Box The Chengjiang Fossil Site

The Chengjiang fossil site in Yunnan Province, south-west China, is exciting because it is one of the oldest sources of exceptionally preserved organisms, falling early in the great Cambrian radiation of animals in the sea. The fossils come from different levels through several hundred metres of mainly fine-grained sediments. When the site was discovered, in 1984, it was thought to correspond to the already well-known Burgess Shale, a Middle Cambrian locality in Canada that has yielded numerous exceptionally preserved arthropods and the putative chordate Pikaia. Chengjiang, however, is older, dating from the middle of the Early Cambrian, some 525-520 Myr ago.

Typical Chengjiang fossils, the vetulicolian Xidazoon (a), and the basal vertebrate Myllokunmingia (b), both facing right. Scale bars In millimetres. Compare with interpretive drawings in Figures 1.6 (b) and 3.1(a). (Courtesy of Shu Degan.)


The Chengjiang site is rich, having produced more than 10,000 specimens, and the fauna consists of 90 or more species, mainly of arthropods (trilobites and trilobite-like forms), sponges, brachiopods, worms, and other groups, including possible basal deuterostomes, such as the vetulicolians and yunnanozoons (Figure 1.7), as well as the first fishes (Shu, 2003). Some of the arthropods are like Burgess Shale animals, but others, such as the basal deuterostomes, seem to be unique. Most of the animals lived on the bottom of the sea-bed, filtering organic matter from the sediment. There were a few floaters and swimmers, and some of the larger arthropods were clearly predators, feeding on the smaller bottom-dwellers.

The Chengjiang beds are grey marine mudstones that preserve soft tissues of many animals in exquisite detail, some replaced by phosphate and others by pyrite. Some soft tissues survive as thin organic films. The grey sediment weathers on contact with the air to a light grey or yellow colour, and the fossils may also be grey, or sometimes reddish, and with internal anatomical details picked out in shades of grey, brown, and black.

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Fig. 1.6 Basal deuterostomes: (a,b) the vetulicolians Didazoon (a) and Xidazoon (b), showing how the body is divided into two sections that are joined by a flexible connection; (c) Haikouella. (Courtesy ofShu Degan.)

mouth-bearing segment, presumably the front part of the body, are five circular structures in a row that have been interpreted as pharyngeal gill slits.

The vetulicolians have been accorded three positions in the phylogenetic tree (Figure 1.7): as basal deuterostomes, as urochordates or as basal chordates. Evidence that vetulicolians are deuterostomes are the gill slits and the possible endostyle. They have been interpreted as basal deuterostomes by Shu et al. (2001) because they apparently lack an atrium, the internal chamber in tunicates into which the gill slits and anus open. In vetulicolians, the intestine terminates at the end of the body, and the gill slits presumably opened directly to the outside through openings in the external body wall. There are, however, some general resemblances to swimming tunicates in the bulbous streamlined body shape, the thin external tunic, and the regularly spaced transverse bands, which might be muscles that ran round the body in rings (Lacalli, 2002). The absence of a notochord in vetulicolians is not critical, since most adult tunicates also have lost this structure.

Additional invertebrate chordates from Chengjiang, the yunnanozoons Yunnanozoon and Haikouella (Figure 1.6(c)), have been interpreted by rival researchers both upwards and downwards in the scheme of things (Figure 1.7). One team identified these animals first as possible cephalochordates (Chen et al., 1995), and then upwards as vertebrates (Chen et al., 1999; Holland and Chen, 2001; Mallatt and Chen, 2003). The other team preferred to regard the yunnanozoons first as hemi-

Fig. 1.7 Phylogenetic tree of the extant deuterostomes, with suggested locations of the major fossil groups.

chordates (Shu et al., 1996), and then downwards as basal deuterostomes allied to the vetulicolians (Shu et al., 2003b). The problems revolve around different interpretations of coloured blobs, lines, and squiggles in the fossils. There are plenty of fossils—literally thousands —but anatomical interpretation is critical.

Haikouella and Yunnanozoon are 25-40 mm long, and preserved as flattened bluish-grey to black films on the rock. Chen et al. (1995) were able to see a notochord, a filter-feeding pharynx with an endostyle, segmented musculature, and branchial arches, all chordate characters. Chen et al. (1999) and Mallatt and Chen (2003) went further, identifying an enlarged, possibly three-part, brain and paired lateral eyes in Haikouella, hence indicating it might have had a distinctive, enlarged head, a key feature of vertebrates. Shu et al. (1996) argued, however, that there is no notochord, and that this tubular structure is actually the gut. In addition, they suggested that the segmented musculature was wrongly identified. In contrast, they claimed to see key hemichordate features in Yunnanozoon, and especially that the body is divided into three parts from front to back, a proboscis, a collar, and a trunk that is divided into a branchial and a gut region, just as in the living acorn worm (Figure 1.4(c)). Shu (2003) and Shu et al. (2003b) subsequently noted similarities between the yunnanozoons and the vetulicolians, and moved them down from the hemichordates to a basal position among deuterostomes (Figure 1.7): they could see no evidence of a notochord, a large brain, lateral eyes, or any of the other chordate features previously reported.

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