Early aculiferan diversification shaped by Ægir-Iapetus palaeogeography: Insights from North Greenland (Cambrian Series 2, Stage 4)

Abstract

Aculifera, comprising Polyplacophora (chitons) and Aplacophora, is one of two major molluscan clades, characterised by a scleritome of mineralised sclerites or plates. Their early evolutionary history is poorly understood due to sparse fossil records bridging possible Terreneuvian (early Cambrian) stem-group taxa and Furongian (late Cambrian) chiton-like articulated representatives. Here, we describe seven species in five genera from the Aftenstjernes & oslash; Formation (Cambrian Series 2, Stage 4) of North Greenland, including a second species, Qaleruaqia bronlundensis sp. nov., of the oldest paleoloricate, Qaleruaqia sp.; the sachitid Hippopharangites groenlandicus, Hippopharangites? sp.; a possible intermediate plate of a chiton-like scleritome, Avannaplax midsommersoensis gen. et sp. nov.; and possible aculiferans Ocruranus? kangerluk and Xianfengella yatesi. Morphological comparisons of the North Greenland assemblage reveal strong affinities with Terreneuvian taxa, including maikhanellids and the Ocruranus-Eohalobia group, supporting their interpretation as stem-group aculiferans. A palaeobiogeographic analysis using 650 global occurrences of Cambro-Ordovician taxa shows that the AE gir Ocean as a principal dispersal corridor for early diversification of aculifera. During the Terreneuvian, aculiferans were widespread along AE gir margins but rare in Laurentia, likely due to isolation by the Iapetus Ocean. From Cambrian Series 2 onward, increased connectivity and expanded low-latitude carbonate platforms fostered diversification in eastern Laurentia, including North Greenland. This assemblage from North Greenland bridges a critical spatio-temporal and morphological gap between the earliest Cambrian taxa and Furongian paleoloricates, offering new insight into aculiferan diversification and palaeobiogeography.