Page 433 - Introduction to Paleobiology and The Fossil Record

420 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Climacograptus

Leptograptus
Dicellograptus
scandent

reclined

horizontal

declined

pendent

Didymograptus extensus
Didymograptus deflexus
Didymograptus 'bifidus'
Figure 15.27 Evolution of stipes.

rhabdopleurids. The fi rst undoubted grapto- ancestors for the planktonic graptolites, and
lites include the dendroids Callograptus, Den- perhaps Staurograptus was in fact the fi rst
drograptus and Dictyonema occurring in planktonic graptolite. The Tremadocian seas
Middle Cambrian rocks of North America. witnessed the radiation of the anisograptids.
But by the Late Cambrian, the diversity of the The explosion of dichograptid genera
dendroid fauna had markedly increased. The during the Floian introduced a variety of sym-
fauna included genera such as Aspidograptus metric graptolites with from about eight to
and Dictyonema, which resembled small two stipes oriented in declined, pendent and
shrubs and were attached to the substrate by scandent attitudes (Fig. 15.27). A twin-stiped
holdfasts or more complex root-like struc- dichograptid was probably ancestral to the
tures. During the Late Cambrian and Early next wave of graptolites, the diplograptids,
Ordovician, some dendroids made the jump which radiated in the Mid Darriwilian (Middle
from the sessile benthos to the plankton; Ordovician).
attachment disks continuous with the nema The single-stiped monograptids dominated
suggest these genera may have hung suspended Silurian graptolite faunas and, despite their
in the surface waters and pursued an epi- apparent simplicity, the group developed a
planktonic life strategy. Both Radiograptus huge variety of forms (Fig. 15.28). The last
and Dictyonema have been cited as possible graptolites, species of Monograptus, disap-