Geology Reference
In-Depth Information
since Late Triassic). In the
irregular echinoids
a sec-
ondary bilateral symmetry is superimposed upon the pri-
mary pentaradiate pattern. This group includes the Cida-
roidea (pencil urchins), Clypasteroidea (sand dollars;
since the Paleocene), Spatangoidea (heart urchins; since
the Cretaceous) and Cassiduolidea (since Jurassic).
Echinoid spines show a great variety in shape and
size. The spines are long and prominent in regular echi-
noids, but very short and stubby, forming an almost
felt-like covering, in irregular echinoids. These spines
are hollow and without a cortex. A spine consists of
three parts, a long shaft, a short neck and a base. The
concave articulation surface that attaches onto the ma-
melon of the tubercle is the acetabulum which may be
either perforate or imperforate. Each spine can be
moved individually. The shaft may be smooth, very
finely striated or ornamented by ribs, thorns or gran-
ules.
Sections of echinoid spines exhibit characteristic in-
ternal radial patterns that are useful taxonomically. In
cross section spines may be hollow (diadematoid type)
Echinoid spines in thin sections:
Echinoid fragments
can be identified using a combination of morphological
criteria (Durham et al. 1966; Smith 1984; Nebelsick
1992). Common constituents in thin sections of lime-
stones are echinoid spines.
Living echinoids are cov-
ered with movable spines, which are anchored in sock-
ets on the top of tubercles in the test and serve for pro-
tection, walking and burrowing.
Plate 96 Echinoderms
The plate displays skeletal elements of non-stalked pelagic crinoids (-> 1-3) and
stalked crinoids (-> 11), echinoid spines (-> 6-10, 12, 13), and a holothurian scler-
ite (-> 4). Taphonomic criteria of echinoderms are shown in -> 14 and 15. The
figure on this page shows a recent non-stalked crinoid. After Black (1988).
1-3 Pelagic 'free-swimming' crinoids. Skeletal elements of planktonic crinoids are important constituents of Mesozoic
open-marine carbonates. Most common elements seen in thin sections are brachial extensions used for swimming.
1
Roveocrinoids.
Oblique sections of thin arm plates (brachialia) of
Osteocrinus rectus
(Frizell and Exline). Late Triassic
(Carnian): Feuerkogel near Aussee, Austria.
2
Microfacies
with
Saccocoma
. Accumulation of antler-like brachial plates and other elements of the non-stalked crinoid.
Note the fine meshwork structure (white arrow) diagnostic for echinoderms. Black arrows point to zoospores of plank-
tonic algae (
Globochaete
; see Sect. 10.2.6). This microfacies is widely distributed in Kimmeridgian to Tithonian basinal
limestones of the Tethys. Late Jurassic: Margin of the Northern Alps near Salzburg, Austria.
3
Saccocoma
. Arm plate. Late Jurassic (
Saccocoma
limestone, Kimmeridgian): Central Apennines, Italy.
4
Holothurian sclerite.
Skeletal elements of holothurians differ from those of other echinoderm groups in that they consist
of isolated microscopic single-crystal calcite pieces. Morphology is highly variable and includes anchors, hooks, tables,
rods, wheels (this figure) and others. Early Cretaceous: Subsurface, Bahamas.
5-10, 12, 13
Echinoid spines:
The tests of echinoids are covered by spines of different size, serving as protection, locomo-
tion, or as means of anchoring. Cross sections exhibit a radial pattern around a hollow or irregularly fabricated center.
Differences in radial patterns may be useful taxonomically and for stratigraphic zonation.
5
Echinoid spine
. Longitudinal section. The spine projects outward from a tubercle (T). Late Triassic: Kuta, Papua New
Guinea.
6-8
Echinoid spines
. Cross sections. The three sections correspond to each other in the perforated thin zone surrounding the
lumen, and the general pattern of the wall zone. The wall zone consists of wedge-shaped radial elements which widen
toward the periphery. The cross sections differ in the shape and the number of the radial septa. The general pattern
corresponds to the
Diadema
type distinguished by Hesse (1900). Cross sections exhibit either low birefringent colors or
extinction under crossed nicols. Same locality as -> 5.
9
Echinoid spine
. Cross-section and oblique section. Same locality as -> 5.
10
Echinoid spine
. Note the syntaxial overgrowth cement. Late Jurassic (Tithonian): Subsurface, Kinsau, Germany.
11
Crinoid arm elements
(brachial fragments), characterized by bifurcated lunate patterns (see Fig. 10.53). Early Jurassic:
Adnet, Salzburg, Austria.
12
Echinoid spine.
Cross section. The spine differs from those shown in -> 6-8 and 13 by the arrangement of the radial
septa, distinct ring-like interseptal connections and by the larger size of the spine. Late Permian: Bergama, western
Anatolia, Turkey.
13
Echinoid spine.
Cross section. Same locality as -> 5.
14
Destruction of echinoderm plates
by boring. Arrows point to cross sections of cricoconarid shells (see Pl. 91). Early
Devonian (Emsian): Erfoud, Anti-Atlas, Morocco.
15
Echinoderms and mollusks compared.
In thin sections, echinoderm (E) elements differ from mollusk (M) shells in color
and net-like microstructure. Mollusk shells are commonly replaced by a coarse calcitic mosaic. Same locality as -> 5.
