Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Harper, Gallagher

Diversity, disparity and distributional patterns amongst the orthide brachiopod groups

Journal of the Czech Geological Society, volume 46 (2001), issue 3-4, 87 - 94

The basic architecture of the deltidiodont articulated brachiopod (strophomenates and early rhynchonellates) was established by the early Cambrian and diversified into a wide variety of morphologies during the early Ordovician radiation, prior to radiations amongst the more advanced cyrtomatodont types. The deltidiodont division includes the pedunculate protorthides (early Cambrian-late Devonian), orthidines (mid Cambrian-mid Devonian) and dalmanellidines (early Ordovician-latest Permian). New classifications for the orthides, presented in the revised Treatise, are analysed using a number of tree metrics: The orthidine tree has a Stratigraphical Consistency Index (SCI), Relative Completeness Index (RCI) and a Gap Excess Ratio of 0.375, 78.79 %, 0.83 respectively whereas the dalmanellidine tree has SCI, RCI and GER values of 0.35, 48.47 %, and 0.395. The relatively low values of tree metrics for the punctate orthides partly reflect a less complete knowledge of dalmanellidine phylogeny. Many orthides originated and developed in shallow-water environments but radiated later into quieter, deeper-water niches or more specialised cryptic habitats. Radiations occured as step-wise waves of diversification simulating ecological displacements by successive individual superfamilies within the Orthida and through the early Palaeozoic; peaks in diversity are matched by expansions in morphological disparity. The early to mid-Cambrian orthide radiation occured at high latitudes; but by the early Ordovician most orthide families had widespread distributions. Few orthides occur in later Palaeozoic faunas. Macroevolutionary divergences presumably during the mid to late Cambrian, reflected at the family level, were apparently decoupled from later generic diversifications during the Ordovician together with abundance patterns of species and ecological events within superfamilial taxa.