Morphological and Taxonomic Diversity in Clade's History: The Blastoid Record and Stochastic Simulations
Year: 1991
Type: article
Abstract: Ordovician-Permian Blastoidea are used to compare morphological and taxonomic diversity.Morphological diversity, based on the locations (in three dimensions) of homologous landmarks on the theca, is compared t o published estimates of generic richness.Rarefaction of morphological distributions with respect to sample size (number of species) is presented as one way to analyze the structure of diversity, i.e., the relationship between morphological and taxonomic diversity.This allows the assessment of morphological diversity per unit taxonomic diversity.Any subset of a clade is less diverse than the clade as a whole.However, Permian blastoids occupy morphospace more extensively than the entire class in the sense that small, random subsamples from the whole class are not as diverse morphologically as are similar samples from the Permian.Whereas maximal generic richness occurs in the Lower Carboniferous, maximal morphological diversity is found in the Permian.Generic richness rises from the Ordovician to the Lower Carboniferous, then drops off, but morphological diversity may increase essentially until the group's demise.Artificially degrading the record so that each stratigraphic interval is represented by a narrow temporal and geographic window does not substantially alter observed diversity patterns.Silurian and Devonian morphological diversification may outpace taxonomic diversification, but this observation depends on how morphological diversity is measured.Use of the center of gravity and the median to describe clade shape suggests that morphological diversity in the Blastoidea is concentrated later in time than taxonomic diversity.Because unequal time intervals bias measures of central tendency, such measures should be compared to the central tendency inherent in the time scale, rather than to the temporal midpoint of a clade's history.Stochastic simulations of taxonomic and morphological evolution yield clades whose morphological diversity is concentrated later in time than their taxonomic diversity.The prevalence of this difference between morphological and taxonomic diversity increases with the duration of the simulated clade and the number of morphological characters.By their very nature, randomly evolving clades exhibit diffusion through morphospace and extinction acting randomly with respect to morphology.This combination allows clades to maintain and even increase their morphospace occupation in the face of declining taxonomic diversity.Although it cannot be concluded that the blastoids represent a randomly evolving clade, smaller-scale directionallties may "cancel out" when the clade is viewed at a sufficiently large scale, so that large-scale cvolutionary patterns are not readily d~stinguishable from randomness.
Author: Mike Foote
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W160601351