Abstract
We herein describe Surusicaris elegans gen. et sp. nov. (in Isoxyidae, amended), a middle (Series 3, Stage 5) Cambrian bivalved arthropod from the new Burgess Shale deposit of Marble Canyon (Kootenay National Park, British Columbia). Surusicaris exhibits 12 simple, partly undivided biramous trunk limbs with long tripartite caeca, which may illustrate a plesiomorphic “fused” condition of exopod and endopod. We construe also that the head is made of five somites (= four segments), including two eyes, one pair of anomalocaridid-like frontalmost appendages, and three pairs of poorly sclerotized uniramous limbs. This fossil may therefore be a candidate for illustrating the origin of the plesiomorphic head condition in euarthropods, and questions the significance of the “two-segmented head” in, e.g., fuxianhuiids. The frontalmost appendage in isoxyids is intriguingly disparate, bearing similarities with both dinocaridids and euarthropods. In order to evaluate the relative importance of bivalved arthropods, such as Surusicaris, in the hypothetical structuro-functional transition between the dinocaridid frontal appendage and the pre-oral—arguably deutocerebral—appendage of euarthropods, we chose a phenetic approach and computed morphospace occupancy for the frontalmost appendages of 36 stem and crown taxa. Results show different levels of evolutionary decoupling between frontalmost appendage disparity and body plans. Variance is greatest in dinocaridids and “stem bivalved” arthropods, but these groups do not occupy the morphospace homogeneously. Rather, the diversity of frontalmost appendages in “stem bivalved” arthropods, distinct in its absence of clear clustering, is found to link the morphologies of “short great appendages,” chelicerae and antennules. This find fits the hypothesis of an increase in disparity of the deutocerebral appendage prior to its diversification in euarthropods, and possibly corresponds to its original time of development. The analysis of this pattern, however, is sensitive to the—still unclear—extent of polyphyly of the “stem bivalved” taxa.
Highlights
Most stem arthropods found in the early Phanerozoic fossil record, despite limited differentiation of cephalic limbs, have diagnosable frontalmost pairs of appendages, some more antenniform, some more spinose and prehensile that are sometimes broadly referred to as “great appendages” [1,2,3,4]
Put forward notably by Gould [24, 113], the application of alternative methods such as morphospaces to aid in understanding the evolutionary significance of the outstanding disparity displayed by Burgess Shale-type communities has had limited resonance so far, they account for the explanatory patterns of body plan evolution [25, 26, 29, 111, 113, 119,120,121]
In light of existing cladistic results, the ordination of disparity across frontalmost appendages is mostly consistent with cladification, but the comparable values of morphological variances do not correspond to the taxonomic levels the systematic classification has recognized so far
Summary
Most stem arthropods found in the early Phanerozoic fossil record, despite limited differentiation of cephalic limbs, have diagnosable frontalmost pairs of appendages, some more antenniform, some more spinose and prehensile that are sometimes broadly referred to as “great appendages” [1,2,3,4]. The homology of the dinocaridid “frontal appendages” with megacheiran “short great appendages” in particular is pivotal in the debate Evolutionary continuity of these appendages would illustrate the early evolution of the chelicerae [4, 8, 11], but, given a deutocerebral homology [7, 15, 16], would highlight the link between the “great appendages” sensu lato and the evolution of antennae/antennules in antennulate clades [7, 17,18,19,20]. By their arguably basal phylogenetic position, “stem bivalved arthropods” and their range of frontalmost appendage morphologies would be expected to yield the relevant evidence clarifying this morphological/topological transition [5, 21, 22]
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