Ch. 2—Key concepts

Fossils & Evolution Ch. 2 1

Ch. 2—Key concepts

• Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding of intraspecific variation is necessary for correct identification

• Ontogenetic variation occurs during an individual’s lifespan

• Population variation occurs among individuals within a given population


Ch. 2—Key terms • Ontogeny; ontogenetic variation• Population variation• Types of skeletal growth

– Addition; accretion; molting; modification; combination

• Isometric vs. allometric growth• Principle of similitude• Ecophenotypic variation• Sexual dimorphism


Ontogenetic variation

• Ontogeny = the life history of an individual (both embryonic and post-natal)

• Understanding ontogeny is important because growth stages of an individual may be so different that they are hardly recognizable as the same species


Types of skeletal growth

1. Accretion (enlargement) of existing parts

2. Addition of new parts

3. Molting

4. Modification

5. Combinations (mixed growth strategies)


Skeletal growth—Accretion

• Accretion = adding new material to an existing shell

• Allows uninterrupted use of shell and more or less continuous growth

• Disadvantage is that adult shape is somewhat constrained by juvenile shape

• Example: bivalve growth


Bivalve accretion


Skeletal growth—Addition of new parts

• Echinoderms may grow simply by adding new plates to their calyx or new columnals to their stalk

• Example: crinoid stalk– Large columnals added just beneath calyx– Smaller columnals added between larger ones– Alternation of different sizes allows increased

flexibility


Crinoid stalk(addition)


Skeletal growth—Molting

• Molting = periodic shedding of an exoskeleton followed by growth of a new, larger one

• Advantage: Shape of adult organism not constrained by shape of juvenile stages

• Disadvantages are (1) vulnerable period during the molt itself; (2) significant metabolic cost of repeatedly replacing entire skeleton

• Example: trilobites


Trilobite molting

Instars = growthstages between molts


Molting (cont.)

Molting produces growth ina series of discrete episodes(not continuous)—Instarsfrom different growth stages form distinct morphologic clusters

instars


Skeletal growth—Modification

• Modification = process of replacement and re-formation of skeletal material, allowing size increase as well as changes in shape and structure

• Skeletal form of adult is not strongly constrained by skeletal form of juvenile

• No vulnerable stage (as in molting)• Example: vertebrate bones


Skeletal growth—Mixed strategies

• Some organisms employ combinations of growth strategies

• Example: coiled cephalopod grows by accretion along leading edge of shell and also by periodic addition of septa


Combined growth strategy(coiled cephalopod)

periodic additionof new septa

continuous accretionof new material alongleading edge of shell


Recognizing and describing ontogenetic change

• Biologists can directly observe ontogenetic change, but paleontologists cannot

• Two main approaches to studying ontogenetic changes in fossil material:– Growth series of specimens representing different

developmental stages (as in successive trilobite instars)

– Adult specimens whose development is recorded by growth lines or newly added parts (as in bivalve example)


Recognizing and describing ontogenetic change

• Approach depends on the kinds of fossils being studied:– Cannot use adult specimens to study ontogeny

in animals that grow through molting or modification


Example 1: Brachiopod ontogeny

• Length and width measurements performed on large (~75) population of specimens of all sizes

• Plot of length vs. width suggests change in shape during growth– Small individuals are wider than long– Large individual are longer than wide


Brachiopod example:Length vs. width

Growth Series:scatter of datapoints suggestschange in shapeduring growth


Example: Brachiopod ontogeny

• A more definitive understanding of brachiopod ontogeny can be achieved by plotting growth curves for individual specimens (by measuring along growth lines)


Brachiopod example:Length vs. width

Individual ontogeny:growth curves for singlespecimens confirm change in shape, ANDallow estimate ofvariation among individuals


Types of growth

• Isometric = no change in shape during ontogeny (ratio between parts does not change as size increases)– Relatively uncommon

• Anisometric (allometric) = change in shape during ontogeny (ratio between parts changes as size increases)– Relatively common


Types of growth (cont.)

• Consider two body parts, X and Y

• As organism grows, relationship between X and Y is given as:

• In isometric growth, a = 1 (linear equation)

• In anisometric growth, a = 1 (curve)

Y = bXa


Isometric growth


Anisometric growth


Why is anisometric growth common?

• Anisometric growth is necessary in most organisms because volume (body mass) increases as the cube of linear size increase

• Example: bone strength is proportional to cross-sectional area of bone– As linear dimensions of bone doubles, cross-sectional

area is squared, but body mass is cubed– Body weight increases faster than relative strength of

supporting bones

• This scaling inequality is “principle of similitude”


“Principle of similitude”

2

10

2

Cross-sectional area = 4Volume = 40

44

20

Cross-sectional area = 16Volume = 320


Anisometry of pelycosaur femurs(note different shapes as well as different sizes)

decreasing size of animal


Population variation

• Variation among individuals within a population is called population variation

• Sources of population variation are:– Genetic differences among individuals– Ecophenotypic variation – Sexual dimorphism– Taphonomic effects


Populations

• Biologic definition of population = “a group of individuals of the same species living close enough together that each individual of a given sex has a chance of mating with an individual of the other sex”– “breeding population”

• Populations are characterized by a single gene pool– Gene flow occurs when two or more populations

interbreed


Genetic variation: Alternation of generations in forams

“megalospheric”(asexually produced)

“microspheric”(sexually produced)


Ecophenotypic variation

• Variation among individuals as a consequence of differences in their environments:– Nutrition– Exposure to sunlight (plants; animals with

phtotsynthesizing symbionts)– Space (crowding)– Environmental stability


Sexual dimorphism in ammonoids

dimorphicpair

dimorphicpair


Fossil populations

• Not as easy to work with as biologic (living) populations

• Sources of difficulty– Sedimentary mixing (reworking; bioturbation)

• Time-averaging; loss of temporal resolution

– Preservation bias• Distortion• Dissolution (reduces observable variation)• Post-mortem sorting


Structural distortion of bivalve shapes

undeformedshape

direction ofrock cleavage


Effects of selectivepost-mortem transport


Fossil populations (cont.)

• Additional example of population “biasing” by selective transport

• Devonian brachiopods– Leptocoelia (879 pedicle; 893 brachial)– Platyorthis (561 pedicle; 548 brachial)– Leptostrophia (378 pedicle; 35 brachial)

untransported, or notselectively transported

selectively transported

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Ch. 2—Key concepts