Page 430 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 417
Figure 15.24 Retiolitid Phorograptus (Middle
Ordovician) (×30). (Courtesy of Denis Bates.)
until the latest Silurian (Kozlowski-Dawid-
´
ziuk 2004). The retiolitids probably represent
a polyphyletic grade of organization where
the rhabdosome of various groups may have
functioned like a sponge, drawing in fl uid and
nutrients through the periderm and expelling
waste upwards.
Growth and ultrastructure of the graptolites
Detailed studies on the ultrastructure of the
graptolite using both scanning and transmis-
sion electron microscopes has identifi ed two
types of skeletal tissue. Fusellar tissue occurs
together with cortical tissue in the form of
longer parallel fibers. Fusellar material was
secreted as a series of half rings with the corti- for particular functions. Swan targeted the
cal tissue overlapping the fusellar layer both
inside and outside the rhabdosome (Fig.
15.25). The cortical tissue itself was secreted
as “bandages”, looking rather like multiple
overlapping band-aids. Secretion may have
been by mobile zooids, free to patrol the exte-
rior of the colony while still attached by a
flexible cord to the rest of the colony, rather
like an astronaut maintaining a space station,
or the entire rhabdosome may have been sur-
rounded by soft tissue.
Although graptolites are abundant and
important fossils in many Early Paleozoic
assemblages, it is notoriously diffi cult to dis-
cover what they were actually made of. Most
assemblages occur in black shales that have
been compacted, diagenetically altered and
often metamorphosed within or around oro-
genic belts. Moreover, graptolite periderm,
when actually preserved, consists mainly of
an aliphatic polymer, immune to base hydro-
lysis. It lacks protein even though both the
structure, as well as chemical analyses, of the
periderm of living Rhabdopleura suggest that
it was originally composed of collagen. Previ-
ous studies suggested that the collagen had
been replaced by macromolecular material
from the surrounding sediment. New analyses
suggest that the aliphatic composition of
graptolite periderm refl ects direct incorpora-
tion of lipids from the organism itself by in
situ polymerization (Gupta et al. 2006). A
similar process may account for the preserva-
tion of many other groups of organic fossils
(see p. 60).
Colonial growth of the graptoloids The
growth of a colony lends itself to graphic and
mathematical simulations. A few authors have
devised computer models based on a set of
simple rules that dictate such growth modes.
These models are usually deterministic and
static. For example, Andrew Swan (1990)
generated a series of theoretical morphotypes
based on a model of dichotomous branching
at given stipe lengths; the orientation of the
bifurcation together with the stipe length and
width was varied. Additionally, soft tissue
could be added to the computer reconstruc-
tions. Swan showed that the shapes of most
graptolite colonies could be simulated using
variations in only a few parameters, and he
was able to test the effi ciency of each colony
efficiency of the graptolite feeding strategy
and tested the efficacy of nutrient capture for
a sequence of computer-generated colonies,
and he showed that known graptolite colonies
pass the test as being the most effi cient shapes