Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution

Abstract

Orthrozanclus is a shell-bearing, sclerite covered Cambrian organism of uncertain taxonomic affinity, seemingly representing an intermediate between its fellow problematica Wiwaxia and Halkieria. Attempts to group these slug-like taxa into a single ‘halwaxiid’ clade nevertheless present structural and evolutionary difficulties. Here we report a new species of Orthrozanclus from the early Cambrian Chengjiang Lagerstätte. The scleritome arrangement and constitution in this material corroborates the link between Orthrozanclus and Halkieria, but not with Wiwaxia — and calls into question its purported relationship with molluscs. Instead, the tripartite construction of the halkieriid scleritome finds a more compelling parallel in the camenellan tommotiids, relatives of the brachiopods and phoronids. Such a phylogenetic position would indicate the presence of a scleritome in the common ancestor of the three major trochozoan lineages, Mollusca, Annelida and Brachiozoa. On this view, the absence of fossil Ediacaran sclerites is evidence against any ‘Precambrian prelude’ to the explosive diversification of these phyla in the Cambrian, c. 540–530 million years ago.

Hyoliths with pedicles illuminate the origin of the brachiopod body plan

“…Deeper in the tree, the tommotiids Eccentrotheca and Dailyatia group with Halkieria as a grade within total-group brachiopods from which hyoliths and crown-group brachiopods were derived, in line with many previous proposals (e.g. [32,33]). Bayesian and Fitch parsimony, perhaps influenced by their mishandling of inapplicable data [23], prefer trees that group these taxa closer to the molluscs; but under both interpretations, tommotiids are reconstructed as a paraphyletic grade from which hyoliths and brachiopods evolved.…”

“…Despite the excellent fossil record of brachiopods [29], a disparate array of hypotheses have been put forward to explain the origin of the brachiopod body plan [28,31,33,34]. Our new observations and phylogenetic framework shed light on the origins of fundamental morphological innovations in the brachiopod total group.…”

Homologous shell microstructures in Cambrian hyoliths and molluscs

“…; Zhao et al . ). However, recent investigation on feeding anatomy and functional morphology of the orthothecid Triplicatella Conway Morris in Bengtson et al ., from the Cambrian Chengjiang Biota of South China demonstrate that the tentaculate feeding organ of this hyolith does not represent a true lophophore or even a suspension feeding life mode (Fan Liu pers.…”

“…(, , ) and Zhao et al . () for tommotiid–brachiopod phylogeny. The presence of biomineralized elements is denoted.…”

The Cambrian molluscs of Australia: overview of taxonomy, biostratigraphy and paleobiogeography

“…сколь-нибудь значимые работы по кембрийской малакофауне Австралии отсутствуют. С массовым внедрением химического препарирования образцов карбонатных пород в практику изучения кембрийских ископаемых (Розанов, Миссаржевский, 1966; Розанов и др., 1969) были открыты богатейшие комплексы так называемых мелкораковинных ис-1 В данной работе мы ограничиваемся анализом таксонов "одностворчатых" моллюсков (гастроподы и моноплакофоры) и двустворок и не рассматриваем халкиериид, сифогонухитид и иные группы склеритомных ископаемых, которые в последнее время стали относить к полиплакофорам (Vinther, Nielsen, 2005;Vinther, 2015;Parkhaev, Demidenko, 2010;Пархаев, 2017б) или к близким группам базальных лофотрохозой (Conway Morris, Caron, 2007;Zhao et al, 2017). копаемых (SSF, small shelly fosssils) в различных регионах мира.…”