PHYLOGENETIC ANALYSIS OF LATE ORDOVICIAN NORTH AMERICAN RHYNCHONELLID BRACHIOPOD GENERA LEPIDOCYCLUS, HYPSIPTYCHA AND HISCOBECCUS

The rhynchonellid brachiopod genera Lepidocyclus, Hypsiptycha and Hiscobeccus have a long history of study and subsequent debate on their phylogenetic affinities. These taxa are found across North America (Laurentia)in Late Ordovician units of the mid-continent. Previous interpretations of evolution within this group have focused on the analysis of distinct morphological characteristics determined by expert opinion largely based on external data. As a result, conflicting interpretations of generic evolution and specific assignment within these genera exist in the literature. Recent publication of enhanced species diagnoses and serial sections data additional diagnostic character states have become available, allowing for a preliminary species level phylogenetic reconstruction.

In this study, a phylogenetic analysis was conducted using parsimony analysis based on a character state matrix of both external and internal characters. To reduce biases resulting from absent character states in taxa both discrete and continuous character states (based on morphometric values) are used in this study. Results from the analysis confirm the monophyly of Lepidocyclus and Hypsiptycha while Hiscobeccus is interpreted to form a paraphyletic clade. Lepidocyclus is distinguished by well developed costae and growth lamellae, a delthyrium closed by medially conjunct deltidial plates, dental plates reduced. Hypsiptycha is distinguished by a closed delthyrium, subpentagonal to subtriganular outline and a nearly obsolete septalium. Within the traditional Hiscobeccus clade, the diagnosis is modified to include an open delthyrium, relatively strong dorsal and ventral valve depths with nearly equal to relatively large width to shell thicknesses ratios; species with reduced dorsal and ventral valve depths are accompanied by relatively small width to shell thicknesses ratios are excluded from the genus and transferred to two new genera.