Post‐metamorphic allometry in the earliest acrotretoid brachiopods from the lower Cambrian (Series 2) of South China, and its implications

Abstract

Abstract:The earliest growth of post-metamorphic (postlarval) shells in two species of Eohadrotreta is described from the Cambrian Shuijingtuo Formation of South China. Two different growth patterns can be observed by quantifying developmental variations in size and shape of successive stages of post-metamorphic shell growth (including the pedicle foramen forming stage, pedicle foramen enclosing stage and intertrough increasing stage) of Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis. The pedicle foramen is never enclosed within the metamorphic shell of E. zhenbaensis, while the enclosed pedicle foramen of E.? zhujiahensis is located directly outside the metamorphic shell after the pedicle foramen enclosing stage. A strongly allometric growth pattern of E. zhenbaensis is demonstrated by the early enclosure of the pedicle foramen; an accelerated lengthening of the ventral intertrough is associated with the development of a more complex dorsal median septum during the intertrough increasing stage. By contrast, E.? zhujiahensis demonstrates possible paedomorphic development by delayed enclosure of pedicle foramen and an associated decreased lengthening of ventral intertrough during the intertrough increasing stage. This ontogenetic developmental sequence represents the marginal accretionary formation and growth of the pedicle foramen, which resembles that of linguloid brachiopods. Furthermore, the developmental process of the pedicle foramen of Eohadrotreta seems to recapitulate the likely evolutionary transition from the Botsfordiidae, with open delthyrium, to the Acrotheloidea, with an enclosed foramen. This study provides a unique opportunity to obtain a complete understanding of the ontogenetic development of the earliest acrotretoids, and casts new light on the phylogeny of lingulate brachiopods.

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

“…All specimens conform to the T2 stage of development sensu Zhang et al . () except U, which conforms to the T3 stage. A–D, specimen SMNH Br150138, ventral valve: A, apical view; B, posterior view; C, lateral view; D, enlarged apical view.…”

“…The ontogeny of the acrotretid taxon Eohadrotreta zhenbaensis Li & Holmer, from the Shackleton Limestone is described in detail, building on the work of previous authors (Zhang et al . ).…”

“…zhenbaensis (Zhang et al . , ). Eohadrotreta zhenbaensis occurs in both the uppermost Shackleton Limestone and Shijingtuo Formation of the Zhenba–Fangxian and Yangtze Blocks, where it co‐occurs with the eodiscid trilobites Tsunyidiscus niutitangensis (Yang et al .…”

“… Eohadrotreta zhenbaensis Li & Holmer, from the Shackleton Limestone articulated valves. All specimens conform to the early parts of T2 stage of development sensu Zhang et al ., () except C–D, which is of the T1 stage. A–B, specimen SMNH Br150152, conjoined valves: A, posteroventral view; B, enlarged view of ventral metamorphic shell, showing pitted micro‐ornamentation; C–D specimen SMNH Br150153: C, posteroventral view, showing open delthyrium; D, apical view; E–F, specimen SMNH Br150154: E, enlarged view of the intertrough, pedicle opening and beak formed by dorsal metamorphic shell; F, apical view.…”

“…The palaeobiology of Eohadrotreta from the Shuijingtuo Formation (Cambrian Series 2) of the Three Gorges Area of Shaanxi, China, has received considerable recent attention (Zhang et al . , , b). Specimens described here do not conform exactly to the diagnosis of Li & Holmer () or to the detailed description provided by Zhang et al .…”