Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps

Abstract

Exceptionally preserved fossils from the Palaeozoic era provide crucial insights into arthropod evolution, with recent discoveries bringing phylogeny and character homology into sharp focus. Integral to such studies are anomalocaridids, a clade of stem arthropods whose remarkable morphology illuminates early arthropod relationships and Cambrian ecology. Although recent work has focused on the anomalocaridid head, the nature of their trunk has been debated widely. Here we describe new anomalocaridid specimens from the Early Ordovician Fezouata Biota of Morocco, which not only show well-preserved head appendages providing key ecological data, but also elucidate the nature of anomalocaridid trunk flaps, resolving their homology with arthropod trunk limbs. The new material shows that each trunk segment bears a separate dorsal and ventral pair of flaps, with a series of setal blades attached at the base of the dorsal flaps. Comparisons with other stem lineage arthropods indicate that anomalocaridid ventral flaps are homologous with lobopodous walking limbs and the endopod of the euarthropod biramous limb, whereas the dorsal flaps and associated setal blades are homologous with the flaps of gilled lobopodians (for example, Kerygmachela kierkegaardi, Pambdelurion whittingtoni) and exites of the 'Cambrian biramous limb'. This evidence shows that anomalocaridids represent a stage before the fusion of exite and endopod into the 'Cambrian biramous limb', confirming their basal placement in the euarthropod stem, rather than in the arthropod crown or with cycloneuralian worms. Unlike other anomalocaridids, the Fezouata taxon combines head appendages convergently adapted for filter-feeding with an unprecedented body length exceeding 2 m, indicating a new direction in the feeding ecology of the clade. The evolution of giant filter-feeding anomalocaridids may reflect the establishment of highly developed planktic ecosystems during the Great Ordovician Biodiversification Event.

Hurdiid radiodontans from the middle Cambrian (Series 3) of Utah

“…This is in contrast to the resolved relationships depicted in Vinther et al (2014) and Van Roy et al (2015), where the two H. sp. B specimens form a clade that is sister to Stanleycaris, rather than to H. victoria.…”

“…Recent phylogenies (Cong et al, 2014;Van Roy et al, 2015) However, H. victoria specimens with straight proximal ventral spines and anteriorly curving distal ends are common (e.g. Daley et al, 2013, fig.…”

“…Hurdia and Peytoia have recently been recovered within Hurdiidae (e.g. Van Roy et al 2015), but these genera differ in a number of ways. Peytoia and Hurdia have a similar overall frontal appendage morphology in that both have elongated ventral spines, but these differ in numerous details including the number and length-width ratio of the podomeres, and the shape, arrangement and number of ventral spines (Daley et al, 2013a).…”

“…Further, we identify H. victoria in the Spence Shale for the first time. A 3D model of an idealized Hurdia appendage potentially allows characters used in previous phylogenetic analyses (e.g., Vinther et al, 2014;Cong et al, 2014;Van Roy et al, 2015) to be visualized and evaluated in the hopes of possibly inferring which characters might be influenced by taphonomic factors.…”

“…This was modified with a subdivision surface, and rendered to a video. A phylogenetic analysis in TNT v. 1.5 (Goloboff & Catalano, 2016) was run using implicit enumeration under equal weighting on a data matrix modified from Van Roy et al (2015) consisting of 33 taxa and 61 characters.…”