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Article: A Burgess Shale mandibulate arthropod with a pygidium: a case of convergent evolution

Papers in Palaeontology - Volume 7 Issue 1 - Cover
Publication: Palaeontology
Volume: 7
Part: 4
Publication Date: November 2021
Page(s): 1877 1894
Author(s): Alejandro Izquierdo-López, and Jean-Bernard Caron
DOI: 10.1002/spp2.1366
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How to Cite

IZQUIERDO-LóPEZ, A., CARON, J.B. 2021. . Papers in Palaeontology, 7, 4, 1877-1894. DOI: /doi/10.1002/spp2.1366

Author Information

  • Alejandro Izquierdo-López - Department of Ecology and Evolutionary Biology University of Toronto 25 Willcocks Street Toronto Ontario M5S 3B2 Canada
  • Alejandro Izquierdo-López - Department of Natural History, Palaeobiology Royal Ontario Museum 100 Queen's Park Toronto Ontario M5S 2C6 Canada
  • Jean-Bernard Caron - Department of Ecology and Evolutionary Biology University of Toronto 25 Willcocks Street Toronto Ontario M5S 3B2 Canada
  • Jean-Bernard Caron - Department of Natural History, Palaeobiology Royal Ontario Museum 100 Queen's Park Toronto Ontario M5S 2C6 Canada
  • Jean-Bernard Caron - Department of Earth Sciences University of Toronto Toronto Ontario M5S 3B1 Canada

Publication History

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    Abstract

    Cambrian bivalved arthropods are a polyphyletic group of carapace-bearing arthropods that includes stem euarthropods, stem mandibulates and crustaceans. Here, we describe Pakucaris apatis gen. et sp. nov., a new stem mandibulate bivalved arthropod from the middle Cambrian (Wuliuan Stage) Burgess Shale (Marble Canyon, Kootenay National Park, British Columbia, Canada). Two morphotypes of this small arthropod (11.6–26.6 mm long) are recognized, which differ mainly in their size and number of segments, possibly reflecting sexual dimorphism or different anamorphic stages. The carapace presents a dorsal crest extending anteriorly into a small recurved rostrum and two anterolateral processes. Around 20% of the posteriormost body segments and limbs are covered by a large spine-bearing shield. The head bears a pair of eyes, a possible pair of unsegmented appendicular projections and two pairs of segmented appendages. The thorax is multisegmented, homonomous, with weakly sclerotized segments bearing biramous limbs, composed of a stenopodous endopod with c. 20 podomeres and a paddle-shaped exopod. Pakucaris is interpreted as a nektobenthic suspension feeder. Bayesian phylogenetic analysis implies a position within Hymenocarina as stem mandibulates. The posterior shield is regarded as a pygidium, and represents a case of morphofunctional convergent evolution between mandibulates, artiopodans and mollisoniids. Furthermore, Pakucaris adds to a growing number of pygidium-bearing arthropods, potentially hinting at a common developmental pattern across early arthropod evolution. This study not only increases our understanding of the early evolution of mandibulates, but also illustrates a unique case of early evolutionary convergence during the Cambrian Explosion.

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