Vetulicolians and Yunnanozoons

Australia: The Land Where Time Began

Vetulicolians and Yunnanozoons

Vetulicolians

The Vetulicolians, a newly named group, named by Shu et al. (2001), based on 3 genera, Vetulicola, Xidazoon and Didazoon, that Benton describes as looking like sausage balloons that are knotted in the middle, the 2-part body having a flexible connection in the middle joining a bulbous section on either side. They are considered by Benton to be one of the most extraordinary finds from the Chengjiang Formation, though they are still highly controversial. They had a strengthened rim around a large mouth, and they are so well preserved that internal structures, such as the guts and a possible endostyle can be discerned, and transverse bands of tissue appear to cross both bulbous sections. There is a row of 5 circular structures on the segment with the mouth, that is presumed to be the front of the body, that have been interpreted as pharyngeal gill slits.

The Vetulicolians have been tentatively assigned to 3 position in the phylogenetic tree, as basal forms in the lines leading to deuterostomes, to urochordates or to chordates. The gill slits, and possible endostyle, have been suggested by Benton to possibly indicate that the vetulicolians may be deuterostomes. As they appear to lack an atrium, in tunicates, the internal chamber into which the gill slits and anus open, they have been interpreted as basal deuterostomes (Shu et al., 2001). The intestine terminates at the end of the body in vetulicolians, and it is assumed that the gill slits opened directly to the outside via openings in the body wall. The bulbous, streamlined body shape, a thin external tunic and the transverse bands that are regularly spaced, that Benton suggests may have been muscles running around the body in rings (Lacalli, 2002), can be regarded as general resemblances to swimming tunicates. As most adult tunicates lose the notochord, the lack of this structure in vetulicolians is not considered to be critical.

Yunnanozoons

Yunnanozoons from Chengjiang, such as Yunnanozoon and Haikouella, are also considered to be invertebrate chordates, though they have been moved both up and down in the phylogenetic tree by other researchers, being originally identified as possible cephalochordates (Chen et al., 1995) then as vertebrates (Chen et al., 1999; Holland & Chen, 2001; Mallatt & Chen, 2003). They have been classified as hemichordates (Shu et al., 1996), and later as basal deuterostomes allied to the vetulicolians (Shu et al., 2003b), the problems with the classification resulting from the interpretations of various coloured blobs, lines and squiggles in the fossils. Though there are thousands of fossils, Benton says anatomical interpretation is critical.

Haikouella and Yunnanozoon are fossilised as bluish-grey to black films, 25-40 mm long, flattened on the rock. A number of chordate features such as a notochord, filter-feeding pharynx with an endostyle, musculature that was segmented, and branchial arches were discerned (Chen et al., 1995), and later the features identified were extended to include an enlarged brain that was possibly 3-part, paired lateral eyes in Haikouella, features that indicate it may have had an enlarged head that was distinctive, which are key features of vertebrates (Chen et al., 1999; Mallatt & Chen, 2003). Other researchers disputed the presence of a notochord, suggesting instead that the feature that had been identified as a notochord was actually the gut, also suggesting the segmental musculature was wrongly identified (Shu et al., 1996). Instead of chordate features they claimed there were actually hemichordate features in Yunnanozoon, especially in regard to the division of the body into a proboscis, a collar and a trunk, the latter being further divided into branchial and gut regions, as is present in the acorn worm of the present. Further study led the same researchers to reassign the yunnanozoons and the vetulicolians from the hemichordates to a basal position in the deuterostome line, as they could see similarities between the vetulicolians and the yunnanozoons. They also failed to distinguish a notochord, lateral eyes, large brain or any of the other chordate features that had been previously reported (Shu, 2003; Shu et al., 2003b)

Sources & Further reading