Cambrian lobopodians and extant onychophorans provide new insights into early cephalization in Panarthropoda

Abstract

Cambrian lobopodians are important for understanding the evolution of arthropods, but despite their soft-bodied preservation, the organization of the cephalic region remains obscure. Here we describe new material of the early Cambrian lobopodian Onychodictyon ferox from southern China, which reveals hitherto unknown head structures. These include a proboscis with a terminal mouth, an anterior arcuate sclerite, a pair of ocellus-like eyes and branched, antenniform appendages associated with this ocular segment. These findings, combined with a comparison with other lobopodians, suggest that the head of the last common ancestor of fossil lobopodians and extant panarthropods comprized a single ocular segment with a proboscis and terminal mouth. The lack of specialized mouthparts in O. ferox and the involvement of non-homologous mouthparts in onychophorans, tardigrades and arthropods argue against a common origin of definitive mouth openings among panarthropods, whereas the embryonic stomodaeum might well be homologous at least in Onychophora and Arthropoda.

Hallucigenia’s onychophoran-like claws and the case for Tactopoda

“…An alternative position for O. ferox in stem-Euarthropoda was also recovered, but only at exceedingly low concavity values. Total-group Euarthropoda includes various disparate forms united by the ancestral presence of fused protocerebral appendages bearing series of spines/spinules, ultimately transformed into the euarthropod labrum 9,20 (Supplementary Note 2, transformation series [12][13][14][15][16][17]. The gradual evolutionary transition from lobopodians with spinose frontal appendages (Jianshanopodia, Megadictyon) through gilled lobopodians (Kerygmachela, Pambdelurion, Opabinia) and anomalocaridid-type taxa (Peytoia, Anomalocaris, Hurdia) to stem euarthropods with full body arthrodization (for example, fuxianhuiids) is in overall agreement with previous reports 5,11,14,20 .…”

“…The Palaeozoic form-taxon Lobopodia encompasses a diverse range of soft-bodied 'legged worms' known from exceptional fossil deposits [1][2][3][4][5][6][7][8][9] . Although lobopodians occupy a deep phylogenetic position within Panarthropoda, a shortage of derived characters obscures their evolutionary relationships with extant phyla (Onychophora, Tardigrada and Euarthropoda) 2,3,5,[10][11][12][13][14][15] .…”

“…Palaeozoic lobopodians feature prominently in discussions about the origins of crown-group panarthropods-the extant velvet worms (Onychophora), water bears (Tardigrada) and euarthropods (Euarthropoda) 5,[9][10][11]20 . Although lobopodians have been regarded as onychophoran ancestors 2,3 , the presence of 'primitive' characters-such as a terminal radial mouth, unsclerotized annulated cuticle, a non-segmented body and terminal claws in the walking legs-suggests a deeper phylogenetic position 1,4,13 .…”

“…Onychophorans lack armature sclerites, but possess two types of appendicular sclerite: paired terminal claws in the walking legs, and denticulate jaws within the mouth cavity 9,23 . As in H. sparsa, claws in E. kanangrensis exhibit a broad base that narrows to a smooth conical point (Fig.…”

“…On one side of this basal split is total-group Onychophora, defined by the limbless posterior extension of the lobopodous trunk, undifferentiated posterior appendages and the loss of radially symmetrical circumoral structures (Supplementary Note 2, transformation series 61, 63, 19). Stem-group Onychophora includes Diania (contra refs 5, 11), Xenusion, Paucipodia, Antennacanthopodia and all lobopodians with sclerotized dorsal elements except Onychodictyon ferox 9 . Luolishania and the Emu Bay Shale ''Collins' monster'' occupy a derived position within a paraphyletic Hallucigenia grade.…”