Anamorphic development and extended parental care in a 520 million-year-old stem-group euarthropod from China

Abstract

Extended parental care (XPC) is a complex reproductive strategy in which progenitors actively look after their offspring up to -or beyond -the first juvenile stage in order to maximize their fitness. Although the euarthropod fossil record has produced several examples of brood-care, the appearance of XPC within this phylum remains poorly constrained given the scarcity of developmental data for Palaeozoic stem-group representatives that would link juvenile and adult forms in an ontogenetic sequence. Here, we describe the post-embryonic growth of Fuxianhuia protensa from the early Cambrian Chengjiang Lagerstätte, and show parental care in this stem-group euarthropod. We recognize fifteen distinct ontogenetic stages based on the number and shape of the trunk tergites, and their allocation between the morphologically distinct thorax and abdomen. Our data demonstrate anamorphic post-embryonic development in F. protensa, in which tergites were sequentially added from a posterior growth zone. A life assemblage consisting of a sexually mature F. protensa adult alongside four ontogenetically coeval juveniles, constitutes the oldest occurrence of XPC in the panarthropod fossil record. These findings provide the most phylogenetically basal evidence of anamorphosis in the evolutionary history of total-group Euarthropoda, and reveal a complex post-embryonic reproductive ecology for its early representatives.

The origin and early evolution of arthropods

“…For the most part, those are undescribable (personal observations), but several post-hatchling stage larvae have already been published (Liu et al, , 2020, some of them differing somewhat in morphology from their adult counterparts, pointing to ontogenetic niche differences. As arthropods were already the most diverse and abundant animals by the Cambrian Stage 3, their larvae must have constituted an important part of planktonic life forms, even if there were many benthic direct developers (Fu et al, 2018).…”

“…They also have been used to investigate early burst models of high Cambrian disparity preceding phenotypic canalization (Hughes, 1991;Webster, 2007), a view that was later refined to point out the variable relaxation of constraints on segment number across lineages, often associated with the co-evolution of adaptive features on a large scale (Hughes, Chapman & Adrain, 1999;Hughes, 2003;Webster & Zelditch, 2011). Reference ontogenetic work on trilobites, especially to reconstruct heterochronic trends (Hughes, 2007), should inspire research on soft-bodied larvae (Zhang et al, 2010;Liu et al, 2016;Fu et al, 2018), because heterochrony is another potentially highly significant explanatory variable of arthropod morphoanatomy over time.…”

Early developmental stages of a Lower Ordovician marrellid from Morocco suggest simple ontogenetic niche differentiation in early euarthropods

“…Even within the famous Cambrian Burgess Shale-type (BST) deposits, including the iconic Burgess Shale of Canada and Chengjiang Biota of China, where preservation of soft tissues and lightly sclerotized taxa occur, small euarthropod specimens are an exception and fossils are biased towards later developmental stages and adults, except for the recently discovered Haiyan locality (Yang et al, 2021). Correspondingly, larval or juvenile stages have been described for only a handful of species (e.g., Garcıá-Bellido and Collins, 2006;Haug et al, 2013;Fu et al, 2014;Liu et al, 2014;Liu et al, 2016;Fu et al, 2018;Liu et al, 2018;Zhai et al, 2019).…”