The first description of the enigmatic Cotyledion was based on only two incomplete specimens and it was accordingly referred to Problematica6 on account of the paucity of characters. Later, an additional well preserved specimen distinctly revealed a circle of tentacles around the upper margin of the calyx12, suggesting an uncertain tentaculate affinity of Cotyledion. Clausen et al.11 re-examined 27 specimens of Cotyledion and concluded that it belonged in the stem group of cnidarians. Our interpretations are based on observations of approximately 400 new specimens collected by the work-team of the Early Life Institute (Prefix: ELI). Well-preserved specimens provide considerable new morphological and anatomical data that form the basis for a major reconsideration of the structure, function and affinities of this peculiar tentaculate animal. The new material shows that Cotyledion was a solitary, sessile and benthic, tentaculate presumed suspension feeder (Fig. 4). Its body consisted of a lower stalk that normally attached (often gregariously) to hard bioclasts and an upper cup-shaped calyx that contained a U-shaped gut and a tentacle crown that surrounded the mouth and anus (Figs. 1, 2). The presence of a U-shaped digestive tract unequivocally rule out an affinity of Cotyledion with the cnidarians11, but rather points to its inclusion in the sessile lophotrochozoans13,14. In contrast filter-feeding deuterostomes usually have bifurcating tentacles15.

The Lophotrochozoa13,16, represents a clade of remarkably diverse and anatomically disparate spiralian animals, comprising a number of animal phyla, specially Annelida, Mollusca and several sessile tentaculate groups (Bryozoa, Cycliophora, Brachiopoda, Entoprocta, Phoronida)10. Anatomically, the Entoprocta differs markedly from other sessile lophophorates in that both the mouth and anus lie inside the “crown” of tentacles. Not least because of differences in cleavage patterns and body cavities17, the Entoprocta is usually placed outside the Superphylum Lophophorata18. Recently, molecular phylogenies confirm that the entoprocts are monophyletic19 and invariably nest within the Lophotrochozoa, with recent results suggesting they are sister group to the cycliophorans and with weak support for forming the Polyzoa together with the bryozoans14,16,20.

Despite the relatively stable composition of the Lophotrochozoa, the precise internal relationships of the clade and its morphological origins remains obscure, largely owing to the widely disparate nature of the phyla contained. As a result, reconstruction of the evolution of fossil stem-groups to extant crown groups is a necessary step to understanding this large group's early history. Such information has largely relied on exceptionally preserved fossils, such as from the Burgess Shale and Chengjiang15,21. Exceptionally preserved tentaculate fossils including eldoniids22, Phlogites6 and the recently reported Herpetogaster15, commonly characterized by a coiled intestine and paired bifurcated tentacles that encircle the mouth but not anus, are best referred to primitive deuterostomes rather than any lophotrochozoans15, let alone entoprocts typified by unbranched tentacles surrounding a mouth and anus in a whole5. Although originally compared to entoprocts3,4, the affinities of Dinomischus were later questioned owing to its lack of pliable and foldable tentacles2 and any further information on its interior anatomy5,15,23.

Although the exact phylogenetic placement of Cotyledion is still open to some speculation, the discovery of a U-shaped gut represents a significant progress in the understanding of these enigmatic Cambrian fossils and demonstrates a bilaterian, lophophorate affinity of Cotyledion with certainty. The morphology (a U-shaped gut with a mouth and anus enclosed within a circle of tentacles) and sessile lifestyle of Cotyledion is compatible with an entoproct affinity1 and accords well with the recent suggestion based on molecular analyses that the entoproct ancestor had a “marine, probably solitary, epizoic adult stage”19. In addition, there is some evidence that the tentacles may have been contractible and could have been retracted into the membranous band where they originate from (Fig. S4a–c), suggesting some degree of retractability of the tentacles that corresponds functionally and structurally to that seen in those of the extant entoprocts1. Other entoproct characters include presence of a slit-like mouth and funnel-like buccal cavity (Fig. 1). By contrast, all extant solitary entoprocts are relatively diminutive, with height of individual zooids ranging from 0.3 to 30 mm and that of calyxes between 0.2 and 1.2 mm1, markedly less than the average calyx (4.3–42 mm) and individual height (8–56 mm) of Cotyledion (Fig. S1, Table S1). In addition, recent entoprocts are pseudocoelomate, with the cavity surrounding the calycal organs and extending into the stalk in-filled by a hydrostatic skeleton of loose mesenchyme cell or narrow primary body cavity1.

It is notable that a significant number of animals that arose in Cambrian explosion have combinations of very unfamiliar morphologies and anatomies21,24, not encountered in extant phyla. Nevertheless, the key utility of fossils in systematics is not supposedly insuperable difference, but shared similarities or homologous characters. Even when weak, such characters provide provisional support for particular stem-group placement of enigmatic taxa such as Cotyledion and assist in reconstruction the sequence of character acquisition in the Cambrian and indeed can polarize hypothesis of character loss or gain in related derived extant crown groups. Thus, although Cotyledion i) is larger; ii) apparently possesses a more extensive body cavity; iii) has an external skeletal organization, it nevertheless shares features with entoprocts such as the holdfast + stalk + calyx bearing tentacles surrounding both mouth and anus, both linked by a U-shaped gut. Together, these are features possessed only by the extant entoprocts and - despite the notable differences – suggest a placement of Cotyledion within the entoproct stem-group.

If correctly interpreted, this placement of Cotyledion has several novel implications for lophotrochozoans. Although living entoprocts are markedly smaller in adult size than Cotyledion, the size decrease could be taken as a result of miniaturization that is a widespread phenomenon in animals25. It is well known that miniaturization involves not only small body size, but also consequent and often dramatic effects of extreme size reduction on anatomy, ecology and life history25. Such consequences were well exemplified in Cotyledion. Along the stalk of Cotyledion (Fig. 1a, b; Fig. S4g), there is a central lineation of darkish color suggesting a central canal, interpreted here as an extension of the calycal cavity. Given that recent molecular results place the entoprocts within a clade of coelomates14,16, it is possible that the body cavity of Cotyledion was indeed a coelom. If so, the loss of a coelom may represent an apomorphic character in living entoprocts, since reduction and structural simplification is a common effect of miniaturization25 and an adaptation that may be associated with a change to colonial life. Moreover, the body wall of living entoprocts is very thin, even transparent on the calyx, solely covered by a glycoprotein cuticle with a trace of chitin25. By contrast, the outer body surface of Cotyledion hosts numerous pronounced sclerites that presumably stiffened the calyx or stalk (Fig. 3,4). Despite providing protection from durophagy26, the strongly armoured scleritome of Cotyledion might thus have precluded the organism from performing the flexible “nodding” motions as seen in recent entoprocts. The single-layered epithelial body wall in extant entoprocts likely represents another consequence of size decrease. It is apparent that Cotyledion had a solitary lifestyle and was attached to the exoskeletons of other organisms (Fig. 1a, c; Fig. S4c, e-g). Taking into account the evolutionary changes of the characters discussed above, solitary life could be a plesiomorphic state in contrast to the colonial one of modern entoprocts. The assumption is supported by the younger colonial entoprocts, so far known from the upper Jurassic rocks in Great Britain2. If so, the interpretation of Cotyledion among stem entoprocts demonstrates that a stolon as attachment structure is an apomorphy for the Order Coloniales within the Entoprocta19. The Cambrian (Stage 3) fossils discussed here are, therefore, a very significant extension of the geological range of total group entoprocts, set a minimum time frame for the divergence of entoprocts from other lophotrochozoans and first shed light on their scleritomous body plan (Fig. 4).

The cuticular sclerites represent the most unique feature of Cotyledion. The discovery that Cotyledion is allied to entoprocts demonstrates that entoprocts developed distinctive armoured bodies early in their history. Comparable to Cotyledion, armoured scleritome-bearing bodyplans were also known from stem groups of annelids27, molluscs21,28,29, brachiopods8 and phoronids9, presumably thought as evidence of a close affinity between lophotrochozoans. Abundant varieties of sclerites commonly found in Cambrian SSF assemblages such as siphogonuchitids, sachitids and halkieriids have been associated with the stem groups of various branches of the lophotrochozoan tree (i.e. Mollusca and Annelida)21,27,28,29 and the phosphatic tommotiids have been placed in the stem groups of the Phoronida9 and Brachiopoda7,8. In terms of morphology, the scleritome of Cotyledion (particulary in the stalk) is most comparable to the tommotiid Eccentrotheca9,30, which has a tubular articulated scleritome composed of highly variable phosphatic sclerites. However, the original mineralogy of the sclerites in Cotyledion cannot be ascertained and although there is some evidence for accretionary growth, the detailed shape and loose configuration of the scleritome of Cotyledion clearly differs from that of the tightly interlocking and organized configuration of Eccentrotheca. In Eccentrotheca, the sclerites are arranged in transverse, ring-shaped units that are eventually fused during ontogeny9,30. This contrasts sharply with the roughly longitudinal arrangement and universal separation of the sclerites in Cotyledion. The sclerites of Cotyledion (Fig. 3) also differ from those of Eccentrotheca and other tommotiids by the existence of a unidirectional mode of growth (Fig. 3f). Based on these differences, Cotyledion cannot be included in the tommotiids and the structural similarities of the stalk in Cotyledion with the scleritome of Eccentrotheca are superficial. The presence of external sclerites in the stem entoproct Cotyledion as well as in the proposed stem groups of phoronids9, brachiopods7,8, annelids27 and molluscs21,28,29 suggests that sclerites themselves are distributed more widely than thought before and may even be plesiomorphic for the Lophotrochozoa. Despite the differences in organization, growth and composition, the discovery of Cotyledion as a sclerite-bearing entoproct strengthens the view that the stems of many of lophotrochozoan phyla may be traced within the early Cambrian Small Skeletal Fauna (SSF).