Page 409 - Introduction to Paleobiology and The Fossil Record

396 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

genera are stalked forms, attached to the genera are recognized with almost two-thirds
seabed. Modern oceans, however, are domi- of known genera extant. Microcrinoids are a
nated by mobile comatulids that move about highly specialized crinoid morphotype devel-
like pneumatic umbrellas, pumping their long oped within both the Inadunata, during the
arms in unison. Antedon is one of nearly 100 Paleozoic, and the Articulata, during the
non-stalked genera that, after a short fi xed Mesozoic. Microcrinoids are minute, never
stage, are free to crawl and swim with the aid more than 2 mm in size; they may be pedo-

of flexible arms and cirri. morphic forms living together with more
typical crinoid communities.

Classification and evolution
Blastozoans
The oldest reported crinoid, Echmatocrinus
brachiatus from the Middle Cambrian Burgess Blastozoans are an informal grouping that
Shale, has uniserial or single brachials, in con- includes three of the more minor, yet never-
trast to the biserial arms of contemporary theless important, echinoderm groups that are
eocrinoids. Echmatocrinus has few other all extinct: the cystoids, blastoids and eocri-
unequivocal crinoid characters and there is noids. These pelmatozoans were usually
wide agreement that it is actually an octocoral. equipped with a short stem but often lacked
More recognizable crinoids with more typi- brachia or arms. Blastomorphs were probably

cally constructed cups and columnal-bearing high-level filter feeders, particularly charac-
stems, such as Dendrocrinus, appear some time terized by pores or brachioles punctuating the
later during the Tremadocian. A major expan- thecal plates. Eocrinoids are included by some
sion in the Early Ordovician tropics marked a authors in the cystoids, appearing near the
period of intense morphological experimenta- base of the Cambrian and becoming extinct
tion and many adaptive radiations. during the Silurian. The eocrinoids, however,
Virtually all Paleozoic crinoids were stalked, probably included ancestors to both the cys-
and traditionally have been grouped into three toids and the crinoids.
subclasses, the Inadunata, Flexibilia and Cam-
erata (Fig. 15.5). Inadunate crinoids comprise Cystoids
a large and varied group, originating in the
Early Ordovician and continuing until the Mid Paleozoic blastozoans with respiratory
Triassic. They have a rigid calyx with either pore structures modifying the thecal plates
free or loosely attached brachials, and mono- have been traditionally placed within the Cys-
cyclic or dicyclic calyx bases. Camerate cri- toidea. This mixed bag includes two classes,
noids are characterized by large cups with the Diploporita and the Rhombifera, that
both monocyclic and dicyclic plate confi gura- became very widespread during the Mid
tions. The uniserial or biserial brachials, deco- Paleozoic. They had spherical or sac-like
rated with pinnules, are firmly attached to the thecae, commonly with 1000 or more irregu-

cup and the tegmen is heavily plated, obscur- larly arranged plates. Moreover the group has
ing the food grooves and mouth, but devel- brachioles lacking pinnules and characteristi-
oped laterally with an anal tube. The Flexibilia, cally the plates are usually equipped with dis-
comprising some 60 genera, have a dicyclic tinctive pore structures. A variety of such pore
plate configuration comprising three infra- structures have been recognized in the cys-

basals. The brachials are uniserial and lacking toids (Fig. 15.7), and they are fundamental in

pinnules, and the tegmen is fl exible with a the higher-level classification of the group.
mosaic of small plates. Their stems have
circular cross-sections and lack cirri. These Diploporita The diploporites had thecal
groups are especially well known in the plates punctuated by pairs of pores either
Carboniferous (Box 15.4). covered with soft tissue (diplopores) or a layer
The fourth subclass of crinoids, the Articu- of stereom with the pore pairs joined by a
lata, with the exception of some Triassic ina- network of minor canals (humatipores). These
dunates, includes all post-Paleozoic crinoids. pores probably held a bulbous respiratory bag
A few Paleozoic forms with articulate simi- and allowed for the efficient entry and exit of

larities such as Ampelocrinus and Cymbiocri- celomic fluid. Both stalked and non-stalked

nus may be stem-group articulates. Over 250 forms are present in this group, suggesting a