Page 168 - Introduction to Paleobiology and The Fossil Record

FOSSIL FORM AND FUNCTION 155

the knee at mid-stance and this would require hypotheses. Clues about the lifestyle of an
the muscles at the back of the leg to act in ancient plant or animal may come from the
order to balance the force in front. Living enclosing rocks, associated fossil remains,
animals do not do this, so there is no reason associated trace fossils and particular features
to assume that extinct ones did. Crouched of the body fossils themselves. These can be
poses are ruled out too because the knee grouped as circumstantial evidence.
moment arm would have been too long and
the knee muscle moment too high: T. rex 1 Fossils are generally preserved in sedimen-
would have had to have muscles relatively tary rocks, and these record all kinds of
much larger than those of a chicken to cope. features about the conditions of deposi-
So the real T. rex probably stood and moved tion. Fossil plants may be found at certain
somewhere between these columnar and levels in a cyclical succession that tells a
crouched extremes (Fig. 6.13g), which still story of the repeated buildup of an ancient

leaves a large area of possibilities that cannot delta as it fingers into the sea, the develop-
be excluded. ment of soils and forests on top, and its

eventual flooding by a particularly high
sea level. Marine invertebrates may be
Circumstantial evidence
found in rocks that indicate deposition in
Paleontologists are inquisitive by nature and a shallow lagoon, offshore from a reef, on
they gather evidence of all kinds to test their the deep abyssal plain or many other

Box 6.5 Finite element analysis of the skull of Tyrannosaurus rex

Emily Rayfield of the University of Bristol (England) had a dream PhD project, to work out how

the skulls of the theropod dinosaurs worked, using finite element analysis (FEA). In FEA, the struc-
ture is modeled in the computer and its strength characteristics entered. Then the whole three-
dimensional shape, however complex, is converted into a network of small triangular or cuboid cells,
or elements. When forces are applied (a side wind on a skyscraper, a bite force on a skull or jaw
bone) the elements respond and the effect can be seen. In Rayfield’s FEA model of a dinosaur skull,

as the bite force increases, the zone of element distortion increases and it becomes clear why the
skull is shaped the way it is.
In one of her studies, Rayfi eld (2004) attacked the skull of T. rex (Fig. 6.12a). She tried to resolve
a paradox that had been noted before: while T. rex is assumed to have been capable of producing

extremely powerful bite forces, the skull bones are quite loosely articulated. Rayfield applied FEA
to assess whether the T. rex skull is optimized for the resistance of large biting forces, and how the
mobile joints between the skull bones functioned. She studied all the available skulls and constructed
a mesh of triangular elements (Fig. 6.12b). Bite forces of 31,000 to 78,060 newtons were applied
to individual teeth, and the distortion of the element mesh observed (Fig. 6.12c). The bite forces had
been taken from calculations by other paleobiologists, and from observations of tooth puncture
marks (a piece of bone bitten by T. rex showed the tooth had penetrated the bone to a depth of
11.5 mm, equivalent to a force of 13,400 newtons or about 1.5 tons).

Rayfield’s results show that the skull is equally adapted to resist biting or tearing forces and therefore
the classic “puncture–pull” feeding hypothesis, in which T. rex bites into fl esh and tears back, is well
supported. Major stresses of biting acted through the pillar-like parts of the skull and the nasal bones
on top of the snout, and the loose connections between the bones in the cheek region allowed small
movements during the bite, acting as “shock absorbers” to protect other skull structures.

Read about dinosaur feeding behavior in Barrett and Rayfield (2006) and about fi nite element
analysis in Rayfield (2007) and at http://www.blackwellpublishing.com/paleobiology/.

Continued