characters exhibited by Yunnanozoon (e.g. Dzik 1995) may indicate a more appropriate
placement in the deuterostome stem-group.
ChordatesFossil representatives have been claimed for all living groups of invertebrate chordates and
jawless vertebrates as far back as the Early Cambrian. Putative fossil tunicates include
Cheungkongella (Shu et al. 2001a), Palaeobotryllus (Müller 1977), and Peltocystis (Jefferies et
al. 1996). Putative acraniate chordates include Lagynocystis (Jefferies 1973), Pikaia
(Conway Morris 1979), Yunnanozoon (Chen et al. 1995; Dzik 1995), and Cathaymyrus (Shu
et al. 1996b). Possible fossil representatives or close relatives of the living jawless
vertebrates include the Cambrian taxa Haikouella (Chen et al. 1999; but see above),
Myllokunmingia and Haikouichthys (Shu et al. 1999), and the Carboniferous taxa Gilpichthys
and Pipiscius (Bardack and Richardson 1977), Mayomyzon (Bardack and Zangerl 1968,
1971), Myxinikela (Bardack 1991, 1998), and Hardistiella (Janvier and Lund 1983; Lund
and Janvier 1986), as well as a number of mitrates such as Mitrocystites (Jefferies 1967) and
Placocystites (Jefferies and Lewis 1978).
In addition, there are a wide variety of fossil jawless vertebrates characterized by an
extensively developed dermal ‘armour’ and historically grouped together as the
‘ostracoderms’. These include the anaspids, galeaspids, heterostracans, osteostracans, and
thelodonts (see Janvier 1996b for an introduction to these various groups). Amongst the
jawed vertebrates, there are also a number of large groups that have no living
representatives, principally including the placoderms and acanthodians. There is also a
swathe of basal chondrichthyans, actinopterygians, and sarcopterygians that belie the
apparent disparity of their living relatives.
The phylogenetic relationships of living and extinct
chordates and their near relativesTo compare palaeontological and molecular estimates for the time of divergence of the
various chordate clades it is first necessary to resolve the phylogenetic relationships of the
living and fossil groups of chordates and their near relatives; palaeontological estimates
can then be provided through calibration of the resulting phylogeny to the stratigraphic
occurrence of the various groups within the geological timescale.
The calcichordate-stylophoran problemNo discussion of early chordate evolution would be complete without a consideration of
the ‘calcichordates’. Jefferies (1967 et seq) identifies an extinct group of calcite-plated
invertebrates, otherwise interpreted as basal echinoderms (Stylophora; e.g. Ubaghs 1968;
Paul and Smith 1984), as paraphyletic suites of lineages that interleave the stems of extant
echinoderms, cephalochordates, tunicates, and vertebrates. This theory has been criticized
on many grounds. Amongst the most substantive of these, independent phylogenetic
analyses resolve tunicates as basal chordates (Garcia-Fernàndez and Holland 1994) rather
than as the sister-group to the vertebrates, which is a requirement of the ‘calcichordate’
198 THE ORIGIN AND EARLY EVOLUTION OF CHORDATES