Sanctacaris uncata: the oldest chelicerate (Arthropoda)

  title={Sanctacaris uncata: the oldest chelicerate (Arthropoda)},
  author={David A. Legg},
The morphology of the arthropod Sanctacaris uncata, from the Middle Cambrian Burgess Shale of Canada, is reinterpreted based on a restudy of previously described material. Although originally considered a chelicerate-like arthropod, these affinities were dismissed based primarily on interpretations of the anterior appendages and hypotheses which considered the megacheirans (‘great-appendage’ arthropods) as putative ancestors of chelicerates. The similarities between megacheirans and… 
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An unusual euchelicerate linking horseshoe crabs and eurypterids, from the Lower Devonian (Lochkovian) of Yunnan, China
Together with bunodids and pseudoniscids, Houia provides evidence for basal dekatriatans persisting into the middle Palaeozoic alongside eurypterids and arachnids and that the morphological diversity of these basal forms was greater than previously thought.
Redescription of the cheloniellid euarthropod Triopus draboviensis from the Upper Ordovician of Bohemia, with comments on the affinities of Parioscorpio venator
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The evolution of feeding within Euchelicerata: data from the fossil groups Eurypterida and Trigonotarbida illustrate possible evolutionary pathways
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The early history and phylogeny of the chelicerates
When fossil taxa are considered, the limits of the Chelicerata become less well constrained, especially true of various problematic arthropods: fossils such as aglaspidids, chasmataspids, Sanctacaris Briggs and Collins, 1988 and other Burgess Shale-type arthropODs.
New ideas about the euchelicerate stem-lineage
In the scenario presented here euchelicerates did not lose (and indeed never had) long, sensory antennae, but probably evolved their chelicerae from a leg-like pair of uniramous appendages.
Cambrian bivalved arthropod reveals origin of arthrodization
A cladistic analysis resolved a new arthropod from the Tulip Beds locality of the Burgess Shale Formation as the basal-most member of a paraphyletic grade of nekto-benthic forms with bivalved carapaces and suggests that arthrodization (sclerotization and jointing of the exoskeleton) evolved to facilitate swimming.
The phylogeny of arachnomorph arthropods and the origin of the Chelicerata
The present study provides a detailed discussion of primary hypotheses of homology and includes a more complete range of terminal taxa than previous analyses, and provides the first convincing synapomorphies for the Arachnomorpha.
Cephalic and appendage morphology of the Cambrian arthropod Sidneyia inexpectans
A restudy of the Burgess Shale (Cambrian) arthropod Emeraldella brocki and reassessment of its affinities
It is shown that the morphology of the Burgess Shale arthropod Emeraldella brocki is more plesiomorphic than previously assumed, particularly regarding tagmosis, and 'Great appendage' arthropods, traditionally included in the Arachnomorpha, are retrieved as sister to the Crustacea sensu lato + Artiopoda clade, which contradicts the arachnomorph concept.
Multi-Segmented Arthropods from the Middle Cambrian of British Columbia (Canada)
The resultant topology indicates that analyses that have resolved megacheirans as stem-chelicerates have done so because they have rooted on inappropriate taxa, e.g., trilobitomorphs and marrellomorphs.
New Middle Cambrian bivalved arthropods from the Burgess Shale (British Columbia, Canada)
Cambrian bivalved arthropods consistently resolved as a paraphyletic assemblage at the base of Arthropoda, with important innovations in arthropod history such as the specialization of the deutocerebral head appendages and a shift from a nekton-benthic deposit feeding habit to a benthic scavenging/predatory habit.
The Arthropod Alalcomenaeus cambricus Simonetta, from the Middle Cambrian Burgess Shale of British Columbia
Alalcomenaeus was probably a predator, moving mainly by swimming, and is now known to be one of the more abundant, widely distributed and longest ranging of Burgess Shale arthropod genera.
Revised systematics of Palaeozoic ‘horseshoe crabs’ and the myth of monophyletic Xiphosura
A number of other characters, including the form of the chelicerae and appendage VII, indicate that xiphosurans may be paraphyletic with respect to a clade consisting of chasmataspidids, eurypterids, and arachnids.