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Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the pharynx and look rather like gill slits; stomochords, similar in composition to notochords, but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord—but also a smaller ventral nerve cord.
The calcichordate hypothesis, formulated by British Museum paleontologist Richard Jefferies, holds that each separate lineage of chordate (Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoderms and the chordates are sister groups, with the hemichordates as an out-group. [1]
Harvard University palaeontologist Stephen Jay Gould popularised Pikaia as an ancestral species of chordates in his 1989 book Wonderful Life: The Burgess Shale and the Nature of History, [13] from which Pikaia became known as the "most famous early chordate fossil," [14] or the earliest chordate, [15] or the oldest ancestor of humans. [16] [17]
Ancestral chordates evolved a post-anal tail, notochord, and endostyle (precursor of thyroid). The pharyngeal slits (or gills) are now supported by connective tissue and used for filter feeding and possibly breathing. The first of these basal chordates to be discovered by science was Pikaia gracilens. [10]
Pikaia gracilens is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia.Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "the most famous early chordate fossil", [1] or "famously known as the earliest described Cambrian chordate". [2]
On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no bone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from the Permian-aged Paleobranchiostoma, trace fossils of the Ordovician colonial tunicate Catellocaula, and various Jurassic-aged and Tertiary-aged ...
The notochord is the defining feature (synapomorphy) of chordates, and was present throughout life in many of the earliest chordates. Although the stomochord of hemichordates was once thought to be homologous or from a common lineal origin, it is now viewed as analogous, convergent , or from a different lineal origin. [ 22 ]
As a subphylum of chordates, all vertebrates have evolved a vertebral column (backbone) based around the embryonic notochord (which becomes the intervertebral discs), forming the core structural support of an internal skeleton, and also serves to enclose and protect the spinal cord.