510 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
All the classic detective stories hinge on a
footprint on the fl owerbed, a used cigarette
end, a crumpled scrap of paper. These are
traces of what happened, and the skilled
detective has an uncanny ability to read clues
from them. Sherlock Holmes astounded his
colleague Mr. Watson by being able to esti-
mate the height of a felon from his footprint;
but is that really so diffi cult?
Trace fossils are the preserved remains of
the activity and behavioral patterns of organ-
isms. Common examples are burrows of
bivalves and worms that live in estuaries and
shallow seas, complex feeding traces of deep-
sea animals on the ocean fl oor, and the foot-
prints of dinosaurs and other land animals
preserved in mud and sand beside rivers and
lakes. At fi rst sight, these remains might seem
rather obscure, but they can tell some remark-
able stories (Box 19.1).
Every trace fossil offers us a vignette of
ancient life, both the life of the organism that
made the marking, as well as the environment
in which it lived. Trace fossils give evidence
about:
• the behavior of organisms – and so are
part of the organisms’ paleobiology;
• sedimentary environments – and so are
like sedimentary structures.
For example, a trackway of dinosaur foot-
prints may tell us about the shape of the soft
parts of the feet of the dinosaur that made
them, the pattern of scales on the skin, the
running speed and the environment in which
the animal lived. The dinosaur tracks can
equally be used to show that the sediments
were deposited on land or in shallow water,
and that the climate was probably warm
(appropriate for dinosaurs).
Trace fossils are common in many sedimen-
tary rocks, and they have been observed by
geologists for centuries. Indeed, many trace
fossils were given zoological and botanical
names from early in the 19th century, since
they were thought to be fossilized seaweeds
or worms. The only trace fossils that were
correctly interpreted from the start were dino-
saur footprints, although many of these were
interpreted at fi rst as the products of fl ocks of
huge birds.
The modern era of trace fossil studies began
in the 1950s with the work of the German
paleontologist Adolf Seilacher. He established
a classifi cation of trace fossils based on
behavior, and discovered that certain assem-
blages of trace fossils indicate particular water
depths in the sea. In addition, trace fossils
have been used widely by exploration geolo-
gists since the 1960s and 1970s when the
study of depositional environments revolu-
tionized understanding of the sedimentary
rock record. These contributions gave a strong
scientifi c basis to the study of trace fossils,
often called ichnology (from the Greek ichnos,
a trace).
UNDERSTANDING TRACE FOSSILS
Types of trace fossils
There are many kinds of trace fossils, and
many of the words used to describe them
(tracks, trails, burrows, borings) are in
common use. There are also a variety of
cryptic fossils and sedimentary structures that
might be regarded as trace fossils, but perhaps
should not. The main trace fossil types are
given in Table 19.1.
Some ichnologists might also include other
examples of biological interaction with sedi-
ments as trace fossils, such as stromatolites
(see p. 191), some kinds of mud mounds,
dinosaur nests, heavily bioturbated or
reworked sediments, and the like. Not
included are eggs, which are body fossils, or
physical sedimentary structures such as tool
marks produced by bouncing and rolling
objects, including shells and pieces of wood.
Naming trace fossils: shapes not
biological species
Trace fossils are given formal names, often
based on Latin and Greek, just like living and
fossil plants and animals (see p. 118). However,
there are some fundamental differences
between the nomenclature of trace fossils and
that of body fossils and modern organisms.
Trace fossil genera are called ichnogenera
(singular, ichnogenus), and trace fossil species
are called ichnospecies.
The key to understanding the naming of
trace fossils produced by invertebrates is to
realize that the names usually say nothing
about the organism that made the trace.
In the early days of ichnology, the common