Page 96 - Introduction to Paleobiology and The Fossil Record

PALEOECOLOGY AND PALEOCLIMATES 83
0 10 20 30 40 50%
Arthropoda 44
Porifera 18
Lophophorata 8
Priapulida 7
Annelida, Polychaeta 6
Chordata, Hemichordata 5
Echinodermata 5
Coelenterata 4
Mollusca 3
Miscellaneous 19
Figure 4.3 Census of organisms preserved in the
Middle Cambrian Burgess Shale. Many groups,
such as the priapulid and annelid worms,
together with the diverse arthropod biota, are
rarely represented in more typical mid-Cambrian
faunas, dominated by phosphatic brachiopods
and trilobites. (From Whiltington 1980.)
study of size–frequency histograms (see
below), the degree of breakage, disarticula-
tion and fragmentation of individuals, together
with the attitude of fossils in sediments,
generate useful criteria to separate auto-
chthonous (in place) from allochthonous
(transported) assemblages (see Chapter 4). A
number of terms have been developed to
describe the fate of a once-living assemblage
on its journey to fossilization. The living
assemblage, or biocoenosis, is transformed
into a thanetocoenosis after death and decay.
The taphocoenosis is the end product that is
finally preserved. In addition life assemblages
still retain the original orientations of their
inhabitants, neighborhood assemblages are
still close to their original habitats, whereas
transported assemblages include broken and
abraded bones and shells that have traveled.
Populations: can groups of
individuals make a difference?
Populations are the building blocks of com-
munities, and can themselves spark dramatic
changes in community and ecosystem struc-
tures. A population is a naturally occurring
assemblage of plants and animals that live in
the same place at the same time and regularly
interbreed. Within an ecosystem – all the pop-
ulations of species living in association – there
may be keystone species, species that help
shape the ecosystem and that can trigger
large-scale changes if they disappear. A classic
keystone species is the elephant: it forms the
landscape in large parts of Africa by knocking
down trees and feeding on certain plants, and
the whole scene looks different when it disap-
pears. Incumbent species can occupy the same
ecological niche for many millions of years,
adding stability to many ecosystems. For
example, although the dinosaurs and the
mammals appeared at roughly the same time,
it was the dinosaurs that dominated the land
throughout the Mesozoic; mammals had
limited niches (insectivores, seed eaters and
small omnivores) until after the extinction of
the incumbent dinosaurs, when they were
able to radiate into vacant ecospaces.
The dynamics and structures of individual
populations can provide us with useful clues
about how the once-living community func-
tioned and whether the assemblage is actually
in place or has been transported. A measure-
ment, such as the length of a brachiopod shell,
is chosen as a proxy for the size (and some-
times for the age) of shells. These data, entered
into a frequency table, based on discrete class
intervals, are plotted as size–frequency histo-
grams, polygons or even cumulative frequency
polygons (Fig. 4.4). Right, positively-skewed
curves generally indicate high infant mortality
and these are typical of most invertebrate
populations. A normal (Gaussian) curve can
indicate a steady-state population or trans-
ported assemblages whereas a left, negatively-
skewed curve indicates high senile mortality.
Mortality patterns are, however, best dis-
played as survivorship curves, where the
number of survivors at each defi ned growth
intervals is plotted (Fig. 4.5). Size–frequency
and survivorship curves store a great deal of
information regarding the lifestyle, habitat
and life history of an individual organism
(Box 4.1). For example, species that mature
early and produce small but numerous off-
spring, many dying before maturity, have been
labeled “r strategists”. “K strategists”, on the
other hand, are long-lived species, with low
reproduction rates. These two strategies are
end members of a spectrum of possibilities
described by the following model:
dN dt = rN K − N K]
)
/
(
/
[