Evolutionary significance of a middle Cambrian (Series 3) in situ occurrence of the pedunculate rhynchonelliform brachiopod Nisusia sulcata

Abstract

Exceptionally preserved, silicified and articulated complete shells of the rhynchonelliform kutorginate brachiopod Nisusia sulcata are redescribed from the middle Cambrian (Series 3) Marjum Limestone, Utah. Cylindroid sausage‐like protrusions, emerging posteriorly between the valves, were originally interpreted as faecal in origin, but restudy under the SEM shows that these features represent silicified pedicles as they are attached in situ to other Nisusia. The Nisusia host most likely was alive at the time of attachment. Restudy of the pedicles of Nisusia provides new phylogenetic information on the anatomy of the earliest rhynchonelliforms. The silicified pedicles differ considerably from the pedicles of living crown group rhynchonelliforms in being strongly annulated, distally tapering, and were likely to have been rather more flexible. The Nisusia pedicles are more similar to the exceptionally preserved pedicles from other Cambrian rhynchonelliform brachiopods, including Kutorgina, Longtancunella and Alisina, but these emerge from the ventral apical foramen rather than from between the valves as in Nisusia. Although generally similar, these two types of pedicles are unlikely to represent homologous structures as Nisusia is provided with both an apical foramen (possibly larval attachment) and a posterior adult pedicle. The similarities may be explained by similar type of accretionary growth from two different types of epithelia. The Nisusia‐like pedicle appeared early within the kutorginates and rhynchonellates. The discovery of hollow spines in Nisusia sulcata further supports the generic assignation of the species.

Hyoliths with pedicles illuminate the origin of the brachiopod body plan

“…Our results instead imply a stepwise transformation from an ancestrally linguliform-like pedicle to the derived state of extant rhynchonelliforms: the pedicle migrated from the hinge to the umbo in early diverging Salanygolina [15] and chileids [41] (character 120), but retained its coelomic cavity (character 32) until the kutorginates had diverged. On this basis, we interpret the two pedicle openings in Nisusia [36] as representing an ontogenetic relocation of the pedicle.…”

“…It has been argued that the pedicles of linguliforms and rhynchonelliforms are not homologous [36][37][38], in part because they are secreted by different parts of the organism (the ventral mantle in linguliforms [39]; a larval pedicle lobe in living rhynchonelliforms [40]). Our results instead imply a stepwise transformation from an ancestrally linguliform-like pedicle to the derived state of extant rhynchonelliforms: the pedicle migrated from the hinge to the umbo in early diverging Salanygolina [15] and chileids [41] (character 120), but retained its coelomic cavity (character 32) until the kutorginates had diverged.…”

Early Cambrian (Stage 4) brachiopods from the Shipai Formation in the Three Gorges area of South China

“…As discussed by Holmer et al (2017Holmer et al ( , 2018, Nisusia has two pedicle openings, an apical foramen and a posterior median opening (between the delthyrium and notothyrium). Nisusia liantuoensis from the Shipai Formation shows a well-developed apical opening (Fig.…”

Cambrian rhynchonelliform nisusioid brachiopods: phylogeny and distribution

“…Both openings have been interpreted as pedicle openings, but cannot represent homologous structures since they both occur in the same specimen, and must have been secreted by different epithelia (Holmer et al . ,b). Soft‐part preservation in Kutorgina from the Chengjiang Biota shows how the pedicle emerges through the ventral umbonal foramen (Zhang et al .…”

“…; Holmer et al . ); in contrast, the pedicle of Nisusia sulcata Rowell & Caruso, emerges from the posterior opening, between the valves, as shown by the silicified complete valves with still‐attached in situ pedicles (Holmer et al . ).…”