Page 422 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 409

Moreover, Jefferies (1986) described struc- however, that they may be interpreted rather
tures indicating a fi sh-like brain, cranial more convincingly as echinoderms, and that
nerves, gill slits and a fi lter-feeding pharynx the calcichordate hypothesis fails (Box 15.7).
similar to that in tunicates (often known as Further, when these redescriptions of the fossil
the sea squirts). In the calcichordate hypoth- material are combined with new molecular

esis, Hemichordata is identified as a sister evidence on phylogeny, the case is lost (Ruta
group to Echinodermata + Chordata, a clade 1999). Molecular phylogenetic analyses
that Jefferies called Dexiothetica. Reapprais- (Winchell et al. 2002; Delsuc et al. 2006)
als of the anatomy of carpoids have shown, show that Hemichordata is the sister group of
Echinodermata, forming together the Ambu-
lacraria, and that Ambulacraria is the sister
group of Chordata. Dexiothetica does not
arm exist.
hydropore
central
gonopore plate HEMICHORDATES
ridge
What was the character of the vegetation
that clothed this earliest prototype of
Europe is a question to which at present
anus

no definite answer is possible. We know,
however, that the shallow sea which
spread from the Atlantic southward and
peripheral fore tail eastward over most of Europe was ten-
gill slit hinge line
mid tail anted by an abundant and characteristic
periproct
series of invertebrate animals – trilobites,
graptolites, cystideans, brachiopods, and
hind tail cephalopods, strangely unlike, on the
whole, to anything living in our waters
now, but which then migrated freely
along the shores of the arctic land
between what are now America and
Europe.

Sir Archibald Geikie from a lecture
delivered to the Royal Geographical
Society (1897)

The hemichordates form a small phylum of
only a few hundred species and are unfamiliar
(a) (b)
to most people, but their importance for the
Figure 15.17 Morphology of the carpoids: study of vertebrate evolution cannot be under-
(a) dorsal and (b) ventral surfaces. (From estimated. Their most common fossil repre-
Jefferies & Daley 1996.) sentatives, the graptolites, were abundant in

Figure 15.18 Reconstruction of a living carpoid: the Devonian Rhenocystis moving across and through
the sediment from left to right. (From Sutcliffe et al. 2000.)