Tentaculate fossils from the Cambrian of Canada (British Columbia) and China (Yunnan) interpreted as primitive deuterostomes

Abstract

Molecular and morphological evidence unite the hemichordates and echinoderms as the Ambulacraria, but their earliest history remains almost entirely conjectural. This is on account of the morphological disparity of the ambulacrarians and a paucity of obvious stem-groups. We describe here a new taxon Herpetogaster collinsi gen. et sp. nov. from the Burgess Shale (Middle Cambrian) Lagerstätte. This soft-bodied vermiform animal has a pair of elongate dendritic oral tentacles, a flexible stolon with an attachment disc, and a re-curved trunk with at least 13 segments that is directed dextrally. A differentiated but un-looped gut is enclosed in a sac suspended by mesenteries. It consists of a short pharynx, a conspicuous lenticular stomach, followed by a narrow intestine sub-equal in length. This new taxon, together with the Lower Cambrian Phlogites and more intriguingly the hitherto enigmatic discoidal eldoniids (Cambrian-Devonian), form a distinctive clade (herein the cambroernids). Although one hypothesis of their relationships would look to the lophotrochozoans (specifically the entoprocts), we suggest that the evidence is more consistent with their being primitive deuterostomes, with specific comparisons being made to the pterobranch hemichordates and pre-radial echinoderms. On this basis some of the earliest ambulacrarians are interpreted as soft-bodied animals with a muscular stalk, and possessing prominent tentacles.

Figure 1. Eldonia ludwigi from the Middle Cambrian Burgess Shale.

A, lectotype, National Museum of Natural History (USNM) 57540 Part, overall view. B–C, USNM 188552; B, Part, overall view; C, detail of the tentacles. D, USNM 201692 Part, overall view, arrows pointing to triangular projections representing evidence of possible segmental mesenteries. Scale bars: 10 mm. an, anus; in, intestine; ph, pharynx; stom, stomach; te, tentacle. (Photos D. Collins).

Figure 2. Herpetogaster collinsi from the Middle Cambrian Burgess Shale.

A–B, Locality maps. C, specimen occurrences.

Figure 3. Herpetogaster collinsi from the Middle Cambrian Burgess Shale.

All specimens are preserved dorso-ventrally, except F. In each photographic figure anterior is indicated by a wide white arrow. A–C, Royal Ontario Museum (ROM) 58051 Holotype. A, Part, overall view; B, Counterpart, detail of tentacles with hydrostatic canal and/or vascular system emphasized by white arrows; C, Camera-lucida drawing of part and counterpart emphasizing the presence of putative segment boundaries and triangular projections along the stomach. D, ROM 58046 Part, with symmetrical tentacles and pharyngeal pores. E, ROM 58039 Counterpart, intestine with putative enclosing tube emphasized by small arrows. F, G, ROM 58037 Part. F, Part, lateral view; G, Detail of terminal disc. H, I, ROM 58047 Part. H, extended stolon and terminal disc; I, Detail of the stolon, small arrows point to a darker central area representing a possible coelomic cavity. Scale bars: A–F, H, 5 mm; G,I, 1 mm. an, anus; hy?, putative hydrostatic canal and/or vascular system; in, intestine; p?, putative pharyngeal pores; ph, pharynx; seg, segment boundary?; st, stolon; stom, stomach; td, terminal disc; te, tentacle.

Figure 4. Reconstruction of Herpetogaster collinsi from the Middle Cambrian Burgess Shale.

© 2010 - Marianne Collins.

Figure 5. Phlogites longus from the Lower Cambrian Chengjiang biota.

A, ELI-Phl-07-001 lateral view, complete specimen with the stolon attached to a fragment of trilobite exoskeleton. B, ELI-Phl-07-002 oral view, specimen showing four tentacles (the broken area suggests the possibility of a fifth set) and three preserved lobes. C–D, ELI-Phl-07-003 lateral view; C, overall view of the gut; D, detail of the anus. Scale bars: A–C, 5 mm; D, 1 mm. Legend, see figure 1; lo?, lobes; ro?, reproductive organs?, tr, trilobite fragment.

Figure 6. Possible positions of the cambroernids within ambulacrarians.

A = stem-group hemichordate, B = stem-group echinoderm, C = stem-group ambulacrarian.