Relationships of Deuterostomes
According to Benton, 3 schemes have been proposed for the
relationships of deuterostomes that are substantially different from
- The hemichordates are basal to the chordates, as they both have
ciliated gill slits and giant nerve cells, plus other features not
present in echinoderms. Enteropneusts were sometimes considered to
be more closely related to chordates, as their gill slits are
similar, their short nerve chord is hollow and dorsal, and they have
gut features that are not present in pterobranchs (Peterson, 1995).
It is most widely believed that amphioxus is the most closely
related to the Vertebrata, based on 15 features that are not present
in tunicates. This is the traditional view (e.g. Maisey, 1986;
Peterson, 1995; Donoghue et al., 1998).
- In the 'calcichordate' model (Jefferies1986, 1997) hemichordates
are basal to echinoderms and urochordates are a sister group to
chordates, based on evidence gained from embryology and fossils.
- Morphological and molecular data support the 3rd view, that is
said by Benton to be widely accepted (Smith et al., in
press). The first molecular studies, in which comparisons were made
of the 18 S rRNA genes of echinoderms, hemichordates and chordates
were said by Benton to be inconclusive, though later work (e.g.
Bromham & Degnan, 1999; Cameron et al., 2000; Peterson &
Eernisse, 2001; Furlong & Holland, 2002; Winchell et al.,
2002) has definitively paired hemichordates and echinoderms, as the
clade Ambulacraria, placing cephalochordates closer to chordates
Sources & Further
- Benton, Michael J., 2005, Vertebrate Palaeontology, 3
rd ed., Blackwell Publishing.