Topic
Orthrozanclus
About: Orthrozanclus is a research topic. Over the lifetime, 3 publications have been published within this topic receiving 94 citations.
Papers
TL;DR: A new early sachitid is described from the Fezouata biota of Morocco, which strongly suggests that the possession of only a single calcareous shell plate and the presence of unmineralised sclerites are plesiomorphic for the molluscan crown.
Abstract: 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.
96 citations
TL;DR: 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.
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 Lagerstatte. 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.
33 citations
TL;DR: Paterson et al. as discussed by the authors considered that Oikozetetes and isolated halkieriid sclerites from a section of the lower Cambrian Mernmerna Formation in the Flinders Ranges, South Australia, are tentatively considered as being derived from the same scleritome.
Abstract: Paterson, J.R., Brock, G.A. & Skovsted, C.B. 2009: Oikozetetes from the early Cambrian of South Australia: implications for halkieriid affinities and functional morphology. Lethaia, Vol. 42, pp. 199-203 Shells of Oikozetetes and isolated halkieriid sclerites from a section of the lower Cambrian Mernmerna Formation in the Flinders Ranges, South Australia, are tentatively considered as being derived from the same scleritome. Details of shell morphology and the possible combination of biomineralized shell and sclerites suggest that Oikozetetes, if interpreted correctly, is closely related to Halkieria. A new interpretation of Oikozetetes shell morphology, in addition to the first report of paired muscle scars on the interior surface, sheds new light on the possible functional morphology of halkieriid shells and the means of attaching the shell to the body. The occurrence of Oikozetetes in South Australia extends its biostratigraphic range to the lower Cambrian and biogeographic range to East Gondwana.
17 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 2017 | 2 |
| 2009 | 1 |