Evolution and systematics

Cephalochordates (also called acraniates or lancelets) possess key features that characterize them as chordates. Those characters include the notochord, dorsal nerve cord, branchial basket, and a post anal tail with fins. It is generally believed that cephalochordates are the closest living relatives to the vertebrates. The two groups share unique traits such as the organization of the main body musculature into separate segments and the organization of the circulatory system. Another feature, which unites cephalochordates and vertebrates, is a caecum that is probably homologous to the vertebrate liver. Thus, the morphological evidence is strong. In fact it is so strong that it was used as evidence against phylogenetic conclusions, which were drawn solely from DNA sequences when a molecular study placed the cephalochordates as relatives of echinoderms, not even within the chordates.

The potential for fossilization of cephalochordates is limited because the animals are soft bodied. Even their main skeletal element, the notochord, consists of specialized muscle tissue. Nevertheless several fossils have been described that could have affinities to cephalochordates.

Pikaia gracilens from the Canadian Burgess Shale (Cambrian, ca. 530 million years old) is a promising candidate for consideration as a cephalochordate fossil. It has the typical tapered body form at both ends, and it has been argued that it possessed a notochord. However, Pikaia also shows characters not found in extant cephalochordates. In some fossils the lines, interpreted as myocommata, are w-shaped rather rem iniscent of vertebrates than of the simpler v-shape of cephalochordates. Also, the anterior end of some fossil remnants displays tentacles, a feature not found in living cephalochordates, but known, for example, from hagfishes.

The nature of several other fossils (e.g., Cathaymyrus di-adexus [Cambrian], Lagenocystis pyramidalis [Ordovician], Palaeobranchiostoma hamatotergum [Permian]) remains more equivocal. An instructive example is the fossil Yunnannozoon lividum (Cambrian). Originally reconstructed as a cephalo-chordate, it was re-described as a worm-like hemichordate only to be interpreted as a sea pen (phylum Cnidaria) shortly thereafter.

Evolutionary relationships among cephalochordate species are unclear. However, it is thought that the genus Epigo-nichthys is more derived compared to Branchiostoma.

The class Cephalochordata (some experts now categorize it as a subphylum) contains one order, Amphioxiformes, and one family, Branchiostomatidae.

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