Australia: The Land Where Time Began

A biography of the Australian continent 

Relationships of Deuterostomes

According to Benton, 3 schemes have been proposed for the relationships of deuterostomes that are substantially different from each other:

• 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 than urochordates.

Sources & Further reading

1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing.