Page 164 - Introduction to Paleobiology and The Fossil Record
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FOSSIL FORM AND FUNCTION 151
the bone, and particular knobs and ridges a great deal about unknown anatomy in a
(processes) that show where the muscles fossil: if crocodiles and birds share particular
attached, and how big they were. Muscle muscles, then dinosaurs had them too. The
size is an indicator of strength, and this kind same goes for all other normally unpreserv-
of observation can show how an animal able organs. So the EPB has considerable
moved. potential to fill in missing anatomy.
But phylogenetic analogs may not be much
use in determining function. Probably a close
Comparison with modern analogs
study of crocodiles and birds will not solve
After the basic anatomy of the fossil organism many problems in dinosaur functional mor-
is understood, the logical next step is to iden- phology. Dinosaurs were so different in size
tify a modern analog. This can be easy if the and shape that a better modern functional
fossil belongs to a modern group, perhaps an analog might be an elephant. Elephants are
Eocene crab or a Cretaceous lily plant. The not closely related to dinosaurs, but they are
paleontologist then just has to look for the large, and their limb shapes show many ana-
most similar living form, and make adjust- tomic parallels. Watching a modern elephant
ments for size and other variations before marching ponderously probably gives the best
determining what the ancient organism live demonstration of how a four-limbed
could do. dinosaur moved.
But what about ancient organisms that do The point of using modern analogs is a
not have obvious close living relatives? In more general one though. Biologists have
trying to understand the functional morphol- learned a great deal about the general princi-
ogy of a dinosaur, for example, should the ples of biomechanics, the physics of how
paleontologist compare the fossil with a croc- organisms move, from observations across
odile or a bird? In former days, paleontolo- the spectrum. So, the scaling principle men-
gists might have begun detailed comparisons tioned earlier (see p. 142), exemplifi ed by the
with a crocodile, but that is not always helpful spindly legs of the antelope and the pillar-like
because crocodiles are different in many legs of the elephant, is a commonsense obser-
aspects of their form and function from dino- vation that clearly applies to extinct forms.
saurs. What about birds? After all, we now And there are many more such commonsense
know that birds are more closely related to observations: among vertebrates carnivores
dinosaurs than are crocodiles (see p. 460). have sharp teeth and herbivores have blunter
Again there are problems because birds are teeth; tall trees require broad bases and deep
much smaller than dinosaurs and they have roots so they do not fall over; vulnerable small
become so adapted to flying that it is hard to creatures survive best if they are camoufl aged;
find common ground. as animals run faster their stride length
There are two issues here: phylogeny and increases (see p. 520); fast-swimming animals
functional analogs. In phylogenetic terms, it tend to be torpedo-shaped; and so on. These
is wrong to compare dinosaurs exclusively observations are not “laws” in the sense of
with crocodiles or with birds. They should be the laws of physics, but they are common-
compared with both. This is because birds sense observations that clearly apply widely
and crocodiles each have their own indepen- across plants and animals, living and extinct.
dent evolutionary histories and there is no Comparison with modern analogs to learn
guarantee that any of their characters were these general rules is the most important tool
also present in dinosaurs. However, if both in the armory of the functional morphologist
birds and crocodiles share a feature, then (Box 6.4).
dinosaurs almost certainly had it too. This is
the concept of the extant phylogenetic bracket
(EPB) (Witmer 1997): even if a fossil form is Biomechanical modeling
distant from living forms, it will be bracketed Increasingly, paleobiologists are turning to
in the phylogenetic tree by some living organ- biomechanical modeling to make interpreta-
isms. That at least provides a starting point tions of movements, especially in feeding and
in identifying some unknown characters, locomotion. Such studies use basic principles
especially of soft tissues. The EPB can reveal of biomechanics and engineering to interpret
