INVESTIGATING THE EVOLUTIONARY HISTORY OF BRACHIOPOD BIOMINERALIZATION

Where and when in metazoan phylogeny does biomineralization originate? To what extent are biomineralization and mineralogy shaped by environmental and evolutionary processes? How do biomineralization products evolve within clades over time? In Brachiopoda, calcium phosphate mineralization has been commonly accepted as the basal state, based largely on decades-old stratigraphic data on Cambrian small shelly fossils thought to represent stem-group brachiopods. New data indicate that phosphate and carbonate mineralogies appear essentially simultaneously in the Cambrian stratigraphic record, meaning the question of biomineral origination must be addressed with phylogenetic methods. Our comprehensive comparative study will address this question using an emerging molecular consensus on broad patterns of phylogenetic relationships within Spiralia.

Transcriptomic data were generated for 21 species of brachiopods and combined with published data on 85 additional spiralians and outgroups. We compiled data on soft-and hard-part morphology, embryology, and development for 17 species of extant spiralians, and on fossil morphology for 32 species of Cambrian tommotiid and non-tommotiid taxa. We analyzed these sources of data separately and in combination using parsimony, Maximum Likelihood, and Bayesian methods, allowing us to compare ancestral state reconstructions relevant to mineralization and mineralogy among the Spiralia.

Our analyses reveal a monophyletic Lophophorata, with Phoronida and Ectoprocta as sister clades together sharing common ancestry with Brachiopoda. Our analyses also suggest that a lack of mineralization is basal for Lophophorata, with mineralization of calcium carbonate evolving more than once independently within this clade. Biomineralization is absent in phoronids, which retain the basal state. Calcite mineralization is basal for Brachiopoda and appears to have evolved twice among bryozoans. Apatitic and siliceous mineralization have evolved once each from the basal calcitic composition in brachiopods, in conflict with the traditional interpretation of ancestral phosphatic mineralogy.

This study highlights the power of combined molecular and paleontological analyses to resolve competing evolutionary hypotheses.