DOI: 10.5061/dryad.pvmcvdnn1
title: Muscle systems and motility of early animals highlighted by cnidarians from the basal Cambrian

description: Although fossil evidence suggests that various animal groups were able to
 move actively through their environment in the early stages of their
 evolution, virtually no direct information is available on the nature of
 their muscle systems. The origin of jellyfish swimming, for example, is of
 great interest to biologists. Exceptionally preserved muscles are
 described here in benthic peridermal olivooid medusozoans from the basal
 Cambrian of China (Kuanchuanpu Formation, ca. 535 Ma) that have direct
 equivalent in modern medusozoans. They consist of circular fibers
 distributed over the bell surface (subumbrella) and most probably have a
 myoepithelial origin. This is the oldest record of a muscle system in
 cnidarians and more generally in animals. This basic system was probably
 co-opted by early Cambrian jellyfish to develop capacities for
 jet-propelled swimming within the water column. Additional lines of fossil
 evidence obtained from ecdysozoans (worms and panarthropods) show that the
 muscle systems of early animals underwent a rapid diversification through
 the early Cambrian and increased their capacity to colonize a wide range
 of habitats both within the water column and sediment at a critical time
 of their evolutionary radiation.
description: All studied fossils come
 from phosphatic limestones collected from the Kuanchuanpu Formation at the
 Shizhonggou section, (Ningqiang County, Shaanxi Province, China; see
 Figure 1—figure
 supplement 4). Biostratigraphy (<em>Anabarites</em> –
 <em>Protohertzina</em> – <em>Arthrochities</em>
 zone, the <em>Siphogonuchites</em> –
 <em>Paragloborilus</em> zone and
 <em>Lapworthella</em> – <em>Tannuolina</em> –
 <em>Sinosachites</em> zone; refs. Qian, 1977; 1999)
 indicates that these rocks correspond to the Meishucunian Stage that is
 the equivalent of the lowermost Cambrian Terreneuvian Stage.
 Radiochronology (U-Pb method; refs. Sawaki et al., 2008; Peng et al.,
 2012) confirms that the Kuanchuanpu Formation is approximately 535 Ma.
 Secondarily phosphatized fossils were extracted from rocks via a standard
 acid digestion in 7% acetic acid. Dried residues with a grain-size
 &gt;60 μm were sorted and picked under a binocular microscope. Twelve
 specimens of Olivooidae (Cnidaria) bearing well-preserved muscle fibers
 were selected for the present study and mounted for SEM (FEI Quanta 400
 FEG scanning electron microscope at Northwest University, China;
 Au-coating, high-vacuum). They belong to <em>Sinaster</em>
 petalon Wang et al. (2017) (ELISN115-39), <em>Hanagyroia orientalis
 </em>Wang et al. (2020) (ELISN107-470) and Olivooidae sp.
 (ELISN150-278, ELISN111-54, ELISN052-33, ELISN045-143, ELISN012-16,
 ELISN061-19, ELISN087-64, ELISN088-48, ELISN087-33 and ELISN098-19). All
 specimens are deposited in the collections of the Shaanxi Key Laboratory
 of Early Life & Environments and the Department of Geology,
 Northwest University, China (‘ELI’ is an acronym of the former Early Life
 Institute that is now replaced by the Shaanxi Key Laboratory of Early Life
 & Environments; ‘SN’ means south Shaanxi Province, China in
 Chinese). Data concerning these specimens are available on request from JH
 and XW. Two-week-old <em>Clytia
 hemisphaerica</em> medusae, newly released <em>Eirene
 sp. </em> medusae, one-month-old <em>Chrysaora
 colorata</em> and <em>Pelagia noctiluca</em> metaephyrae
 were raised in the laboratory (Villefranche-sur-mer) following Lechable et
 al. (2020) and Ramondenc et al. (2017) culture protocols. Fixation
 followed by Phalloidin (actin) and Hoechst (nuclei) staining was performed
 on the four species as described for <em>Clytia
 hemisphaerica</em> in Sinigaglia et al. (2020). Samples were mounted
 in 50% Citifluor AF1 antifade mountant and imaged using Leica SP8 confocal
 and Zeiss Axio-Observer microscopes. Extant priapulid worms
 (<em>Priapulus caudatus</em>) (see Figure 4—figure supplement
 1) were collected (JV) from near the Kristineberg Marine Station (Sweden),
 fixed with glutaraldehyde and dried (Critical Point) for SEM observations
 (Univ. Lyon).