Page 431 - Introduction to Paleobiology and The Fossil Record

418 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(b)

(a) (c)
Figure 15.25 Graptolite ultrastructure: (a) collage of Geniculograptus rhabdosome showing banded
fusellar tissue (×50); (b) detailed section through part of a rhabdosome showing relatively thin, parallel
sheet fabric (top) and criss-cross fusellar fabric (below) (×1000); and (c) detail of aperture exterior of
Geniculograptus showing the development of bandages (×500). (Courtesy of Denis Bates.)

for capturing most food in the shortest time or even by vane-like extensions to the nema.
from a given water volume. But they clearly occupied different levels in
the water column (Underwood 1994).
Ecology: modes of life and feeding strategy The suggestion by Nancy Kirk (1969) that
far from being passive members of the plank-
There is little doubt that the earliest bush-like ton, the graptolites were automobile, moving
dendroids were attached to the seabed and up and down in the water column, has stimu-
functioned as part of the sessile benthos. lated considerable and continued interest and
Detachment in various benthic genera occurred research on the life habits of these extinct
at the beginning of the Ordovician with genera organisms. During intervals of intense feeding,
such as Rhabdinopora entering the plankton. a reactive upward movement of the colony in
More controversial is the mode of life of the the water column would have occurred. At
various graptoloid groups (Fig. 15.26). Con- night the colony could move vertically into
ventionally the graptoloids were considered the nutrient-rich photic zone and later, when
to be passive drifters, their fl otation being replete, the rhabdosomes would sink to posi-
aided by fat and gas bubbles in their tissues tions in the water column where the specifi c