Page 432 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 419

1 2 3
3a
1a

2a

3c

2b
3b
1b
4 5
4a

5a

4b 5b
Figure 15.26 Graptolite life modes: 1, conical forms with spiral motion; 2, flat or slightly conical forms

with slow, slightly spiral velocities; 3, mono- or biramous forms with spiral movement due to
asymmetry; 4, forms with high angles between stipes having linear movement; 5, straight forms with
mainly linear descent. (Based on Underwood 1994.)

gravity of the colony matched that of the sur- 1 The transition from sessile to planktonic
rounding seawater. strategies in the dendroids during the Late
Computer models and physical models, Cambrian and Early Ordovician.
including exposure to wind-tunnel conditions 2 At the end of the Tremadocian (early Early
that mimic the effects of water currents, have Ordovician), the appearance of the single-
emphasized the importance of harvesting type thecae of the graptoloids.
strategies for the success of the colony. These 3 The development of the biserial rhabdo-
probably exerted an important infl uence on some in the Floian (late Early
the evolutionary pathways that the grapto- Ordoviaian).
loids followed. 4 Finally, the origin of the uniserial
monograptids.
Evolution: graptolite stipes and thecae
The small, stick-like benthic organisms
Graptolite evolution has been described in reported from Middle Cambrian rocks on
terms of four main stages of morphological the Siberian platform and ascribed to the
development: graptolites may be better assigned to the