Palaeotreta

Description. Shell ventribiconvex, sub-circular to transversely oval, ornamented with evenly distributed pits, about 1 M m in diameter covering the metamorphic shell, and with finely concentric growth lines on the post-metamorphic shell. Ventral valve cap-like to low conical, pseudointerarea vestigial, from catacline to apsacline, with very short intertrough. Incipient pedicle notch is open during the long pedicle foramen-developing stage. Adult pedicle foramen located outside of the metamorphic shell. Growth lines developed on a separate area between the metamorphic shell and pedicle foramen. Apical process vestigial, very close to the pedicle foramen. Cardinal muscle scars and vascula lateralia weakly impressed. Dorsal valve slightly convex. Pseudointerarea orthocline, short with narrow subtriangular median groove. Median buttress generally well developed, while median septum vestigial. Cardinal muscle scars weakly impressed. Secondary shell layer columns relatively short.

Remarks. The new genus is established for acrotretide species that are similar to Eohadrotreta and Linnarssonia. However, the new genus differs from all other (especially from Cambrian Epoch 2) acrotretides in having the following characters: (1) a catacline to apsacline inclination of ventral pseudointerarea with short intertrough; (2) a pedicle foramen that is located mostly outside the metamorphic shell; (3) a vestigial apical process and median septum; and (4) weakly impressed cardinal muscle scars. Eohadrotreta, Prototreta, Vandalotreta, Hadrotreta and Kotylotreta mainly have procline pseudointerarea with a relatively long intertrough. Although the apsacline ventral pseudointerarea and internal structures (apical process, median septum) of Palaeotreta can be compared with Cambrian Epoch 3 taxa, such as Aphelotreta and the Furongian Linnarssonella, the positions of pedicle foramen and ventral pseudointerarea are different. The ventral apical process and dorsal septum of Palaeotreta are very weakly defined, while Linnarssonia develops a high, boss-like apical process and has a high median septum. Eohadrotreta has a relatively more well-developed apical process and median septum. Vandalotreta has an apical process that forms a boss-like thickening anterior to the internal foramen and Hadrotreta develops apical pits that are situated close to the apical process (Holmer & Popov 2000). Kuangshanotreta is difficult to compare with the new genus in the absence of data on shell morphology and ornamentation (Wang et al. 2012). Palaeotreta is the third genus of the Acrotretida that has been described from the Cambrian Series 2 of South China, and this region has a higher diversity of coeval acrotretides as compared to South Australia, Laurentia, Antarctica and Siberia. The most unique character of Palaeotreta is the position of the pedicle foramen, which is located mostly outside of the metamorphic shell from the T 2 ontogenetic stage. In this feature it has similarities to some Cambrian Ceratretidae and Ordovician Scaphelasmatidae species (Holmer 1989; Holmer & Popov 2000); but to date, this character is not known from early Cambrian Acrotretidae.

Type species. Palaeotreta shannanensis gen. et sp. nov.; Cambrian Series 2 of southern Shaanxi, South China. Derivation of name. Palaeo, ‘ old’, from the Greek palai ́ os, describing an ancestral representative of acrotretide brachiopods, with the ending – treta, ‘ perforated’ from the Greek tr ̄ etos.