Saccorhytus is an early ecdysozoan and not the earliest deuterostome

Abstract

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New Qinscyphus material from the Fortunian of South China

“…The lithostratigraphy and biostratigraphy of the Zhangjiagou section were studied by Li (1984) and Steiner et al (2004a), Steiner et al (2007). The same locality and horizon have previously yielded abundant and beautifully preserved microfossils, such as specimens of Olivooides (Liu et al, 2014a;Steiner et al, 2014;Liu et al, 2017;Shao et al, 2018b;Shao et al, 2019a), Pseudooides, the oldest known priapulid-like cycloneuralians as well as other scalidophorans (Liu et al, 2014b;Shao et al, 2016;Shao et al, 2018b;Shao et al, 2019b;Wang et al, 2019;Wang D et al, 2020) and possibe Saccorhytus (Han et al, 2017;Liu et al, 2022). The fossil-bearing horizon correlates with the Anabarites trisulcatus-Protohertzina anabarica assemblage zone (Steiner et al, 2007;Steiner et al, 2014), which has an estimated age of 531.8-536.4 Ma and falls within the Fortunian Stage (Peng et al, 2020).…”

Early evolution of the ecdysozoan body plan

“…Saccorhytida first appeared in the literature as a new stem-group Deuterostomia that accommodated a single species, Saccorhytus coronarius (Han et al, 2017). Since Saccorhytus is no longer considered a primitive deuterostome and, instead, more likely belongs to ecdysozoans, Saccorhytida became an extinct Order of Ecdysozoa (Liu et al, 2022; Shu and Han, 2020b). Because both Saccorhytus and Beretella display major morphological differences with all other known ecdysozoan phyla (Nematoida, Scalidophora, and Panarthropoda), Saccorhytida is tentatively elevated here to the rank of phylum within Ecdysozoa.…”

“…Beretella spinosa has no exact equivalent in any Cambrian animals except Saccorhytus coronarius , an enigmatic, sac-like ecdysozoan (Han et al, 2017; Liu et al, 2022; Shu and Han, 2020b). Both forms share a tiny, poorly differentiated ellipsoidal body, and a set of prominent bilaterally arranged spiny sclerites.…”

“…535 Ma (Sawaki et al, 2008)) that is anything but a worm sowed doubt among scientists. Saccorhytus is a sac-like secondarily phosphatized microscopic animal spiked with conical sclerites and a single opening that was first seen as the earliest known deuterostome (Han et al, 2017) but is now considered as an ecdysozoan on more solid grounds (Liu et al, 2022;Shu and Han, 2020b), thus broadening the anatomical spectrum of the group and its disparity in the Cambrian and reopening the debate on the nature of LCAE. We describe here Beretella spinosa gen. et sp.…”

“…Current reconstruction based on fossil and developmental evidence features the ancestral ecdysozoan as a millimeter-sized worm (Budd, 2001; Valentine and Collins, 2000) with a terminal (Ortega-Hernandez et al, 2019) or ventral mouth (Martín-Durán and Hejnol, 2015; Nielsen, 2019). Clearly, the discovery of Saccorhytus (Han et al, 2017; Liu et al, 2022; Shu and Han, 2020b) in the basal Cambrian of China (Kuanchuanpu Formation; ca. 535 Ma (Sawaki et al, 2008)) that is anything but a worm sowed doubt among scientists.…”