Page 412 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 399
Echinosphaerites
rhombiferan
Sphaeronites
diploporite
Pleurocystites
rhombiferan
Haplosphaeronis
diploporite
Figure 15.7 Some Ordovician cystoid genera:
Echinosphaerites and Sphaeronites, (×0.75),
Haplosphaeronis and Pleurocystites (×1.5).
(Based on Treatise on Invertebrate Paleontology,
Part S. Geol. Soc. Am. and Univ. Kansas Press.) Paleontology, Part S. Geol. Soc. Am. and Univ.
a few horizons are packed with blastoids, par- basal Fissiculata characterized by hydrospire
ticularly when the diversity of the group
peaked in the Early Carboniferous. Viséan
reefal facies in northern England yield abun-
dant blastoids, as do the Permian limestones
on the island of Timor where, for example,
Timoroblastus and Schizoblastus occur.
The blastoids first appeared during the
Silurian, probably evolving from an Ordovi-
cian ancestor with brachioles and a reduced
number of plates. They initially competed,
ecologically, with the rhombiferan cystoids.
The evolutionary history of the group was
marked by changes in the shape of the theca
and variations in the length of the ambulacra.
Two main groups are recognized: the more
lateral view aboral view oral view
Timoroblastus Permian
lateral view oral view lateral view
Pentremites Schizoblastus
Carboniferous Carboniferous-Permian
Figure 15.8 Some blastoid genera. Magnifi cation
×0.6 for all. (Redrawn from various sources.)
(a) (b)
Figure 15.9 (a) An eocrinoid, and (b) a
paracrinoid. (Based on Treatise on Invertebrate
Kansas Press.)
folds, and the Spiraculata with, as the name
suggests, well-developed spiracles.
Eocrinoids
The eocrinoids were the earliest of the brachi-
ole-bearing echinoderms. They had a huge
range of thecal shapes with primitive hold-
fasts and an irregular to regular arrangement
of plates (Fig. 15.9a). Sutural pores rather
than thecal pores, along the joins between the
plates, were characteristic of the earliest eocri-
noids; in others there is a total lack of respira-
tory structures. Eocrinoids differ from the
crinoid groups in having biserial brachial