Mandibulate convergence in an armoured Cambrian stem chelicerate

BMC Evolutionary Biology

Research Article

Jean-Bernard Caron¹  Cédric Aria² 

[1] Department of Ecology and Evolutionary Biology, University of Toronto, M5S3B2, Toronto, ON, Canada;Department of Natural History (Palaeobiology Section), Royal Ontario Museum, M5S2C6, Toronto, ON, Canada;Department of Earth Sciences, University of Toronto, M5S3B1, Toronto, ON, Canada;Department of Ecology and Evolutionary Biology, University of Toronto, M5S3B2, Toronto, ON, Canada;Present address: State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 210008, Nanjing, China;

关键词: Arthropoda;    Chelicerata;    Convergence;    Macroevolution;    Cambrian;    Burgess Shale;   

DOI  :  10.1186/s12862-017-1088-7

 received in 2017-07-17, accepted in 2017-11-21,  发布年份 2017

来源: Springer

【 摘 要 】

BackgroundChelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile “panchelicerate” body plan.ResultsIn this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates.ConclusionsAlthough habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheiran Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods—Chelicerata and Mandibulata—and implies that this diversification was facilitated by the expansion of durophagous niches.

【 授权许可】

CC BY   
© The Author(s). 2017