New reconstruction of the Wiwaxia scleritome, with data from Chengjiang juveniles

Abstract

Wiwaxiids are a problematic group of scale-covered lophotrochozoans known from Cambrian Stages 3–5. Their imbricating dorsal scleritome of leaf-like scales has prompted comparison with various annelids and molluscs, and has been used as a template to reconstruct the articulation pattern of isolated Small Shelly Fossils. The first articulated specimens of Wiwaxia from the Cambrian Stage 3 Chengjiang Konservat-Lagerstätte show that the Wiwaxia scleritome comprised nine equivalent transverse rows associated with outgrowths of soft tissue, but did not possess a separate zone of anterior sclerites. This serial construction is fundamentally incompatible with the circumferential disposition of sclerites in early molluscs, but does closely resemble the armature of certain annelids. A deep homology with the annelid scleritome must be reconciled with Wiwaxia’s mollusc-like mouthparts and foot; together these point to a deep phylogenetic position, close to the common ancestor of annelids and molluscs.

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

“…An equivalent sclerite arrangement was once envisaged in Wiwaxia 15,23 , but recent studies 8,9 have shown that the Wiwaxia scleritome conforms to a metameric architecture, comprising 8–9 transverse rows (Fig. 3c,g).…”

“…If, on the other hand, brachiozoans evolved from a halkieriid-like ancestor, then multi-element scleritomes characterise the earliest brachiozoans as well as molluscs and annelids 9 (Fig. 4).…”

“…3c,g). Even though the most lateral sclerites are morphologically distinct in certain Wiwaxia species, they belong to the same transverse rows as the medial sclerites, rather than forming a distinct peripheral zone that surrounds the entire circumference of the organism 9,24 (Fig. 3c).…”

“…(A potentially radula-like structure evident in a single specimen of Halkieria 16 corresponds in angle and dimensions with diagonal displacements of sclerites elsewhere in the scleritome, and is not associated with any diagnostically radular characteristics, such as teeth 38 ; its identification as a radula must be considered unproven.) As a radula was present in the ancestral mollusc 39 , and perhaps deeper in the trochozoan lineage 9 , its absence in halkieriids is difficult to reconcile with a molluscan affinity.…”

“…Such taxa often represent long-extinct combinations of characters, offering a unique perspective on the early origin of modern body plans – presuming, of course, that relationships with modern groups can be established 1 . The reconstructed origins of the molluscan lineage, for example, have been overhauled in order to accommodate two emblematic Cambrian taxa, Halkieria and Wiwaxia 2–9 . These two genera bear superficially similar sclerites, which occur the world over as carbonaceous and mineralized microfossils 10–13 ; the grouping Sachitida was erected to reflect this perceived commonality 14 .…”