Page 186 - Sedimentology and Stratigraphy

Trace Fossils 173

the depositional environment. In the younger Ceno- of benthic to planktonic organisms present: if the
zoic strata the fossils may be of organisms so similar to proportion of benthic organisms is high the water
those alive today that determining the likely environ- was probably shallow, whereas a high count of plank-
ment in which they lived is quite straightforward. tonic organisms indicates deeper water. This method
Farther back in geological time this task becomes normally only provides a very rough guide to relative
more difficult. Groups of organisms such as trilobites water depth but is applied in a semi-quantitative
and graptolites, which were abundant in the Lower way in Cenozoic and Mesozoic strata by considering
Palaeozoic seas, have no modern representatives for the proportions of benthic and planktonic forms of
direct comparison of lifestyle. Clues as to the ecologi- foraminifers.
cal niche occupied by a fossil organism are provided
by considering the functional morphology of the
body fossil. All organisms are in some way adapted 11.7 TRACE FOSSILS
to their environment so if these adaptations can be
recognised the lifestyle of the organisms can be deter- Although body fossils provide physical evidence of
mined to some extent. In trilobites, for example, it has an organism having lived in the past, trace fossils
been recognised that some types had well-developed are evidence of the activity of an organism. Traces
eyes whereas in others they were very poorly devel- include tracks of walking animals, trails of worms,
oped: one interpretation of this would be that the burrows of molluscs and crustaceans, and are collec-
trilobites with eyes needed them to help move around tively called ichnofauna. Trace fossils are usually
on the sea floor but those that lived buried in the found on or within sediment that was unconsolidated
sediment had no need of sight. but with sufficient strength to retain the shape of the
Some organisms are thought to have occupied animal’s trace. Contrasts in sediment type between a
very specific niches and can provide quite precise burrow and the host sediment are a considerable aid
information about the environment of deposition. to recognition. A distinction is made between bur-
Some algae and hermatypic corals require clear rows formed in soft sediment and borings made by
water and sunlight to thrive, so they are indicators organisms into hard substrate.
of shallow, mud-free shelf environments. Other The different forms of trace fossils are given names
organisms (certain bivalves, for instance) are more similar to those used in the classification of animals
tolerant of different environments and can live in a and body fossils: so, for example, smaller vertical
range of conditions and water depths provided that a tubes in sands are called Skolithos and a crawling
supply of nutrients are available. In general, the trail produced by a multilimbed organism is known as
abundance of benthic organisms decreases as the Cruziana. Comparison of the form of Cruziana traces
water depth increases. Shoreface environments with body fossils provides very strong evidence that
usually have the most diverse assemblages of benthic trilobites formed these features, but this link between
fauna and flora due to the well-oxygenated conditions ichnofauna and body fossils is the exception rather
of the wave-agitated water and the availability of light than the rule. For the majority of trace fossils, we can
(provided that it is not too muddy). The abundance of only guess at the nature of the animal that formed
organisms living on the sea floor decreases in the them: other exceptions are Ophiomorpha, a pellet-
offshore transition and offshore parts of the shelf. lined burrow which has a morphology identical to
In the deep oceans only a few specialised organisms burrows made by modern callianassid shrimps, and
live on the sea floor adjacent to areas of hydrothermal Trypanites, a boring made in rock or solid substrate
activity. that can be seen in modern seas as being made by
The abundance of planktonic organisms is con- bivalve molluscs such as Lithophaga.
trolled by the supply of nutrients and the surface Ichnofossils are classified according to the inferred
temperature of the water. The hard parts of plank- manner in which they were formed, for example, by
tonic organisms may be distributed in sediments of movement of an animal over a surface, feeding, crea-
any water depth, although dissolution of calcium car- tion of a shelter, and so on (Fig. 11.11) (Simpson
bonate occurs in very deep water (16.5.2). One 1975; Ekdale et al. 1984). However, there is consider-
approach to the problem of determining the depth at able variation within these categories as dinosaur
which sediment was deposited is to consider the ratio footprints and trilobite tracks classify as the same