Wellerella girtyi

Introduction

The diversity of life has long fascinated scientists. The theory of natural selection provides a unifying framework in which to consider the evolution of diversity through time; however, consensus is still lacking in our understanding of how organisms have explored their potential morphospaces in such a way to produce the plethora of species on Earth.

The idea of a morphospace, and adaptive landscapes therein, provides quantitative rigor in addressing questions of morphologic diversity, but here again little consensus exists, even on model predictions of evolution given assumed adaptive scenarios.

In the current project we seek to bring a phylogenetic perspective to the question of diversification within a morphospace, namely a generalized morphospace for pseudo-logarithmically expanding shells, in order to test hypotheses about how diversity evolves, whether it is similar to random diffusion, driven by adaptive "basins of attraction," or the result of genetic and developmental constraints on organism structure.

Here we present preliminary results achieved towards this goal. Specifically, a phylogenetic tree based on 35 discrete morphologic multistate characters was recreated for 46 species in the subfamily Wellerellinae (Brachiopoda: Wellerellidae). These species largely originated and went extinct in the Permian of West Texas.

Methods

The Guadalupe and Glass Mountains of West Texas contain a rich silicified fossil record of Permian brachiopods.

We selected the subfamily Wellerellinae for phylogenetic and morphometric study because of its high species diversity (n = 46), well-defined taxonomy, and interesting ontogenically varying shell morphology.

35 discrete multi-state characters were identified based on published phylogenies and taxonomic accounts (Cooper and Grant 1976a; Leighton and Maples 2002) and from personal observation. First and last occurrences where obtained from Cooper and Grant (1977) and stratigraphic ranges were estimated using the maximum likelihood method of Foote (1997). Using MacClade v.4 (Maddison and Maddison 2003) we reconstructed a most parsimonious stratigraphically-explicit phylogenetic tree using these 35 characters.

Whorl expansion rate was calculated as proposed by McGhee (1980) using R (r-project.org) and ImageJ (rsbweb.nih.gov/ij) to analyze digitized photographs were collected by Cooper and Grant (1976b).

Results

The phylogeny inferred from Cooper and Grant?s (1976a) taxonomy is fairly comparable to the most parsimonious tree as compared to randomly generated equiprobable trees.

The Wellerellinae experienced substantial diversification during the middle to late Permian. Homoplasy is quite common, particularly in the two younger clades. With a preservation rate of R = 0.69, this fossil record is reliably complete. The likelihood surface is notably flattened near its maximum, and as such values within the ranges R:[0.57,0.79], and mu:[0.53,0.67] are all approximately equiprobable.

The Wellerellinae are characterized by a non-uniform whorl expansion rate described by a piecewise continuous function. Early in ontogeny the brachial valve expands moderately quickly, followed rapid expansion, and ending with slow expansion approaching W = 1.

While only a subset of species have been analyzed, variation exists in the onset of these various stages, as well as the disparity between fast and slow expansion.

Discussion and Future Work

The comparability of Cooper and Grant's (1976a) taxonomy and our stratigraphically-explicit phylogeny is surprising and encouraging. That pre-phylogenetic classification could approach the replicablity of recent cladistic methods was not expected, yet perhaps it means that older classifications are not entirely unreliable. The high level of homoplasy in our phylogenetic reconstruction begs for further analysis; particularly a consideration of character exhaustion is warranted to see if the concentration of homoplasies in derived clades is a result of their being derived, or of radiation about a potentially adaptive phenotype.

With estimates of preservation and extinction rates (R and mu, respectively) it will be possible to estimate the duration of each lineage and evaluate whether duration varies across the Permian or is predicted by morphologic characteristics; i.e. more adaptive traits should persist the longest. Such an analysis will be particularly interesting in conjunction with a consideration of how the phylogeny occupies the morphospace.

In order to understand what evolutionary mechanisms drive the diversification of the Wellerellinae in their theoretical morphospace, we will model three unique hypotheses:

(1) that evolution is the result of random diffusion

(2) that evolution is driven by basins of attraction within the morphospace (i.e. adaptive peaks)

(3) that genetic and developmental constraints determine extent and direction of diversification

Acknowledgements

We wish to thank M. Sangrey for coordinating the logistics of this project, G. Hunt for advice and commentary on quantitative methods, and the summer 2008 Natural History interns for their scholarly commentary and camaraderie. This project was made possible by a grant from the National Science Foundation to D.H.E.
grant number EAR 0451802.

References

Cooper GA, and Grant RE. 1976a. Permian Brachiopods of West Texas, IV (Part 1 - Text). Smithsonian Contributions to Paleobiology 21:1923-2283.

Cooper GA, and Grant RE. 1976b. Permian Brachiopods of West Texas, IV (Part 2 - Figures). Smithsonian Contributions to Paleobiology 21:2288-2607.

Cooper GA, and Grant RE. 1977. Permian Brachiopods of West Texas, VI. Smithsonian Contributions to Paleobiology 32:1923-2283.

Foote M. 1997. Estimating taxonomic duration and preservation probability. Paleobiology 23:278-300.

Leighton LR, and Maples CG. 2002. Evaluating internal versus external characters: Phylogenetic analyses of the Echinoconchidae, Buctoniinae, and Juresaniinae (Phylum Brachiopoda). Journal of Paleontology 76:659-671.

Maddison WP, and Maddison DR. 2003. MacClade: Analysis of Phylogeny and Character Evolution v.4.06. Sinauer Associates, Sunderland, MA.

McGhee GR. 1980. Shell form in biconvex articulated Brachiopoda: A geometric analysis. Paleobiology 6:57-76.