Brachiopods from the Byrd Group (Cambrian Series 2, Stage 4) Central Transantarctic Mountains, East Antarctica: biostratigraphy, phylogeny and systematics

Abstract

Brachiopods from Cambrian Series 2, Stage 4 carbonate strata of the Byrd Group in the Central Transantarctic Mountains, East Antarctica, are described for the first time. These include six lingulate, one paterinate and one rhynchonelliform taxon, including the new lingulate brachiopod Plicarmus wildi gen. et sp. nov. The biostratigraphy correlates closely to the brachiopods recently reported from the Xinji Formation (Shuiyu section) in North China, as well as brachiopods recovered from the Dailyatia odyssei Zone across the Arrowie Basin of South Australia. These findings also support the previously identified close palaeobiogeography of these regions. The first unambiguous example of the acrotretid brachiopod Eohadrotreta zhenbaensis Li & Holmer outside South China is also identified in the context of its ontogenetic stages. Well-preserved specimens of the acrotheloid Schizopholis yorkensis (Holmer & Ushatinskaya) facilitate a new reconstruction of its musculature and visceral region. These data are synthesized into a new cladistic analysis that resolves Acrotheloidea as a well-supported monophyletic clade and supports previous hypotheses of a morphocline in acrotheloid evolution.

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

“…This also revealed that Palaeotreta zhujiahensis experienced three stages during the entire ontogenetic sequence, which can be well compared with the almost coeval E. zhenbaensis ( Fig. 10; Z.-L. Zhang et al 2018a;Claybourn et al 2020). When compared to the ontogeny of E. zhenbaensis, however, only two ontogenetic stages (T1-T2) can be distinguished during the growth of P. shannanensis (Figs 2, 6, 7).…”

“…Compared with the other two species P. zhujiahensis and E. zhenbaensis (Z.-L. Zhang et al 2018a), P. shannanensis has a more restricted distribution and lower abundance, which may indicate this ancestral acrotretide was not suited to life in distal mixed clastic platforms. By contrast, E. zhenbaensis has more derived characters exemplified by faster growth rates during accretionary shell growth, especially during stage T3, enabling a wider geographical distribution and increased abundance in shallow carbonate environments (Li & Holmer 2004;Betts et al 2017;Z.-L. Zhang et al 2018a;Claybourn et al 2020). The evidence presented here suggests evolutionary modification to ontogenetic changes resulted in the early divergence of Palaeotreta and Eohadrotreta, and played an important role in the early evolution of acrotretides.…”

“…The new evidence presented here supports the hypothesis that the ventral intertrough in acrotretide brachiopods evolved by the gradual and variable 'rolling up' of the lingulide propareas along the posterior margin of the ventral valve in association with changes in the inclination of the ventral intertrough (Popov 1992;Holmer & Popov 2000;Li & Holmer 2004;Z.-L. Zhang et al 2018a). A distinct evolutionary lineage from basal botsfordiids that developed an open delthyrium and vestigial ventral pseudointerarea to more derived acrothelids which have an enclosed pedicle foramen and raised ventral pseudointerarea probably indicates that this monophyletic clade is derived from the lingulides (Popov et al 2015;Smith et al 2016;Claybourn et al 2020;Zhang et al 2020).…”

“…To date, only six acrotretide genera, including Eohadrotreta, Linnarssonia, Prototreta, Vandalotreta, Kuangshanotreta and Hadrotreta, are known from Cambrian Series 2 (Holmer & Popov 2000;Li & Holmer 2004;Ushatinskaya 2010). Recently published works show that Eohadrotreta rapidly achieved a wide distribution over key palaeocontinents (South China, East Gondwana, West Gondwana, North China), soon after their first appearance in Cambrian Epoch 2 (Wang et al 2012;Popov et al 2015;Z.-L. Zhang et al , 2017Z.-L. Zhang et al , 2018aPan et al 2019;Claybourn et al 2020). Two species of Eohadrotreta (E. zhenbaensis and E.?…”

First report of acrotretoid brachiopod shell beds in the lower Cambrian (Stage 4) Guanshan Biota of eastern Yunnan, South China

“…By the Devonian, this group diminished and finally went extinct (Bassett et al, 1999; Holmer and Popov, 2000). Our present knowledge of Cambrian acrotretoid brachiopods is largely based on material obtained by dissolving carbonate rocks in Kazakhstan, Scandinavia, the Ural Mountains (Popov and Holmer, 1994; Holmer et al, 2001), United States (Streng and Holmer, 2006), Canada (Robson et al, 2003), Antarctic (Holmer et al, 1996; Claybourn et al, 2020), Morocco (Streng, 1999), south Australia (Betts et al, 2017, 2019), Himalaya (Popov et al, 2015), and China (Li and Holmer, 2004; Zhang et al, 2016; Pan et al, 2019). These acrotretoid faunas are well preserved in three-dimension, including fine details of ornamentation and delicate shell structures.…”