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Yunnanozoans are a distinctive clade of Lower Cambrian metazoans. Although widely accepted as deuterostomes, their exact placement within this superphylum is controversial. Here we describe a new species of Haikouella (H. jianshanensis) from the Chengjiang Lagerstatte (Yunnan, China) with exceptional preservation of a number of features. These include external gills, which suggest that the origin of the pharyngeal clefts was independent of the gills. The diagnostic bronchial arches of chordates may, therefore, be composite structures. No evidence was found for the chordate-like structures that have been described in other yunnanozoans. We propose that yunnanozoans are stem-group deuterostomes, allied to the vetuticolians.
Yunnanozoans are a distinctive group of metazoans, occurring only in the Chengjiang fossil-Lagerstatte located near Kunming, in southwestern China (1, 2). They are known from the type species Yunnanozoon lividum (3), Haikouella lanceolata (4), and H. jianshanensis, a new species described here. The body plan of yunnanozoans is defined by a relatively slender anterior bearing metamerically arranged gills and an expanded posterior section that dorsally bears a prominent segmented unit with a cuticular covering. Yunnanozoans have been described as deuterostomes but, alternatively, as some sort of stem group (5), hemichordates (6), cephalochordates (7), or craniates (4, 8). These divergences of opinion have arisen on account of the distinctiveness of the yunnanozoan body plan and because postmortem crushing makes crucial details at the anterior difficult to discern (3, 4, 6, 8). Here, on the basis of more than 1420 specimens of Haikouella jianshanensis (520 are well preserved), which predate the other two yunnanozoan species (3, 4, 7, 8), we report a number of previously unidentified features.
The anatomy of H. jianshanensis differs from that of the other two known species in the size of the gills. Unlike other yunnanozoans (3, 4, 5-8), in which the dorsal segmented unit is well preserved, this region is only occasionally preserved (Fig. 1, A and B). Possibly, in H. jianshanensis, the cuticle was thinner, thereby decreasing the preservation potential of the posterior region. The anterior of the main body consists of two discrete units, dorsal and ventral, each in the approximate form of a compressed half cylinder (Fig. 1, G to J). These units are separated by a median zone (Fig. 1, A to F and K). The...