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Nat Commun
2019 Mar 25;101:1366. doi: 10.1038/s41467-019-09059-3.
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A stem group echinoderm from the basal Cambrian of China and the origins of Ambulacraria.
Topper TP
,
Guo J
,
Clausen S
,
Skovsted CB
,
Zhang Z
.
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Deuterostomes are a morphologically disparate clade, encompassing the chordates (including vertebrates), the hemichordates (the vermiform enteropneusts and the colonial tube-dwelling pterobranchs) and the echinoderms (including starfish). Although deuterostomes are considered monophyletic, the inter-relationships between the three clades remain highly contentious. Here we report, Yanjiahella biscarpa, a bilaterally symmetrical, solitary metazoan from the early Cambrian (Fortunian) of China with a characteristic echinoderm-like plated theca, a muscular stalk reminiscent of the hemichordates and a pair of feeding appendages. Our phylogenetic analysis indicates that Y. biscarpa is a stem-echinoderm and not only is this species the oldest and most basal echinoderm, but it also predates all known hemichordates, and is among the earliest deuterostomes. This taxon confirms that echinoderms acquired plating before pentaradial symmetry and that their history is rooted in bilateral forms. Yanjiahella biscarpa shares morphological similarities with both enteropneusts and echinoderms, indicating that the enteropneust body plan is ancestral within hemichordates.
Fig. 1. Yanjiahella biscarpa from the lower Cambrian (Fortunian) Yanjiahe Formation in China. Specimen ELI-HS18. a Specimen with plated theca, a muscular stalk, and a single feeding appendage. b Line drawing of specimen. c Detail of globular theca with plates, a feeding appendage and proximal stalk with distinct transverse ridges. d Detail of proximal stalk showing transverse ridges and a digestive tract. e Details of the stalk showing transition from the transverse ridged proximal stalk to the striated distal stalk. f Scanning electron microscopy (SEM) image of the digestive tract, with transverse ridges visible. g SEM image of the transition from the proximal to distal stalk. Scale bars, 5 mm (a, b), 1 mm (c–e), and 200 μm (f, g). ds, distal stalk; dt, digestive tract; fa, feeding appendage; p, plate; ps, proximal stalk; s, muscular stalk; t, theca
Fig. 2. Yanjiahella biscarpa from the lower Cambrian Yanjiahe Formation in China. Specimen ELI-HS84 (a–c). a Specimen with plated theca, a muscular stalk and feeding appendages. b Line drawing of specimen. c Detail of theca, and ornamented plates. Specimen G21 (d, part and e, counterpart). d Specimen with theca, stalk, and a feeding appendage. e Image of ruptured stalk. f Specimen ELI-HS998B, Specimen with two feeding appendages, and a muscular stalk showing longitudinal folds. Specimen ELI-HS999 (e, f). e Specimen with slightly distorted theca, a stalk and feeding appendages. f. Detail of stalk with digestive tract. Scale bars 1 mm (a–c, e), 2 mm (e), and 5 mm (d, g). ds, distal stalk; dt, digestive tract; fa, feeding appendage; m, mouth; p, plate; ps, proximal stalk; paao, post-anal anchoring organ; s, muscular stalk; t, theca
Fig. 3. Phylogenetic position of Yanjiahella biscarpa within the Ambulacraria. Topology based on implicit enumeration with autocollapse search off and resampling via standard bootstrap methods. Bootstrap values shown under each node. Fifteen trees were found with a total length of 71 steps (Consistency Index = 0.648, Rentention Index = 0.821). Taxa illustrated by Nobumichi Tamura
Fig. 4. Reconstruction of Yanjiahella biscarpa on the Cambrian seafloor. The precise arrangement of the plates on the theca is unknown. Artwork by Nobumichi Tamura
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