Ancestral morphology of crown-group molluscs revealed by a new Ordovician stem aculiferan

Abstract

Exceptionally preserved fossils provide crucial insights into extinct body plans and organismal evolution(1). Molluscs, one of the most disparate animal phyla, radiated rapidly during the early Cambrian period (approximately 535-520 million years ago (Ma))(2). The problematic fossil taxa Halkieria(3) and Orthrozanclus(4) (grouped in Sachitida) have been assigned variously to stem-group annelids, brachiopods(4,5), stem-group molluscs(4) or stem-group aculiferans (Polyplacophora and Aplacophora)(6), but their affinities have remained controversial owing to a lack of preserved diagnostic characters. Here we describe a new early sachitid, Calvapilosa kroegeri gen. et sp. nov. from the Fezouata biota of Morocco(7,8) (Early Ordovician epoch, around 478 Ma). The new taxon is characterized by the presence of a single large anterior shell plate and polystichous radula bearing a median tooth and several lateral and uncinal teeth in more than 125 rows. Its flattened body is covered by hollow spinose sclerites, and a smooth, ventral girdle flanks an extensive mantle cavity. Phylogenetic analyses resolve C. kroegeri as a stem-group aculiferan together with other single-plated forms such as Maikhanella (Siphogonuchites) and Orthrozanclus; Halkieria is recovered closer to the aculiferan crown. These genera document the stepwise evolution of the aculiferan body plan from forms with a single, almost conchiferan-like shell through two-plated taxa such as Halkieria, to the eight-plated crown-group aculiferans. C. kroegeri therefore provides key evidence concerning the long debate about the crown molluscan affinities of sachitids. This new discovery strongly suggests that the possession of only a single calcareous shell plate and the presence of unmineralised sclerites are plesiomorphic (an ancestral trait) for the molluscan crown

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

“…This position has most recently been propounded based on the Ordovician aculiferan Calvapilosa , which has been interpreted as a close relative of halkieriids 28 . The evidence that Calvapilosa is an aculiferan is strong; the evidence that it is a halkieriid warrants more careful consideration.…”

“…At a broader taxonomic level, perceived similarities in scleritome construction are said to indicate a close relationship between halkieriids and aculiferan molluscs 4,6,19,26–28 . This position has most recently been propounded based on the Ordovician aculiferan Calvapilosa , which has been interpreted as a close relative of halkieriids 28 .…”

“…Halkieriid sclerites exhibit a broad range of morphologies, but none resemble the slender, spinose sclerites of Calvapilosa 28 . The central cavity present in both halkieriid and Calvapilosa sclerites has little taxonomic value (discussed in ref.…”

Ichnological evidence for meiofaunal bilaterians from the terminal Ediacaran and earliest Cambrian of Brazil

“…Body fossils of late Ediacaran macro-organisms include at least some early animals 3 , but crucially, most plausible claims for metazoans lie within the diploblasts rather than the Bilateria 3 . Kimberella, which is potentially a stem mollusc 7 , is a notable exception, but some authors suggest that it can only be reliably considered as a member of total group Bilateria 3 .…”

Three Cambrian fossils assembled into an extinct body plan of cnidarian affinity

“…2 and 3), ctenophores (e.g., ref. 4), molluscs (5,6), annelids (7), panarthropods (e.g., refs. 8 and 9), lophophorates (e.g., ref.…”