high pro-
portion of, for example, annelid and priapulid
worms, adds a different dimension to
the more typical reconstructions of mid-
Cambrian communities (Fig. 4.3).
These important taphonomic constraints
must be addressed and built into any paleo-
ecological analysis and may be partly coun-
tered by a careful selection of sampling