Geology Reference
In-Depth Information
Bryozoans in thin sections
(Pl. 85): Bryozoan sec-
tions can be distinguished from other fossils consisting
of calcareous tubes (e.g. chaetetid sponges) by the pres-
ence of polymorphic tubes of various widths, and by a
characteristic fanlike pattern in longitudinal and oblique
sections, resulting from a different arrangement of the
zooecia in the inner axial zone of the colony and near
the surface (Pl. 85/2, 7). Paleozoic trepostome bryozo-
ans may display morphologically different tubes in ad-
dition to the zooecia, differing in the diameter and the
development of horizontal elements. Small acantho-
pores have a laminated cone-in-cone structure point-
ing outward within the laminated wall. Mesopores of
Early Paleozoic trepostome bryozoan contain numer-
ous transverse plates, in contrast to the mesopores of
Late Paleozoic taxa. Many Paleozoic trepostome and
cryptostome tubular bryozoans exhibit zooecia walls
that are thin in the inner zone of the colony and thick in
the outer zone. Paleozoic cyclostome bryozoans are
characterized by simple tests with walls of constant
thickness and generally without internal structures.
Post-Paleozoic cyclostomes exhibit a thick finely lami-
nated wall structure in the outer zone of the colony and
minutely porous walls. Paleozoic cystoporate bryozo-
ans have a vesicular to cystose framework between the
zooecia and simple tubes which are commonly modi-
fied by a hood (lunarium) at their outer ends. This struc-
ture is reflected internally in tangential sections by a
crescent-shaped notch in the wall and a thickening of
the wall tissue behind the notch (Pl. 85/4).
Bryozoan wall microstructure may be laminated,
granular or fibrous. The laminated structure occurs in
many bryozoan groups and is one of the distinctive fea-
tures of bryozoan debris (Pl. 85/4). Details of the mi-
crostructural types can be found in Horowitz and Pot-
ter (1971).
Plate 85 Bryozoans
The plate displays thin sections of bryozoans belonging to different systematic units. Characteristic thin-section
features of bryozoans are (a) colonies consisting of variously sized tubes (-> 1), (b) different arrangement of
tubes in the inner and outer part of the colony (-> 3) resulting in a fan-like pattern (-> 7), (c) window-frame
network structures (-> 10), and (d) finely laminated wall structures (-> 4, 10). See also Pl. 4/5; Pl. 9/5, 6; Pl. 51/
6; Pl. 106/1-3; Pl. 107/1, 2; Pl. 144/7, and Pl. 147/2, 3 for further photographs.
1
Trepostome bryozoans
(
Reptonoditrypa cautica
Schäfer and Fois), characterized by polymorph tubes (zooecia). Oblique
sections. Sections of trepostome bryozoans are characterized by robust massive or dendroid colonies and small tubes
between larger ones. Middle Triassic (Anisian): Olang Dolomites, Southern Alps, Italy.
2
Cystoporid bryozoan
are characterized by encrusting or massive colonies consisting of large zooecia (Z) embedded
within a framework of smaller tubes (cystopores (C). Oblique section. Early Carboniferous: Nötsch, Austria.
3
Bifoliate cyclostomid bryozoan.
Note the differentiation of central and peripheral parts of the colonies. The vinculariid
erect growth form with dichotomously branched zooecia indicates a protected environment. Late Tertiary (Miocene):
Subsurface, Suez region, Egypt.
4 Broken branch of a cystoporid bryozoan. The cross section of zooecia (Z) showing the typical crescent-shaped notch in
the wall is infilled with micrite and embedded in a thickened wall exhibiting a lamellar structure (arrow). Mississippian:
Turtle Mountains, Frank Slide, southeastern Alberta, Canadian Rocky Mountains.
5
Cryptostome bryozoan
. Cryptostome bryozoans are characterized by delicate dendroid or bifoliate colonies, axial zooecia
bundles and in cross sections short elliptical zooecia in the peripheral zones. Oblique section. Ordovician (Caradoc,
Macrourus limestone): Öland Island, Sweden.
6
Fenestrate bryozoan
exhibiting cross sections of zooecia and lamellar wall structure. Middle Permian: Straza, Slovenia.
7
Cryptostome bryozoan.
Transverse (T) and longitudinal (L) section. Note radial distribution and the curvature of zooecia
toward the outer margin and the thickened wall structure at the exterior margin (arrow). Cold-water carbonate. Permian
(Berriedale limestone): Southeastern Tasmania, Australia.
8
Fenestrate bryozoan
with serially arranged zooecia on the surface of a broken frond. Mississippian (Tournaisian): Mouse
Mountains near Calgary, Alberta, Canada.
9
Cystoporid
bryozoan
(
Fistulipora
). Oblique section of a broken colony. Note zooecia (Z) separated by vesicular tissue
(cystopores, C). Early Permian: Powow Canyon, Hueco Mountains, Texas, U.S.A.
10
Fenestrate bryozoan.
Outer part of a fan-shaped colony. Zooecia occurring on the surface of the colony (inner part of the
fan) can not be seen in this section. Fenestrate bryozoans are characterized by erect rigid fronds connected by cross bars
forming a characteristic network with open meshes (fenestrulae, F). Note lamellar wall structure. Most members of this
group preferred quiet sheltered environments. Mississippian: Turtle Mountains, Frank Slide, Alberta, Canada.
11
Bryozoan debris
consisting of fragments of fenestrate bryozoans. Reef facies. Late Permian (Zechstein): Bartolsfelde,
Lower Saxony, Germany.
