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Cambrian Explosion - Other Possible Deuterostomes

Recognisable body plans are maintained by the Crown phyla as they are tracked back to the Early Cambrian, though the stem groups commonly have unusual synapomorphies unknown among crown groups, or in some cases are mixtures of characters that are not shared by any known crown groups.

One such form from the Burgess Shale is a soft-bodied fossil with tentacles, Herpetogaster collinsi, which has been grouped tentatively with other fossils from the Cambrian with which they share general morphological features but have uncertain affinities. This group has been interpreted as possibly being stem Ambulacraria (Caron, Conway Morris & Shu, 2010), i.e., they branched following the split between chordates and ambulacararians, though before the split between hemichordates and echinoderms. According to the authors1 a segmented region that, if coelomate, is unlike those of deuterostomes, appears to be included in the trunk of Herpetogaster. There is a trace in the tentacles that has been interpreted as either a hydrostatic canal or a vascular system. Pharyngotremy had been evolved by the ambulacrarian/chordate LCA, and a possible location for gill pores is behind the head, though on Herpetogaster gill pores are not present equivalently. According to the authors1 interpretation of this animal is very difficult, though as fossils go, its preservation is reasonably good, which epitomises the problems encountered when dealing with stem groups.

Vetulicolids and yunnanozoans are 2 other possible deuterostome groups that are often referred tentatively to the chordate branch, or alternatively as more basic positions as stem deuterostomes, as well as other phylogenetic positions that have been proposed. Vetulicolids have bipartite bodies, with an anterior section that is carapace-like that may be subdivided into 2 or more units, which sometimes appear to be segmented, and a trunk or posterior section that in some genera appears to be narrowly segmented in a fashion reminiscent of arthropods. They have been recovered from both the Chengjiang Shale Fauna and Burgess Shale Fauna (Aldridge et al., 2007; Caron, 2006; Shu et al., 2001). There are structures on some of the Chengjiang forms that have been interpreted as gill slits, which suggests an affinity with chordates, whereas the form from the Burgess Shale Fauna, Banffia, which is known from hundreds of specimens, shows structures that can be interpreted as midgut diverticulae (Caron, 2006), a feature that is not found in crown chordates but is common in protostomes. The authors1 suggest that forms assigned to Vetulicolida represent more than 1 group; at the very least, the morphology of veticulids is sending a mixed phylogenetic message.

Another form present in the Chengjiang Fauna that is also known from many hundreds of specimens is Yunnanozoans, though they are described as being quite enigmatic, and have been reconstructed as stem hemichordates (Shu, Zhang & Chen, 1996) or stem chordates (J.Y. Chen et al., 1995). They have a body that is fish-shaped or lancelet-shaped with a series of anterolateral structures that make sense as gills, and they have a remarkable dorsal fin that is seriated or segmented which at first glance suggests a relationship with the cephalochordate or vertebrate body plans, though it doesn’t seem likely it functioned in an analogous manner. Also, there is no notochord structure that has been confirmed, which weakens an assignment to the chordates. (see Shu, Zhang & Chen, 1996).

Important reasons for the uncertainties about the phylogenetic position of these soft-bodied fossils are taphonomic loss and blurring of morphological characters. Studied of decay patterns among modern chordates have revealed a progressive loss of phylogenetically informative characters, with the result that as decay progresses, a specimen appears similar to progressively earlier phylogenetic stages (Sansom, Gabbott, & Purnell, 2010). Therefore decay is not random with respect to the phylogenetic sequence of chordate characters. It is suggested by these modern taphonomic experiments that the interpretation of fossils from the Cambrian may be affected by such sequential taphonomic biases.

Sources & Further reading

  1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado
Author: M. H. Monroe
Last Updated 10/05/2014
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