EVOLUTION AND MORPHOLOGICAL DIFFERENTIATION OF ORDOVICIAN BRACHIOPODS EOCHONETES AND THAERODONTA

The genus Thaerodonta Wang, 1949 was erected to include a suite of strophomenid brachiopods from Middle to Late Ordovician of North America characterized by small, elongate valves with hinge-line denticles and dorsal lateral septa. Thaerodonta has been synonymized with the Baltic genus Eochonetes Reed, 1917, re-recognized as a valid taxon, and subsequently synonymized again several times. The ambiguous relationship between these genera is promoted by the high degree of intraspecific variation within species and the general sympatry of “Thaerodonta” and Eochonetes sensu stricto species. Although the relationship between Thaerodonta and Eochonetes has previously been assessed based on character data, these comparisons have lacked an explicit phylogenetic framework.

In this study, we test whether Thaerodonta is an evolutionary lineage distinct from Eochonetes within a phylogenetic and morphometric context. Type and references specimens from museum collections were examined for 18 species assigned to Eochonetes or Thaearodonta. Prior to phylogenetic analysis, species validity was assessed using multivariate morphometric analysis of 9 characters. Approximately forty internal and external morphological characters were examined for each species. Both discrete and continuous characters were used. Continuous characters states were differentiated through morphometric analysis and coded as qualitative states. Species-level phylogenetic hyoptheses were generated using both parsimony and Bayesian analysis.

The resulting well-resolved phylogenetic topologies support the differentiation of Eochonetes and Thaerodonta as discrete evolutionary lineages. The monophyly of Thaerodonta is supported by multiple synapomorphies. These results indicate the importance of generic and species level characters in examining evolutionary differentiation. The trends in morphological evolution within these lineages also highlight the importance of biogeography as a driver of evolutionary patterns.