TRACE FOSSILS 531
• Colonization styles (softgrounds may be
colonized from above or below,
hardgrounds generally from above).
Putting this information together with sedi-
mentological observations may allow the ich-
nologist to interpret the original modes of
deposition throughout a core in some detail.
Petroleum geologists are often able to inter-
pret sedimentary facies from the limited infor-
mation available in a narrow core.
Ichnofabric studies may provide critical
evidence for the identifi cation of key strati-
graphic surfaces, the basis of sequence stratig-
raphy (see pp. 34–7). For example, marine
fl ooding surfaces or exposed erosive surfaces
may be indicated by distinctive ichnofabrics
that can be traced laterally across a whole
basin.
Trace fossils may also be important in
determining oil reservoir quality. The key
factors are the porosity (void space between
grains) and permeability (ability to transmit
fl uids) of the sediments. The economic value
of an oil reserve may be reduced or enhanced
by trace fossil activity: porosity and permea-
bility may be reduced by intense burrowing
and mixing of sand and clay grains, and so
the value is lowered; or organisms may burrow
through impermeable clay layers and link
porous sandy layers, so increasing the value
of the oil reserve. These interactions of trace
fossils and sediments are only now coming to
be understood and used extensively in the oil
industry. Who would have thought the price
of a barrel of crude oil, such a key factor in
driving the world economy, might depend on
the activities of some ancient worms!
Review questions
1 What kinds of trace fossils could a human
being leave behind? Think of examples of
all the main categories shown in Fig.
19.10.
2 Do big dinosaurs run faster than small
dinosaurs? Find 10 dinosaur track photo-
graphs on the web, check the scale and
measure the stride lengths. Use the Alex-
ander formula (see Box 19.5) to work out
speeds, and plot these against estimated
body sizes for the dinosaur track makers.
3 How well do the classic marine ichnofa-
cies work (see Fig. 19.12)? Look up exam-
ples of the indicator trace fossils, such as
Nereites and Zoophycos, from lakes and
other settings. How many exceptions
invalidate the general usefulness of a
scheme like Seilacher’s ichnofacies
model?
4 If life has diversifi ed through time, you
might expect trace fossil diversity to
increase too. Find 10 papers about deep-
sea Nereites ichnofacies, trace fossil assem-
blages from the Cambrian, Ordovician,
Silurian, Carboniferous, Jurassic, Creta-
ceous and Tertiary, and count the number
of named trace fossils. Plot these against
time: does deep-sea trace fossil diversity
increase, decrease or stay the same through
the last 500 million years? Why would
your result need further work to be com-
pletely convincing?
5 Find examples of where trace fossil study
has been useful in the oil industry. What
ages of rocks and sedimentary facies
most benefi t from trace fossil studies of
boreholes?
Further reading
Bromley, R.G. 1996. Trace Fossils: Biology, taphonomy
and applications, 2nd edn. Chapman and Hall,
London.
Donovan, S.K. (ed.) 1994. The Palaeobiology of Trace
Fossils. Wiley, Chichester.
Ekdale, A.A., Bromley, R.G. & Pemberton, S.G. 1984.
Ichnology; The use of trace fossils in sedimentology
and stratigraphy. Society of Economic Paleontolo-
gists and Mineralogists, Tulsa, OK.
Lockley, M.G. 1991. Tracking Dinosaurs. Cambridge
University Press, Cambridge.
Maples, C.G. & West, R.R. (eds) 1992. Trace Fossils.
Short Courses in Paleontology, No. 5. Paleontologi-
cal Society, Tulsa, OK.
McIlroy, D. (ed.) 2004. The Application of Ichnology
to Palaeoenvironmental and Stratigraphic Analysis.
Special Publication No. 228. Geological Society,
London.
Miller III, W. 2005. Trace Fossils; Concepts, problems,
prospects. Elsevier, Amsterdam.
Seilacher, A. 2007. Trace Fossil Analysis, Springer, New
York.
References
Alexander, R.M. 1976. Estimates of speeds of dino-
saurs. Nature 261, 129–30.
Crimes, T.P. & Crossley, J.D. 1991. A diverse ichno-
fauna from Silurian fl ysch of the Aberystwyth Grits