The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod

Abstract

Biramous appendages are a common feature among modern marine arthropods that evolved deep in arthropod phylogeny. The branched appendage of Cambrian arthropods has long been considered as the ancient biramous limb, sparking numerous investigations on its origin and evolution. Here, we report a new arthropod, Erratus sperare gen. et sp. nov., from the Lower Cambrian (Stage 3, 520 Ma) Chengjiang biota of Yunnan, China, with unique trunk appendages formed of lateral anomalocaridid-type flaps and ventral subconical endopods. These appendages represent an intermediate stage of biramous limb evolution, i.e. from ‘two pairs of flap appendages' in radiodonts to ‘flap + endopod’ in Erratus , to ‘exopod + endopod’ in the rest of carapace-bearing arthropods that populate the basal region of the upper-stem lineage arthropods (deuteropods). The new species occupies a phylogenetic position at the first node closer to deuteropods than to radiodonts, and therefore pinpoints the earliest occurrence of the endopod within Deuteropoda. The primitive endopod is weakly sclerotized, and has unspecialized segments without endites or claw. The findings might support previous claims that the outer branch of the biramous limb of fossil marine arthropods, such as trilobites, is not a true exopod, but is instead a modified exite. This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.

The origin and early evolution of arthropods

“…Recent studies have confirmed previous observations (Fu et al ., 2014) that the trunk appendages in isoxyids likely covered various levels of articulation, some endopods being segmented but not necessarily articulating via arthrodial membranes (Fu et al ., 2022), while in certain species the whole biramous appendage morphology seems close to that of hymenocarines (Zhang et al ., 2021). According to these data, arthrodization of post‐frontal appendages could have originally arisen from inter‐specific variations.…”

“…3D). Remarkably, megacheirans point to a euarthropod ground pattern of seven podomeres ['heptapodomerous' condition , see Table 1] for post-frontal endopods, with variations in certain species to eight or nine (Liu, Hou & Bergstrom, 2007b), which also characterized isoxyids (Fu et al, 2011(Fu et al, , 2014(Fu et al, , 2022Zhang et al, 2021).…”

A Cambrian fossil from the Chengjiang fauna sharing characteristics with gilled lobopodians, opabiniids and radiodonts

“…Van Roy et al (2015), see also Edgecombe (2015), proposed that radiodonts actually possessed both dorsal and ventral flaps: the dorsal flaps are homologous with the flaps of the gilled lobopodians, the ventral flaps are homologous with their stubby lobopodian limbs. In this hypothesis, these dorsal/ ventral structures may eventually have fused to form the typical arthropod biramous limb (Van Roy et al, 2015;Figure 4); but compare this with the alternative interpretations of biramous limb origins in, e.g., Fu et al (2022) as outlined in the Introduction. Given that P. atavus gen. et sp.…”

“…Ortega-Hernández (2016) cautioned that gilled lobopodians probably represent a grade, rather than an explicit clade. This is reflected in most of the recently published phylogenies (e.g., Daley et al, 2018;Aria 2022;Fu et al, 2022;Pates et al, 2022) in which the opabiniids (i.e., Opabinia and Utaurora) usually resolve as a sister-group to a wider clade including radiodonts and the remaining arthropods, with the gilled lobopodians (Pambdelurion and Kerygmachela) as their immediate outgroups.…”

“…The transition series from lobopodian to gilled lobopodian and eventually to radiodonts and arthropods is still being actively investigated. Drawing on the sequence of character acquisition in Fu et al (2022), the lobopodian condition involves the appearance of walking limbs (lobopods) and the gilled lobopodian grade includes the addition of respiratory exites (the flaps). An important question (see also Aria 2022) is whether these original lobopods' walking limbs were retained beyond the gilled lobopodian grade-and are thus homologous with the endopods of modern arthropods-or whether the lobopod limbs were lost during the transition to Radiodonta.…”