Altaethyrella(Brachiopoda) from the Late Ordovician of the Tarim Basin, Northwest China, and its significance

Abstract

Altaethyrella tarimensis, a new species of rhynchonellide brachiopod, is described from the late Katian (Late Ordovician) Hadabulaktag Formation in the Kuruktag region of Xinjiang, Northwest China on the northeastern edge of the Tarim Basin. Serial sections of the shell clearly show no dorsal median septum or septalium in the dorsal valve, and no spiralia or atrypide-style crura. Like other species of the genus, A. tarimensis n. sp. exhibits a high degree of intraspecific variation, including variations in shell shape and size, number of ribs in the sulcus at the anterior, and degree of asymmetry. The discovery of Altaethyrella in Tarim has important paleogeographic implications, indicating a close relationship between the Late Ordovician brachiopod faunas of Tarim and those of the Kazakh terranes and North and South China paleoplates, supporting a recently published paleogeographic projection that places Tarim near the Chu-Ili terrane during the Late Ordovician. The abundant large biconvex shells of A. tarimensis n. sp. would have provided a firm substrate for encrusting filter feeders like bryozoans to establish on the Kuruktag Platform.UUID: http://zoobank.org/df8843cd-4db0-48e7-ba03-bf0ce81c4f01

Paleogeographic and paleoecological significance of Schachriomonia (Brachiopoda) from the Upper Ordovician of the Tarim Basin, Northwest China

“…Such a relationship agrees with the findings from the rhynchonellides already published. The ancistrorhynchide Altaethyrella tarimensis Sproat and Zhan, 2018 occurs alongside S. pentagonia at the collection locality in Tarim and is very similar in terms of shell shape and structures to Altaethyrella otarica (Rukavishnikova, 1956) found in the Chu-Ili Range, further solidifying the paleogeographic ties between these blocks.…”

“…The main goal of this study is to systematically document Schachriomonia pentagonia (Fang in Liu and Fang, 1990) from this locality, but the size and diversity of the collection also provides an opportunity to discuss several important aspects of this species from Tarim. Along with the previously reported rhynchonellide Altaethyrella tarimensis (Sproat and Zhan, 2018), finding Schachriomonia on the Tarim paleoplate has paleogeographic implications as the precise position of the plate during the Late Ordovician remains uncertain. The unusually high abundance of these shells at a single sampling horizon may indicate a short-term disruption of the benthic environment that allowed this species to proliferate as an opportunistic colonizer of the seafloor or may reflect a substrate selectivity during the larval stage of these brachiopods.…”

“…In the description, Fang noted a prominent dorsal groove on the fold and a sulcus with a single prominent rib, indicating that those shells are not Rhynchotrema, which almost always has four ribs on the dorsal fold and three in the ventral sulcus (Sproat et al, 2014). Many species that were described as Rhynchotrema in China are now being recognized as belonging to one of the atrypide genera (as in here) or the externally similar rhynchonellide Altaethyrella (Zhan and Li, 1998;Sproat and Zhan, 2018). The dimensions of the shells in the collection of Liu and Fang (1990) are within the range of the specimens we describe here (see Table 1 for summary statistics of each measurement), although our collection additionally includes shells that are much smaller than what they reported.…”

“…Nonetheless, the distinctive ‘tongue’ formed by the dorsal fold and ventral sulcus at the anterior with a single prominent rib in the sulcus and a median furrow flanked by two ribs on the dorsal fold allows for easy identification of these specimens as Schachriomonia . Some specimens can be mistaken for the similar Altaethyrella tarimensis Sproat and Zhan, 2018, but the rhynchonellides are generally larger and more globose and always feature several prominent costae in the sulcus. The other atrypides from Central Asia and China have different ribbing patterns and shell shapes except for the very similar Sulcatospira , which may yet be proven to be congeneric with this genus (see discussion in Systematic paleontology).…”

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Figure 2.Upper Ordovician stratigraphy in the study region (adapted from Zhang and Munnecke, 2016) and photos of the outcrop. Modified from Sproat and Zhan (2018). ( 1 ) Outcrop of the Hadabulaktag Formation showing typical lithology.

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