Page 425 - Introduction to Paleobiology and The Fossil Record

412 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

tubes budded from a basal disk. Moreover, in largely unknown until the 1940s, when the
further contrast to Rhabdopleura, species of Polish paleontologist Roman Kozłowski iden-

Cephalodiscus usually have five pairs of cili- tified a notochord in three-dimensional mate-
ated feeding arms. Individual zooids in the rial isolated from limestones. There are several
Cephalodiscus colony can actually crawl groups of graptolites and graptolite-like
outside the colony along its exterior and often animals (Box 15.8).

farther afield onto adjacent surfaces. The
zooids of living Cephalodiscus, with their
considerable freedom of mobility, can even Morphology: the graptolite colony
construct external spines from outside the
skeleton. The basic graptolite architecture consists of a
probably collagenous skeleton characterized
by a growth pattern of half rings of periderm
Graptolites
interfaced by zigzag sutures, similar to the
The graptolites, or Graptolithina, are gener- construction of the pterobranchs (Fig. 15.21).
ally stick-like fossils, very common in many Each colony or rhabdosome grew from a
Lower Paleozoic black shales. In fact the small cone, the sicula, as one or a series of
group is so prevalent that it has proved to be branches or stipes. The stipes may be isolated
of key importance in correlating Lower Paleo- or linked together by lateral struts to resemble
zoic strata. The majority of Ordovician and a reticulate lattice. A series of variably cylin-
Silurian biozones are based on graptolite drical tubes are developed along the stipes;
species or assemblages. Graptolites, from the these thecae house the individual zooids of the
Greek “stone writing”, usually occur in black colony. Aggregates of rhabdosomes, synrhab-
shales as fl attened carbonized fi lms resem- dosomes, have been documented for some
bling hieroglyphics. Graptolite fossils often species. These complex structures have gener-
show evidence of having been transported ally been explained by asexual budding or
by currents, although fortunately complete, common attachment to a single fl oat or patch
unflattened specimens have been extracted of substrate. A more recent taphonomic expla-

from cherts and limestone by acid-etching nation, however, suggests they formed by
techniques. The affinities of the group were entrapment of clusters of rhabdosomes using

sicula
distal
dissepiment
nema
stipe
growth lines
theca
lateral view nema ventral view

(a) prosicula thecal aperture

autotheca sicula initial bud

bitheca sicula aperture virgula
proximal
(c)

dissepiment
(b)
Figure 15.21 Graptolite morphology: (a) dendroid morphology with a detail of the thecae (b), and
(c) graptoloid morphology.