Geology Reference
In-Depth Information
Plate 89 Gastropods
The gastropod shell corresponds to a conical tube, closed at the pointed end and open at the wide end (apertura).
Whorls are wrapped about the preceding whorls forming a central pillar (columella, -> 5) or an axial cavity. The
exterior of the shells is smooth or variously ornamented (-> 5). Many gastropod shells are infilled with sedi-
ment. Infilling may occur (a) within the environment of the deposition of the shells (as shown by the sediment
type within the shell and in the matrix; -> 1, 2, 5), or (b) prior to the deposition of the shells (different sediment
in the shell and the matrix; -> 3, 4). Some small, encrusting tubelike fossils, formerly assigned to 'vermetid
gastropods', are now included within a separate mollusk group - the Microconchida (-> 9, 10).
1
Gastropod wackestone.
Longitudinal section (LS) parallel to the axis of coiling, transverse section through the upper
whorls (TS), and oblique sections (OS). Black coating of the shells corresponds to the exceptionally well-preserved
external organic cover (black arrow). Note different preservation of originally calcitic outer part and the primarily arago-
nitic and now recrystallized part (white arrow) of the mollusk shell. The sample is an example of a 'loosely packed,
bimodal sorted‚ biofabric', following the classification of Kidwell and Holland (1991). The fabric is characterized by
matrix-support, closely spaced coarse shells and large bioclasts associated with finer bioclasts. SMF 8. Early Triassic
(Scythian, Werfen Formation): Gartnerkofel, Carnic Alps, Austria.
2
Preservation.
High-spired gastropod. Note double-layered shell construction (white arrow) and displacement of the shell
along microfractures (black arrow). Longitudinal section. Late Triassic: Northern Alps, Austria.
3
Preservation.
Longitudinal section of a high-spired gastropod. Outlines of the shell are well preserved due to the enclo-
sure within an oncoid. The facies corresponds to a peri-reef environment with some terrigenous influx. Note the angular
quartz grains within the gastropod infilling. Late Tertiary (Sarmatian): Prut Valley, Simleu Basin, Romania.
4
Preservation
. Oblique section. Note the still preserved growth lines (arrow). Silurian (Ludlowian): Hobuggen, Gotland
Island, Sweden.
5
Shell exterior.
Longitudinal section. The shell is coiled tightly about its axis, forming a solid central pillar (columella,
CO). Note outline of ornamentation (arrow) consisting of strong spiral ribs. Jurassic: Northern margin of the Alps near
Salzburg, Austria.
6
Actaeonellid gastropod.
Cross section. Actaeonellids are common in brackish-influenced Cretaceous sediments. Early
Cretaceous: Subsurface, southern Bavaria, Germany.
7
Low-spired shell.
Note entire holostome apertura (A).
Arrow points to relicts of prismatic microstructure.
Note the geopetal structure. Late Jurassic: Northern
margin of the Alps near Salzburg, Austria.
8
Diagnostic criteria.
Typical appearance of a gastro-
pod section perpendicular to the axis of coiling char-
acterized by a 'projection' of the wall into the central
cavity. Cross sections of high-spired gastropod shells
have been used to differentiate Tertiary gastropod gen-
era (Strausz 1959). The original wall structure was
obliterated during inversion to calcite, but the diag-
nostic morphological criteria (spire) are still recog-
nizable. The mold is filled by drusy calcite cement.
The sediment is pelmicrite or peloid packstone, re-
spectively. Lagoonal facies (see Pl. 117/2). Early Ter-
tiary (Eocene): Monte Bolca,Verona, Italy.
9-10
'Microconchids'
, formerly considered to be
'vermetid gastropods' or spirorbid worms, are tiny
coiled tubes with differentiated walls. Erect, helicoi-
dal, gregarious forms are frame builders in peritidal
schizohaline lagoonal settings (Burchette and Riding
1977). Prostrate, discoidal forms are encrusters, com-
monly associated with porostromate algae, forming
oncoids in high-energy settings.
Microconchus
is now
assigned to a separate group of mollusks (Micro-
conchida). The group is known from Silurian to the
Jurassic (Dreesen and Jux 1995), but more commonly
found in Devonian and Early Carboniferous lime-
stones. Late Jurassic: Argentina.
This figure shows a gastropod limestone composed of densely
packed shells still exhibiting the original layered microstructure
preserved as aragonite. Late Triassic Zlambach beds, a near-reef
facies, Austrian Northern Calcareous Alps. Scale is 5 mm.
