动物界的发展史中尚未解决的重要问题之一是棘皮类动物的起源问题。分子数据表明，它们是半索动物纲的一个姐妹群体，后者是蠕虫状海洋动物，有一个原始的脊索。但现代棘皮类动物（放射状对称的无脊椎动物，包括海胆、海参和海星）的身体构造已经发生了根本性的重组，因此难以设想它们的共同祖先是什么样子。现在，来自中国云南澄江化石产地的新发现为这一问题提供了线索。这些化石被划分为一个新类别，称为Vetulocystids。它们有腮，但缺少在后来的进化中形成的一些构造细节，因此符合棘皮类动物进化树的根部特征，是真正的棘皮类动物和半索动物纲之间的一个姐妹群体。
Ancestral echinoderms from the Chengjiang deposits of China
Deuterostomes are a remarkably diverse super-phylum, including not only the chordates (to which we belong) but groups as disparate as the echinoderms and the hemichordates. The phylogeny of deuterostomes is now achieving some degree of stability, especially on account of new molecular data, but this leaves as conjectural the appearance of extinct intermediate forms that would throw light on the sequence of evolutionary events leading to the extant groups. Such data can be supplied from the fossil record, notably those deposits with exceptional soft-part preservation. Excavations near Kunming in southwestern China have revealed a variety of remarkable early deuterostomes, including the vetulicolians and yunnanozoans. Here we describe a new group, the vetulocystids. They appear to have similarities not only to the vetulicolians but also to the homalozoans, a bizarre group of primitive echinoderms whose phylogenetic position has been highly controversial.
Figure 1 Two specimens of Vetulocystis catenata from Anning, Kunming, Yunnan. a–e, ELI-Ech-04-001A; a, entire specimen, note anterior cone is partially obscured by thecal surface; b, detail of respiratory organ, note cuticle missing from left-hand side; c, detail of the posterior cone; d, interpretative drawing; e, detail of posterior strand and sediment infill (?faeces) adjacent to ?anus. f, g, ELI-Ech-04-002; f, entire specimen, apparently interior view of theca; g, interpretative drawing. Scale bars where shown on photographs are millimetric. Abbreviations: Ac, anterior cone; RO, respiratory organ; Psc, posterior cone; ?Seg, ?segment boundary; Int, possible intestine (strand); Ta, tail; Th, theca. Question marks indicate that identification of body part is tentative.
Figure 2 Four specimens of D. jianshanensis from Haikou, Kunming, Yunnan. a, b, i, j, ELI-Ech-04-003A; a, entire specimen; b, interpretative drawing; i, detail of anterior cone; j, detail of respiratory organ and posterior cone in counterpart. c, d, ELI-Ech-04-004A; c, entire specimen; d, interpretative drawing. e, f, k, ELI-Ech-04-005; e, entire specimen; f, interpretative drawing; k, detail of respiratory organ and tail with possible strand. g, h, l, ELI-Ech-04-006; g, entire specimen; h, interpretative drawing; l, detail of anterior cone. Scale bars where shown on photographs are millimetric. Abbreviations: as in Fig. 1 with the addition of ?Tu, possible tube.
Figure 3 Form A (a–c, e, f) and form B (d, g), both from Haikou, Kunming, Yunnan. a, b, e, ELI-Ech-04-007; a, entire specimen, composite photograph of part (anterior section) and counterpart; b, detail of respiratory organ; e, interpretative drawing, composite of both part and counterpart. c, f, ELI-Ech-04-008; c, entire specimen; f, interpretative drawing. d, g, ELI-Ech-04-009A; d, entire specimen; g, interpretative drawing. Scale bars where shown on photographs are millimetric. Abbreviations: as in Fig. 1 with the additions, ?Org, unknown organ; ?Sto, striated organ.
Figure 4 Phylogeny of early deuterostomes. Plesiomorphic to all deuterostomes are segmentation25,43 and a bipartite body, the anterior of which possesses gill slits24,25,44. The posterior is a tail-like structure, segmented with an intestine and terminal anus13,14. Vetulicolians13,14 may be the most primitive known deuterostomes, showing segmentation of the entire body, and an anterior with five pairs of gill slits. Extant members of the Ambulacraria24,25 are the echinoderms and hemichordates. Vetulocystids are regarded as more derived than the hemichordates, but retain the bipartite body and a respiratory organ that also characterize the most primitive echinoderms (homalozoans8). In more primitive Ambulacraria the gut extended along the posterior tail, but either just before or after the bifurcation leading to the vetulocystids the gut became restricted to the anterior body. Here we depict the former possibility. All echinoderms, including homalozoans, possess a stereom, but the most primitive representatives retain gill slits. The acquisition of a water-vascular system and ambulacra was a subsequent development. The position of the extinct yunnanozoans remains controversial. Here we indicate two alternatives, either closer to the chordates17 or the hemichordates16,21,41,45. In support of the latter hypothesis is the lack of evidence for key craniate features, including eyes, a complex brain, a notochord and myomeres, but the possible presence of both dorsal and ventral nerve cords.
