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

Nature volume 542, pages 471–474 (2017)Cite this article

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Abstract

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.

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Figure 1: Calvapilosa kroegeri, holotype YPM 237255, from the Lower Ordovician epoch (Tremadocian age) Fezouata formation, near Zagora, Morocco.
Figure 2: Details and additional specimens of C. kroegeri.
Figure 3: Reconstruction of Calvapilosa kroegeri, juvenile.
Figure 4: Time tree of aculiferan evolution based on a Bayesian total evidence analysis.

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Acknowledgements

M., B. and L. ‘Ou Said’ Ben Moula collected the specimens and provided support in the field. B. Kröger ‘discovered’ the holotype in 2014 among freshly collected and, at the time uninventoried, material at the YPM. L. Ben Moula and B. Tahiri provided practical assistance. S. Butts and J. Utrup curated specimens and facilitated access to the collections at the YPM. C. Graham provided support and access to digital imaging facilities at the Yale Digital Collections Center (YDC2). The drawings of C. kroegeri were made by Z. Dutta and the physical reconstruction was done by E. Horn (http://www.10tons.dk/). The research was supported by National Science Foundation Grant EAR-1053247 and by the Division of Invertebrate Paleontology, YPM.

Author information

Author notes

  1. Jakob Vinther and Luke Parry: These authors contributed equally to this work.

Authors and Affiliations

  1. School of Biological Sciences, University of Bristol, Life Sciences, Building, 24 Tyndall Avenue, Bristol, BS8 1TQ, UK

    Jakob Vinther & Luke Parry

  2. School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK

    Jakob Vinther

  3. Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK

    Luke Parry

  4. Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, 06511, Connecticut, USA

    Derek E. G. Briggs & Peter Van Roy

  5. Yale Peabody Museum of Natural History, 170 Whitney Avenue, New Haven, 06520, Connecticut, USA

    Derek E. G. Briggs

  6. Department of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, Ghent, B-9000, Belgium

    Peter Van Roy

Authors
  1. Jakob Vinther
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  3. Derek E. G. Briggs
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  4. Peter Van Roy
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Contributions

J.V. initially analysed the fossils and wrote the first manuscript draft with substantial input to both from all other authors. L.P. and J.V. assembled the character matrix. L.P. did the phylogenetic analyses and produced the interpretative drawings. P.V.R., J.V. and L.P. photographed the fossils. P.V.R. obtained the specimens and locality information through M. ‘Ou Said’ Ben Moula, and prepared the fossils. D.E.G.B. facilitated curation and accession of material to YPM.

Corresponding author

Correspondence to Jakob Vinther.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks S. Bengtson and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 C. kroegeri YPM 237255, holotype.

a, Part, illuminated from the northwest. b, Counterpart, illuminated from the southwest and then mirrored horizontally. c, Part, submerged in water.

Extended Data Figure 2 C. kroegeri YPM 227515, paratype.

a, Part, illuminated from the northwest. b, Counterpart, illuminated from the southwest and then mirrored horizontally. c, Interpretative drawing of YPM 227515 with part and counterpart combined. Interpretations below the stippled line are derived from the counterpart. Boxes denote position of details shown in d, e, g. Areas shown in green are of the exposed ventral mantle, areas within dashed lines denote gaps in preservation, dark blue shows the teeth of the radula and light blue denotes the shell. Sclerites are shown in orange but are only drawn where lying flat or covering the shell. d, Exposed ventral mantle, which is preserved in darker colouration but shows no visible sclerites. e, Folded dorsal mantle which preserves dorsal sclerites in longitudinal section. Boxed area is magnified in f. f, Dorsal sclerites. g, Dorsal sclerites in cross-section and marginal sclerites in longitudinal section.

Extended Data Figure 3 The radula of C. kroegeri.

a, YPM 530837. b, YPM 227515. c, YPM 237255. d, YPM 530836. e, detail of d (bottom) with explanatory drawing (top) showing basal attachment points of radula teeth. Ur, uncinal region; Mc, major cusp; La, lateral area; Mt, median tooth. Dark grey areas areas obscured by overlying mineral.

Extended Data Figure 4 Additional specimens of C. kroegeri and Calvapilosa sp.

a, Part of YPM 530837, isolated head valve with radula preserved. b, Counterpart of YPM 530837, mirrored. c, Part of YPM 530836, isolated head valve with radula preserved. d, Counterpart of YPM 530836, mirrored. e, Part of YPM 530835, isolated head valve. f, Counterpart of YPM 530835, mirrored. g, Part of YPM 227641, Calvapilosa ?kroegeri, isolated head valve. h, Counterpart of YPM 227641, mirrored.

Extended Data Figure 5 SEM images of C. kroegeri sclerites and radula.

a, YPM 530836 indicating regions of photomicrographs b–e. b, Left major cusps and adjacent uncinal teeth. c, Left uncinal region. d, Overview showing median teeth (a single tooth indicated by the white arrow), broad flanking region with lateral teeth (a single tooth indicated by the black arrow) and major cusps. e, Right uncinal teeth. f, YPM 237255 indicating regions of SEM photomicrographs g–i. g, Sclerites with diagenetic infill white arrows. h, Sclerites in longitudinal section. i, Sclerite in longitudinal section preserved as a void. b–h and g–i were taken using backscatter and secondary electron imaging, respectively.

Extended Data Figure 6 Reconstruction of C. kroegeri.

a, Juvenile. b, Adult, dorsal view. c, Adult, lateral view. d, Adult ventral view. Drawing by Z. Dutta.

Extended Data Figure 7 Cartoon explaining preservation of YPM 227515, cross-sectional view.

a, Prior to burial. b, Burial. c, Decay and replacement of the mantle and sclerites by pyrite. d, Compaction. e, Weathering and uneven splitting along the dorsal mantle, cross cutting sclerites and along smooth, ventral mantle.

Extended Data Figure 8 Morphological phylogenetic analyses of molluscs incorporating C. kroegeri.

a, Strict consensus of 369 trees of length 209 steps from parsimony analysis under equal character weighting, numbers at the nodes are from 1,000 bootstrap replicates, 1,000 Jacknife replicates and Bremer support, respectively. b, Parsimony analysis under implied weighting (k = 3), numbers at the nodes are from 1,000 replicates of symmetric resampling. c, Results of Bayesian analysis using the mkv + Γ morphology model, numbers at the nodes are posterior probabilities.

Extended Data Figure 9 Bayesian total evidence dating analysis incorporating C. kroegeri.

Analysis performed using the uniform tree prior and LG + Γ and mkv + Γ for the molecular and morphological data, respectively. The topology shown is for analysis using a 549 Ma maximum for the age of the molluscan crown group. Error bars on the node ages are 95% HPD intervals for both unconstrained analyses (red) and analyses with the maximum age constraint on molluscs (purple). Numbers at the nodes are posterior probabilities from the constrained analysis.

Supplementary information

Supplementary Information

This file contains a Supplementary Discussion giving the historical context for the interpretations of the Cambrian fossils in the ‘Sachtida’ (i.e. Halkieria and similar taxa) and Wiwaxia (Section 1), Supplementary Methods giving details of the phylogenetic analyses and character codings (Section 2) and additional references. (PDF 729 kb)

Supplementary Data

This file contains the morphology character matrix. (TXT 19 kb)

Supplementary Data

This file contains the morphology and molecular matrix. (TXT 135 kb)

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Vinther, J., Parry, L., Briggs, D. et al. Ancestral morphology of crown-group molluscs revealed by a new Ordovician stem aculiferan. Nature 542, 471–474 (2017). https://doi.org/10.1038/nature21055

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