Page 421 - Introduction to Paleobiology and The Fossil Record

408 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

arm
pedicellariae
madreporite
disk ambulacral spines

ambulacral
mouth groove anus

(a) (b)
Figure 15.16 Morphology of the asterozoans: (a) ventral and (b) dorsal surfaces. (Based on Treatise on
Invertebrate Paleontology, Part U. Geol. Soc. Am. and Univ. Kansas Press.)

long, thin, flexible arms. The mouth is situ- Variably described as carpoids, homalozoans

ated centrally on the lower surface of the disk. or calcichordates, depending on preference,

Most of the disk is filled by the stomach and, most authorities consider the group to be very
in the absence of an anus, waste products are different to the radiate Echinodermata; indeed,
regurgitated through the mouth. The arms carpoids show some puzzling similarities to
consist of highly specialized ossicles or verte- the chordates.
brae. Ophiuroids are common in modern seas The carpoids were marine animals ranging
and oceans, preferring deeper-water environ- in age from Mid Cambrian to possibly Late
ments below 500 m. Their basic architecture Carboniferous, with a calcitic, echinoderm-
differs little from some of the fi rst members type skeleton lacking radial symmetry (Fig.
of the group, for example Taeniaster from the 15.17). Two main types of carpoid are recog-
Middle Ordovician of the United States. nized: the cornutes and the mitrates. The cor-
A few modern starfi sh are vicious and vora- nutes were often boot-shaped and appear to
cious predators enjoying a diet of shellfi sh. have a series of gill slits on the left side of the
Asteroids can prize apart the shells of bivalves roof of the head, whereas the mitrates, derived
with their sucker-armored tube feet far enough from a cornute ancestor, were more bilaterally
to evert their stomachs through their mouths symmetric with covered gill slits on both
and into the mantle cavity of the animal, where sides.
digestion of the soft parts takes place. Stephen It might seem unexpected, but the carpoids
Donovan and Andrew Gale (1990) suggested have featured at the center of a long-running
that this predatory life mode signifi cantly and heated debate that has hit the headlines
inhibited the post-Permian diversifi cation of over the past 50 years. After much careful
some brachiopod groups. The strophomenides, study, Richard Jefferies (1986) presented
the most diverse Permian brachiopods, largely detailed evidence that carpoids and chordates
pursued a reclined, quasi-infaunal life strategy share many characters, the so-called “cal-
and they may have presented an easy kill for cichordate hypothesis”. He based his conclu-
the predatory asteroids. sion on painstaking studies of their anatomy
and the anatomy of embryos of modern echi-
noderms and chordates. A chordate-implied
Carpoidea
reconstruction of carpoids suggests that the
The carpoids include some of the most bizarre body consists of a head and a tail used for
and controversial fossil animals ever described. locomotion (Sutcliffe et al. 2000) (Fig. 15.18).