Ontogeny and evolutionary significance of a new acrotretide brachiopod genus from Cambrian Series 2 of South China

Abstract

Micromorphic acrotretide brachiopods first appeared during Cambrian Epoch 2 and subsequently experienced a rapid diversification. However, our knowledge of acrotretide origins and early evolution is hampered by our poor understanding of their earliest representatives. Here, we describe one of the oldest known acrotretides from the Cambrian Series 2 Shuijingtuo Formation of southern Shaanxi and western Hubei, South China. The new genus Palaeotreta gen. nov. can be distinguished from all other genera, mainly by its unusual ontogeny and morphology; in Palaeotreta the pedicle foramen is located mostly outside of the metamorphic shell, the ventral intertrough is very short and the inclination of the ventral pseudointerarea changes from catacline to apsacline. Two ontogenetic stages (T1-T2) of post-metamorphic shell growth can be distinguished in the low cap-like Palaeotreta shannanensis gen. et sp. nov.; this type of development with a relatively long pedicle foramen forming stage (T1), a simple pedicle opening, weakly developed ventral apical process and dorsal median septum, can be considered to represent a plesiomorphy for the acrotretides. By contrast, the more derived Palaeotreta zhujiahensis (Li & Holmer, 2004) is characterized by paedomorphosis, expressed in an early enclosure of pedicle foramen and greatly decreased growth of the ventral intertrough during the intertrough-increasing stage (T3), producing a cap-like shape and retaining catacline inclination of the ventral pseudointerarea. Thus, heterochrony may have played an important role in the morphological divergence of acrotretides during the Cambrian evolutionary radiation, and promoted the subsequent radiation of acrotretides. The ontogenetic study of Palaeotreta gen. nov. also supports the notion that the ventral intertrough of acrotretide brachiopods may have formed by the 'rolling up' of the lingulide propareas.

Go large or go conical: allometric trajectory of an early Cambrian acrotretide brachiopod

“…7G; Ushatinskaya 2010; Zhang 2018; Zhang et al . 2020 b ). The development of this conical shape together with the adaptive fitness of acrotretides across the early Palaeozoic strengthens the evidence for a similar, homologous basic body plan across the Acrotretida, which was rapidly derived from a common ancestor through different heterochronic pathways in different taxa (Fig.…”

“…Although, the majority of acrotretide taxa possess a similar, homologous basic body plan (Ushatinskaya 2010; Zhang 2018; Zhang et al . 2020 b ), characterized by their conical ventral valve, our study of E . zhenbaensis demonstrates that a conical ventral valve is formed through two successive phases of the entire ontogeny of the species.…”

“…This hypothesis, known as the ‘rolled‐up’ lingulide hypothesis (Popov 1992; Holmer & Popov 2000; Zhang et al . 2020 b ) suggests that a raised ventral lingulide valve is the source of the conical valve, with the reduced ventral propareas partly forming the acrotretide intertrough, the acrotretide enclosed pedicle foramen modified from the lingulide pedicle groove, and the advanced median septum derived from lingulide median ridge (Popov 1992; Holmer & Popov 2000; Zhang et al . 2018 a ).…”

“…zhenbaensis and 5 ventral valves of Palaeotreta zhujiahensis (Zhang et al . 2020 b ), a taxon similar to E . zhenbaensis , were added to our dataset, sourced from other Shuijingtuo Formation sections at Aijiahe and Wangzishi (Zhang et al .…”

“…2020; Zhang et al . 2020 b ), little attention has been paid to ontogenetic development in Cambrian animals and the potential importance of shape variation as an evolutionary driver in Cambrian communities has rarely been quantitatively assessed. One recent example is the work of Zhang et al .…”