Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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


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.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Get just this article for as long as you need it


Prices may be subject to local taxes which are calculated during checkout

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.


  1. Briggs, D. E. G. Extraordinary fossils. Am. Sci . 79, 130–141 (1991)

    ADS  Google Scholar 

  2. Vinther, J. The origins of molluscs. Palaeontology 58, 19–34 (2015)

    Article  Google Scholar 

  3. Conway Morris, S. & Peel, J. S. Articulated halkieriids from the Lower Cambrian of north Greenland. Nature 345, 802–805 (1990)

    Article  ADS  Google Scholar 

  4. Morris, S. C. & Caron, J.-B. Halwaxiids and the early evolution of the lophotrochozoans. Science 315, 1255–1258 (2007)

    Article  CAS  ADS  Google Scholar 

  5. Conway Morris, S. & Peel, J. S. Articulated halkieriids from the Lower Cambrian of north Greenland and their role in early protostome evolution. Phil. Trans. R. Soc. Lond. B 347, 305–358 (1995)

    Article  ADS  Google Scholar 

  6. Vinther, J. The canal system in sclerites of Lower Cambrian Sinosachites (Halkieriidae: Sachitida): significance for the molluscan affinities of the sachitids. Palaeontology 52, 689–712 (2009)

    Article  Google Scholar 

  7. Van Roy, P. et al. Ordovician faunas of Burgess Shale type. Nature 465, 215–218 (2010)

    Article  CAS  ADS  Google Scholar 

  8. Van Roy, P., Briggs, D. E. G. & Gaines, R. R. The Fezouata fossils of Morocco; an extraordinary record of marine life in the Early Ordovician. J. Geol. Soc. Lond. 172, 541–549 (2015)

    Article  Google Scholar 

  9. Schwabe, E. Illustrated summary of chiton terminology. Spixiana 33, 171–194 (2010)

    Google Scholar 

  10. Williams, A. & Holmer, L. E. Shell structure and inferred growth, functions and affinities of the sclerites of the problematic Micrina. Palaeontology 45, 845–873 (2002)

    Article  Google Scholar 

  11. Scheltema, A. H. & Ivanov, D. L. An aplacophoran postlarva with iterated dorsal groups of spicules and skeletal similarities to Paleozoic fossils. Invertebr. Biol . 121, 1–10 (2002)

    Article  Google Scholar 

  12. Sigwart, J. D. & Sutton, M. D. Deep molluscan phylogeny: synthesis of palaeontological and neontological data. Proc. R. Soc.B 274, 2413–2419 (2007)

    Article  Google Scholar 

  13. Bengtson, S. The cap-shaped Cambrian fossil Maikhanella and the relationship between coeloscleritophorans and molluscs. Lethaia 25, 401–420 (1992)

    Article  Google Scholar 

  14. Ronquist, F. et al. A total-evidence approach to dating with fossils, applied to the early radiation of the hymenoptera. Syst. Biol . 61, 973–999 (2012)

    Article  Google Scholar 

  15. Vinther, J., Sperling, E. A., Briggs, D. E. G. & Peterson, K. J. A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors. Proc. R. Soc. B 279, 1259–1268 (2012)

    Article  Google Scholar 

  16. Sutton, M. D., Briggs, D. E. G., Siveter, D. J. & Siveter, D. J. An exceptionally preserved vermiform mollusc from the Silurian of England. Nature 410, 461–463 (2001)

    Article  CAS  ADS  Google Scholar 

  17. Sutton, M. D., Briggs, D. E. G., Siveter, D. J., Siveter, D. J. & Sigwart, J. D. A Silurian armoured aplacophoran and implications for molluscan phylogeny. Nature 490, 94–97 (2012)

    Article  CAS  ADS  Google Scholar 

  18. Nielsen, C., Haszprunar, G., Ruthensteiner, B. & Wanninger, A. Early development of the aplacophoran mollusc Chaetoderma. Acta Zool . 88, 231–247 (2007)

    Article  Google Scholar 

  19. Eernisse, D. J. & Reynolds, P. D. in Microscopic Anatomy of the Invertebrates Vol. 5 (eds Harrison, F. W. & Kohn, A. J. ) 55–110 (Wiley-Liss, 1994)

    Google Scholar 

  20. Scheltema, A. H. Aplacophora as progenetic aculiferans and the coelomate origin of mollusks as the sister taxon of Sipuncula. Biol. Bull . 184, 57–78 (1993)

    Article  CAS  Google Scholar 

  21. Vendrasco, M. J., Wood, T. E. & Runnegar, B. N. Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons. Nature 429, 288–291 (2004)

    Article  CAS  ADS  Google Scholar 

  22. Vinther, J. et al. The origin of multiplacophorans—convergent evolution in aculiferan molluscs. Palaeontology 55, 1007–1019 (2012)

    Article  Google Scholar 

  23. Salvini-Plawen, L. & Steiner, G. in Origin and Evolutionary Radiation of the Mollusca (ed. Taylor, J. D. ) 29–51 (Oxford Univ. Press, 1996)

  24. Wingstrand, K. G. On the anatomy and relationships of recent Monoplacophora. Galathea Reports 16, 7–94 (1985)

    Google Scholar 

  25. Smith, S. A. et al. Resolving the evolutionary relationships of molluscs with phylogenomic tools. Nature 480, 364–367 (2011)

    Article  CAS  ADS  Google Scholar 

  26. Kröger, B. & Mutvei, H. Nautiloids with multiple paired muscle scars from Lower–Middle Ordovician of Baltoscandinavia. Palaeontology 48, 781–791 (2005)

    Article  Google Scholar 

  27. Vendrasco, M. J. & Runnegar, B. Late Cambrian and Early Ordovician stem group chitons (Mollusca: Polyplacophora) from Utah and Missouri. J. Paleontol . 78, 675–689 (2004)

    Article  Google Scholar 

  28. Smith, M. R. Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: implications for the ancestral molluscan radula. Proc. R. Soc. B 279, 4287–4295 (2012)

    Article  Google Scholar 

  29. Scheltema, A. H. The radula of the chaetodermatidae (Mollusca, Aplacophora). Z. Morph. Tiere 72, 361–370 (1972)

    Article  Google Scholar 

  30. Schiemann, S. M. et al. Clustered brachiopod Hox genes are not expressed collinearly and are associated with lophotrochozoan novelties. Preprint at (2016)

  31. Goloboff, P. A., Farris, J. S. & Nixon, K. C. TNT, a free program for phylogenetic analysis. Cladistics 24, 774–786 (2008)

    Article  Google Scholar 

  32. Ronquist, F. et al. MrBayes 3.2: efficient Bayesian phylogenetic inference and model choice across a large model space. Syst. Biol . 61, 539–542 (2012)

    Article  Google Scholar 

  33. O’Reilly, J. E. & Donoghue, P. C. Tips and nodes are complementary not competing approaches to the calibration of molecular clocks. Biol. Lett . 12, 20150975 (2016)

    Article  Google Scholar 

  34. Boag, T. H., Darroch, S. A. & Laflamme, M. Ediacaran distributions in space and time: testing assemblage concepts of earliest macroscopic body fossils. Paleobiology 42, 574–594 (2016)

    Article  Google Scholar 

  35. Waggoner, B. The ediacaran biotas in space and time. Integr. Comp. Biol . 43, 104–113 (2003)

    Article  Google Scholar 

  36. Keane, T. M., Creevey, C. J., Pentony, M. M., Naughton, T. J. & Mclnerney, J. O. Assessment of methods for amino acid matrix selection and their use on empirical data shows that ad hoc assumptions for choice of matrix are not justified. BMC Evol. Biol . 6, 29 (2006)

    Article  Google Scholar 

  37. Benton, M. J. et al. Constraints on the timescale of animal evolutionary history. Palaeontol. Electronica 18, 1–106 (2015)

    Google Scholar 

Download references


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 ( The research was supported by National Science Foundation Grant EAR-1053247 and by the Division of Invertebrate Paleontology, YPM.

Author information

Authors and Affiliations



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.

Ethics declarations

Competing interests

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 be. 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 gi. g, Sclerites with diagenetic infill white arrows. h, Sclerites in longitudinal section. i, Sclerite in longitudinal section preserved as a void. bh and gi 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)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing