Page 244 - Introduction to Paleobiology and The Fossil Record

PROTISTS 231

aperture
recessed operculum
aperture
neck
lip
operculum
neck outer prosome
wall flange
inner wall
body
chamber
spine filled with
spongy matter
base spines
(a) (b)
Figure 9.21 Descriptive morphology of the chitinozoans: (a) Operculatifera (simplexoperculate),
Lagenochitina, and (b) Prosomatifera (complexoperculate), Ancyrochitina.

correlations and is a key part of global bio- echinoderms, gastropods and graptolites, but
stratigraphic schemes for the Ordovician and they were probably the products of some soft-
Silurian systems. bodied, worm-like animal during a pelagic
life stage.
Morphology and classifi cation Two main groups of chitinozoans have
been established based on the way the vesicle
Chitinozoans are small (between 50 and is sealed, and are further subdivided accord-
2000 μm), flask- to vase-shaped, hollow vesi- ing to the outline or silhouette of the vesicle

cles with smooth or ornamented surfaces (Fig. together with modifications of the neck. The
9.21). The vesicles were thought to have con- Operculatifera have a relatively simple oper-
sisted of a protein called pseudochitin similar culum and they lack a neck (including the
in composition to the graptolite rhabdosome, Desmochitinidae with small subspherical ves-
but recent research suggests that they are icles), whereas the Prosomatifera have a more
actually composed of networks of kerogen, complex opercula with a prosoma and a well-
and chitin is in fact absent from the pyroly- developed neck (including the Conochitinida
sates vaporized from the vesicle (Jacob et al. and the Lagenochitinidae, the second with a
2007). The vesicle encloses a chamber that recessed operculum) (Fig. 9.23).
ranges in shape from spherical through ovoid
to cylindrical and conical forms. The chamber Evolution and distribution
opens through an aperture at the oral end,
either directly or at the end of a neck with a Possible chitinozoans, in the form of Desmo-
collar. The aperture is closed by an operculum chitina-like sacs, have been reported from the
that may be supported by the prosome. The Upper Proterozoic of Arizona, but the fi rst
base of the vesicle may be flat or extended as true chitinozoans appeared during the

a variety of structures, for example a copula Tremadocian (Early Ordovician) and subse-

(long hollow tube), mucron (short hollow quently diversified rapidly during the Early
tube), siphon (bulb-like process) or peduncle Ordovician, evolving hundreds of different
(solid process). There are nearly 60 genera of species spread across at least 50 genera. This
chitinozoans. diversity continued through the Silurian with

The precise affinity of the group remains all the three main groups represented. They
uncertain (Box 9.12). Chitinozoan vesicles declined during the Devonian, disappearing
were probably tightly sealed, and they occur finally at the top of the Famennian, when the

as chains and clusters that suggest they may last remaining lagenochitinid went extinct.
have been eggs or egg capsules or even Through time the group developed smaller,
dormant cysts. Chitinozoans have, in fact, self-contained chambers with an increased
been interpreted in the past as egg cases of a complexity of ornament and a greater degree
huge range of invertebrates, such as annelids, of apparent coloniality (Fig. 9.23).