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  • chelicerate neural ground pattern in a cambrian great appendage arthropod
    Nature, 2013
    Co-Authors: Gengo Tanaka, Gregory D. Edgecombe, Nicholas J. Strausfeld
    Abstract:

    Cambrian great appendage arthropods had heads that bore a claw-like appendage pair; neuroanatomical and phylogenetic analysis of a well-preserved Alalcomenaeus fossil reveals the relationship of great appendage arthropods to Chelicerata. 'Great appendage' arthropods are extinct jointed-legged creatures from the Cambrian period equipped with often large claw-like appendages of an arrangement not seen in modern arthropods. Their evolutionary relationships are much debated. Gregory Edgecombe and colleagues use micro-computed tomography to reconstruct the neuroanatomy of Alalcomenaeus, an exquisitely preserved great-appendage arthropod from China. Several characters of the nervous system are uniquely shared with the chelicerates — spiders, scorpions, mites and horseshoe crabs — placing the fossils firmly on the arthropod tree and demonstrating that chelicerate neuroanatomy had evolved by 520 million years ago. Preservation of neural tissue in early Cambrian arthropods has recently been demonstrated1, to a degree that segmental structures of the head can be associated with individual brain neuromeres. This association provides novel data for addressing long-standing controversies about the segmental identities of specialized head appendages in fossil taxa2,3. Here we document neuroanatomy in the head and trunk of a ‘great appendage’ arthropod, Alalcomenaeus sp., from the Chengjiang biota, southwest China, providing the most complete neuroanatomical profile known from a Cambrian animal. Micro-computed tomography reveals a configuration of one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. Arrangements of optic neuropils, the brain and ganglia correspond most closely to the nervous system of Chelicerata of all extant arthropods, supporting the assignment of ‘great appendage’ arthropods to the chelicerate total group4,5. The position of the deutocerebral neuromere aligns with the insertion of the great appendage, indicating its deutocerebral innervation and corroborating a homology between the ‘great appendage’ and chelicera indicated by morphological similarities4,6,7. Alalcomenaeus and Fuxianhuia protensa1 demonstrate that the two main configurations of the brain observed in modern arthropods, those of Chelicerata and Mandibulata, respectively8, had evolved by the early Cambrian.

  • chelicerate neural ground pattern in a cambrian great appendage arthropod
    Nature, 2013
    Co-Authors: Gengo Tanaka, Gregory D. Edgecombe, Xianguang Hou, Nicholas J. Strausfeld
    Abstract:

    Preservation of neural tissue in early Cambrian arthropods has recently been demonstrated, to a degree that segmental structures of the head can be associated with individual brain neuromeres. This association provides novel data for addressing long-standing controversies about the segmental identities of specialized head appendages in fossil taxa. Here we document neuroanatomy in the head and trunk of a 'great appendage' arthropod, Alalcomenaeus sp., from the Chengjiang biota, southwest China, providing the most complete neuroanatomical profile known from a Cambrian animal. Micro-computed tomography reveals a configuration of one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. Arrangements of optic neuropils, the brain and ganglia correspond most closely to the nervous system of Chelicerata of all extant arthropods, supporting the assignment of 'great appendage' arthropods to the chelicerate total group. The position of the deutocerebral neuromere aligns with the insertion of the great appendage, indicating its deutocerebral innervation and corroborating a homology between the 'great appendage' and chelicera indicated by morphological similarities. Alalcomenaeus and Fuxianhuia protensa demonstrate that the two main configurations of the brain observed in modern arthropods, those of Chelicerata and Mandibulata, respectively, had evolved by the early Cambrian.