518 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Box 19.4 Dinosaur behavior
Dinosaur tracks are probably the most familiar trace fossils, and they can tell us a great deal about
how the dinosaurs lived. Some dinosaur track sites cover huge areas, and may reveal hundreds or
thousands of footprints, often in long trackways, sometimes representing numerous different species.
It is fascinating to use these trackways to speculate about ancient behaviors – but you have to be
careful! It is important to check whether all the tracks were made at the same time – do they overlap
each other or not? A busy-looking track site might have been produced by just one hyperactive
dinosaur trotting back and forwards around a water hole.
Three-dimensional dinosaur prints are quite rare. Normally, the dinosaur trots across fi rm mud
or sand, and you are left with simple impressions on the top surface. In some cases, though, the
dinosaurs got bogged down in soft sediment, and their feet went in a meter or more. Then, when
they wanted to move on, they had to haul their feet out of the gloop, leaving odd-shaped closure
traces behind.
A remarkable fi nd from the Late Triassic of Greenland (Gatesy et al. 1999) shows this. Stephen
Gatesy, from Brown University, Rhode Island, and his collaborators found strange, narrow, bird-like
prints (Fig. 19.9a). Had they been made by a theropod dinosaur with feet made from wire? When
they pulled apart the rock layers, they could see that the dinosaur foot had gone in, and sunk through
layers of mud, so that the mud fl owed back around its ankles. Then, in moving forward and pulling
the foot out, the mud fl owed back around the exit trace, leaving a long forward trail made by the
long middle toe. Computer animations (Fig. 19.9b) demonstrated how the foot may have moved as
it went into the mud, and then pulled out at the end of the stride.
Read more about dinosaur tracks on web sites linked to http://www.blackwellpublishing.com/
paleobiology/.
have pinned the water depth, tide and storm
conditions, salinity and oxygen levels. And
this works whatever the age of the rocks,
whether Cambrian or Cretaceous. The trace
fossils remained remarkably constant in
appearance, even if their producers might
have been quite different.
This paleoenvironmental scheme of trace
fossils presented by Seilacher (1964, 1967b)
has been modifi ed and enlarged since then
(Frey et al. 1990), but in principle it divides
trace fossil assemblages into a number of
ichnofacies (Fig. 19.12). The ichnofacies are
named after a characteristic trace fossil, and
they indicate particular sedimentary facies
(Box 19.6). The ichnofacies is identifi ed on
the basis of an assemblage of trace fossils, and
it may be recognized even if the name-bearing
form is absent.
The classic marine ichnofacies, those
named for Nereites, Zoophycos, Cruziana
and Skolithos, are not simply depth-related,
as Seilacher fi rst proposed, but are associated
with particular sedimentary regimes, combin-
ing aspects of water energy, bottom sediment
type, temperature, chemistry and food supply.
These four ichnofacies include assemblages of
trace fossils typical of fair-weather, normal
conditions of deposition, and those character-
istic of exceptional storm and turbidite event
beds. The complexity of controls on the
marine ichnofacies is shown in many fi eld-
based studies where alternations between ich-
nofacies may be found at a single location
(Box 19.7).
The Scoyenia ichnofacies is one of several
continental trace fossil facies, and depends on
the presence of shallow freshwater, while the
Psilonichnus ichnofacies is controlled by
coastal marine infl uence on a terrestrial
setting. Not included here are some additional
terrestrial ichnofacies (see McIlroy 2004).
Since 1990, several ichnologists have pro-
posed ichnofacies in ancient soils, paleosols,
to characterize different kinds of insect
burrows, nesting chambers and the like, and