Page 407 - Introduction to Paleobiology and The Fossil Record

394 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 15.3 Columnal classifi cation

The majority of crinoid assemblages are represented by disarticulated ossicles. Conventional taxon-
omy based on a description of complete, articulated specimens is thus not possible. Nevertheless,

ossicles have many distinctive features, arguably with more well-defined characteristics than many
groups of macrofossils (Fig. 15.3). Single stems consist of many ossicles with a central canal or lumen
usually carrying nerve fibers. Both the ossicles and lumens have distinctive shapes that are the basis

of a form taxonomy of the group. Form taxonomy helps us classify the shapes of fossils, in the same
way that we can classify nuts and bolts. It is a useful method of organizing our data, but since it is
not biologically meaningful, cannot be used in phylogenetic analyses. Stems may be either homeo-
morphic, composed of similarly shaped ossicles, or heteromorphic with a variety of different-shaped
ossicles. Moreover stems may be subdivided into zones that may be internally homeomorphic or
heteromorphic. Columnal taxonomy has proved useful in describing taxa (so-called col. taxa) of
pelmatozoan, particularly crinoid, ossicles of stratigraphic signifi cance.

(a) (b)

(c)
Figure 15.3 Some crinoid ossicle types. (a) Articular facet of a columnal of the bourgueticrinid
Democrinus (?) sp., with a fulcral ridge of the synarthrial articulation; the lumen opens at the
bottom of the “8”-shaped depression (×15). (b) Cirral scar on a nodal of the isocrinoid Neocrinus
with well-preserved stereom microstructure and knob-like synarthrial fulcrum (×18). (c) Articular
facet of a columnal of the isocrinoid Neocrinus with symplectial articulation around the fi ve
petal-like areola areas (×9). (Courtesy of Stephen Donovan.)

units within a deep-water mudstone sequence.
Entire crinoids, cystoids, echinoids and cal- Crinoidea
cichordates were carried downslope and Although the crinoids, famously called “sea
rapidly buried on the unstable slopes of a lilies”, look more like plants than animals,
submarine fan system. there is no doubt they are animals, and echi-