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.
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- Cambrian Epoch 2
- micromorphic brachiopods
- pedicle foramen