Evolution of the Late Ordovician Plaesiomyid Brachiopod Lineage in Laurentia

During the Late Ordovician, a transgression flooded much of Laurentia. The fauna of these intracratonic basins became differentiated from the fauna of the pericratonic shelves and platforms, typically displaying gigantism and coarser shell ornamentation. In this study, 509 specimens from 11 species of the Plaesiomyidae brachiopod family from the Katian and Hirnantian were measured, of which 198 included in principal component analysis to quantify morphological changes over this interval. Three trends were revealed: 1) increasing globosity and dorsal convexity from the early to late Katian, 2) coarser, but fewer ribs on species from the paleoequatorial intracratonic seas compared to species from the mid- to high-tropical pericratonic shelves and platforms, and 3) enlargement of the cardinal process from early to late Katian and predominance of a trilobed cardinal process in the intracratonic species versus bilobed cardinal process in pericratonic species. Two new species were assigned, and two existing species were reassigned

Evolution, Paleoecology, and Paleobiogeography of the Late Ordovician Brachiopod Fauna of Laurentia

During the early Katian (Late Ordovician), the North American craton was being inundated due to a major eustatic sea level rise and regional subsidence associated with the ongoing Taconic orogeny. The Trentonian brachiopod fauna, as a dominant group of the marine shelly benthos at that time, evolved and invaded the expanding epicontinental seas. Three Trentonian brachiopod lineages were studied to trace their evolution. The Rostricellula-Rhynchotrema-Hiscobeccus lineage was characterized by an increase in shell size, globosity, and frilled lamellae, with Hiscobeccus becoming a prominent component of the Late Ordovician epicontinental brachiopod fauna. Parastrophina is a widely reported but non-dominant taxon of the Trentonian brachiopod fauna in eastern North America. This study clarified details of the internal structures of the type species, P. hemiplicata, and quantified the high degree of intraspecific variability, particularly the development of ribs, fold, and sulcus. This has led to the recognition of Parastrophina tarimensis from northwest China as a new species. Plectorthis is another key taxa of the Trentonian fauna, previously reported from several tectonic paleoplates. By examining the type species, P. plicatella, from the Cincinnati area in this study, the diagnostic characters of the genus were defined, which provided a basis for evaluating the North American and global distribution and diversity of this genus and a more accurate paleobiogeographic analysis. Multivariate analyses of the Trentonian brachiopod collections from Manitoulin Island and the Lake Simcoe area in Ontario, in conjunction with other Trentonian faunas from North America, demonstrated that during the early stage of the Late Ordovician (early Katian) sea level rise and marine inundation of the continental interior, the epicontinental sea brachiopod fauna began to separate from the older pericratonic faunas. In Ontario, this is reflected by the brachiopod fauna of Lake Simcoe being more closely related to the Scoto-Appalachian fauna, and that of Manitoulin Island most similar to those in the intracratonic basins (e.g. Minnesota)

LATE ORDOVICIAN BRACHIOPOD RONGATRYPA XICHUANENSIS FROM XICHUAN, HENAN PROVINCE, CENTRAL CHINA

The atrypide brachiopod Rongatrypa Popov & Cocks, 2014 is one of the early members of the subfamily Clintonellinae. This genus was previously known only from the Kazakh terranes. Here, we reassign a species to the genus, Rongatrypa xichuanensis (Xu, 1996), from the Shiyanhe Formation (Katian, Upper Ordovician) of Xichuan, Henan Province, central China. A wide range of shell sizes was found and measured to investigate the ontogeny of the species, and several specimens were selected for serial sectioning to examine the internal morphology. The linear regression results of natural logarithms of length vs. width and depth vs. width revealed an allometric growth pattern, perhaps influenced by the development of the lophophore. Rongatrypa xichuanensis inhabited a shallow marine oxygenated environment in the South China palaeoplate near the palaeo-equator. The distribution of Rongatrypa across South China and Kazakh terranes reflects the proximity of these blocks in the Late Ordovician