GEOS 26400/36400/BIOS 23255/EVOL 32300: Principles of Paleontology

Morphology, I.

I. Introduction to form
      -Three components of form:
      1. size
      2. shape: dimensionless combination of size measures
      3. dependence between size and shape (largely expressed via ontogeny)
	      -ontogeny: growth and development of individual organism
	      -astogeny: development in colonial organisms

II. Types of growth
      -accretionary
      -addition of parts
      -molting
      -modification
      -mixed
      *Adult "stuck" with juvenile form to different extent, depending on kind of growth.

III. Types of growth study
      -longitudinal (desirable)
      -cross-sectional (often our only choice in fossil studies)
            -often approximates longitudinal pattern, but need not do so
      -obtaining longitudinal data from organisms with accretionary growth
      *Age can be difficult to assess; we often use a proxy such as size.
        -see Gryphaea example under Heterochrony (below) for age estimation.

IV. Isometry and Allometry 
      -isometric (same shape) vs. anisometric (changing shape) growth
      -reasons for anisometric growth
            -change in function (e.g., metamorphosis)
            -retention of function as size changes (Principle of Similitude)
                  -area:volume(mass) relations as examples
      -allometry as special kind of anisometry
            -x&y are two size measures
             -y=bx^a (b=relative growth coefficient; a=allometric coeff.)
             -equivalently: ln(y)=+a*ln(x)+ln(b)
            -scales at which allometry is studied
		-ontogenetic (longitudinal)
		-intraspecific (cross-sectional)
		-evolutionary (ancestor-descendant; species-level analog of longitudinal study)
		-interspecific (species-level analog of cross-sectional)
            -brain:body size example
                  -larger organisms with absolutely larger but relatively smaller brains
                  -ontogenetic changes
                  -changes in encephalization
      -use of predicted allometric relationships to test functional hypotheses
            *example: body size versus respiratory area in pleurocystitid echinoderm

V. Heterochrony
      -changes in rate and timing of development
        -examples of timing: absolute growth rate; relative rate of 
		somatic development vs. sexual maturation
      -may allow large evolutionary change from small genetic change,
            especially if many traits are coupled or coordinated
      -example: paedomorphosis (descendant adult resembles juvenile ancestor)
            -Paedomorphosis is a pattern that could have different
            underlying causes (faster growth rate vs. prolonged growth).
            -Jones and Gould (1999) study of Jurassic Gryphaea (bivalves)
                  -possible advantage of paedomorphosis: larger shells and
                  juvenile-shaped shells more stable on soft substrates (experimental flow-tank studies)
                  -exact chronological dating by sclerochronology and oxygen isotopes
                  -age/size relationship suggests descendants grew faster rather than longer

      *"Global" heterochrony probably less common than has often been claimed
	-Example: Olenelloid trilobites (Webster and Zelditch 2005)
	-"Overall" appearance suggests evolution of Nephrolenellus geniculatus
		from N. multinodus by peramorphosis (extended development
		of descendant along same ontogenetic trajectory as
		ancestor) [Webster and Zeldith, Fig. 3]
	-This is but one example of many "classic" cases of heterochrony in trilobites.
	-Analysis of individual landmarks suggests localized differences in
		ontogenetic trajectory and how it varies from ancestor to descendant.