Ch. 2—Key concepts

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 Fossils & Evolution Ch. 2

2. 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 Fossils & Evolution Ch. 2

3. 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 Fossils & Evolution Ch. 2

4. Types of skeletal growth • Accretion (enlargement) of existing parts • Addition of new parts • Molting • Modification • Combinations (mixed growth strategies) Fossils & Evolution Ch. 2

5. 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 Fossils & Evolution Ch. 2

6. Bivalve accretion Fossils & Evolution Ch. 2

7. 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 Fossils & Evolution Ch. 2

8. Crinoid stalk(addition) Fossils & Evolution Ch. 2

9. 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 Fossils & Evolution Ch. 2

10. Trilobite molting Instars = growth stages between molts Fossils & Evolution Ch. 2

11. Molting (cont.) Molting produces growth in a series of discrete episodes (not continuous)—Instars from different growth stages form distinct morphologic clusters instars Fossils & Evolution Ch. 2

12. 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 Fossils & Evolution Ch. 2

13. 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 Fossils & Evolution Ch. 2

14. Combined growth strategy(coiled cephalopod) continuous accretion of new material along leading edge of shell periodic addition of new septa Fossils & Evolution Ch. 2

15. Recognizing and describing ontogenetic change • Biologists can directly observe ontogenetic change, but paleontologistscannot • 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) Fossils & Evolution Ch. 2

16. 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 Fossils & Evolution Ch. 2

17. 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 Fossils & Evolution Ch. 2

18. Brachiopod example:Length vs. width Growth Series: scatter of data points suggests change in shape during growth Fossils & Evolution Ch. 2

19. 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) Fossils & Evolution Ch. 2

20. Brachiopod example:Length vs. width Individual ontogeny: growth curves for single specimens confirm change in shape, AND allow estimate of variation among individuals Fossils & Evolution Ch. 2

21. 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 Fossils & Evolution Ch. 2

22. 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 Fossils & Evolution Ch. 2

23. Isometric growth Fossils & Evolution Ch. 2

24. Anisometric growth Fossils & Evolution Ch. 2

25. 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” Fossils & Evolution Ch. 2

26. “Principle of similitude” 10 20 2 2 Cross-sectional area = 16 Volume = 320 Cross-sectional area = 4 Volume = 40 4 4 Fossils & Evolution Ch. 2

27. Anisometry of pelycosaur femurs(note different shapes as well as different sizes) Fossils & Evolution Ch. 2 decreasing size of animal

28. 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 Fossils & Evolution Ch. 2

29. 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 Fossils & Evolution Ch. 2

30. Genetic variation: Alternation of generations in forams “megalospheric” (asexually produced) “microspheric” (sexually produced) Fossils & Evolution Ch. 2

31. 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 Fossils & Evolution Ch. 2

32. Sexual dimorphism in ammonoids dimorphic pair dimorphic pair Fossils & Evolution Ch. 2

33. 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 Fossils & Evolution Ch. 2

34. Structural distortion of bivalve shapes direction of rock cleavage undeformed shape Fossils & Evolution Ch. 2

35. Effects of selectivepost-mortem transport Fossils & Evolution Ch. 2

36. 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 not selectively transported selectively transported Fossils & Evolution Ch. 2