Discussion of Phylogenetic Relationships
As noted below, the relationships of some of the presumed fossil chordates is based on scant evidence and there is debate about the position of especially the calcichordates and conodonts (see references cited below and Chen et al. 1995). There is strong morphological, especially embryological, evidence for monophyly of the Urochordata, Cephalochordata and Craniata, with the latter two being sister taxa. Schaeffer (1987) details several embyological synapomorphies, in addition to those noted here, that support these same relations among and monophyly of the three living chordate groups.
- Calcichordata. Jeffries (1986) provides descriptions and comparisons and argues for the placement of calcichordates near the Chordata. Other workers believe that calcichordates are closer to echinoderms. Reconstructions of these fossil organisms include visceral (pharyngeal or gill) slits that would suggest chordate affinities, but the mineralized skeleton was composed of calcite, like echinoderms, not bone as in many chordates.
- Urochordata. Evidence that tunicates are chordates comes clearly from the larval "tadpole" stage which shows pharyngeal slits and arches, dorsal hollow nerve cord, notochord and post-anal muscular (unsegmented) tail. Adults of most members are sessile filter feeders with an expanded pharynx and, like cephalochordates and larval lampreys, with an endostyle, a mucous food trap in the pharyngeal floor that is homologous with the thyroid gland of vertebrates.
- Cephalochordata. Among the living chordates there is little doubt that lancelets are most closely related to the Craniates based on synapomorphies such as segmented axial muscles and metameric organization of the visceral (pharyngeal) arches. Uniquely, the notochord of cephalochordates extends to the tip of the snout, the gonads are segmentally organized, adults have a high number (50+) gill arches, and there is a hood-like atrium covering the pharyngeal region. The Early Cambrian fossil Yunnanozoon possesses the extended notochord and segmental gonads, but lack the atrium and increased number of gill arches.
- Craniata. Because hagfishes (Myxini) lack all traces of vertebrae, i.e. a backbone, Janvier (1981) groups the Myxini with all other vertebrates in the higher taxon Craniata (referring to the presence of a head skeleton). The taxon Vertebrata is, therefore, in a strict sense, applied to those animals known or believed to possess at least a simple backbone of neural arches. Synapomorphies of the Craniata include: presence of a cartilaginous (and often bony) head skeleton; relatively large brain plus a unique set of sensory and motor cranial nerves; nephrons as the functional excretory unit; neural crest embryonic tissue.
The traditional taxa Agnatha (jawless fishes), Ostracodermi (fossil jawless fishes) and Cyclostomata (living lampreys and hagfishes) are non-monophyletic assemblages that are no longer recommended. Details of jawless fish relationships are introduced on the Craniata and Vertebrata pages.
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