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

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138 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Charles Darwin gave us phylogenetic trees

(see p. 117) and he gave us biodiversity (see

p. 535); he also gave us an evolutionary view

of form. He was astonished by the variety of

external appearances of plants and animals,

and by their wonderful adaptations to life. He

discovered many remarkable examples of

extraordinary bodily appearance and func-

tion in insects, birds and plants. He was

intrigued by the specialized beaks and tongues

some birds and moths have for feeding on

nectar from particular plants. He was fasci-

nated by species where the males and females

look utterly different because of the rigors

of pre-mating displays. He tried to under-

stand how such remarkable adaptations

could be honed and sustained through the

generations.

The form, or external appearance, of any

microbe, plant or animal is shaped by evolu-

tion. The wings of birds are adapted for ﬂ ight;

the long beak and tongue of the hummingbird

is adapted for feeding on nectar deep in a

ﬂ ower. The amazing tail of the male peacock

is adapted through sexual selection to attract

a mate. Sexual selection is the set of evolu-

tionary processes that depend on interactions

between the sexes. The most familiar exam-

ples are the astonishing tails and colors of

male birds, the antlers and horns of male deer

and antelope, the mane of the male lion, and

other structures that are there to impress

females. These structures generally have little

to contribute in protecting the animal from

predators or in helping it to ﬁ nd food: in

many cases they are a considerable handicap.

So sexual selection can act counter to natural

selection; but the beneﬁ t for a male peacock

in ﬁ nding a willing female, or females, and in

mating (sexual selection) is clearly greater

than the disadvantage of carrying such a huge

tail when trying to avoid a predator (natural

selection).

Plants and animals have adaptations that

function in the context of both natural and

sexual selection. An adaptation is an aspect

of form that performs a physical or behav-

ioral function. It may not perform that func-

tion terribly well, merely well enough. So,

human beings are adapted to walking upright,

and this has changed many aspects of our

body shape. But we are not enormously good

at it, and we betray some of our quadrupedal

ancestry: humans still get bad backs, arthritic

joints and cannot run very fast. In the early

years of bipedalism on the African plains 5

million years ago, perhaps speed was not

always essential. A human could never outrun

a lion or a cheetah, but could perhaps climb

a tree or hide in a cave, or act with other

humans to distract the predator.

Paleontologists have always been fascinated

by the form and function of fossils. Not only

are fossil forms often startlingly beautiful,

they may be puzzling. So many fossils belong

to groups without modern analogs that it

becomes an intriguing exercise to determine

why they had the forms they had, and what

their functions may have been. The form of

fossils is important for paleontologists for

three reasons:

1 Form is the only evidence we have in

fossils for identifying species and wider

relationships to reconstruct the tree of life

(see below and p. 128).

2 Form can tell us about behavior and

ecology (see p. 80).

3 Variations in form are commonplace

within a species, and the study of changes

in form through time informs us about

evolution (see p. 121).

GROWTH AND FORM

Recognizing ancient species

Paleontologists must interpret fossil species,

and their ranges of variation, solely from the

morphology, or external shape, of the speci-

mens. There are problems in deciding where

one species ends and another begins. When

there are close living relatives, it may be pos-

sible to compare the modern species with

the fossils. But how are paleontologists to

decide just what is a species of dinosaur or

trilobite?

For modern plants and animals, system-

atists ideally apply the biological species

concept (see p. 121). Clearly paleontologists

cannot test whether fossil species can or

cannot interbreed. So, paleontologists use the

morphological species concept, judging the

bounds of a species entirely on form. The

assumption is that all members of a species

should look similar, and that a few simple

statistical observations should deﬁ ne the mean

or average characteristics of members of a

FOSSIL FORM AND FUNCTION 139

species and the range of variation around that

mean. In practice, this usually seems to be the

case (Box 6.1). Critics are correct to point out

that the morphological species concept suffers

from many problems. Not least is the concern

that the true species boundaries might be

missed: how do you cope with a single species

that has many different forms; how do you

recognize species that differ in color, song or

smell; how can you distinguish cases where

Box 6.1 Phenetics and variation within populations

Frequently, paleontologists are faced with problems that require the simpliﬁ cation of a great mass

of measurements. For example, a paleontologist may have a large sample of fossils from a single

rock horizon and may wish to determine whether these represent one or more species. It might be

sufﬁ cient to plot univariate frequency histograms (see p. 16) of particular measures, such as width,

length and depth of the shells, as well as the hinge width, the diameter of the pedicle foramen, and

the length and width of internal muscle scars. In addition, bivariate plots could be prepared, in which

various measures are plotted against each other. However, it might still be difﬁ cult to differentiate

clusters of points, and this approach means the paleontologist has many separate graphs to

compare.

Multivariate techniques can help solve these problems by dealing with all the measured variates

together. Two common techniques are cluster analysis and principal components analysis (PCA). In

PCA, the maximum direction of variation is determined from the table of raw measurements of

many characters, and this direction is termed eigenvector 1. Further eigenvectors are then plotted in

sequence perpendicular to the ﬁ rst, representing successively less variation in the sample. The ﬁ rst

eigenvector usually reﬂ ects growth-related or size-dependent variation, and it is typically ignored in

taxonomic studies. Species are usually plotted against the second and third eigenvectors, and tests

can then be applied to determine whether there are separate clusters of points.

As an example, a comparison may be made between specimens of two species of brachiopod,

Dicoelosia biloba from the Early Silurian of Sweden, and D. hibernica from rocks of the same age

in Ireland. Four measurements were made on samples of both species and a PCA was performed.

Both species were then plotted against the second and third eigenvectors (Fig. 6.1). Although both

samples overlap, in general the Irish specimens have lower scores on eigenvector 2, showing that D.

hibernica is wider and less deep than D. biloba, and strongly suggesting that there are two

species.

0.6

0.2

–0.2

–0.6

10.60.2–0.2–0.6–1

narrow form

wide form

Eigenvector 2

Eigenvector 3

Figure 6.1 Variation in the Early Silurian brachiopod species Dicoelosia biloba from Sweden (o)

and D. hibernica from Ireland (+), based upon numerous measurements. A principal components

analysis plot separates wide and narrow forms along eigenvector 2, so there may truly be two

species, although there is considerable overlap between the two.

140 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

the male and female look very different from

each other?

Actually paleontologists need not be too

downhearted. Where the biological and mor-

phological species concepts have been applied

to particular groups, both generally give the

same answers. Further, it would be wrong for

a systematist of modern organisms to be too

smug. Most decisions on the species bounds

of living plants and animals are based on

assessments of the morphologies of dead spec-

imens in museums: it is impractical to carry

out extensive crossbreeding tests even with

living organisms.

Problems with fossil species usually arise

from the added dimension of time. If a pale-

ontologist ﬁ nds a long evolving lineage, where

should the dividing line be drawn between

one species and the next? Decisions are often

made easier by gaps in the fossil record that

create artiﬁ cial divisions within evolving lin-

eages. Where gaps are not present, splitt-

ing events clearly mark off new species. If

there are few of these, an evolving lineage is

divided somewhat arbitrarily into chronospe-

cies (“time species”), each being deﬁ ned by

particular morphological features.

Variations in form within species

Within a species, there may be a range of

morphologies; think of the variation among

humans, or more dramatically, among domes-

tic dogs. In naturally occurring species, mor-

phology may vary as a result of several

factors:

1 Individual variation, the normal differ-

ences between any pair of individuals of a

species that are not identical twins; this

base level variation is the stuff of natural

selection, as Darwin stressed.

2 Geographic variation and physical differ-

ences between populations or subspecies

in different parts of the overall species

range.

3 Sexual dimorphism, in which males and

females may show different sizes, and dif-

ferent specialized features (horns, antlers,

tail feathers) often related to sexual

selection.

4 Growth stages, where there may be quite

different larval and adult stages, or where

body form alters during growth.

5 Ecophenotypic effects, where local eco-

logical conditions affect the form of an

organism during its lifetime (see p. 123).

Geographic variation may be substantial

among members of modern species, particu-

larly those distributed over wide ranges.

Sexual dimorphism is seen in living animals,

particularly in those where males engage in

ritualized displays, or where females have

special reproductive activities. Sexual dimor-

phism is also common in fossils, and it has

often caused serious problems of identiﬁ ca-

tion where males and females look very dif-

ferent. For example, many ammonites show

sexual dimorphism, where the postulated

females are much larger than the males, and

the males possess unusual lappets on either

side of the aperture (Fig. 6.2).

Most organisms change substantially in

form as they grow from egg to adult, and

these growth stages will be explored next.

Ecophenotypic variation was introduced in

Chapter 5 (see p. 123) and this includes all

the changes in form that may occur through

an individual’s lifetime, but that are not coded

genetically. Ecophenotypic variation in a

human might include minor features such as

the acquisition of powerful arm muscles

through work or exercise or the loss of liver

function through alcohol abuse. Major eco-

phenotypic changes might include the loss of

a limb in an accident or a carefully main-

tained Mohican haircut. None of these

changes can be passed on genetically to a son

or daughter by the legless, muscular or unusu-

ally coiffed individual.

Allometry

Changes in form during growth are common.

Think of human growth: babies have rela-

tively large heads and eyes, and small limbs.

Similar features are found in fossil examples

too. Juvenile vertebrates, not just humans,

usually have large eyes and heads in propor-

tion to overall body size. A tiny embryo of an

ichthyosaur (Fig. 6.3) shows just these fea-

tures. If measurements of the variable parts

(eye diameter, head length) are scaled against

a standard measure of the animal (total body

length, for example), it is evident that the

proportions change as the animal grows older

(Fig. 6.4). In the case of the ichthyosaur, the

1 cm(b)(a)

Figure 6.2 Sexual dimorphism in ammonites, the Jurassic Kosmoceras. The larger shell (a) was

probably the female, the smaller (b) the male. (Courtesy of Jim Kennedy and Peter Skelton.)

(a)

(b)

Figure 6.3 Adult female Ichthyosaurus (a) from the Lower Jurassic of Somerset, England, showing an

embryo that has just been born (arrow), and detail of the curled embryo (b). (Courtesy of Makoto

Manabe.)

142 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

ratio of eye diameter to body length dimin-

ishes as the animal approaches adulthood.

This is an example of allometric (“different

measure”) growth. If there is no change in

proportions during growth, the feature is said

to show isometric (“same measure”) growth.

Allometric growth is commoner than iso-

metric. Positive allometry is when the organ

or feature of interest increases faster than the

isometric expectation, and negative allometry

is when growth of the structure of interest is

slower than isometry. Head and eye size

usually show negative allometry, starting rela-

tively large in the juvenile, and becoming rela-

tively smaller in the adult. An example of

positive allometry is in the antlers of the Irish

deer (Box 6.2), and indeed in many other

sexually selected features that are minute or

absent in the juvenile, but very large in the

adult.

Allometry is commonly considered only in

the context of ontogeny, the growth from egg

or embryo through juvenile to adult. But

studies of form may compare species, and

shape variation can be accounted for in an

evolutionary context too. For example, a

comparison of species of antelope would

show positive allometry in leg width: scaled

against body length, the sturdiness or width

of the leg increases positively allometrically.

This is because of the well-known biological

scaling principle: some organs and functions

relate to the mass of an animal (a three-dimen-

sional measure), whereas others relate to body

length or body outline (one- and two-dimen-

sional measures). As body mass (three-dimen-

sional) increases, the diameter of the legs

(two-dimensional) increases in proportion to

support the added weight. So, in body outline,

small antelope have extremely slender legs,

and larger ones have relatively more massive

legs.

These aspects of allometry may be under-

stood in terms of the allometric equation,

y = kx

where y is the measurement of interest (e.g.

head length, eye diameter), x is the standard

of comparison (e.g. body length), k is a con-

stant and a is the allometric coefﬁ cient. The

constant k is calculated using the allometric

equation for each particular case. The allome-

tric coefﬁ cient a deﬁ nes the nature of the

slope: if a = 1, the slope is at 45˚ and this

deﬁ nes a case of isometric growth; if a > 1,

we have positive allometry, and if a < 1, we

have negative allometry (see Fig. 6.4).

After the nature of any allometric change

of parts or organs has been established quan-

titatively, it is possible to investigate why such

changes might occur. The large eyes and small

noses of babies are said to make them look

cute so their parents will look after them, and

feed them. But the fundamental reason is pre-

sumably because the eye is complex and is at

nearly adult size in the baby for functional

reasons, and the relatively large head of a

human baby is to accommodate the large

(a)

(b)

Orbit length (cm)

Skull length (cm)

100

100101

Skull length (cm)

Backbone length (cm)

10010

Figure 6.4 Tests of allometry in the ichthyosaur

Ichthyosaurus. (a) Plot of orbit length against

skull length, and (b) plot of skull length against

backbone length. The Somerset embryo (Fig.

6.3b) is indicated by a solid circle. Both graphs

show negative allometry (orbit diameter = 0.355

(skull length)

0.987

; skull length = 1.162 (backbone

length)

0.933

), conﬁ rming that embryos and

juveniles had relatively large heads and eyes.

(Courtesy of Makoto Manabe.)

FOSSIL FORM AND FUNCTION 143

Box 6.2 The Irish deer: too big to survive?

The Irish deer Megaloceros, formerly called the Irish elk, is one of the most evocative of the Ice Age

mammals (Fig. 6.5a), and one of the most misunderstood. When the ﬁ rst fossils were dug out of the

Irish bog, Thomas Molyneux wrote of them in 1697, “Should we compare the fairest buck with the

symmetry of this mighty beast, it must certainly fall as much short of its proportions as the smallest

young fawn, compared to the largest over-grown buck.”

This was a large deer, some 2.1 m tall at the shoulders, and it famously had massive antlers, the

largest spanning 3.6 m. The old story was that this deer simply died out because its antlers became

too large. Paleontologists understood that the antlers were subject to sexual selection and, as in

modern deer, the male with the largest antlers and the scariest display probably gathered the largest

harem of females and so passed on his genes most successfully. But can a species really be driven to

extinction by sexual selection?

In a classic paper, the young Steve Gould (1974) showed that this was clearly nonsense. He measured

the body lengths and antler dimensions of dozens of specimens and showed that they fell precisely on

an allometric curve, and that the allometric curve was the same as for other relatives such as the smaller,

living red deer and wapiti (Fig. 6.5b). It is clear that sexual selection and natural selection were at odds

in this case, as often happens, but the balance was maintained and indeed the Irish deer was successful

throughout Europe, existing until 11,000 years ago in Ireland and 8000 years ago in Siberia. It proba-

bly died out because of climate change at the end of the Pleistocene and hunting by early humans, rather

than by collapsing beneath the weight of its overgrown antlers.

It is worth reading Gould’s (1974) classic study of positive allometry in the Irish deer, and a

broader review of positive allometry in sexually-selected traits by Kodric-Brown et al. (2006). Read

more and see color illustrations at http://www.blackwellpublishing.com/paleobiology/.

100

40 60 80

Height of shoulder (cm)

Maximum length of antler (cm)

(b)(a)

Figure 6.5 Positive allometry in the antlers of the giant Irish deer Megaloceros. (a) A famous

photograph of an Irish deer skeleton mounted in Dublin in Victorian times. (b) Positive allometry

in the antlers of modern deer, showing that Megaloceros (M) falls precisely on the expected trend

of its closest living relatives. Note that the fallow deer (D) plots above the slope (i.e. antlers are

larger than expected from its height), and the European and American moose (A) plot below the

line (i.e. antlers are smaller than expected from their height). Two regression lines, the reduced

major axis (steeper) and least squares regression, are shown. The allometric equation is antler

length = 0.463 (shoulder height)

1.74

. (Based on information in Gould 1974.)

144 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

brain, which again is rather well developed

at birth.

Ichthyosaurs (see Figs 6.3, 6.4) were born

live underwater, as shown by remarkable

fossils (see p. 462), and did not hatch from

eggs laid onshore, as is the case with most

other marine reptiles. Their large head at birth

would have allowed them to feed on ﬁ shes

and ammonites as soon as they were born.

The large eyes were perhaps necessary also for

hunting in murky water, and had to be near-

adult size from the start. Or, perhaps, it made

them look cute and encouraged parental

care!

Shape variation between species

Within any clade there are many forms.

Related plants and animals usually show some

common aspects of form, and species and

genera vary around a theme. For example,

gastropods all have coiled shells and the three-

dimensional shape can be thought of as a

result of variation in four parameters (see p.

333). When form can be reduced to a small

number of parameters like this, then the whole

range of possible forms governed by those

parameters may be deﬁ ned – the theoretical

morphospace for the clade. Studies of the

theoretical morphospace for gastropods,

ammonoids and early vascular plants show

that known species have only exploited a

selection of possible morphologies. Some

zones of morphospace may represent impos-

sible forms – such as gastropods or ammo-

noids with a minute aperture, with no room

for the living animal – but others have simply

not been exploited by chance, or they cannot

be reached by normal evolutionary change

because of the impossibility of intervening

stages.

The range of forms within a clade may also

be described as disparity, the sum of morpho-

logical variation. Disparity may be quantiﬁ ed

as the range of values for all possible shape

parameters seen in species in a clade. All the

measures of shape may be combined in a mul-

tivariate analysis that can simplify dozens of

shape measures to a smaller number of prin-

cipal coordinates or eigenvectors (see p. 139)

so that size and other general principles may

be separated. It is possible to compare the

disparity of different clades, or to look at how

disparity varies through time. Disparity is

generally high early in the history of a clade

as the species “try out” all the possibilities of

their new body form, and then the disparity

of the group remains rather constant for the

rest of its history. Changes in disparity through

time may roughly mimic changes in diversity

(as diversity increases, so too does disparity),

but the correlation is usually not perfect, and

shape change often goes ahead of diversity

increase.

EVOLUTION AND DEVELOPMENT

Ontogeny and phylogeny

Biologists have long sought a link between

ontogeny (development) and phylogeny (evo-

lutionary history). In 1866, Ernst Haeckel, a

German evolutionist, announced his Biogene-

tic Law, that “ontogeny recapitulates phylog-

eny”. His idea was that the sequence of

embryonic stages mimicked the past evolu-

tionary history of an animal. So, in humans,

he argued, the earliest embryonic stages were

rather ﬁ sh-like, with gill pouches in the neck

region. Next, he argued was an “amphibian”

stage and a “reptile” stage, when the human

embryo retained a tail and had a small head,

and ﬁ nally came the “mammal” stage, with

growth of a large brain and a pelt of ﬁ ne

hair.

Haeckel’s view was attractive at the time,

but too simple. Haeckel had drawn on earlier

work, including Von Baer’s Law, presented in

1828, and this law can be matched with

current cladistic models. Von Baer interpreted

the embryology of vertebrates as showing that

“general characters appear ﬁ rst in ontogeny,

special characters later”. Early embryos are

virtually indistinguishable: they all have a

backbone, a head and a tail (vertebrate char-

acters). A little later, ﬁ ns appear in the ﬁ sh

embryo, legs in the tetrapods. More special-

ized characters appear later: ﬁ n rays in the

ﬁ sh, beak and feather buds in the chick, snout

and hooves in the calf, and large brain and

tail loss in the human embryo.

“General characters appearing before

special characters” has taken on a new

meaning with the establishment of a cladistic

view of phylogeny (see p. 129). Von Baer’s

Law draws a parallel between the sequence of

development, and the structure of a clado-

gram. In human development, the embryo

FOSSIL FORM AND FUNCTION 145

passes through the major nodes of the clado-

gram of vertebrates. The synapomorphies (see

p. 130) of vertebrates appear ﬁ rst, then those

of tetrapods, then those of amniotes, then

those of mammals, of primates, and of the

species Homo sapiens last.

Three other aspects of development throw

light on phylogeny. Certain developmental

abnormalities called atavisms, or throw-

backs, show former stages of evolution, such

as human babies with small tails or excessive

hair, or horses with extra side toes (Fig. 6.6a),

showing how earlier horses had ﬁ ve, four or

three toes, compared to the modern one.

Vestigial structures tell similar phylogenetic

stories. These are structures retained in living

organisms that have no clear function, and

may simply be there because they represent

something that was once used. So, modern

whales have, deep within their bodies, small

bones in the hip region that are remnants

of their hindlegs (Fig. 6.6b). Whales last

had functioning hindlegs over 50 Ma in the

Eocene, and the vestigial remnants are still

there, even though they serve no further

purpose in locomotion, and only support

some muscles associated with the penis.

The third aspect of development that forms

links with phylogeny is the observation that

ontogenetic patterns themselves have evolved.

In particular the timing and rate of develop-

mental events has varied between ancestors

and descendants, often with profound effects.

This phenomenon is termed heterochrony.

Heterochrony: are human adults juvenile apes?

Heterochrony means “different time”, and

includes all aspects of changes of timing and

rates of development. There are two forms

of heterochronic change, pedomorphosis

(“juvenile formation”), or sexual maturity in

a juvenile body, and peramorphosis (“overde-

velopment”), where sexual maturity occurs

relatively late. These changes can each occur

in three ways, by variation in timing of the

beginning of body growth, the timing of

sexual maturation or the rate of morphologi-

cal development (Table 6.1).

(a)

(b)

femur

pelvis

Figure 6.6 Hints of ancestry in modern animals.

(a) Extra toes in a horse, an example of an

atavistic abnormality in development, or a

throw-back, to earlier horses which had more

than one toe; normal horse leg (left), extra toes

(right). (b) The vestigial hip girdle and hindlimb

of a whale; the rudimentary limb is the rudiment

of a hindlimb that functioned 50 Ma.

Table 6.1 The processes of heterochrony: differences in the relative timing and rates of development.

Onset of growth Sexual maturation Rate of morphological development

Pedomorphosis

Progenesis – Early –

Neoteny – – Reduced

Postdisplacement Delayed – –

Peramorphosis

Hypermorphosis – Delayed –

Acceleration – – Increased

Predisplacement Early – –

146 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

In studying heterochrony, it is necessary to

have a robust phylogeny of the organisms

in question, an adequate fossil record of

the group, and a sound set of ontogenetic

sequences for each species. This allows the

paleontologist to compare juveniles and adults

throughout the phylogeny. A classic example

is human evolution. It seems obvious that

human adults look like juvenile apes, with

their ﬂ at faces, large brains and lack of body

hair. These would imply a pedomorphic

change in humans with respect to the human/

ape ancestor. However, other characters do

not ﬁ t this pattern. For example, developmen-

tal time in humans is far longer than in apes

and ancestral forms, a feature of peramorpho-

sis, and hyperomorphosis in particular (devel-

opmental time is longer, but rate of morpho-

logical development is not faster). Thus,

heterochronic changes can occur in different

directions in different characters, a phenome-

non called mosaic evolution.

In a classic study, McNamara (1976) sug-

gested that species of the Cenozoic brachio-

pod Tegulorhynchia evolved into Notosaria

by a process of heterochrony (Fig. 6.7). The

main changes were a narrowing of the shell,

a reduction in the number of ribs in the shell

ornament, a smoothing of the lower margin,

and an enlargement of the pedicle foramen

(the opening through which a ﬂ eshy stalk

attaches the animal to a rock). These changes

Paleocene Eocene

Oligocene

Miocene Pliocene

Ple.

Rec.

N. nigricans

N. antipoda

T. thomsoni

T. doederleini

T. coelata

T. squamosa

adult

juvenile

ontogeny

pedomorphocline

T. boongeroodaensis

Figure 6.7 Heterochronic evolution in the Cenozoic brachiopods Tegulorhynchia and Notosaria.

Adults of more recent species are like juveniles of the ancestor. Hence, pedomorphosis (“juvenile

formation”) is expressed in this example. (Based on McNamara 1976.)

FOSSIL FORM AND FUNCTION 147

related to a shift of habitats from deep to

shallow high-energy waters: the large pedicle

allowed the brachiopod to hold tight in

rougher conditions, and the other changes

helped stabilize the shell. The developmental

sequence of the ancestral species T. boongeroo-

daensis shows that its descendants are like the

juvenile stage. Hence, pedomorphosis has

taken place along a pedomorphocline (“child

formation slope”). It is harder here to deter-

mine which type of pedomorphosis has taken

place; perhaps it was neoteny.

A second example illustrates a peramor-

phic trend. Rhynchosaurs were a group of

Triassic herbivorous reptiles. Later species

had exceptionally broad skulls as adults,

which gave them vast muscle power to chop

tough vegetation. Juvenile examples of these

Late Triassic rhynchosaurs retain the rather

narrower skulls of the ancestral adult forms

(Fig. 6.8). Hence, the evolution of the broad

skull is an example of peramorphosis,

along a peramorphocline (“overdevelopment

slope”). The adult Late Triassic rhynchosaurs

are larger than earlier forms, which implies

that sexual maturation was delayed while the

body continued to grow (hypermorphosis)

or the rate of morphological development

increased in the same duration of ontogeny

(acceleration).

Developmental genes

It has been understood since the time of

Darwin that the external form, or phenotype,

of an organism is controlled by the genotype,

the genetic code (see p. 121), but the exact

mechanisms have been unclear. At one time

people thought there was roughly one gene

for each morphological attribute. Some char-

acters seem to be inherited in a unitary manner

– you inherit blond, black or red hair from

one or the other or both of your parents, and

so it might be reasonable to assume that there

is a gene variant for each color. But most

phenotypic characters are inherited in a much

more complex manner, and it is clear that

there is no single gene that controls the shape

of your nose, the length of your legs or your

mathematical ability.

Some clarity has now been shed on how

genes control form. There are a number of

Triassic

LateMidEarly

(A)

(?A)

50 mm

(J)

ontogeny

peramorphocline

Rhynchosaurus

Stenaulorhynchus

Mesosuchus

Scaphonyx

Figure 6.8 Heterochronic evolution in the Triassic rhynchosaurs. The skull of adult (A) Late Triassic

forms developed beyond the size and shape limits seen in earlier Triassic adult forms. Here, the

juveniles (J) of the descendants resemble the ancestral adults, and this is thus an example of

peramorphosis (“beyond formation”). (Based on Benton & Kirkpatrick 1989.)