Page 165 - Introduction to Paleobiology and The Fossil Record

152 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

modern and ancient biological structures (Fig. principles of movement, clarify the nature
6.11). A simple example is to consider the of the joint, and the positioning and relative
vertebrate jaw as a lever, with the jaw joint as forces of the muscles. Such real-life
the fulcrum (Fig. 6.11c). Simple mechanics models may also form the basis for educa-
shows that the bite will be strongest nearest tional demonstrations and museum recon-
to the fulcrum, and weakest towards the far structions. More commonly now, however,
end: that is why we bite food off at the front paleobiologists do their modeling on the
of our jaws but chew at the back. Subtle computer.
changes to the positions of the jaw muscles Some computer modeling has been very
and the relative position of the jaw margin effective in studying the mechanical strength
with respect to the fulcrum can then improve of ancient structures. In particular, paleobiol-

the efficiency of the bite. The vertebrate limb ogists have begun looking at the skulls of
can be modeled as a series of cranks, each ancient vertebrates to assess how the structure
with a characteristic range of movement at was shaped by the normal stresses and strains
the joints. This kind of model allows the of feeding and head-butting. A useful model-

analyst to work out the maximum forwards ing approach is finite element analysis (FEA),
and backwards bend of the limb and the rela- a well-established method used by engineers
tive scaling of muscles, for example. to assess the strength of bridges and buildings
Biomechanical models may be real, three- before they are built, and now applied to
dimensional models made out of steel rods, dinosaur skulls (Box 6.5), among other fossil
bolts and rubber bands. Such models can problems. FEA is one of many methods of
provide powerful confirmation of the basic modeling how forces act on biological struc-

Box 6.4 The Triassic tow-net

For a century or more, fossil hunters had been aware of some astonishing fossils from the Jurassic
of Germany that showed long, slender crinoids (see p. 395) attached to driftwood. In life, these cri-
noids must have dangled beneath the driftwood, and their mode of life was a mystery. Driftwood
crinoids have now been identified in many parts of the world, from the Devonian onwards.

Crinoids today can live attached to the seabed, as most of their fossil ancestors did, fi ltering food
particles from the bottom waters. Most living crinoids are free-swimmers, but they do not seem to
attach to driftwood. So why did the fossil forms do it, and how did they live?
New discoveries from China (Hagdorn et al. 2007) give some clues. Numerous pieces of driftwood

have been identified in the Late Triassic Xiaowa Formation of Guizhou, southwest China, each car-
rying 10 or more beautiful specimens of the crinoid Traumatocrinus (Fig. 6.10a). The juveniles were
presumably free-swimming microscopic plankton, as with other echinoderms, and they settled on
driftwood logs. Many juveniles have been found on the logs. The crinoids then matured and became
very long. Their feeding arms were longer than in seabed crinoids, perhaps to capture more food.

This floating mode of life has been termed pseudoplanktonic, meaning that the crinoids are living
like “fake plankton”. They probably fared better up in the oxygenated surface waters than in the
black anoxic seabed ooze.
The functional interpretation of a Traumatocrinus colony (Fig. 6.10b) is that it worked like a

tow-net (Fig. 6.10c), a standard kind of fishing net towed in the open sea. As the boat moves forward,
the tow-net hangs passively behind and billows outward. Any fishes encountered are caught. The

Traumatocrinus colony similarly spread its feeding arms passively as the log moved forward in the
gentle Triassic sea currents. Any food particle encountered by the crinoid net would be captured and
eaten. Paleontologists have to use their imaginations and intellects in fi nding plausible functional
models for some ancient organisms!