動(dòng)物界的發(fā)展史中尚未解決的重要問(wèn)題之一是棘皮類(lèi)動(dòng)物的起源問(wèn)題,。分子數(shù)據(jù)表明，它們是半索動(dòng)物綱的一個(gè)姐妹群體,，后者是蠕蟲(chóng)狀海洋動(dòng)物,，有一個(gè)原始的脊索。但現(xiàn)代棘皮類(lèi)動(dòng)物（放射狀對(duì)稱(chēng)的無(wú)脊椎動(dòng)物,，包括海膽,、海參和海星）的身體構(gòu)造已經(jīng)發(fā)生了根本性的重組，因此難以設(shè)想它們的共同祖先是什么樣子?，F(xiàn)在,，來(lái)自中國(guó)云南澄江化石產(chǎn)地的新發(fā)現(xiàn)為這一問(wèn)題提供了線索。這些化石被劃分為一個(gè)新類(lèi)別,，稱(chēng)為Vetulocystids,。它們有腮，但缺少在后來(lái)的進(jìn)化中形成的一些構(gòu)造細(xì)節(jié),，因此符合棘皮類(lèi)動(dòng)物進(jìn)化樹(shù)的根部特征,，是真正的棘皮類(lèi)動(dòng)物和半索動(dòng)物綱之間的一個(gè)姐妹群體。
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.
