Benton M.J. Introduction to paleobiology and the fossil record

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148 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

developmental genes that are widely shared

among organisms and that determine funda-

mental aspects of form such as symmetry,

anteroposterior orientation and limb differen-

tiation. Since the 1980s a major new research

ﬁ eld has emerged, sometimes called “evo-

devo” (short for evolution–development),

that investigates these developmental genes.

This ﬁ eld is exciting for paleontologists

because the developmental genes control

aspects of form on a macroevolutionary scale,

and so major evolutionary transitions can be

interpreted successfully in terms of develop-

mental genes.

The most famous developmental genes are

the homeobox genes, identiﬁ ed ﬁ rst in the

experimental geneticist’s greatest ally, the fruit

ﬂ y Drosophila, but since found in a wide range

of eukaryotes from slime molds to humans,

and yeast to daffodils. Homeobox genes

contain a conserved region that is 180 base

pairs long (see p. 186) and encodes transcrip-

tion factors, proteins that switch on cascades

of other genes, for example all the genes

required to make an arm or a leg. In this sense

homeobox genes are regulatory genes; they act

early in development and regulate many other

genes that have more specialist functions.

The Hox genes are a speciﬁ c set of homeo-

box genes that are found in a special gene

cluster, the Hox cluster or complex that is

physically located in one region within a chro-

mosome. Hox genes function in patterning

the body axis by ﬁ xing the anteroposterior

orientation of the early embryo (which is

front and which is back?), they specify posi-

tions along the anteroposterior axis, marking

where other regulatory genes determine the

segmentation of the body, especially seen in

arthropods (see p. 362), and they also mark

the position and sequence of differentiation

of the limbs (Box 6.3).

Box 6.3 Hox genes and the vertebrate limb

One of the greatest transitions of form in vertebrate evolution was the remodeling of a ﬁ sh into a

tetrapod, a process that occurred more than 400 Ma in the Devonian (see p. 442). The fossils show

how the internal skeleton of a swimming ﬁ n was transformed into a walking limb. A crucial part

of this repatterning from ﬁ n to limb seemed to be the pentadactyl limb, the classic arm or leg with

ﬁ ve ﬁ ngers or toes seen in humans and most other tetrapods. But then paleontologists began to ﬁ nd

Late Devonian tetrapods with six, seven or eight digits. How could this be explained in a world

where there was supposed to be a gene for each digit, and ﬁ ve was the norm?

The tetrapod limb can be divided into three portions that appear in the embryo one after the

other, and that appeared in evolutionary history in the same sequence. First is the proximal portion

of the limb, the stylopod (the upper arm or thigh), then the middle portion of the limb, the zeugopod

(the forearm or calf), and ﬁ nally the distal portion, the autopod (the hand and wrist or foot and

ankle).

This evolutionary sequence is replicated during development of the embryo (Shubin et al. 1997;

Coates et al. 2002; Tickle 2006; Zakany & Duboule 2007). At an early phase, the limb is represented

simply by a limb bud, a small lateral outgrowth from the body wall. Limb growth is controlled by

Hox genes. Early in ﬁ sh evolution, ﬁ ve of the 13 Hox genes, numbered 9–13, were coopted to control

limb bud development. Manipulation of embryos during three phases of development has shown

how this works (Fig. 6.9a). In phase I, the stylopod in the limb bud sprouts, and this is associated

with expression of the genes HoxD-9 and HoxD-10. In phase II, the zeugopod sprouts at the end

of the limb bud, and the tissues are mapped into ﬁ ve zones from back to front by different nested

clusters of all the limb bud genes HoxD-9 to HoxD-13. Finally, in phase III, the distal tip of the

lengthening limb bud is divided into three anteroposterior zones, each associated with a different

combination of genes HoxD-10 to HoxD-13. Phases I and II have been observed in bony ﬁ sh devel-

opment, but phase III appears to be unique to tetrapods.

In the development of vertebrate embryos, there is no ﬁ xed plan for every detail of the limb. A

developmental axis runs from the side of the body through the limb, and cartilages condense from

FOSSIL FORM AND FUNCTION 149

Developmental

axis

Radial

(b)

(a)

(c)

Knockout phenotype

Phase I expression

9 10111213

Knockout phenotype

Phase II expression

9 10111213

Knockout phenotype

Phase III expression

9 10111213

Paralogs

Phase I • HoxD-9, D-10

Phase II • HoxD-9

• HoxD-9, D-10

• HoxD-9, D-10, D-11

• HoxD-9, D-10, D-11, D-12

• HoxD-9, D-10, D-11, D-12, D-13

Phase III • HoxA-13

• HoxA-13, D-13

• HoxA-13, D-13, D-12, D-12, D-11, D-10

Stylopod

Zeugopod

Autopod

Developmental time

Figure 6.9 Hox genes and the development of the tetrapod limb. (a) The sequence of growth of a

tetrapod limb bud, reading from top to bottom, showing how the stylopod (humerus/femur),

zeugopod (forearm/calf) and autopod (hand/foot) differentiate. The pattern is determined by

turning on (ﬁ lled squares) and off (open squares) of Hox genes D-9 to D-13. (b, c) Interpretation

of the forelimbs of the osteolepiform ﬁ sh Eusthenopteron (b) and the tetrapod Acanthostega (c)

in terms of development. The developmental axis (solid line) branches radial elements (dashed

lines) in a pre-axial (anterior) direction in both forms, and the digits of tetrapods condense in a

post-axial direction. (a, based on Shubin et al. 1997; b, c, courtesy of Mike Coates.)

soft tissues in sequence from the body outwards to the tips of the ﬁ ngers. In an osteolepiform ﬁ sh

(Fig. 6.9b), the developmental axis presumably ran through the main bony elements, and additional

bones, radials, developed in front of the axis (pre-axial side). In tetrapods (Fig. 6.9c), the axis in the

leg (arm) runs through the femur (humerus), ﬁ bula (ulna) and ankle (wrist) and then swings through

the distal carpals (tarsals). Radials condense pre-axially at ﬁ rst, as in the osteolepiform, forming the

tibia (radius) and various ankle (wrist) bones. The developmental process then switches sides to

sprout digits post-axially (behind the axis). This reversal of limb-bud growth direction in the hand/

foot is matched by a reversal of the expression of the Hox genes. In the zeugopod, HoxD-9 is

expressed in all ﬁ ve zones, HoxD-10 in the posterior four zones, down to HoxD-13 only in the

posterior of the ﬁ ve. In the autopod, on the other hand, HoxA-13 is present in all zones, HoxD-13

in the posterior two zones, and HoxD-10 to HoxD-12 only in the posterior zone.

In Late Devonian tetrapods, six, seven or eight digits were freely produced, and it was only at

the beginning of the Carboniferous that tetrapods seem to have ﬁ xed on ﬁ ve digits fore and aft.

Since then, digital reduction has commonly occurred, down to four (frogs), three (many dinosaurs),

two (cows and sheep) or one (horses) ﬁ ngers and toes. Systematists must beware of interpreting such

events as unique, however: the new evo-devo perspective suggests that loss of digits has happened

many times in tetrapod evolution, and by the same processes of switching Hox genes on and off.

Read more about Hox genes and limb-bud development at http://www.blackwellpublishing.com/

palaeo/, and about evo-devo topics in general in Carroll (2005) and Shubin (2008).

150 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

In early studies of the Hox genes of Dro-

sophila, experimenters were amazed to dis-

cover that mutations in particular Hox genes

might cause the insect to develop a walking

leg on its head in place of an antenna. The

mutations were not simple changes of the

base-pair sequence, but knockouts or dele-

tions of entire functional portions and replace-

ment of their expression domains by more

posterior Hox genes. Study of such knockouts

showed how each Hox gene worked; in this

case the Hox gene acted on the limb bud, the

small group of cells on the side of the body

that appears early in development and eventu-

ally becomes a limb. A particular Hox gene

determines how many limb buds there are and

where they are located, and other Hox genes

determine whether the limb bud becomes a

walking leg, a mouthpart or an antenna. If

experimenters induce a knockout within a

Hox gene, it works its magic in the wrong

place, giving the ﬂ y extra legs or legs in the

wrong place. Mutations of Hox genes in ver-

tebrates normally do not produce these spec-

tacular effects; the embryo often fails and is

aborted.

Such mutations need not always result in

damage. Duplication of homeobox genes can

produce new body segments, and such dupli-

cations may have been important in the evolu-

tion of arthropods and other segmented

animals. The new evo-devo perspective allows

us to understand that an arthropod with

numerous body segments and 10 or 100 legs

may have evolved by a single evolutionary

event, perhaps a relatively straightforward

mutation of homeobox genes, rather than an

elaborate multistep process of gradual addi-

tion of segments and legs through many sepa-

rate evolutionary events. The evo-devo

revolution is beginning to explain some of the

most mysterious aspects of evolution.

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 ﬂ y? 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 proba-

bly ﬂ ew 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 ﬁ rst, 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 efﬁ cient 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 ﬂ ower of

an angiosperm is colorful to attract pollinat-

ing insects, and the nectar is located deep in

the ﬂ ower 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 funda-

mental evidence of value in interpreting func-

tion. 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 ﬂ eshy 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 ﬁ rst 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

FOSSIL FORM AND FUNCTION 151

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 iden-

tify 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 adjust-

ments 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 morphol-

ogy of a dinosaur, for example, should the

paleontologist compare the fossil with a croc-

odile or a bird? In former days, paleontolo-

gists 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 dino-

saurs. 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 ﬂ ying that it is hard to

ﬁ nd 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 indepen-

dent 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 organ-

isms. 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 ﬁ ll 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 mor-

phology. 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 ana-

tomic 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 princi-

ples of biomechanics, the physics of how

organisms move, from observations across

the spectrum. So, the scaling principle men-

tioned earlier (see p. 142), exempliﬁ ed by the

spindly legs of the antelope and the pillar-like

legs of the elephant, is a commonsense obser-

vation 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 camouﬂ aged;

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 interpreta-

tions of movements, especially in feeding and

locomotion. Such studies use basic principles

of biomechanics and engineering to interpret

152 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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 efﬁ ciency 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 rela-

tive 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 conﬁ rmation 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 educa-

tional demonstrations and museum recon-

structions. 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 model-

ing approach is ﬁ nite 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-

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 identiﬁ ed 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 identiﬁ ed 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 ﬂ oating 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 ﬁ shing net towed in the open sea. As the boat moves forward,

the tow-net hangs passively behind and billows outward. Any ﬁ shes 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!

FOSSIL FORM AND FUNCTION 153

(a)

(b)

(c)

Wind-driven float

Plankton

Depth (m)

Figure 6.10 The use of a modern analog to interpret a mysterious fossil. (a) A colony of the

pseudoplanktonic crinoid Traumatocrinus attached to a fossil piece of driftwood, from the Late

Triassic of China. (b) Reconstruction of the crinoids in life, showing how the wind pulled the log

to the left, and the dangling crinoids captured plankton like a net. (c) A tow-net used to

maximize catches of ﬁ sh, a possible modern analog that explains the feeding mode of the fossil

colony. (Courtesy of Wang Xiaofeng.)

tures, while other modeling methods seek to

establish how ancient organisms moved.

A number of attempts have been made to

understand how dinosaurs ran, and of course

everyone focuses on Tyrannosaurus rex. At

one level, we all know how T. rex ran – we

have seen it on Jurassic Park and Walking

with Dinosaurs, so what is the problem? The

locomotion in those movies was based on

study of the limb bones, calculation of their

ranges of movement, observation of modern

ostriches at speed, and computer animations

that rendered a reasonable swing of the leg,

and that prevented the animal from falling

over. But Hutchinson and Gatesy (2006) have

urged caution. They argue that the style of

locomotion shown in those ﬁ lms is probably

near enough right, but that the animators

chose only one out of many possible positions

for the limbs.

In the most likely running gait (Fig. 6.13a)

the backbone is horizontal and the legs rela-

tively straight and long. Whatever happens,

the animal must not fall over, so the ﬁ rst thing

in reconstructing locomotion is to determine

the center of mass, the central point in the

core of the body. This can be found crudely

by dangling a plastic model from a string and

ﬁ nding the three-dimensional central point of

balance – or a more elaborate set of calcula-

tions can be done in the computer. In T. rex

the center of mass lay just in front of the hips,

and the tail balanced the body over the hips

that acted as a fulcrum.

154 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Figure 6.11 Basic mechanical models for biological structures. There are different kinds of levers in use

in everyday appliances, and these styles may be seen in biological structures. (a) In a class 1 lever the

effort and load are on opposite sides of the fulcrum. (b, c) In class 2 and 3 levers the effort and load

are on the same side of the fulcrum, with the effort furthest away in a class 2 lever (b), and closest in a

class 3 lever (c).

Load

Effort

Load

Effort

Load

Effort

Fulcrum

(a) Class 1

(b) Class 2

The running cycle of any animal can be

divided into the stance phase, when the foot

touches the ground, and the swing phase,

when the foot is off the ground (Fig. 6.13a).

The limb swings through three extreme pos-

tures during the stance phase, from the point

at which the foot touches the ground, through

mid-stance as the body moves forwards to

late stance just before the foot leaves the

ground (Fig. 6.13b–d). An animal in contact

with the ground produces a ground reaction

force (GRF) that is the reaction to its body

mass and the force of the limb hitting the

ground during movement. The GRF swings

its line of action as the limb shifts its position,

and the point of maximum stress on the knee

is at the mid-stance position (Fig. 6.13c) when

the knee is bent, the knee moment arm is

longest, and the muscle moment about the

knee acting against gravity is at its highest.

Hutchinson and Gatesy (2006) showed

that this is only one of many other possible

poses for the limbs. Could T. rex have run in

a high ballet-dancer pose or an extreme crouch

(Fig. 6.13e–g)? The ballet-dancer pose is ruled

out because the line of the GRF is in front of

FOSSIL FORM AND FUNCTION 155

Box 6.5 Finite element analysis of the skull of Tyrannosaurus rex

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

the skulls of the theropod dinosaurs worked, using ﬁ nite 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 Rayﬁ eld’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. Rayﬁ eld 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).

Rayﬁ eld’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 Rayﬁ eld (2006) and about ﬁ nite element

analysis in Rayﬁ eld (2007) and at http://www.blackwellpublishing.com/paleobiology/.

the knee at mid-stance and this would require

the muscles at the back of the leg to act in

order to balance the force in front. Living

animals do not do this, so there is no reason

to assume that extinct ones did. Crouched

poses are ruled out too because the knee

moment arm would have been too long and

the knee muscle moment too high: T. rex

would have had to have muscles relatively

much larger than those of a chicken to cope.

So the real T. rex probably stood and moved

somewhere between these columnar and

crouched extremes (Fig. 6.13g), which still

leaves a large area of possibilities that cannot

be excluded.

Circumstantial evidence

Paleontologists are inquisitive by nature and

they gather evidence of all kinds to test their

hypotheses. Clues about the lifestyle of an

ancient plant or animal may come from the

enclosing rocks, associated fossil remains,

associated trace fossils and particular features

of the body fossils themselves. These can be

grouped as circumstantial evidence.

1 Fossils are generally preserved in sedimen-

tary rocks, and these record all kinds of

features about the conditions of deposi-

tion. Fossil plants may be found at certain

levels in a cyclical succession that tells a

story of the repeated buildup of an ancient

delta as it ﬁ ngers into the sea, the develop-

ment of soils and forests on top, and its

eventual ﬂ ooding by a particularly high

sea level. Marine invertebrates may be

found in rocks that indicate deposition in

a shallow lagoon, offshore from a reef, on

the deep abyssal plain or many other

Continued

156 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a)

(b)

(c)

Figure 6.12 Finite element analysis of the skull of Tyrannosaurus rex. The skull (a) was

converted into a cell mesh (b), and biting forces applied (c). In the stress visualization (c), high

stresses are indicated by pale colors, low stresses by black. Each bite, depending on its strength

and location, sends stress patterns through the skull mesh and these allow the paleobiologist to

understand the construction of the skull, but also the maximum forces possible before the

structure fails. (Courtesy of Emily Rayﬁ eld.)

situations. Moreover if they had a wide-

spread geographic distribution, perhaps

they were planktonic. Dinosaurs or fossil

mammals may be found in sandstones

deposited in an ancient river or desert. All

these clues from sedimentary rocks guide

the paleontologist in interpreting the envi-

ronment of deposition, and in turn can

reveal clues about climates and other

physical conditions.

2 Associated fossils also give clues. They can

show where the organism of interest sits

in a food web (see p. 88) – who ate it, and

what did it eat? Sometimes groups of

fossils may be associated in death in such

a way that they indicate life habits. For

FOSSIL FORM AND FUNCTION 157

example, fossil reefs may be killed off in

a particular crisis, and all the organisms

that lived together are found in life posi-

tion; some corals, bryozoans and crinoids

may be ﬁ xed to the substrate in their

normal growth position, and mobile

organisms like gastropods or trilobites

may be preserved among the thickets of

benthic sessile organisms. Similarly, a

paleosol (see p. 518) may preserve roots

and stems of dozens of plants in life posi-

tion, together with burrows of insects and

worms that lived among them. Associa-

tions of fossils can also be more intimate,

where for example parasites may be found

attached to their hosts, or fossils of one

species may be found in the stomach

region of another.

3 Associated trace fossils can sometimes be

linked to their producers, but not always.

There are some rare examples of arthro-

pods preserved at the ends of their trails.

The link between trace fossil and producer

is usually a little less clear: dinosaur foot-

prints may be found at certain levels within

a particular geological formation, and the

skeletons of likely producers at other

levels. The bones of ﬁ shes and marine rep-

tiles may be found associated with phos-

phatic coprolites (fossil dung) in certain

marine beds – it is likely that the copro-

lites were dropped by one or other of the

associated animals. If a link can be made

between a trace fossil and its maker, then

a great deal of additional paleobiologi-

cal information can be established (see

Chapter 19).

4 Close study of the body fossils themselves

is also warranted. Skeletal fossils regularly

preserve evidence of soft tissues and other

Figure 6.13 The running stride of Tyrannosaurus rex. (a) The main components of a stride, showing

the stance phase when the foot touches the ground, and the swing phase. (b–d) Three positions of the

limb in early stance, mid-stance and late stance, as the body moves forward, and showing the main

forces, including the ground reaction force (GRF). (e–g) Three alternative postures for the limb, with

the body held high or low. Read more, and see the movies at http://www.rvc.ac.uk/AboutUs/Staff/

jhutchinson/ResearchInterests/beyond/Index.cfm. (Courtesy of John Hutchinson.)

(a)

(b)

(e)

(f)

(g)

GRF

early stance

mid-stance

muscle

moment

late stance

knee

moment

arm

stance phase swing phase

{