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

Abstract

Early euarthropod evolution involved a major transition from lobopodian-like taxa to organisms featuring a segmented, well-sclerotized trunk (arthrodization) and limbs (arthropodization). However, the precise origin of a completely arthrodized trunk and arthropodized ventral biramous appendages remain controversial, as well as the early onset of anterior–posterior limb differentiation in stem-group euarthropods. New fossil material and micro-computed tomography inform the detailed morphology of the arthropodized biramous appendages in the carapace-bearing euarthropod Isoxys curvirostratus from the early Cambrian Chengjiang biota. In addition to well-developed grasping frontal appendages, I. curvirostratus possesses two batches of morphologically and functionally distinct biramous limbs. The first batch consists of four pairs of short cephalic appendages with robust endites with a feeding function, whereas the second batch has more elongate trunk appendages for locomotion. Critically, our new material shows that the trunk of I. curvirostratus was not arthrodized. The results of our phylogenetic analyses recover isoxyids as some of the earliest branching sclerotized euarthropods, and strengthens the hypothesis that arthropodized biramous appendages evolved before full body arthrodization.

Large euarthropod carapaces from a high latitude Cambrian (Drumian) deposit in Spain

“…The close packed nature of the soft tissues, their proximal curvature, and the change in angle at the same distance from the dorsal margin of the carapace indicates that these features probably represent thoracic segments with pleurae or ventral appendages. Comparable features, with ventral appendages extending as far or slightly beyond the carapace margin, have been reported from a range of Cambrian euarthropods with bivalved carapaces from different deposits including Balhuticaris voltae (Burgess Shale; [ 56 ]) , Clypecaris pteroidea (Chengjiang; [ 57 ]), Clypecaris serrata (Xiaoshiba; [ 58 ]) , Isoxys curvirostratus (Chengjiang; [ 59 , 60 ]); Pauloterminus spinodorsalis (Sirius Passet; [ 61 ]), Vermontcaris montcalmi (Parker Quarry; [ 62 ]). In Tuzoia , appendages do not appear to extend as far as the margin of the carapace (e.g.…”

Multiple origins of dorsal ecdysial sutures in trilobites and their relatives

“…Such ventral head structures are unknown from other fossil arthropods, or even extant arthropods (Edgecombe, 2020), e.g. radiodonts (Potin and Daley 2023), fuxianhuiids (Yang et al, 2018), Cambrian bivalved arthropods (Zhang et al, 2023), megacheirans (Liu et al, 2020), artiopodans excluding Acanthomeridion (Ortega Hernández et al, 2013; Jiao et al, 2022), chelicerates and mandibulates (Brusca et al, 2016). One ecological interpretation is that Acanthomeridion may have used its head to plough through the sediment, using endopodites bearing long endites to catch food items, while the large ventral plates and hypostome protected the non-biomineralized structures and directed sediment.…”