Phylum Chordata

Chordates are placed in the superphylum Deuterostomia. The possible relationships of the chordates and deuterostomes to other metazoans are discussed in Halanych (2004). He restricts the taxon of deuterostomes to the chordates and their proposed immediate sister group, a taxon comprising the hemichordates, echinoderms, and the wormlike Xenoturbella.

The phylum Chordata has been used by most recent workers to encompass members of the subphyla Urochordata (tunicates or sea-squirts), Cephalochordata (lancelets), and Craniata (fishes, amphibians, reptiles, birds, and mammals). The Cephalochordata and Craniata form a mono-phyletic group (e.g., Cameron et al., 2000; Halanych, 2004). Much disagreement exists concerning the interrelationships and classification of the Chordata, and the inclusion of the urochordates as sister to the cephalochor-dates and craniates is not as broadly held as the sister-group relationship of cephalochordates and craniates (Halanych, 2004).

Many exciting fossil finds in recent years reveal what the first fishes may have looked like, and these finds push the fossil record of fishes back into the early Cambrian, far further back than previously known. There is still much difference of opinion on the phylogenetic position of these new Cambrian species, and many new discoveries and changes in early fish systematics may be expected over the next decade. As noted by Halanych (2004), D.-G. (D.) Shu and collaborators have discovered fossil ascidians (e.g., Cheungkongella), cephalochordate-like yunnanozoans (Haikouella and Yunnanozoon), and jaw-less craniates (Myllokunmingia, and its junior synonym Haikouichthys) over the last few years that push the origins of these three major taxa at least into the Lower Cambrian (approximately 530-540 million years ago). The Lower Cambrian jawless (agnathan) vertebrate specimens, of about 530 million years age, lacking bone but with well-preserved soft anatomy, were found in Yunnan, China (Janvier, 1999; Shu et al., 1999). Shu et al. (1999), in reporting this discovery, presented a phylogeny suggesting that Myllokunmingia is sister to the remaining vertebrates and Haikouichthys is sister to a clade with lampreys. Shu et al. (2003a), in describing additional detail from more specimens of Haikouichthys ercaicunensis, felt it either formed a trichotomy with hag-fishes and all other vertebrates (and possibly is a stem craniate), or that it is the sister group to all other vertebrates except hagfishes, in a position similar to that of Myllokunmingia. In further clarification, Xian-guang et al. (2002) described details of a new specimen co-occurring with the nominal Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis and concluded that all are conspecific; the oldest name Myllokunmingia fengjiaoa is appropriate. Characters include filamentous gills, V-shaped myomeres, and a distinct dorsal fin (the latter indicating a more derived condition than in the hagfish). Their phylogenetic analysis suggested that Myllokunmingia is either the sister group to the lampreys, or the sister group to the lampreys plus skeletonized vertebrates. Shu et al. (2003b) and Shu and Morris (2003) proposed that the Lower Cambrian yunnanozoans, Haikouella and Yunnanozoon, are stem-group deuterostomes, and questionably placed them in the phylum Vetulicolia, class Yunnanozoa, family Yunnanozoidae (= Yunnanozoonidae) (with the relationship to fossil calcichordates being unknown). However, in presenting a different interpretation of the possible phylogenetic position of Haikouella, Mallatt et al. (2003) interpreted it as not just a nonchordate stem-group deuterostome, but as the immediate sister group of vertebrates.

A classification of the major taxa of the phylum Chordata, as an overview of what follows, is as follows:

Phylum Chordata

Subphylum Urochordata Subphylum Cephalochordata Subphylum Craniata

Superclass Myxinomorphi (with their sister group being the vertebrates, which comprise the following additional six jawless craniate or agnathan taxa and the gnathostomes, each ranked at the same level and sequenced as follows): Superclass Petromyzontomorphi fSuperclass Conodonta fSuperclass Pteraspidomorphi fSuperclass Anaspida fSuperclass Thelodonti fSuperclass Osteostracomorphi (possible sister group being the gnathostomes, as given below) Superclass Gnathostomata (jawed vertebrates) fClass Placodermi

Class Chondrichthyes (cartilaginous fishes, e.g., chimaeras, sharks, and rays) fClass Acanthodii

Class Actinopterygii (ray-finned fishes)

Class Sarcopterygii (includes coelacanths, lungfishes, and tetrapods)

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