Ordovician opabiniid-like animals and the role of the proboscis in euarthropod head evolution

Abstract

A crucial step in the evolution of Euarthropoda (chelicerates, myriapods, pancrustaceans) was the transition between fossil groups that possessed frontal appendages innervated by the first segment of the brain (protocerebrum), and living groups with a protocerebral labrum and paired appendages innervated by the second brain segment (deutocerebrum). Appendage homologies between the groups are controversial. Here we describe two specimens of opabiniid-like euarthropods, each bearing an anterior proboscis (a fused protocerebral appendage), from the Middle Ordovician Castle Bank Biota, Wales, UK. Phylogenetic analyses support a paraphyletic grade of stem-group euarthropods with fused protocerebral appendages and a posterior-facing mouth, as in the iconic Cambrian panarthropod Opabinia. These results suggest that the labrum may have reduced from an already-fused proboscis, rather than a pair of arthropodized appendages. If some shared features between the Castle Bank specimens and radiodonts are considered convergent rather than homologous, phylogenetic analyses retrieve them as opabiniids, substantially extending the geographic and temporal range of Opabiniidae.

A large pelagic lobopodian from the Cambrian Pioche Shale of Nevada

“…3.3). The preserved portion of the trunk appears to bear at least 18 or 20 ventrolateral trunk flap pairs, more than observed in any other known gilled lobopodian (Budd, 1997), radiodont (Pates et al, 2021a; Zeng et al, 2023), or opabiniid (Pates et al, 2022a). Like the holotype, the total length of the body in this specimen cannot be reliably estimated, but there were likely more flap pairs on the remainder of the body that are not preserved here.…”

“…Studies on preserved musculature in panarthropods from the Sirius Passet of Greenland have suggested that musculature was absent in the flaps of Pambdelurion but present in the lobopodous ventral limbs (Young and Vinther, 2017). This casts doubt on the swimming capabilities of Pambdelurion and, by extension, the Opabiniidae, which share homologous dorsal flaps and the controversial ventral lobopods (Budd and Daley, 2012); recent discoveries, including the ventral lobopods of Mieridduryn bonniae Pates et al, 2022b, and small, enigmatic structures possibly attributable to lobopodous legs in the holotype of Utaurora comosa Pates et al, 2022a, suggest that the lobopods of Opabinia are genuine. That the dorsal flaps are homologous to the dorsal flaps of Pambdelurion also suggests Opabinia may have moved primarily by walking on its lobopod legs rather than as an active swimmer, as previously believed.…”

“…These animals, known from only a small number of species, include the siberiids (three or four species), the "gilled lobopodians" (three species), and the opabiniids (three or four species). Siberiids, which essentially resemble their onychophoran-branch relatives but with more obvious adaptations for predation, may have been some of the first raptorial animals on the planet according to phylogenetic bracketing (Howard et al, 2022), while opabiniids, known only from a single species before the recent descriptions of Utaurora comosa Pates et al, 2022(Pates et al, 2022a and two specimens from the Ordovician of Wales (Pates et al, 2022b), are famously unique, even among other Cambrian taxa. The gilled lobopodians are perhaps some of the least well understood, being known only from two species from the Sirius Passet Lagerstätte of Greenland and, questionably, the recently described Utahnax vannieri Lerosey-Aubril and Ortega-Hernandez, 2022.…”

Three-dimensional morphology of the biramous appendages in Isoxys from the early Cambrian of South China, and its implications for early euarthropod evolution

“…To assess the phylogenetic position of Isoxys , we analysed an updated version of the published morphological matrix of Pates et al . [ 35 ]. We coded I. curvirostratus according to new anatomical data herein, and added I. acutangulus , I. auritus , I. volucris , Surusicaris elegans and E. sperare from previous literature .…”

“…We analysed the morphological dataset in MrBayes v.3.2.7 [ 35 ], implementing the Mk model of character evolution [ 37 ] with gamma distributed among-character rate variation for four runs of four chains and 10 million generations, with 25% burnin. Convergence was assessed based on standard deviations of split frequencies less than 0.01, reaching effective sample size greater than 200 for every parameter, and by comparing posterior distributions in Tracer v.1.7.1 [ 38 ].…”