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

Morphology, II: Evolutionary Morphology

I. Constructional Morphology
      -"Seilacher's Triangle"
            -interplay among three factors
                  -historical (phylogenetic) 
                  -functional (adaptive)
                  -structural (morphogenetic, architectural, fabricational)
            -historical corner as "holding bag"; put traits there depending on scale of analysis

II. Functional Morphology
      -homology as key to function (e.g., wings in extinct birds)
      -analogy as key to function  (e.g., wings in pterosaurs)
      -Biomechanical analysis
	A. Paradigm approach
            1. propose function(s)
            2. design optimal structure for function
            3. assess match between actual and optimal structure
            -e.g. trilobite lens doublets
	B. Experimental approach
            1. propose function(s)
            2. make a model of actual structure (exact or simplified, physical or numerical)
            3. experimentally determine whether structure capable of function
            -e.g. wing plates in Pterotocrinus (crinoid)
      -Advantages of Biomechanical approaches
                  -universality of physical and chemical properties
                  -allows inference where homologues or analogues unknown
      -Disadvantage: limited by the range of potential functions considered
      -Combined functional and statistical arguments
            -E.g., Statistical distribution of dinosaur leg morphology
            compared with distributions for birds and mammals (Carrano 1998).
      -Taphonomic evidence (e.g. platyceratid gastropods on crinoids)
      -Other evidence, e.g. geographic distribution in nektonic trilobites (Fortey 1989)

III. Theoretical Morphology
      -comparison between spectrum of theoretically possible forms and distribution of realized forms
      -incomplete and non-random occupation of morphological space
      -coiling parameters as example
            -logarithmic spiral: allows constant shape as size increases
            -theoretical upper and/or lower limits of coiling parameters
            -generation of theoretical shells vs. measurement of actual shells
      -some questions illustrated with coiling models
	1. alternative modes of life
            -"the problem of bivalveness"
	    -"the problem of univalveness"
            -distribution of ammonoids in W-D space
                  -closed coiling ==> confinement of ammonoids below W=1/D    
	2. POSSIBLY: relationship between size of developmental change and size of phenotypic change
                  -two kinds of limpets
      --Branching models used to illustrate additional questions
        3. bryozoan example: trade-offs and limits to optimality
      	4. land plant example: trade-offs and multiple adaptive modes
      -Outstanding question: How can we increase the taxonomic scope of
	studies in theoretical morphology?
	-Skeleton Space (Thomas and Reif 1993) as one possible step in this direction