Turritelline mass accumulations from the Lower Miocene of southern Germany: implications for tidal currents and nutrient transport within the North Alpine Foreland Basin

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Mass accumulations of fossils are useful indicators of both local and regional environmental conditions during deposition.In this study, a classic locality, the Ermingen Turritellenplatte (Fig. 1), with mass accumulations of turritelline gastropod from the Miocene of the Upper Marine Molasse, is studied in detail.The purpose of this investigation is to show changes in sedimentation, orientation and other taphonomic features of the fossil assemblage from the base to the top of the accumulation; to use these data to interpret water movement and relative depth; and to draw possible inferences with respect to basin-wide water movement within the North Alpine Foreland Basin.

The North Alpine Foreland Basin
The locality is Early Miocene in age (Fig. 2) and lies within the North Alpine Foreland Basin (NAFB) (Fig. 3), an asymmetric, peripheral foreland basin produced by alpine tectonics during the Palaeogene.It extends from the Rhone Valley in France across Switzerland, southern Germany and Austria to what is now the Vienna Basin (Fig. 3).The NAFB was formed in the Eocene by the Alpine orogeny with the underthrusting of the European Plate beneath the Adriatic-African Plate (Kuhlemann & Kempf 2001;Bieg 2005;Bieg et al. 2008).The NAFB has a complex history of sedimentation resulting from global eustatic sea-level changes, climate oscillations, isostatic reaction to the overthrusting Alpine chain to the south and variations in sediment input primarily from the emerging Alps, as well as sediment transport along its axis (see discussion in Pippèrr & Reichenbacher 2017).These variations in sea level and sedimentation patterns led to two major transgressive-regressive cycles, and thus repetitious transitions between marine and non-marine conditions.
In southern Germany, this led to the well-known succession of the Lower Marine Molasse, Lower Freshwater Molasse, Upper Marine Molasse and, following a brackish water phase, the Upper Freshwater Molasse which essentially continues to the presentday.
Sedimentological data and modelling have shown that the Upper Marine Molasse in southern Germany was a macrotidally influenced shallow marine basin (Homewood & Allen 1981;Allen & Homewood 1984;Allen et al. 1985;Roetzel et al. 2006;Bieg et al. 2008;Frieling et al. 2011), connecting the Mediterranean to the more extensive, and in part deeper water basinal configuration towards the east in Austria.Marginal marine sediments are preserved on the northern rim of the basin.These can be highly fossiliferous, yielding microfossils, trace fossils, invertebrates and vertebrate faunas (Pippèrr & Reichenbacher 2009;Schneider et al.   2011b), primarily in siliciclastic settings.In some restricted areas, however, the high contributions of bioclastic material have led to the production of bryomoltype carbonates (Nebelsick 1989(Nebelsick , 1992;;Kaiser et al. 2001;Bieg et al. 2007).Tidal currents and amplitudes have been reconstructed within this basin using nonlinear, three-dimensional hydrodynamic modelling (Bieg et al. 2008) indicating a complex system of convergent and divergent currents in the seaway within a high mesotidal ranges and current velocities reaching up to 8 ms −1 (Fig. 4).

Fossil orientation data
The orientation of components and their inclusion in directional statistical analysis have been shown to be useful for reconstructing transport mechanisms and palaeocurrent directions (Nagle 1967;Futterer 1978a,b;Wendt 1995 and literature therein).Orientation is influenced by numerous factors including the shape, density and ornamentation of the biogenic components, as well as coarseness and traction of the substrate over which the shells are transported.As shown by flume experiments and field observation, the movement of even simple shapes can be complex depending on current velocity, grain size characteristic and bed forms resulting in a variety of transport phenomena (Trussheim 1931;Menard & Boucot 1951;Ager 1963;Nagle 1967;Futterer 1978a,b).Various elongate and cone-shaped biotic components have been used as current indicators including crinoid stems (Schwarzacker 1963), belemnites (Futterer 1978a;Vörös 2011), bivalves and brachiopods (Nagle 1967), orthocerid cephalopods (Wendt 1995) and others.Directional data for turritelline gastropods have also been measured and interpreted with ambient current directions (Allmon & Dockery 1992;Allmon & Knight 1993;Crawford et al. 2008;Anderson et al. 2017).

Mass occurrences of gastropods and palaeoecology of Turritella
Mass occurrences of gastropods are known from both marine and non-marine settings (Price et al. 1985;Cohen 1989;Geary & Allmon 1990;Waite et al. 2008;Waite & Strasser 2011) as well as turritelline gastropods as in this study.Turritella is a highly speciose taxon present on or in mobile substrates in fully marine, normal salinity conditions (see Allmon 2011).These gastropods represent semi-infaunal suspension feeders which use a modified operculum to acquire food (Yonge 1946;Waite & Allmon 2013) though some are deposit feeders.They can occur in very high numbers either buried in the substrate with the apertures exposed or lying on the sediment surface.Studies on recent (Allmon 1988;Allmon et al. 1992;Teusch et al. 2002;Waite & Allmon 2013) and fossil (Allmon et al. 1995;Allmon 2007;Crawford et al. 2008;Anderson et al. 2017) turritelline gastropods have stressed their importance as indicative of high nutrient conditions such as shallow water areas with coastal upwelling and increased phytoplankton, though there are exceptions (Allmon et al. 1994;Allmon 2011).They are also sensitive to disturbance and hypoxia conditions (Gallmetzer et al. 2017;Tomašových et al. 2018).In turritelline gastropoddominated assemblages (Allmon 2007), local concentrations can occur at such high densities as to exclude the presence of other benthic taxa.Predation on Turritella has been reported from various predators including naticid gastropods, crabs and starfish in some cases leading to recognizable traces (Dudley & Vermeij 1978;Allmon et al. 1990;Tull & Bohning-Gaese 1993;Cadée et al. 1997;Hagadorn & Boyajian 1997;Allmon 2011).In living specimens of Turritella, the thin outer lip is seldom complete, even in mature examples (Bosch et al. 1995).

Materials and methods
The Erminger Turritellenplatte (ETP) locality is well known as a collection site and source of museum specimens (Baier et al. 2004;Baier 2008), but has not been studied in detail with respect to taphonomy and sedimentology.It occurs close to the northern boundary of the NAFB (Figs 3, 4), which transgressed across sediments of the Lower Freshwater Molasse and Late Jurassic limestones at its northern border.These Lower Miocene sediments have been correlated with the Ottnangian (Fig. 2) of the regional Paratethys stages (Vennemann & Hegner 1998).
The turritelline gastropod from this locality has been identified as Turritella cf.eryna communiformis Voorthuysen 1944 (Fig. 5C).It possesses 13 weakly convex whorls that increase steadily in size.The whorls are separated by a clear suture.The number of spiral lines increases from 3 to 5 to 7 in the last whorls.Finer sculpturing is commonly difficult to ascertain due to poor preservation (see below).The aperture is slightly oval in shape.See Höltke (2009) for discussion of species identification and historical treatment of the molluscan fauna found in this locality.An excavation was conducted at the edge of a quarry wall of the ETP, using a mini-excavator (Fig. 5 A), and then cleaned (Fig. 5B).Sedimentological features were assessed, and samples were taken for fossil material and thin sections.Taphonomic features were noted and snails were measured for length if complete from the apex to aperture (Fig. 6).The compass bearings of apex direction, as well as horizontal inclinations of turritelline gastropod specimens, were measured in situ as well as on excavated, oriented blocks using a geological compass to collect orientation data (Fig. 7).An unlithified clay deposit underlying the ETP was drilled using a manual piston core.Thin sections measuring 48 × 48 mm. were studied for detailed sedimentary, fabric and component analysis (Figs 8,9).Microfacies photographs were taken using a Wild Microscope and digital Nikon VI camera and processed using Adobe Photoshop CC2018.
Specimen orientations were plotted, and statistical tests of uniformity were run using the Circular package (Agostinelli & Lund 2013) in the statistical computing software R (R Core Team 2015).Trend measurements for specimens in each of the measured beds were plotted using rose diagrams with bin size equal to 20°and fried egg plots using kernel density estimation with a bandwidth of 25 (Fig. 7).Rao's spacing test, with results shown in Table 1, was used to determine whether the orientation data were not uniformly distributed.Size and orientation data were plotted and analysed in R. The length of complete specimens was plotted for all bedding planes pooled together (Fig. 6) and for each bed (Fig. 10).Additionally, for the three beds with more than twenty individuals, a Kruskal-Wallis test was used to assess whether or not the length-frequency distribution of individuals in the beds were significantly different.

Fossils and stratigraphical succession
The ETP is dominated by the turritelline gastropod Turritella cf.eryna communiformis Voorthuysen 1944, which occurs in rock-forming quantities (Fig. 5).Thick-shelled ostreids and the venerid bivalve Tapes helvetica Mayer 1853, can also occur, though less common (see Höltke 2009).Other molluscs, including both gastropods and bivalves, are rare and occur along with vertebrate remains, including shark teeth, abraded bone fragments and scarce teeth of cervids.The recovered turritelline specimens range between 10 and 70 mm in length with a mean of 36 mm.(Fig. 6).The distribution of lengths for the three beds where more than twenty complete individuals were measured (Beds 11, 17 and 18) were not shown to be significantly different (Kruskal-Wallis test, χ 2 = 0.78, two degrees of freedom, P = 0.68).
The excavated succession of the ETP is ca.3.5 m thick (Fig. 7) and consists of poorly consolidated sediments and rocks including coarse-grained fossiliferous sandstones and mass occurrences of turritelline gastropods.Isolated fossil specimens can be recovered, but most specimens are firmly cemented within a bioclastic sandstone with calcareous matrix which can pass into quartz-rich limestones.The fossiliferous sandstones of the ETP are underlain by unconsolidated, light blue to grey calcareous muds, lacking macrofossils with a thickness of 190 cm, preceded by an impenetrable, hard, calcareous horizon.The ETP starts abruptly with thin beds consisting of a basal gravel layer with well-rounded quarzitic pebbles, silty marlstone (Bed 2), well-rounded, coarsegrained sandstone (Bed 3) and an oblique thin layer of silty marls with sands.These thin beds, which include isolated turritelline specimens and bivalves, are followed by more massive, coarse-grained, poorly consolidated rocks (Beds 5-14) rich in fossils, which make up the bulk of the ETP.
The turritelline specimens are concentrated in and mostly accumulated along distinct, several cm thick layers that represent bedding planes.Areas with densely packed components occur adjacent to or laterally grade into clean sandstones.The gastropods can also completely dominate the sedimentary succession in decimetre thick accumulations (especially Bed 8) where the specimens are often bunched together sometimes with opposing, though parallel orientations.The coarse-grained bioclastic sandstone succession is interrupted by thin discontinuous mudstone intercalations, as well as by a 30-cm-thick sedimentary layer dominated by isolated oyster valves.Some beds are slightly inclined (Beds 11,15).Towards the top of the succession, more unconsolidated sandstones rich in isolated gastropods are present (Beds 16, 17) followed by large blocks in situ.Among the non-carbonate components, rounded coarse quartz grains up to 40 mm also occur along with dark siliciclastic pebbles up to several cm size.Well-rounded mudstone clasts composed of the material from the above-mentioned silty mudstone layers are distributed throughout the section, reaching sizes up to 100 mm in Bed 6.

Microfacies analysis and taphonomy
Thin section analysis shows sedimentary rocks dominated by poorly sorted, angular to subrounded quartz with an admixture of bioclasts (Figs 8,9).The most common bioclastic components are high-spired gastropods attributed to turritellines (Figs 8, 9) which totally dominates the macrofauna.Bivalves are disarticulated and can be highly fragmented.Oyster fragments are typically highly rounded while balanid barnacles are present as disarticulated single wall  plates (Fig. 8A).Rare echinoid spines, bryozoans and isolated fish teeth are also present.Well-rounded clasts of Jurassic limestone consisting either of mudstones or sponge limestones are present, though rare (Fig. 8B).
The highly spired shells are often broken at both the aperture and the apex, especially at the base of the section with more complete shells towards the top (Table 1).Cylindrical, bevelled boreholes typical for naticid predation can be present (Oichnus paraboloides Bromley 1981), but were not quantified as most specimens recovered during the excavation could not be readily extracted from the sedimentary matrix.The surface preservation of the turritelline gastropod shells often is somewhat irregular with respect to surface sculpturing.
Sediment infilling of the high-spired shells is mostly incomplete and often restricted to the whorls closest to the aperture (Fig. 9A).Whorls in the middle part of the shells show geopetal distribution of sediment grains, while the apical whorls are mostly empty of sediment.These apical whorls either remain empty or are filled by sparite.The lithified sediment within the shells, when present, shows the same characteristics as the sedimentary matrix between components, consisting predominately of poorly sorted, angular to subrounded quartz grains (Figs 8,9).
As revealed by optical microscopy, there is a complex diagenetic pathway which includes the dominant turritelline shells and other primarily aragonitic components such as infaunal bivalves.The shells are micritized, which is visible as a thin, opaque layer (Figs 8D,9C).This micritized layer is present on the outer shell surface, but can be missing within more inner whorls of the snail as seen in longitudinal sections (Fig. 9A).Otherwise, the shell is replaced by sparite crystals, though faint growth lines parallel to the shell surface can, in part, still be preserved (Figs 8 C, 9B).In addition, the replacement by sparite is often incomplete leading to the production of secondary vugs (Figs 8D, 9B).Coarse calcite crystals can also grow into the primary pore space of the shell interior as well as on the exterior of the shell if not blocked by matrix.This crystal growth is often irregular and does not necessarily faithfully follow the fine sculpturing of the outer shell surface (Fig. 9C).Both former shell surfaces as well as quartz grains can be covered by a thin layer of equant sparite crystals (Fig. 9C).

Analysis of directional data and size-frequency distribution
Rose diagrams and fried egg plots showing the distribution of specimen trends in each bed are shown in Figure 7.In all cases, the null hypothesis of uniformly distributed data is rejected (Table 1), and, indeed, the non-uniformity of the data is evident in the rose diagrams, which show clear directionality (Fig. 7).The directional data show a shift of shell orientations and dip from the base to the top of the section (Beds 5 to 17).A broader distribution of orientations is often accompanied by a wider distribution of dip directions.The main direction of snail apices varies between south-east to north-east (and corresponding opposite orientations).The base of the section is dominated by poorly constrained orientations, varying dip directions and often poorly preserved specimens.In the middle part of the section, more bimodal distributions are present with major axis 180°to one another (Beds 7, 11 to 14).Some of these beds also contain a few shells with a 90°offset (especially Bed 12).These shells also show high dips and are generally poorly preserved.Clear unimodal orientations are in Beds 16 and 17 where the snail shells are loosely distributed and show flattened dip directions.These shells are well preserved, and most specimens are complete as in Bed 17. Bed 18 reverts to a wide distribution of orientations, while the specimens are well separated from one another as well as showing only very slight inclination, if at all.As in Bed 17, there are numerous wellpreserved specimens.

Preservation and diagenesis
The shells often show damage to the apex and aperture.It is not clear whether damage to the aperture is due to predation, pagurisation or abrasion due to transport (compare with Tull & Bohning-Gaese 1993;Cadée et al. 1997;Hagadorn & Boyajian 1997;Allmon 2011).In general, however, the preservation of specimens is better towards the top of the section.Clear predatory events are seen in rare borehole (O.paraboloides) interpreted as the result of naticid gastropod predation, but could not be quantified as specimens were well cemented within the rock and could not be removed without destroying the shell.An outstanding feature of the ETP is the preservation of originally aragonitic components as calcitic replacements in highly permeable, coarse-grained, siliciclastic sedimentary rocks.This is interpreted as a neomorphic transformation of aragonite by calcite, resulting in replacement of the original aragonitic skeleton by drusy calcite while maintaining, to a large extent, the original morphology of the skeleton (see discussion in Perrin et al. 2014).An important role is played by the micritization of the shell surface which serves as a template for drusy calcite crystals growing both into the internal space and on the   outside of the shell (Flügel 2010).The transformation of aragonite to calcite was rapid as reflected by the preservation of growth lines within the sparite crystals.A further implication of the presence of drusy calcite growing outward from the shell surface is the masking of surface details.This may have implications for the identification of fossil turritelline species, which are often differentiated based on fine sculpturing of the outer shell surface.
The observed characteristics of transformation of aragonite to calcite are functions of climate and hydrology (Dodd 1966) with the calcium carbonate derived from aragonite dissolution reprecipitated as calcite either in the mixed marine/freshwater zone or in freshwater lenses (Budd 1988;Brachert & Dullo 2000).This rapid transformation of gastropod shells and other aragonitic molluscs in three dimensions was most probably aided the impermeable layers of the muddy sediments below the ETP.

Palaeoenvironmental gradient within the ETP
A polymodal origin of shell accumulations is present in the ETP and can be interpreted with respect to sedimentary structures, orientation data and shell preservation.The basal clays can be correlated with sediments of the Lower Freshwater Molasse (LFM), which are represented by isolated occurrences on the slope south of the excavation site.The basal gravels represent a transgression horizon over the preceding muds of the LFM.This succession of the ETP, dominated by turritelline gastropods, ranges from four different associations (Fig. 7) including (from bottom to top): A High energy, chaotically arranged, densely packed, transported assemblages possibly representing storm deposits.These represent the thickest and most highly dense shell beds.B Tidally influenced deposits showing bimodal shell orientations, in a cross-bedded sandstone texture, with concentrated, transported gastropod shells following the sedimentary inclination.C Lower energy conditions with slightly finegrained sedimentary rocks and widely dispersed, parautochthonous, flat-lying shells which were subjected to enough water energy to result in a uni-directional orientation.D Flat-lying, dispersed specimens showing a poorly constrained orientation representing the lowest energy conditions which were not high enough to substantially influence shell orientation.The reduction in energy conditions is accompanied by the presence of better-preserved, complete specimens.
These developments point to a generally deepening of palaeoenvironment in line with the ongoing transgression of the Upper Marine Molasse as reconstructed in Figure 11.The turritelline gastropods are interpreted to have lived as semi-infaunal suspension-feeding gastropods in associations C and D (see above).The shells were then transported to shallower, higher energy environments where they were then concentrated at higher densities.

Current directions and origin of nutrients
The high density of turritelline gastropods together with the presence of other filter-feeding molluscs (bivalves), bryozoans and barnacles of the ETP are indicative of nutrient-rich palaeoenvironments.This is present despite the fact that sediments of the Molasse Sea generally transgressed over a karstified Upper Jurassic limestone landscape, represented by rounded components, from which little nutrient input could be expected.Upwelling conditions in the North Alpine Foreland Basin have been reported for various time-slices of the Paratethys Sea, notably during the formation of the Paratethys at the Eocene/ Oligocene transition (Schulz et al. 2005) as well as during the Late Oligocene and Early Miocene (Wagner 1996(Wagner , 1998)).Upwelling systems driven by wind leading to oxygen deficiency and resulting oxygenpoor sediments were suggested by Wagner (1996) on the northern margin of the North Alpine Foreland basin.Grunert et al. (2010Grunert et al. ( , 2012) ) used dinoflagellates and planktonic foraminifera to reconstruct upwelling conditions along the Bohemian Massif of the Central Paratethys during the Mid-Burdigalian with a sea surface temperature between 10°and 14°related to north-east trade winds and strong tidal currents.Nutrient-rich conditions at the northern NAFB margin are also reflected by bryomol-type carbonates dominated by filter-feeding organisms along with coralline algae and a restricted larger foraminiferal fauna (Nebelsick 1989(Nebelsick , 1992;;Kaiser et al. 2001;Bieg et al. 2007).
The present study confirms the high degree of water movement within the Molasse Sea with nutrients either derived from Central Paratethyan upwelling some 400 km to the east or from the emerging alpine chain on the southern coastline of the North Alpine Foreland Basin.Caution must be exerted in making any general conclusion on the general direction of basinal water movement from the directional data as near-coastal water movements can differ highly from that of more offshore currents.Bieg et al. (2008) reconstructed tidal generated water movements within the Molasse Sea, whereas current movements change responding to tidal cycles.This study shows that there are differences in current velocities at different depths, although, at least in the study area, a general direction from the east is assumed.

Conclusions
The following conclusions can be drawn: 1 Detailed investigations of a mass occurrence of turritelline gastropods in the Lower Miocene Ermingen Turritellenplatte from the North Alpine Foreland Basin shows 3.5-m-thick sections dominated by coarse-grained sandstones containing variously packed fossils.This succession follows directly over unconsolidated muds of the Lower Marine Molasse. 2 Aragonitic shells have been completely replaced by calcite resulting in vugs within the shells.In some specimens, calcite growth from the surface outward has resulted in a slight modulation of surface morphologies.3 Sedimentary characteristics, shell preservation and orientation data of the gastropod specimens indicate a shallowing-upward sequence ranging from the following: A) high energy, storm-induced and winnowed sediments with variously dipping, densely packed non-directional shells; B) variously dipping, commonly bimodal-distributed shells in cross-bedded sands; C) tidally influenced, strictly uni-directional, isolated, well-preserved, flat-lying specimens; and finally D) variously oriented, loosely distributed, flat-lying specimens in depths not affected by strong current.4 Possible sources of nutrients are seen as being transported from the east where deeper water upwelling has been reconstructed or coastal input from the emerging Alps to the south.

Fig. 3 .
Fig. 3. Central part of the North Alpine Foreland Basin with palaeogeographical and bathymetric reconstruction of the Upper Marine Molasse during the Early Miocene (after Bieg et al. 2008, fig.4).

Fig. 5 .
Fig. 5. A, excavation and recovery of sedimentary blocks of the Erminger Turritellenplatte using a mini-excavator.B, overview and sampling within the exposed section.C, isolated Turritella cf.eryna communiformis.D, typical preservation and sedimentary context of fossils from the Erminger Turritellenplatte from Bed 17 showing generally isolated, uni-directionally orientated specimens.E, F, side and plan views of Bed 5 with densely packed, variously orientated and slightly dipping specimens.

Fig. 7 .
Fig. 7. Stratigraphical section of the ETP with orientation data and schematic representation of turritelline gastropod presence within the section.Orientation data include azimuth ('fried egg' diagram) and dip directions.Key: N = number of measured specimens; C = % complete specimens; LFM = Lower Freshwater Molasse; UMM = Upper Marine Molasse.A-D = palaeoenvironmental associations (see Fig. 11).

Fig. 8 .
Fig. 8. Thin sections: A, ERMBX-6, Bed 7. B-D, ERMBX-6, Bed 5. Key: BP = Barnacle plate: CF = columellar fold; IC = isopachous calcitic cement; JC = Jurassic component; MR = micritized rim; OS = oyster shell; PP = primary porosity; QG = quartz grains; SP = secondary porosity.A, overview showing poorly quartzose sandstone rich in bioclasts including mollusc shells replaced by calcite, rounded calcitic oyster shell fragments and isolated balanid barnacle plates showing tree-like structures and growth lines.The angular to subrounded quartz grains are poorly sorted.Thin section, scale bar = 2 mm.B, overview showing cross-sections of gastropods and a wellrounded, upper Jurassic clast consisting of a sponge limestone.Scale bar = 0.5 mm.C, circular cross-section of snail shell showing prominent columellar fold.Thin rim of isopachous sparite covers both the shell surface and quartz grains.Scale bar = 2 mm.D, close up of Figure 5C showing prominent micritization on both the outer and inner surface of the shell.Incomplete infilling of calcite leads to secondary porosity.Isopachous sparite grown both into the shell volume and away from the surface.The interior of the shell is incompletely filled by sparite leading to secondary porosity.Scale bar = 0.2 mm.

Fig. 9 .
Fig.9.Thin section ERMBX-26, Bed 9. Key: A = Artefact; GB = Growth bands; GS = Geopetal infill; MZ = micritized rim; PP = primary porosity; SP = secondary porosity; RZ = rim cement; SC = sparite cement.A, longitudinal section of an elongate, many-whorled turritelline gastropod shell.Anterior whorls filled with sparite cement, followed by a whorl free of both sediment and cement, followed by posterior whorls partially filled with geopedal quartz grains, otherwise showing primary porosity.Scale bar = 2 mm.B, detail of Figure9Ashowing shell preservation.Growth bands within the shell are partially preserved.Outer shell surface shows micritization present as a dark band, which is missing from the inner surface.Primary porosity is present within the empty whorl of the snail, secondary porosity within the shell.Scale bar = 0.5 mm.C, detail showing growth of sparite crystals away from the micritized rim.Differential growth of the sparite obscures ornamental details of the shell surface.Scale bar = 0.25 mm.

Fig. 10 .
Fig. 10.Box plots showing the distribution of shell lengths from each bed.

Fig. 11 .
Fig. 11.Reconstruction of depositional environments of turritelline gastropods in the ETP along a depth gradient with representative orientation data (see Fig. 4).A, jumbled, poorly preserved, inclined shells indicating strong transport.B, bidirectional, poorly preserved, inclined shells on lee surfaces of crossbedded sandstones indicating stronger current conditions.C, uniform orientation of disjunct, well-preserved, flat-lying shells indicating moderate uniform current.D, disjunct, well-preserved, autochthonous, flat-lying shells on the sedimentary bed surface with no clear orientation.

Table 1 .
Results of Rao's spacing test to determine if orientation data is not uniformly distributed; number of measured taxa and % recorded complete specimens