Morphology: how does it vary and what does it reveal about evolution? Ontogenetic and phylogenetic variation in extant brachiopod crura and loops

University Of California-Davis, Davis CA

Investigators

Abstract

Morphology? how does it vary and what does it reveal about evolution? Ontogenetic and phylogenetic variation in extant brachiopod crura and loops Abstract Organism shape is the medium through which the underlying genetic makeup of the organism interacts with the environment. Only by studying shape and how and why it varies, can evolutionary biologists begin to understand the product of underlying genes/environment interaction. Because shape is the primary source of heritable information from extinct (no longer living) organisms, paleontologists assume that the boundaries of shape variation correspond closely to clades (evolutionary entities), as implied by the hierarchy of rank-based taxa. This assumption is seldom tested. This research project will test the clade status of taxa recognized in extant Brachiopoda, one of the most paleontologically significant metazoan groups in terms of diversity and abundance for over half a billion years. Roughly 95% of species are extinct; with the 5% of rhynchonellide and terebratulide brachiopod species extant (still living), we have a rare opportunity (1) to test molecular and morphological phylogenies at different hierarchical levels, never before attempted for this clade; (2) to compare the size and shape of functionally significant features, primarily features called crura and loops, among adults within and among species, and across growth stages; (3) to use this information to produce quantitative comparisons among adults and growth stages in crura and loops, and place the growth transformations in an evolutionary context informed by both shape and molecules. If paleontologists cannot reliably distinguish evolutionarily informative from significantly misleading shape variation, evolutionary interpretations drawn from the observed variation are fundamentally untestable. The results of this study on living brachiopods can have profound implications for macroevolutionary interpretations derived from taxonomic data on any group of extinct metazoans, in revealing problems associated with uncritically accepting untested assumptions of homology (shared common ancestry of features). Analyses will be conducted on representative species of all living rhynchonellide and terebratulidine genera, and all lving terebratellidine genera for which molecular sequences have been made available, using parsimony and Bayesian analysis. Growth variation in crura and loops will be quantified using geometric shape analysis on 3D images obtained from microCT scans of calcareous lophophore supports in living brachiopods. Growth character polarity in crural and loop development will be compared with phylogenetic polarity using outgroup analysis. Tree comparison methods will be utilized to evaluate patterns of qualitative and quantitative morphological character distribution, and to compare results from molecular and morphological analyses. Total evidence analyses will also be conducted and compared with those derived from morphology alone and molecular sequence data alone. Although the word brachiopod is not a household word, as is clam or snail, brachiopods provide an excellent opportunity to study changes in morphology over lifespans and lineages, with enormous extinct diversity preserved in the fossil record and a smaller, more manageable diversity living today. This research project will contribute to the education of undergraduate and graduate students through coursework in evolutionary biology and paleontology at UC Davis, and will support the training and mentoring of a female graduate student and female post-doctoral researcher. We will take advantage of opportunities to share our research results with the public at Picnic Day (a UC Davis campus-wide annual open house) and the Explorit Science Center (City of Davis). Three-dimensional images of brachiopods will be fully accessible on a website created to share the visual results of our research. We are beginning to develop a more sophisticated understanding of the development and evolution of morphology in vertebrate animals, yet our understanding of evolutionary morphology across a much more diverse array of marine invertebrate animals lags well behind. Our research will strive to introduce and explain a portion of the fascinating paleobiological complexity of one group of marine invertebrates to the general public, and engage the public in discussions about the nature of evolution: recognizing, characterizing, and interpreting change over time.