New insight into the soft anatomy and shell microstructures of early Cambrian orthothecids (Hyolitha)

Abstract

Hyoliths (hyolithids and orthothecids) were one of the most successful early biomineralizing lophotrochozoans and were a key component of the Cambrian evolutionary fauna. However, the morphology, skeletogenesis and anatomy of earliest members of this enigmatic clade, as well as its relationship with other lophotrochozoan phyla remain contentious. Here, we present a new orthothecid, Longxiantheca mira gen. et sp. nov. preserved as part of secondarily phosphatized small shelly fossil assemblage from the lower Cambrian Xinji Formation of North China. Longxiantheca mira retains some ancestral traits of the clade with an undifferentiated disc-shaped operculum, a simple conical conch with apical septa and a two-layered microstructure of aragonitic fibrous bundles. The operculum interior exhibits impressions of soft tissues, including muscle attachment scars, mantle epithelial cells and a central kidney-shaped platform interpreted as a support structure in association with its presumptive feeding apparatus. The muscular system in orthothecids appears to be similar to that in hyolithids, suggesting a consistent anatomical configuration among the total group of hyoliths. The new finding of shell secreting cells demonstrates a mantle regulating the mode of growth for the operculum. Investigations of shell microstructures support the placement of hyoliths as total group molluscs with an unsettled position within the phylum Mollusca.

First Report of Small Shelly Fossils from the Cambrian Miaolingian Limestones (Zhangxia and Hsuzhuang Formations) in Yiyang County, Henan Province of North China

“…In recent years, SSFs from the Xinji Formation have been studied in detail. Among the SSFs, hyoliths and mollusks have a high diversity and disparity and are the typical representatives of SSFs from North China Platform (NCP) [3][4][5][6][7]. Brachiopods, though less abundant, have also been reported recently by Pan et al from the basal Cambrian of North China [8], in connection with studies of the Shuiyu Section, Shanxi Province.…”

A potential cephalopod from the early Cambrian of eastern Newfoundland, Canada

“…An affinity with the lophophorates has recently been proposed, based on the discovery of rare softbody features interpreted as lophophores in Haplophrentis carinatus 51,52 . By contrast, the relationship with molluscs has also been emphasised on the basis of the inner shell structure and biomineralisation of Allatheca, Microcornus, Parakorilithes, Conotheca and Cupitheca 53,54 . Hyoliths are distinguished from cephalopods by several features (Landing and Kröger 25 and references therein), such as the operculum with its muscles 47,55 or differences in septum morphology 48 .…”

Hyolithid-like hyoliths without helens from the early Cambrian of South China, and their implications for the evolution of hyoliths

“…But Paramicrocornus is not a typical hyolithid, notably the taxon is lacking helens, lateral sinuses on the conch and rooflets on the operculum. It was suggested that these features indicated that Paramicrocornus could possibly be a member of a sister group of hyolithids rather than a hyolithid as typically defined [ 7 , 19 , 20 , 38 ]. The closely comparable Protomicrocornus reported by Pan et al, [ 37 ] from the early Cambrian of North China , shows a similar hyolithid-like morphology without any evidence of helens, but can be distinguished from Paramicrocornus by the lack of a gap between the cardinal processes and clavicles on the operculum.…”

“…The presence of helens is the key character to distinguish the hyolithids [ 3 , 12 , 20 ]. To clarify the phylogenetic placement of Paramicrocornidae and evaluate the rationality of the hypothetical phylogenetic position of the family [ 7 , 19 , 20 , 37 ], we performed a preliminary phylogenetic analysis based on a dataset of 25 hyolith taxa and scored for 33 characters (Figs. 6 , 7 ).…”