Beretella spinosa gen. et sp. nov. from Member 5 of the Yanjiahe Formation (Cambrian Stage 2), Yichang, Hubei Province, China.

a‒c, Holotype, CUBar138-12. a, Dorsal view showing the external ornament: (five sclerites at the midline in yellow (S1); flanked by two rows of sclerites in blue (S2); large broad-based conical sclerites in two dorsolateral pairs and one antero-posterior pairs in green (S3)); white arrows indicate lateral constriction. b, Right lateral view showing two additional rows of six sclerites (S4 and S5, in light blue and pink, respectively). c, Ventral view showing a large opening that may have accommodated the mouth (see the text) and an empty body cavity. d‒g, CUBar75-45. d, Dorsal view showing a broken S3. e, Micro-CT image, right lateral view displaying S4. f, Ventral view depicting a tiny projection in purple. g, An enlargement of the projection of f. h‒i, Paratype, CUBar171-5. h, Right dorsal view showing S1‒S4. i, Right-lateral view showing S4 and S5. j‒k, Paratype CUBar121-8. j, Dorsal view showing poorly preserved S1 and S2. k, Right-lateral view showing S3‒S5. A, assumed anterior end (see text); ef, exotic fragment; D, assumed dorsal side; L, left; P, posterior end; R, right; tp, tiny spine; V, ventral side. The same abbreviations are used throughout the manuscript including Supplementary materials.

Beretella spinosa gen. et sp. nov.

a, CUBar99-19, dorsal view showing an ornament S1‒S4. b‒c, CUBar136-9, general dorsal view and details. d, CUBar136-11, dorsal view showing S1‒S5. e‒f, CUBar73-15 general view and details of the cuticular polygonal reticulation in black. g‒j, CUBar128-27. g, general view. h, i, details of outer and inner surface of the bi-layered structure of the cuticular wall as seen in broken conical sclerites. j, Micro-CT section showing possibly sclerite infilling. k-l, CUBar99-18, cuticular fragment, general view and details of large sclerite (central feature represents possible phosphatic infilling). is, infilling sclerite; pr, polygonal reticulation; sb, sclerite base.

Position of Beretella spinosa in the animal tree based on cladistic analysis.

a-c, artistic three-dimensional reconstructions of Beretella spinosa in the anterolateral (a), dorsal (b), and posterolateral views (c). d, Phylogenetic tree obtained from cladistic analyses using maximum likelihood. Saccorhytus and Beretella join in a clade (new phylum Saccorhytida) resolved as the sister-group of all other ecdysozoans; numbers at key nodes denote probability. Fossil and extant taxa are in italics and bold, respectively. Known fossil record indicated by thicker vertical bars (after (Shu and Han, 2020a)).

Possible evolutionary scenario to explain the origin and early evolution of ecdysozoans.

a, Summary tree (see Supplementary Figs. 6-9) showing saccorhytids as a sister-group of Cycloneuralia (Nematoida plus Scalidophora) + Panarthropoda; main morphological features of each group listed along each branch. b, Potential evolutionary pathway to evolve Saccorhytida and crown-group Ecdysozoa. Numbers in green, red and blue circles designate pre-ecdysozoan (Spiralia), Saccorhytida and Cycloneuralia, respectively. Light brown gradient (circle) to emphasize ecdysis and sclerite secretion seen as key evolutionary steps. 1, Hypothetical pre-ecdysozoan animal with a ciliated epidermis and glycocalyx. 2, Saccorhytid exemplified by Beretella with a cuticle bearing sclerites. 3, Crown-group ecdysozoan exemplified by a scalidophoran worm with an elongated shape, a differentiated head (introvert) and trunk, sclerites, a through gut, a terminal mouth and abilities to burrow into bottom sediment. Animals not to scale. Abbreviations: a, anus; a?, uncertain status of anus; ci, cilia; cu, cuticle; ec, epidermal cell; gl, glycocalyx (mucous layer); m, mouth; in, introvert; sc, sclerite; se, sediment; TGE, total-group Ecdysozoa. Silhouettes from phylopic.org.