Palaeotreta shannanensis Zhang & Holmer & Chen & Brock 2020, gen. et sp. nov.
Creators: name: Zhang, Zhiliang
name: Holmer, Lars E.
name: Chen, Feiyang
name: Brock, Glenn A.
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Brachiopoda
Lingulata
Acrotretida
Acrotretidae
Palaeotreta
Palaeotreta shannanensis

description: Palaeotreta shannanensis gen. et sp. nov.
(Figs 2–7; Table 1)
Derivation of name. After the occurrence in Shannan, southern Shaanxi Province.
Holotype. ELI-XYB S4-3 AU-01, ventral valve (Fig. 2I–L, length [L] = 1196 M m, width [W] = 1331 M m).
Paratype. ELI-XYB S4-3 AV-18, dorsal valve (Fig. 3H–K, L = 1008 M m, W = 1226 M m).
Type locality. Shuijingtuo Formation (layer S4-3, 0.7 m above the base) at Xiaoyangba section (32 Ǫ 29, 29 ,, N, 107 Ǫ 57, 2 ,, E), Zhenba County, south-eastern Shaanxi, South China. Cambrian Series 2.
Other material. A total of 21 ventral valves and nine dorsal valves were recovered from the same layer (S4-3) in the Shuijingtuo Formation.
Description. Shell ventribiconvex, sub-circular in outline with round posterior margin (Figs 2, 3). 1.1 M m hemispherical pits evenly distributed on the whole metamorphic shell surface without overlapping (Fig. 4F, G), while the post-metamorphic shell is covered by finely circular growth lines and drape structures (Fig. 4H). Shell structure consists of thin-lamella (2 M m) primary layer and thin-lamina (5 M m) secondary columnar layers (Fig. 5H–M).
Ventral valve sub-circular, on average 89% as long as wide with maximum width at mid-length (Table 1). The valve is convex, with a cap-like shape (Fig. 2), on average 21% as deep as long, with maximum height almost at mid-valve. Metamorphic shell pronounced at the apex (Fig. 2C, N), occupying 13% of valve length. Pseudointerarea weakly developed, catacline to apsacline. Intertrough poorly defined, very short, occupying on average about 2% of the length and 6% of the width of the valve (Fig. 2I–P). Apical process vestigial, only observed in adult specimens, close to pedicle foramen, occupying 20% of valve length (Fig. 4B, C). Enclosed pedicle foramen almost circular, about 50 M m in diameter, located mostly directly outside of the metamorphic shell, until valve reaches about 1100 M m in length. Growth lines developed at the posterior margin of the metamorphic shell and the lateral side of the pedicle foramen (Fig. 4C). Cardinal muscle scars and vascula lateralia weakly impressed.
Dorsal valve sub-circular, on average 87% as long as wide, with maximum width almost at mid-valve (Table 1). It is slightly convex (Fig. 5), on average about 19% as deep as long. Pseudointerarea small, orthocline, occupying 6% of the valve length and 36% of valve width (Fig. 5E, H). Median groove sub-triangular, short, about 31% of the pseudointerarea width (Fig. 4E). Median buttress generally developed, fading anteriorly (Fig. 4E, F). Median septum vestigial, only developed in adult valve, extending anteriorly for 59% of valve length (Fig. 5H–K). Cardinal muscle scars weakly impressed, occupying 17% of the length and 45% of the width of the valve (Fig. 5D).
Remarks. Palaeotreta shannanensis gen. et sp. nov. shares a similar morphology in outline with Eohadrotreta zhenbaensis (from the same locality). However, the new species has a low cap-like shape, small pseudointerarea with very short intertrough, apsacline pseudointerarea in the adult valve, prolonged pedicle foramen-forming stage and a pedicle foramen that is located outside of the metamorphic shell (Figs 6, 7). Furthermore, the new species has a less developed intertrough as compared with E. zhenbaensis (Z.-L. Zhang et al. 2018a); in the latter species the intertrough is becoming remarkably prominent in stage T 3, which does not happen in the new species. The shell structure of P. shannanensis is similar to that of E. zhenbaensis. However, the former species has a very thin primary layer about 2 M m thick, and very thin secondary columnar layers. The thickness of columns in P. shannanensis is variable in different parts of the shell, ranging from 4 M m to 8 M m, and are quite short compared to the columns of E. zhenbaensis. The secondary layer insignificantly increases the overall thickness of the ventral valve (Z.-L. Zhang et al. 2016), being composed of one to two columnar laminae, resulting in a low cap-like shape in P. shannanensis.
Palaeotreta shannanensis differs from Vandalotreta djagoran (Kruse, 1990) and Linnarssonia rowelli (Pelman, 1973) in having a lower ventral valve, weakly developed apical process and median septum, apsacline ventral pseudointerarea, as well as a pedicle foramen that is mostly located outside the metamorphic shell (Holmer et al. 1996, pl. 13; Ushatinskaya & Korovnikov 2019, pl. 4). P. shannanensis has a similar inclination of the ventral pseudointerarea as Aphelotreta khemangarensis (Popov et al. 2015), while the ornamentation of the metamorphic shell, pedicle foramen size and the median groove allow the distinction of the two species (Popov et al. 2015, fig. 21).
At the Xiaoyangba section, P. shannanensis occurs at the base of the Shuijingtuo Formation, while E. zhenbaensis first occurs 60 cm higher. Therefore, P. shannanensis represents the oldest known acrotretide brachiopod in southern Shaanxi. The diachronous nature of the stratigraphic hiatus at the base of Cambrian Series 2 across southern Shaanxi and western Hubei makes the biostratigraphic correlation between these regions imprecise, and more index fossils from new sections are required to better constrain the time gap and better resolve correlation.
description: Published as part of Zhang, Zhiliang, Holmer, Lars E., Chen, Feiyang & Brock, Glenn A., 2020, Ontogeny and evolutionary significance of a new acrotretide brachiopod genus from Cambrian Series 2 of South China, pp. 1569-1588 in Journal of Systematic Palaeontology 18 (19) on pages 1572-1578, DOI: 10.1080/14772019.2020.1794991, http://zenodo.org/record/10932614