Portal de Periódicos da CAPES

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

2017; Nature Portfolio; Volume: 542; Issue: 7642 Linguagem: Inglês

10.1038/nature21055

1476-4687

Jakob Vinther, Luke A. Parry, Derek E. G. Briggs, Peter Van Roy,

Mollusks and Parasites Studies

Presence of a radula in Calvapilosa kroegeri confirms the molluscan affinity of sachitids, and the single shell plate reveals the ancestral condition for all crown molluscs and early evolution of the multi-plated body plan characteristic of Aculifera. From the snails in your garden to giant squid that wrestle with sperm whales, the molluscs are one of the most disparate and successful animal phyla. But because they evolved rapidly in the Cambrian period, some 500 million years ago, there is still much debate about their early history—in particular, what the earliest molluscs looked like. A fossil from the Ordovician Fezouata formation in Morocco (known for relic Burgess-Shales-type animals) might shed light on the issue. The creature is a flattened slug-like animal with a distinct, single shell on its head, the rest of the body being covered with spines. The exciting part is that the animal has a radula, the distinctive rasping tongue that is a defining character for molluscs and is the reason slugs can demolish your lettuces so effectively. This phylogenetic analysis shows that the newly discovered creature groups with some other forms variously classified as molluscs, stem brachiopods or similar, but places it within the molluscs, at the base of the Aculifera (the multiple-shelled chitons plus the shell-free aplacophora), as opposed to the Conchifera (all other molluscs). This finding suggests that early molluscs had only a single shell. Exceptionally preserved fossils provide crucial insights into extinct body plans and organismal evolution1. 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 Halkieria3 and Orthrozanclus4 (grouped in Sachitida) have been assigned variously to stem-group annelids, brachiopods4,5, stem-group molluscs4 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 Morocco7,8 (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.