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Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, U.S.A.
Climacograptus typicals and its varieties, along with the closely related C. pygmaeus, are abundant at many horizons in North American Caradoc strata. Morphometric analysis of colony form in C. typicalis and C. pygmaeus, from large collections of isolated, three-dimensionally preserved material, reveals low levels of variation for a colony. The analysis of astrogeny and its relationship to colony form suggests that many aspects of overall form are highly correlated with astogeny and that differences in the astogenetic pattern of C. typicalis and C. pygmaeus are correlated with some of the differences in colony form that distinguish the two species.
A theoretical framework is needed for the incorporation of astogenetic data into systematic and phylogenetic practice. The graptoloid colony was an integrated, functional entity and a unit of selection. Astogeny was a colonial character that displayed a morphometric and evolutionary pattern of change that suggested an analogy with von Baer’s Law and its evolutionary interpretation: morphogenesis during astogeny proceeds from generalized states to specific stages and shared astogenetic pattern is indicative of shared descent. This is proposed as a working hypothesis in the formulation of systematic relationships and phylogenies.
