Page 424 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 411

pneusts that lived in burrows mainly in sub- Rhabdotubus from the Middle Cambrian and
tidal environments. Graptovermis from the Tremadocian. The
The hemichordates have a mixture of char- living genera Cephalodiscus and Rhabdo-
acters suggesting links with the lophopho- pleura (Fig. 15.20) have been used as analogs
rates, the echinoderms and the chordates. for many aspects of graptolite morphology,
They have been closely related to the cepha- ontogeny and paleoecology. These living
lochordates and urochordates or tunicates, genera and the graptolites both have a peri-
but molecular and other data suggest that the derm, or skin, with fusellar tissue, while the
latter two groups are more closely related to dendroid stolon, a tube that connects the
the chordates than the hemichordates. thecae to each other, may be related to the
Although the notochord is now known to be pterobranch pectocaulus. Rhabdopleura is

unrelated to a true backbone, the hemichor- known first from the Middle Cambrian and
dates have, nevertheless, gill slits and a nerve occurs in oceans today mainly at depths of a
cord. few thousand meters. The genus is minute
with a creeping colony hosting a series of
exoskeletal tubes, each containing a zooid
Modern hemichordate analogs
with its own lophophore-like feeding organ
Pterobranchs superficially resemble the bryo- comprising a pair of arms. The zooids are

zoans – both are colonial animals and the budded from a stolon and interconnected by
individual zooids feed with tentaculate, cili- a contractile stalk, the pectocaulus.
ated arms (Fig. 15.20). The group has a long Cephalodiscus, however, is rather different,
geological history with early records such as being constructed from clusters of stalked

lophophore
stomochord

preoral disk

mouth
bud 1
stomach
intestine
bud 2
peduncle
bud 3
(a) (c)
extended zooid

contractile stalk terminal bud

bud 4
retracted zooid
bud 3
bud 2
bud 1

pectocaulus
(b)
Figure 15.20 Rhabdopleurid morphology: (a, b) Rhabdopleura and (c) Cephalodiscus. (Based on
Treatise on Invertebrate Paleontology, Part V. Geol. Soc. Am. and Univ. Kansas Press.)