Paleo 2002 Sept 10, 2002
Pinkie variation and sources of variation
Taphonomic (preservational) effects
Determining age in fossils
Growth rates and patterns
-determinate and indeterminate growth
-environmental controls
Relative growth (size and shape)
-isometric growth and allometric growth
Sources of variation
(consider pinkies)
Overall body size (generically based, parental lineages, watusi vs bantu)
Age (thus size)
Gender (often correlated with overall size) – sexual dimorphism: when different genders have
different appearances
Environmental effects (nutrition, injury)
Taphonomic (preservational) effects. Damage and distortion
Individual organisms vary, both within a species (look around room), between species (look around) and individuals look different depending on their state of growth, gender, ethnic heritage and other factors.
Major source of variation between individuals is a function of growth and how organisms grow is a major influence on their size and shape, so first:
Types of growth
-accretion adding new material to existing skeleton; mollusks, (example with growth rings), trees, fish scales, some vertebrate teeth
-adding new parts. Additional segments in trilobites, adding new plates in echinoderms, new chambers in cephalopods and forams
-molting. periodic shedding of skeleton and formation of a new one after a burst of rapid growth. Arthropods molt. Trilobite cephalon measurements
diarticulated bits - arthropod molts. More than one potential fossil per individual. Some antlers are shed each year in some vertebrates
-modification. re-formation and re-shaping of original material as size increases. Vertebrate bone
-mixed. trilobites add segments while molting. Echinoderm plates accrete as new ones added; cephalopods accrete and add walls between chambers
Determining age in
fossils (ontogenetic age: age since
birth, not geologic age)
-growth rings: if growth is accretionary and somewhat discontinuous (slows or stops during some part of the year), the counting rings will estimate growth Examples: trees, mollusk shells
-key morphological features that appear at some stage in development.
In mammals, the ends of the bone are fused to the rest at the adult size (fused epiphyses).
Onset of sexual maturity. Example: antlers and horns in some mammals; many soft part features, but often not preservable
allows dating as “older than” or “younger than” - two categories
-tooth wear - in mammals, where tooth generation is limited, tooth wear a measure of age (don’t look a gift horse in the mouth). Doesn’t work with sharks or dinosaurs - because tooth replacement is continuous through life.
-size. bigger is usually older, in general, but approach is limited because not all organisms grow continuously through life (mammals ands birds don’t), and in others, growth rates vary within the lifespan.
Growth rates and patterns
Typically, organisms grow rapidly when young, and then slow down when large: two basic patterns of growth are common.
-determinate growth: rapid growth to an adult size, with no growth thereafter (birds, mammals)
-indeterminate growth: rapid growth followed by very slow growth throughout the rest of the lifespan. Most invertebrates, other vertebrates (besides mammals and birds), trees.
Growth can, of course be either continuous or discontinuous. When we talk about growth in spurts, we are talking about slightly discontinuous growth, so these are really end-members of a broad spectrum. Examples:
-Gulf of California clams. Growth starts in late March, speeds up in spring and early summer, slows or stops, speeds up and then shuts down in late November.
-Trilobite growth. Rapid growth between instars (molts)
Relative growth (size
and shape)
Typically, organisms change shape as they increase in size. This is the topic of relative growth - the change in one dimension of size relative to another dimension of size.
When individuals change in shape with an increase in size, as in humans or dogs, for example, growth is said to be allometric: shown graphically
When there is no change in shape with an increase in size, (as in lizards), growth is said to be isometric: shown graphically
Size-required allometry A lot of allometric growth can be said to be size-required allometry. Two examples:
1. Bone strength. In vert bone, larger limb (support) bones look different than the same bones when smaller or in smaller species. Why?
Consider this: the mass or weight of an organism is a function of its volume. Volume is the cube of a linear dimension.
Bone strength is a function of the cross-sectional area of the bone. Area is the square of a linear dimension. So..., if an animal doubles in size, its mass or weight increases eight-fold (2 to the 3rd power). However, without a change in shape, the cross-sectional area of bone would only increase four-fold (2 to the 2nd power). Thus, bones must get disproportionately thicker as size increases.
Problem with movie monsters - giant insects. Lizards. Limb strength, respiration (surface area).
Thermoregulation in pelycosaurs
function of the sail? Protection, sexual display, thermoregulation?
If thermoregulation, sail area would increase disproportionately in larger and larger pelycosaurs.
It does (other functions for sail also still possible).
Other sources of variation
-sexual dimorphism
not just in soft parts. Often size and shape differences between genders
example: Jurassic ammonites Callomon=s work (same range, same distribution, same initial shapes in whorls. Which male , which female?
-environmental effects (nutrition, injury)
-taphonomic effects
post-mortem damage, compression
-genetic (within species and between species)