Fossils in Thin Section: It is Not That Difficult - Microfacies of Carbonate Rocks - page 546

Fig. 10.49. Cross sections of ostracod shells. A : Shell showing the duplicature. The excellent preservation is due to an
early diagenetic silicification of the test. Late Carboniferous (Gzhelian): Auernig, Carnic Alps, Austria/Italy. B : Slightly
displaced smooth-shelled test exhibiting the finely prismatic microstructure. Early Carboniferous: Poland. C : Sculptured
shell of an entomozoan pelagic ostracod. Late Devonian: Menorca Island, Spain.

ture is layered, homogeneous or finely prismatic. The
microstructures commonly form only one shell layer,
except in some Paleozoic ostracods (e.g. Leperdit-
iidae).

Distribution: Ostracods appeared in the Cambrian,
diversified in the Ordovician and were abundant in the
Silurian and Devonian, where they are successfully
used for biozonations. Late Devonian and lowermost
Carboniferous sediments yield the entomozoan ostra-
cods that can also be recognized in thin sections. New
groups also including deep-water ostracods (Fig. 10.50)
appeared in the Late Paleozoic, Mesozoic and Ceno-
zoic.

Ecology: Most ostracods are found crawling on or
burrowing into the sediments at the bottoms of the
ocean (Fig. 10.50) or lakes. A few modern ostracods
have a pelagic swimming life style.

Ostracods in thin sections : Ostracods are common
in many thin sections due to the repeated shedding that
result in accumulations of high numbers of disarticu-
lated valves (Pl. 93/1). Complete shells are small; they
rarely exceed a length of 1 mm. Criteria in thin sec-
tions include shape, size, shell overlap (Pl. 93/2), ex-
terior ornamentation (Pl. 93/4; Fig. 10.49C) and the
prismatic shell microstructure (Pl. 93/2). The dup-
licature is rarely shown (Fig. 10.49B). In general, these
criteria do not allow systematic assignments, except
where characteristic shell ornamentation patterns can
be recognized.

Balanids
Balanids or barnacles (Pl. 93/6, 7) are sessile crus-
tacean arthropods belonging to the Cirripedia. Cirri-
pedians are highly unusual crustaceans that loose many
of their typical arthropod features during ontogeny.
All adults are sessile. The carapace of the larvae is
replaced by a series of calcified plates that form a pro-
tective compartment for the body and the appendages.
Balanids occur attached to various substrates includ-
ing rocky coasts, shells, drifting wood or swimming
organisms. Attachment is made by a broad basal plate
(Pl. 93/6) that is a part of the calcitic test. The shell
forms a truncated cone consisting of up to eight cal-
careous wall plates and other plates that can close the
aperture. Modern balanids are abundant in temperate
and cold-water environments and occur from intertidal

Fig. 10.50. Deep-marine ostracods associated with benthic
foraminifera, mollusk debris and echinoderm fragments
exhibiting the characteristic fine net-like structure. The
ostracod near the top (infilled with red micrite) displays the
duplicature. The large particle near the bottom is a hard-
ground intraclast (see Sect. 4.2.8.1). Early Jurassic: Adnet,
south of Salzburg, Austria. Scale is 1 mm.