The earliest known Stegerhynchus (Rhynchonellida, Brachiopoda) from the Hirnantian strata (uppermost Ordovician) at Borenshult, Östergötland, Sweden (original) (raw)
Related papers
Biodiversity, biogeography and phylogeography of Ordovician rhynchonelliform brachiopods
2013
The phylogeographical evolution and the consequent changing distribution and diversity of rhynchonelliform brachiopods through the Ordovician are linked to the dynamic palaeogeography of the period. The Early Ordovician (Tremadocian and Floian) is characterized by globally low-diversity faunas with local biodiversity epicentres, notably on the South China Palaeoplate; low-latitude porambonitoid-dominated faunas with early plectambonitoid and clitambonitoid representatives, as well as high-latitude assemblages mostly dominated by orthoids, can be recognized, but many taxa are rooted in Late Cambrian stocks. The Early Ordovician displays a steady increase in rhynchonelliformean biodiversity, which was mostly driven by the increasing success of the Porambonitoidea and Orthoidea, but the billingsellids and early plectambonitoids also contributed to this expansion. During the Early to Mid Ordovician (Dapingian-Darriwilian), marine life experienced an unprecedented hike in diversity at the species, genus and family levels that firmly installed the suspension-feeding benthos as the main component of the Palaeozoic fauna. However, this may have occurred in response to an early Darriwilian annihilation of existing clades, some of which had been most successful during the Early Ordovician. New clades rapidly expanded. The continents were widely dispersed together with a large number of microcontinents and volcanic arcs related to intense magmatic and tectonic activity. Climates were warm and sea-levels were high. Pivotal to the entire diversification is the role of gamma (inter-provincial) diversity and by implication the spread of the continents and frequency of island arcs and microcontinents. The phylogeographical analysis demonstrates that this new palaeogeographical configuration was particularly well explored and utilized by the strophomenides, especially the Plectambonitoidea, which radiated rapidly during this interval. The porambonitoids, on the other hand, were still in recovery following the early Darriwilian extinctions. Orthides remained dominant, particularly at high latitudes. Biodiversity epicentres were located on most of the larger palaeoplates, as well as within the Iapetus Ocean. Provincial patterns were disrupted during the Sandbian and early Katian with the migration of many elements of the benthos into deeper-water regimes, enjoying a more cosmopolitan distribution. Later Katian faunas exhibit a partition between carbonate and clastic environments. During the latest Katian, biogeographical patterns were disrupted by polewards migrations of warm-water taxa in response to the changing climate; possibly as a consequence of low-latitude cradles being developed in, for instance, carbonate reef settings. Many clades were well established with especially the strophomenides beginning to outnumber the previously successful orthides, although this process had already begun, regionally, in the mid to late Darriwilian. At the same time, atrypoid and pentameroid clades also began to radiate in low-latitude faunas, anticipating their dominance in Silurian faunas. The Hirnantian was marked by severe extinctions particularly across orthidestrophomenide clades within the context of few, but well-defined, climatically controlled provincial belts.
Palaeozoic brachiopod extinctions, survival and recovery: patterns within the rhynchonelliformeans
Geological Journal, 2001
The brachiopod-dominated seabeds of the Palaeozoic were modi®ed by a series of biotic extinctions and recoveries. The ecologic and taxonomic dynamics of the Cambrian and Paleozoic Evolutionary Faunas were interrupted during the late Cambrian, end-Ordovician, late Devonian and end-Permian by a multiple series of extinctions. While the balance between the deltidiodont and cyrtomatodont rhynchonelliformeans was maintained during the Palaeozoic, the dominant role of the orthides and strophomenides during the early to mid-Palaeozoic was usurped by the productides during the late Palaeozoic. The deltidiodont morphology did not survive the end-Permian extinctions; the modern articulated brachiopod fauna is dominated by cyrtomatodonts with crurae and loops. Nevertheless, a recurring trend is the long-term success of brachiopod taxa with widespread distributions and the demise of the more specialized and highly adapted morphs having more restricted distributions commonly evolved during interregna between phases of extinction.
Juvenile shells of the strophomenate brachiopods Antigonambonites (Polytoechioidea, Billingsellida), Plectella , Bilobia and Ujukella (Plectambonitoidea, Strophomenida), Leptaena (Strophomenoidea, Strophomenida) and Valdaria (Ortothetoidea, Orthotet-ida) from the Lower to Middle Ordovician of the East Baltic and the Silurian of Gotland preserve exceptional morphology of the umbonal region in both valves, which allows reconstruction of early stages of ontogeny and inferred larval morphology. The first formed shell in all taxa is about 350–600 μ m wide, marking a growth stage very soon after metamorphosis when secretion of calcite was initiated. Dorsal valves preserve two pairs of lateral lobes, which can be interpreted as sets of two pairs of larval setal sacs, and a posteromedian mound resembling the 'alimentary mound' of the paterinide larval shell. Ventral valves at that stage possess a well-developed pseudodeltidium and a short, wide pedicle sheath formed by accretionary growth involving the ventral mantle lobe. The delthyrium is completely sealed at the earliest observed growth stages, and there is no gap between the valves along the hinge line. The position of the pedicle opening suggests that it was not formed by a larval pedicle lobe, but represents a modification of the ventral mantle lobe. This can be compared most closely with the pedicle of chileides and dictyonellides, but not of rhynchonellates. Brachiopoda, strophomenata, Ordovician, Silurian, East Baltic, Gotland, early ontogeny. Michael G. Bassett [mike.bassett@museumwales.ac.uk] and Leonid E. Popov [leonid.popov@museumwales.ac.uk],
Journal of Paleontology, 2004
Holorhynchus giganteus Kiaer, 1902, a common Late Ordovician (mid-Ashgill) pentameride brachiopod in the Baltic region, Kazakhstan, and southern Tien Shan, is documented for the first time from the Badanjilin Formation (mid-Ashgill) of western Inner Mongolia (Alxa block), North China. Serial sections of the Chinese material confirm the presence of a vestigial ventral median septum in the early growth stage of H. giganteus, but the septum becomes embedded in the secondary shell thickening at the adult growth stage. A survey of the type material from Norway and additional material from other regions indicates that the incipient ventral median septum is a much more commonly developed structure than was previously believed. The presence of a well-developed pseudodeltidium in the Tien Shan material of H. giganteus and the absence of such a structure in conspecific material from many other regions require a systematic revision of the generic group. Holorhynchus has rodlike crura (ϭbrachial processes) that do not form flanges at their junctions with the inner hinge plates (ϭouter plates ϭ crural plates) and outer hinge plates (inner plates). This, together with the development of a crude spondylial comb structure, points to its affinity to the Virgianidae rather than to the Stricklandiidae. Holorhynchus can be regarded as a Lazarus taxon because of its absence during the crisis (Hirnantian) and survival (early-middle Rhuddanian) intervals associated with the Late Ordovician mass extinction and its reappearance in Kazakhstan and North China during the Early Silurian (late Rhuddanian-early Aeronian). The mid-Ashgill Holorhynchus fauna, typified by a number of large-shelled pentamerides, was common in the Baltic region, the Urals, Kazakhstan, Tien Shan, Alxa, Qaidam, Kolyma, and east-central Alaska, but largely absent from Laurentia and Siberia (except for Taimyr) in the ancient tropical-subtropical regions. This paleobiogeographic pattern agrees with the general pattern of the Late Ordovician brachiopod provincialism.
Journal of Paleontology, 2004
Stegerhynchus was perhaps the most common rhynchonellide brachiopod in the Early Silurian (Llandovery) postextinction brachiopod fauna of North America. In shell morphology, it closely resembles the Late Ordovician rhynchonellide Rhynchotrema in having a rostrate shell posterior and simple, strong costae. It differs internally from Rhynchotrema primarily in having a much-reduced septalium (ϭcruralium). Superficially Stegerhynchus has a conservative, typically simple, rhynchonellide morphology, but detailed biometric analysis of large samples from the Ͼ400 m-thick Llandovery (mid-Aeronian to mid-Telychian) tropical carbonate succession of Anticosti Island reveals that the genus underwent significant evolutionary and ecophenotypic changes over about five million years. The shell length/width ratios of Stegerhynchus peneborealis from sediments of a low-energy, muddy substrate tend to be lower than those of S. deltolingulatus new species from high-energy, reef or inter-reef settings. A wider shell and hingeline probably had a greater stability on the soft seafloor. Also a wider shell appears to be associated with a larger (relative to shell size) sulcus size at the anterior margin. The changes in relative sulcus size either represent ecophenotypic variation in response to environmental conditions, or a random effect of evolutionary selection through time. Both multivariate and single-character analyses demonstrate that two important biometric features, the apical angle and shell flank rib number, separate all forms of Anticosti Island Stegerhynchus (middle Aeronian to middle Telychian) from the widely cited European species Stegerhynchus borealis (latest Telychian-Wenlock) from the type area of Gotland, Sweden. The temporal changes of these two characters are interpreted to be evolutionary modifications.