Paper No. 4
Presentation Time: 1:50 PM
NEW STEM ARTHROPODS FROM THE BURGESS SHALE AND IMPLICATIONS FOR THE MORPHOLOGICAL DISPARITY OF THE "GREAT APPENDAGES"
Feeding and sensory specialisations are central in the – interlaced – ecological and phylogenetic characterisations of arthropods. This evolutionary drive is within stem-group taxa typified by the variety of forms of the plesiomorphic frontal apparatus known as yet as the "great appendage." Although some of them are usually confounded within the class Megacheira Hou and Bergström, the relationships between all taxa sharing such an appendage are nevertheless obscure, notably because the homology and extent of disparity of this trait remain unresolved. We hereto report, from a new phyllopod bed-like assemblage in Kootenay National Park, Burgess Shale, the discovery of three new Cambrian "megacheiran" arthropods; two of which are bivalved and one leanchoiliid. The leanchoiliid frontal appendage notably presents a pronounced spinose condition in comparison to other described forms, as well as a dense spherical organ within its peduncular portion; the latter trait seems in fact to be widespread in "great appendages" and is possibly diagnostic. Additional evidence exposed by the 41 specimens collected points to a possibly overlooked complexity of cephalic appendages in previous – and smaller – morphotypes, and revigorates the question of crustacean/artiopod affinities within 'pleureomorph megacheirans.' The appendages of the two other, bivalved, morphotypes, are respectively suggestive of plesiomorphic and derived conditions: in one (single specimen), whose bodyplan is very reminiscent of Isoxys Walcott, the general yohoiid aspect of the frontal appendage is associated with Hurdia-like ventral spinose projections (a morphology we associate with a reinterpretation of the frontal appendage in Captopodus poschmanni Kühl and Rust from the Devonian Hunsrück Slate Lagerstätte); in the second (two specimens), the appendage is a 'can opener-like' pseudo-chelicera. Altogether, the appendage morphologies and overall bodyplans of these new bivalved arthropods (1) further support the rooting of yohoiids within stem bivalved taxa, and (2) show the early appearance of a very chelicera-like appendage as part of the Cambrian ecological disparity. In light of this new information, we reevaluate the disparity of the "great appendages" and review phylogenetic scenarii for the radiation of basal euarthropods.