Page 423 - Introduction to Paleobiology and The Fossil Record

410 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 15.7 The oldest stylophorans: echinoderms with a locomotory organ

How much further can we go with the debate on the affinities of the carpoids? New material and

new investigative techniques will always help. The oldest stylophoran carpoid is from the Middle
Cambrian rocks of Morocco. Sebastien Clausen and Andrew Smith (2005) have analyzed the mor-
phology of the animal in great detail, particularly its microstructure with the scanning electron
microscope. Ceratocystis in fact has a stereom microstructure typical of most echinoderms, but its

appendage was covered by articulating plates and filled with muscle tissue and ligaments (Fig. 15.19).
It seems that this bizarre asymmetric animal has an echinoderm skeletal structure but also possessed
a muscular locomotory appendage rather similar to its sister taxon, the pterobranchs. This group
has all the features of a stem-group echinoderm prior to acquiring five-fold symmetry and presum-

ably a water vascular system, and it rather conclusively disproves a close phylogenetic association
between carpoids and chordates.

Metacoel
Fixed plates
Nerve
Stylocone
Body Muscle
(b)
Notochord Fixed plates
Gill slits?
Body Brain
Stylocone
Body Muscle somites
(c)
Water vascular system
Appendage
Mouth (with tube feet)
Stylocone
Moveable
Gut plates
Stylocone
Lines of section Body
Muscle
(a) (d)

Figure 15.19 Ceratocystis from North Africa. (a) Basic anatomic features. (b–d) Three current
interpretations of the soft-tissue anatomy of the stylophoran appendage in proximal longitudinal
(left) and distal transverse (right) section: (b) primitive echinoderm model, (c) calcichordate model
and (d) crinozoan model. (Based on Clausen & Smith 2005.)

the ancient seas of the Early Paleozoic, in about 100 living species characterized by a
communities and environments quite differ- rod-like structure, the notochord. They were
ent from those of today. Graptolites are widely small, soft-bodied animals with bilateral sym-
used for correlation because of their abun- metry and a lack of segmentation. The phylum
dance, widespread distribution and rapid evo- contains two very different classes: fi rst, the
lution. Although graptolites are extinct, and tiny, mainly colonial, pterobranchs that lived

their life styles are difficult to interpret, they in the sessile benthos and, second, the larger
were hemichordates – a phylum containing infaunal acorn or tongue worms, the entero-