Download - Ch. 2—Key concepts
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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
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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
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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
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Types of skeletal growth
1. Accretion (enlargement) of existing parts
2. Addition of new parts
3. Molting
4. Modification
5. Combinations (mixed growth strategies)
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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
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Bivalve accretion
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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
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Crinoid stalk(addition)
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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
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Trilobite molting
Instars = growthstages between molts
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Molting (cont.)
Molting produces growth ina series of discrete episodes(not continuous)—Instarsfrom different growth stages form distinct morphologic clusters
instars
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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
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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
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Combined growth strategy(coiled cephalopod)
periodic additionof new septa
continuous accretionof new material alongleading edge of shell
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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)
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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
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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
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Brachiopod example:Length vs. width
Growth Series:scatter of datapoints suggestschange in shapeduring growth
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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)
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Brachiopod example:Length vs. width
Individual ontogeny:growth curves for singlespecimens confirm change in shape, ANDallow estimate ofvariation among individuals
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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
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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
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Isometric growth
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Anisometric growth
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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”
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“Principle of similitude”
2
10
2
Cross-sectional area = 4Volume = 40
44
20
Cross-sectional area = 16Volume = 320
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Anisometry of pelycosaur femurs(note different shapes as well as different sizes)
decreasing size of animal
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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
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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
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Genetic variation: Alternation of generations in forams
“megalospheric”(asexually produced)
“microspheric”(sexually produced)
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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
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Sexual dimorphism in ammonoids
dimorphicpair
dimorphicpair
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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
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Structural distortion of bivalve shapes
undeformedshape
direction ofrock cleavage
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Effects of selectivepost-mortem transport
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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