Investigating heterochrony in the fossil record: A geochemical, morphometric, and phylogenetic study of Thecideida (Brachiopoda), Triassic-recent

This is an empirical study of the relationship between development and evolution in fossil and living thecideide brachiopods. Thecideide brachiopods are an order of marine, minute (1–15 mm), cemented, cryptic, pan-tropical to warm temperate articulated brachiopods possessing a unique brachidium and a long fossil history (Triassic to Recent). Heterochrony is an evolutionary change in the timing of development, and can be a change in rate, duration or timing of onset or the offset of a developmental process. The study of heterochrony requires information on growth rate, phylogeny, shape change. In a three part study, I combine phylogenetic, stable isotope sclerochronologic, and morphometric analyses to determine the role of changing development in the origin and evolution of thecideides.; Species-level morphological phylogenetic analysis of thecideides reveals that thecideide brachiopods are monophyletic. I constructed a 58 character by 48 species matrix, and analyzed the data using parsimony and a heuristic search in PAUP*. The sister taxon to thecideides remains elusive, two extant subclades containing are resolved, and many Triassic and Jurassic species are basal to the extant subclades. Phylogenetic resolution increases with increasing taxon sampling density; support indices values decrease with increasing taxon sampling densities. The relationships and character polarities indicated by phylogenetic results reveal a mosaic of heterochrony within Thecideida, and a role of evolutionary novelty in the origin of the dorsal median septum, the primary lophophore support in thecideides.; Stable isotope sclerochronology, the use of shell oxygen isotope ratios to determine seasonality, in combination with thecideide phylogeny, reconstructs the ancestral condition of thecideide longevity to be only a few years. Thecideides secrete calcite in isotopic equilibrium. All thecideide groups have short lived individuals only. A single evolutionary, progenetic (foreshortened growth) event is responsible for thecideide small size.; Outline morphometrics of over 500 thecideide brachidial outlines helps reveal a complex history of heterochrony within thecideides as a group, and within single evolutionary transitions. Juvinilization combines with extended development in the monoseptate subclade, and in the more diverse multiseptate subclade, both juvemlization and extension occur. There is a linear relationship between lophophore length and lobe number. Thecideides are not paedomorphic.