Interpreting The Function Of Fossils

Functional morphology_

Inferring the function of ancient organisms is hard, and yet it is the main reason many people are interested in paleobiology. Just how fast could a trilobite crawl? Why did some brachiopods and bivalves mimic corals? How did that huge seed fern support itself in a storm? How well could pterosaurs fly? Why did sabertoothed cats have such massive fangs? The most fascinating questions concern those fossil organisms that are most different from living plants and animals. This is because it is easy to work out that a fossil bat probably flew and behaved like a modern bat. But what about a pterosaur: so different, and yet similar in certain ways?

There are three approaches to interpreting the function of fossils: comparison with modern analogs, biomechanical modeling and circumstantial evidence. Let us look at some general assumptions first, and then each of those approaches in turn.

The main assumption behind functional morphology is that biological structures are adapted in some way and that they have evolved to be reasonably efficient at doing something. So, an elephant's trunk has evolved to act as a grasping and sucking organ to allow the huge animal to reach the ground, and to gather food and drink. The flower of an angiosperm is colorful to attract pollinating insects, and the nectar is located deep in the flower so the insect has to pick up pollen as it enters. The siphons of a burrowing mollusk are the right length so it can circulate water and nutrients when it is buried at its favored depth.

Fossils can provide a great deal of fundamental evidence of value in interpreting function. For example, the hard skeleton of a fossil arthropod reveals the number and shape of the limbs, the nature of each joint in each limb, perhaps also the mouthparts and other structures relating to locomotion and feeding (see p. 362). Even a fossil bivalve shell gives some functional information in the hinge mechanism, the pallial line (which marks the extent of the fleshy mantle) and the muscle scars (see p. 334). Exceptionally preserved fossils may reveal additional structures such as the outline of the tentacles of a belemnite or ammonite (see p. 344), muscle tissue (see p. 64) or sensory organs. The first step in interpreting function then is to consider the morphology, or anatomy, of the fossil.

The vertebrate skeleton can provide a great deal of information about function. The maximum amount of rotation and hinging at each joint can be assessed because this depends on the shapes of the ends of the limb bones. There may be muscle scars on the surface of the bone, and particular knobs and ridges (processes) that show where the muscles attached, and how big they were. Muscle size is an indicator of strength, and this kind of observation can show how an animal moved.

Comparison with modern analogs_

After the basic anatomy of the fossil organism is understood, the logical next step is to identify a modern analog. This can be easy if the fossil belongs to a modern group, perhaps an Eocene crab or a Cretaceous lily plant. The paleontologist then just has to look for the most similar living form, and make adjustments for size and other variations before determining what the ancient organism could do.

But what about ancient organisms that do not have obvious close living relatives? In trying to understand the functional morphology of a dinosaur, for example, should the paleontologist compare the fossil with a crocodile or a bird? In former days, paleontologists might have begun detailed comparisons with a crocodile, but that is not always helpful because crocodiles are different in many aspects of their form and function from dinosaurs. What about birds? After all, we now know that birds are more closely related to dinosaurs than are crocodiles (see p. 460). Again there are problems because birds are much smaller than dinosaurs and they have become so adapted to flying that it is hard to find common ground.

There are two issues here: phylogeny and functional analogs. In phylogenetic terms, it is wrong to compare dinosaurs exclusively with crocodiles or with birds. They should be compared with both. This is because birds and crocodiles each have their own independent evolutionary histories and there is no guarantee that any of their characters were also present in dinosaurs. However, if both birds and crocodiles share a feature, then dinosaurs almost certainly had it too. This is the concept of the extant phylogenetic bracket (EPB) (Witmer 1997): even if a fossil form is distant from living forms, it will be bracketed in the phylogenetic tree by some living organisms. That at least provides a starting point in identifying some unknown characters, especially of soft tissues. The EPB can reveal a great deal about unknown anatomy in a fossil: if crocodiles and birds share particular muscles, then dinosaurs had them too. The same goes for all other normally unpreserv-able organs. So the EPB has considerable potential to fill in missing anatomy.

But phylogenetic analogs may not be much use in determining function. Probably a close study of crocodiles and birds will not solve many problems in dinosaur functional morphology. Dinosaurs were so different in size and shape that a better modern functional analog might be an elephant. Elephants are not closely related to dinosaurs, but they are large, and their limb shapes show many anatomic parallels. Watching a modern elephant marching ponderously probably gives the best live demonstration of how a four-limbed dinosaur moved.

The point of using modern analogs is a more general one though. Biologists have learned a great deal about the general principles of biomechanics, the physics of how organisms move, from observations across the spectrum. So, the scaling principle mentioned earlier (see p. 142), exemplified by the spindly legs of the antelope and the pillar-like legs of the elephant, is a commonsense observation that clearly applies to extinct forms. And there are many more such commonsense observations: among vertebrates carnivores have sharp teeth and herbivores have blunter teeth; tall trees require broad bases and deep roots so they do not fall over; vulnerable small creatures survive best if they are camouflaged; as animals run faster their stride length increases (see p. 520); fast-swimming animals tend to be torpedo-shaped; and so on. These observations are not "laws" in the sense of the laws of physics, but they are common-sense observations that clearly apply widely across plants and animals, living and extinct. Comparison with modern analogs to learn these general rules is the most important tool in the armory of the functional morphologist (Box 6.4).

Biomechanical modeling_

Increasingly, paleobiologists are turning to biomechanical modeling to make interpretations of movements, especially in feeding and locomotion. Such studies use basic principles of biomechanics and engineering to interpret modern and ancient biological structures (Fig. 6.11). A simple example is to consider the vertebrate jaw as a lever, with the jaw joint as the fulcrum (Fig. 6.11c). Simple mechanics shows that the bite will be strongest nearest to the fulcrum, and weakest towards the far end: that is why we bite food off at the front of our jaws but chew at the back. Subtle changes to the positions of the jaw muscles and the relative position of the jaw margin with respect to the fulcrum can then improve the efficiency of the bite. The vertebrate limb can be modeled as a series of cranks, each with a characteristic range of movement at the joints. This kind of model allows the analyst to work out the maximum forwards and backwards bend of the limb and the relative scaling of muscles, for example.

Biomechanical models may be real, three-dimensional models made out of steel rods, bolts and rubber bands. Such models can provide powerful confirmation of the basic principles of movement, clarify the nature of the joint, and the positioning and relative forces of the muscles. Such real-life models may also form the basis for educational demonstrations and museum reconstructions. More commonly now, however, paleobiologists do their modeling on the computer.

Some computer modeling has been very effective in studying the mechanical strength of ancient structures. In particular, paleobiol-ogists have begun looking at the skulls of ancient vertebrates to assess how the structure was shaped by the normal stresses and strains of feeding and head-butting. A useful modeling approach is finite element analysis (FEA), a well-established method used by engineers to assess the strength of bridges and buildings before they are built, and now applied to dinosaur skulls (Box 6.5), among other fossil problems. FEA is one of many methods of modeling how forces act on biological struc-

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