The development of the mollusc fauna in the Cenomanian of the stratigraphie sequence of Visogliano (Karst of Trieste, Italy)

The stratigraphie sequence of Visogliano (Karst of Trieste) belongs to lower part of the Zolla Member of the still informal and provisional "Trieste Karst Limestone Formation" and has been attributed to Middle-Late Cenomanian. This sequence records an environmental evolution of more or less restricted settings, where a rich mollusc fauna is present. Among the molluscs, six radiolitid species occur {Radiolites carsicus n. sp., Radiolites peroni, Praeradiolites acutilamellosus n. sp., Praeradiolites cf. P. fleuriausus, Sauvagesia sharpei, and Durania acuticostata n. sp.). Of these, three species are described as new (Radiolites carsicus n. sp., Praeradiolites acutilamellosus n. sp., and Durania acuticostata n. sp.). In addition, attention has been focussed on the link between radiolitids and environmental changes to refine the previously described environmental evolution. In this regard, a hierarchy of tolerability to environmental changes can be also suggested. The most tolerant radiolitids species are Praeradiolites cf. P. ßeuriausus and Sauvagesia sharpei, meanwhile the least tolerant ones are Radiolites carsicus n. sp., Radiolites peroni, Praeradiolites acutilamellosus n. sp., and Durania acuticostata.


Introduction
In agreement with Cucchi et al. (1987), the lithotypes outcropping in the Karst of Trieste pertain to the stiH informal and provisional Trieste Karst Limestones Formation w^hich ranges from early Cretaceous to early Eocene. Within this formation, the low^er part of the Zolla Member, w^hich has been up to now attributed to Cenomanian-early Turonian, corresponds to the begirming of a clear development of the mollusc fauna and, particularly, of the radiolitids. Thus, the paleontological study concerning this member can be considered very useful to increase the knowledge of the stratigraphy and palaeoenvironmental evolution of the Karst of the Trieste.
This research focuses on the outcrops of Visogliano, near Sistiana (western Trieste Karst, F°40 111 NE of the IGM; Fig. 1), where the geological survey highlights a thick sequence with very fossiliferous beds wdthin the lower part of the ZoUa Member. The fossils here found mainly consist of bivalves and, particularly, of radiolitids.
Purposes of this work are 1) the chronological and palaeoenvironmental study of the stratigraphie sequence of Visogliano, and 2) the identification of the mollusc fauna and, particularly, of the radiolitids here found in order to refine the interpretation of the environmental evolution. Cucchi et al. (1987) include the ZoUa Member between the underlying dolomitic Rupingrande Member (Albian p.p.-Cenomanian p.p.) and the overlying Borgo Grotta Gigante Member (early Turonian p.p.-Senonian p.p.). They subdivide it into two parts: the lower part, which is mainly characterized by blackish limestones, sometimes with a very rich radiolitid fauna and, at the basis, interbedded dolostones; the upper part, which is characterized by grey limestones, with rudist fragments and Pithonella specimens. The fossils of the lower part consist of radiolitids, such as Sauvagesia nicaisei and Sauvagesia sharpei, and microfossils, such as Nubeculariidae, Miliolidae, and ostracods. Because of these radiolitids, and of the lack of the microfossils, which are significant from the chronological point of view, these authors attribute the lower part of this member to an undefined time-span from Cenomanian to early Turonian. Besides, they attribute the upper part of this member to early Turonian for the finding of the Pithonella specimens, in agreement v^ith Polšak (1979).

Historical setting
Other papers concerning the outcrops of the Zolla Member are due to Cucchi et al. (1986) andC o lizza et al. (1987), who studied the stratigraphy of localities of the Karst of Trieste, such as Zolla-Monrupino and San Pelagio, respectively. These papers confirmed the stratigraphie data already presented in the above cited studies. where all the studied mollusc species were represented. Visogliano SI. L Situacijska karta; raziskano območje je omejeno s pravokotnikom. Zvezdica kaže lego šestega intervala stratigrafskega zaporedja pri Vižovljah, iz katerega so vse predstavljene vrste moluskov. Vižovlje

The stratigraphie sequence
In the Visogliano area the lithotypes of the Zolla Member shovv^ a monoclinal asset with WNW-ESE main axial direction and southward main dip of 22°. In detail, on the basis of the mollusc fauna and the microfacies analyses, the stratigraphie sequence here recognized can be subdivided into nine intervals (Fig. 2). For each interval, data regarding lithology, palaeontology, microfacies, thickness, and palaeoenvironment are reported. The nine intervals are described below, from bottom to top.

Interval 1
This interval consists of foetid very fossiliferous black limestones and interbedded dark greyish dolostones, both with bedding of decimetric period.
In the limestones, the fossils mainly consist of radiolitids (Praeradiolites cf. P. ßeuriausus and Sauvagesia sharpei), which are present in lenses of isooriented displaced shells. These lenses are horded by isolated specimens, w^hich seem to be in situ. On the contrary, the dolostones are devoid of fossils, save for some rare gastropod ghosts.
This interval overlies the dolostones of the Rupingrande Member and is 71 m thick. From the palaeoenvironmental point of view, it testifies very restricted settings, sometimes characterized by local biostromes (lenses of displaced radiolitids) and the first isolated radiolitid specimens. The latter are probably able to colonize such restricted settings taking advantage of temporary permissive conditions for benthic life. These conditions degradate in correspondence with very restricted environment, as testified by the almost unfossiliferous dolostones.
Interval 2 This interval is characterized by dark-grey fossiliferous limestones, with bedding of decimetric period. The fossils consist of rare fragmented radiolitid specimens pertaining to Praeradiolites cf. P. ßeuriausus and Sauvagesia sharpei, together with Neitheaßeuriausiana.
The corresponding microfacies are represented by wackestones and packstones with bioclasts (radiolitids) and very common Nubeculariidae, Miliolidae, and subordinate algae {Thaumatoporella parvovesiculifera, Cayeuxia sp.), Chrysalidina gradata, Broeckina balcanica, Cuneolina sp., Nezzazata simplex, and ostracods. This interval is 12m thick. It indicates episodes of an increased bottom energy in a restricted environment, as demonstrated by the presence of very abundant radiolitid fragments.

Interval 3
This interval is characterized by interbeddings of grey dolostones and blackish fossiliferous limestones, with bedding of decimetric period. The dolostones are devoid of fossils, meanwhile the limestones include fossils such as Chondrodontajoannae, Neitheaßeuriausiana, and scattered fragments of Praeradiolites acutilamellosus n. sp. and Radiolites carsicus n. sp.
The corresponding microfacies are respectively represented by euehedral/subeuehedral dolomitic mosaic, wàth gastropod ghosts, and bioclastic mudstones or wackestones with rare intraclasts and pellets; the microfossils are represented by very common Nubeculariidae  This interval, vi^hich is 65m thick, indicates very restricted settings (dolostones) with a moderately increased bottom energy (bioclastic limestones), where lamellar shells of Chondrodonta joannae colonize the substrate, also in correspondence with a low rate of sedimentation.

Interval 4
This interval is constituted by grey, sometimes dolomitic, limestones, with a fossiliferous level (in the basal part), which is from 1.80 to 3m thick and characterized by shells of Chondrodonta joannae and rare fragments of radioHtids.
This interval is 18m thick. In the lower part, it records the colonization of the substrate due to the specimens of Chondrodonta joannae. Because of the lamellar morphology of the shell of Chondrodonta joannae, this colonization probably occurs in correspondence v^dth episodes of moderate-high bottom energy and low sedimentation rate. In the mid-upper part of the interval, the mollusc fauna is substituted by a large number of microorganisms, probably in relation to environmental conditions shovvdng a decreased bottom energy and an increasing sedimentation rate.

Interval 5
This interval is characterized by brovm dolomitic limestones with not evident bedding. The corresponding microfacies are characterized by very abundant euhedral dolomitic crystals included in a carbonatic mud, wdth rare Miliolidae and fragments of Chrysalidina gradata.
The thickness of this interval is 5m. It testifies very restricted settings, which become adverse for the benthic life.

Interval 6
This interval consists of grey-brovm very fossiliferous limestones with bedding of decimetric period. The fossils are represented by requienids, and very common specimens of thickshelled radiolitids (Radiolites carsicus n. sp., Radiolites peroni, Praeradiolites acutilamellosus n. sp., Praeradiolites cf. P. ßeuriausus, Sauvagesia sharpei, and Durania acuticostata n. sp.), very and Chondrodonta joannae; in addition, rare specimens of Neithea ßeuriausiana are also present. Among the molluscs, the radiolitids are often present in clusters consisting of from 2 to 10 specimens in situ.
This interval is 8.5m thick. It records restricted settings characterized by favourable environmental conditions for the benthic life, for both the growth of the build-ups and the increased specific diversity of the microfossils. This episode can correspond to a dicreased bottom energy and an increased sedimentation rate.

Interval 7
This interval is characterized by grey limestones vvith bedding of decimetric period. The fossils are rare and consist of fragments of radiolitids, such as Radiolites carsicus n. sp.
This interval is 6m thick. It shows a decreased specific diversity for both the radiolitids and the microfossils. The radiolitids are represented by rare small specimens and, over all, by fragments. The latter also testify episodes of high bottom energy in a restricted setting.
Interval 8 This interval consists of from light grey to grey-brovm dolomitic limestones, wdth bedding of decimetric period.
The corresponding microfacies are characterized by very abundant euhedral dolomitic crystals included in a carbonatic mud, with rare Miliolidae.
This interval is 6m thick and indicates very restricted settings, which are adverse for the benthic life.

Interval 9
This interval is characterized by from dark-grey to blackish foetid limestones, with rare fragments of radiolitids and bedding of centimetric period.
This interval tops the sequence studied and it has been analysed in the first meters only. It indicates restricted settings affected by episodes of increased bottom energy, probably linked to tractive currents, as demonstrated by the plane-parallel laminations.
Chondrodonta joannae, Neithea ßeuriausiana, and requienids have been already signalled in the Karst of Trieste by several Authors in beds which are Cenomanian and/or Turonian in age, as reported by Forti and Mas oli (1969). Recently, Chondrodonta joannae and Neithea ßeuriausiana are reported as Chondrodonta and Neithea, respectively, and occur in the limestones of Borgo Grotta Gigante Member (Cucchi et al., 1987), meanwhile requienids have been signalled by M as o li and Ulcigrai (1969) and Cucchi et al. (1987) in black limestones of Aptian-Albian age.
Among the radiolitids, Praeradiolites acutilamellosus, Radiolites carsicits, and Durania acuticostata are considered new. Thus, these species have been signalled for the first time in the Karst of Trieste, meanwhile Radiolites peroni and Sauvagesia sharpei had already been signalled in the Turonian beds by Par ona (1932) and in the Zolla Member by Cucchi et al. (1987), respectively. In addition, Praeradiolites cf. P. ßeuriausus and Radiolites carsicus are also present in the Cenomanian of the Archi locality (Karst of Gorizia) by Caffau and Pleničar (1991).
All the radiolitid species are described in the following sections, where data concerning their morphology, stratigraphie distribution, and geographical diffusion are reported. Basic source for the stratigraphie and geographic data is Sanchéz' catalogue (1981 fig. 1 Derivation of name: The specific name acutilamellosus is due to large and strongly dissociated lamellae. Type-series: Holotype, MCV 104 (PL 1, fig. 1). Paratype, MCV 105 (PI. 1, fig. 2), deposited in the rudist collection of the Institute of Geology and Paleontology, University of Trieste.
Examined material: Lower valves of 15 specimens from MCV 104 to MCV 118, deposited in the rudist collection of the Institute of Geology and Paleontology, University of Trieste.
Diagnosis: The right valve shows strong external lamellae turned towards the commissure and proyecting outwards in correspondence of the cardinal zone (PI. 2, fig. 2). A considerable variability characterizes the siphonal ridges which are protruding, flat (PI. 1, figs. 1, 2) or rounded (PI. 1, fig. 4; PI. 2, fig. 1). The ridge "E" is wider than ridge "S". The interband is slightly concave; in its middle part it shows a weak rib, which does not appear in all the specimens. Where present, the rib is differently pronounced, mostly in the lower part of the valve. (PI. 1, fig. 2; PI. 2, fig. 2). The ligament ridge is thin. The valve layer is considerably broader in the cardinal zone than in the siphonal one.
Description: The valves are from 25 to 90mm long, with a commissural diameter of 30-60mm. The structure of the layer is characterized by parallel rows of rectangular prisms along the entire valve periphery, save for the siphonal zone where the rows of prisms bend and form a syncline-like arrangement (PI. 7, fig. 1). External lamellae are both almost horizontal in correspondence of the cardinal side, and erected in the siphonal one.
Description: The lower valves are cylindrical-conical, from 40 to 70mm long, with commissural diamater of 30-39mm. The lower valve has strong and protruding lamellae. In the lower part of the valve, in correspondence with the foot fold "V", the lamellae are pronouncedly inverted (PI. 4, fig. 4). The siphonal zone is not entirely preserved in each valve examined. This part is damaged in all the specimens, and as a consequence, the species could not be exactly determined. Because of this, a dubitative taxonomic attribution is to be preferred. The celluloprismatic structure of the layer is very evident in eros section (PI. 7, fig. 2). The layer is 1-2 mm thick in the siphonal side, and 3-4mm in the cardinal one.
Stratigraphie distribution and geographic diffusion: Even if this species is dubitatively determined, we recall that Praeradiolites ßeuriausus (d'Orbigny) is characteristic of Cenomanian. It may be found in Italy, Istria (Croatia), France and Romania.  fig. 2), deposited in the rudist collection of the Institute of Geology and Paleontology, University of Trieste.
Examined material: Lower valves of 20 specimens; from MCV 126 to MCV 145, deposited in the rudist collection of the Institute of Geology and Paleontology, University of Trieste.
Type-locality: Visogliano (Vižovlje) near Sistiana (Sesljan). Diagnosis: The lower valve is from 41 to 110mm long, abruptly narrowing towards the basis, ornamented with fine longitudinal ribs. The external lamellae are numerous and well expressed. They also pass through the siphonal region. The latter consists of two protruding siphonal ridges. In some specimens, the ridge "E" is slightly stronger than ridge "S". The intersiphonal area is characterized by two different types of ornamentation. The most frequent ornamentation is constituted by three longitudinal ribs, which run along the shell, or by V-shaped protruding lamellae.
Description: Twenty lower valves have been studied. They are 40 to 112mm long, with a commissural diameter of 25-50mm. The shell is traversed by rounded and well expressed ribs interrupted by weU developed neighbouring megacycles. The "E" siphonal band is rounded, protruding and similar to the "S" band. The interband consists of both three ribs, very similar to the others of the shell (PL 3, fig. 2) and by "V"-shaped lamellae (PL 4, figs. 1, 2). In a transversal section of the lower valve the siphonal zone shows a structure of pseudopillars. The "E" siphonal zone shows two small different-sized lobes, meanwhile the "S" zone presents only one prominent lobe (PL 8,figs. 1,2). In cross section the cellular structure of the shell is evident (PL 8, figs. 1, 2, 3) The ligament ridge is triangular.
Similarities and differences: Radiolites carsicus n. sp. differs ivom Radiolites lusitanicus (Bayle) and Radiolites trigeri (Coquand) because of the siphonal area morphology. "E-S" bands are very protruding and rounded in Radiolites carsicus n. sp., while in the other mentioned species these bands are slightly pronounced and flattened.
Examined material: Lov^^er valves of 3 specimens, from MCV29 to MCV 31, deposited in the rudist collection of the Geology and Paleontology Institute of Trieste university.
Description: The low^er valves examined are from 30 to 45nun long with a conical shape. The outer part of the shell consists of a series of protruding and neighbouring megacycles. The lamellae are traversed by wdde longitudinal ribs which determine an undulating outline. The "E-S" bands are flattened, slightly protruding, and have similar sizes. The interband is concave and traversed by a pleat which vanishes towards the basis of the shell. The ligament ridge is small and triangular. The inner part of the layer shows a poligonal structure, which is sometimes seen as a quadrangular network. This effect should be due to oblique sections as reported byAlencaster and Pons (1992).
Examined material: Lower valves of 6 specimens, from MCV 20 to MCV 25, deposited in the rudist collection of the Institute of Geology and Paleontology, University of Trieste.
Description: The lower valves are cylindrical-conical and slightly curved, crossed by thin longitudinal ribs. The valves are from 45 to 100mm long wdth a commissural diameter of 25-50mm. The "E" siphonal band is three times wdder than the "S" one. Along the "E" band there are 17 thin ribs, and other 7 can be seen in the "8" band. The interband is concave. The triangular ligament ridge is weUpronounced. The shell structure is prismatic, consisting of hexagonal prisms (PL 9, fig. 1).
Stratigraphie distribution and geographic diiixxsion-. Sauvagesia sharpei (Bayle) is characteristic of late Cenomanian-early Turonian. It may be found in Italy, Croatia, Portugal, Spain, France and Algeria.
Examined material: Lower valves of 15 specimens from MCVI to MCV 15, deposited in the rudist collection of the Institute of Geology and Paleontology, University of Trieste.
Type-locality: Visogliano (Vižovlje) near Sistiana (Sesljan). Diagnosis: The low^er valve is conical-cylindrical and characterized by a commissural diameter which is 0.5-1 times longer than the shell. 13-16 triangular and well pronounced costae run longitudinally along the thick shell. The "E" siphonal band area is flattened and traversed by 9-11 thin ribs while the "S" area shows 5-7 ones. A concave interband, as wide as the "S" band, is traversed by a protruding thin rib.
Description: These specimens are characterized by graceless lower valves, v^th a development in length once or twice the commissural diameter. In the correspondence with the commissure, the thickness of the layer varies from 6 to 10mm, in the dorsal zone, and from 1 to 2.5mm, in the ventral one (PI. 9, fig. 2). The "E" and "S" siphonal band areas are flattened, slightly protruding, and located in the thinner part of the shell (ventral zone).
The "E" band is three times wdder than the "S" band. Both are separated by a concave 1.8mm deep and 4mm vvide interband, traversed by a thin 0.4mm wide and 0.6mm high rib (PI. 6, figs. 1-2). Thin sections of lower valves show the celluloprismatic structure of the shell. In addition, a great difference in thickness between the dorsal and ventral areas is evident. The former is thicker than the latter. The shell ornamentation consists of triangular and well pronounced ribs, wáth wide basis and acute apex. The distance between the ribs corresponds to the width of each rib.
Similarities and differences: Durania acuticostata n. sp. presents significant differences wdth respect to all the species of this genus so far described in literature, mainly for the morphology of siphonal area (presence of a concave interband) and the shell ornamentation (strong and longitudinal costae).

Discussion
Save for the interval 1, which should be generically attributed to middle-late Cenomanian for the presence of Chrysalidina gradata (PL 10, fig. 1), in agreement vdth De Castro (in Schroeder & Neumann, 1985), the Visogliano sequence can be entirely attributed to late Cenomanian p.p. This chronological attribution is due to Broeckina balcanica (PL 10, figs. 2, and 3), which is present in all the Visogliano sequence, and over all to Chondrodonta joannae and Neithea fleuriausiana. Really, the foraminiferal species is considered as middle-late Cenomanian in age by Cherchi and Schroeder (in Schroeder & Neumann, 1985), meanwhile the molluscs are considered as late Cenomanian in age by Dhondt and Die ni (1993). In agreement with the above described data, it is possible to refine the previous chronological attribution of the majority of the lower part of the ZoUa Member by dating back it from Cenomanian-Turonian to late Cenomanian p.p. Thus, the previously described environmental evolution mainly occurs during the late Cenomanian p.p. Within a frame of an evolution of restricted settings, it is possible to record a very important biological event: the development of the mollusc fauna and, particularly, of the radiolitids. Really, after a first very brief appearance of requienids within the Aptian-Albian limestones of the Monte Coste Member, the moUuscs found in the Visogliano sequence are able to colonize the substrates.
In the Visogliano sequence, the development of the mollusc fauna begins with the occurrence of two radiolitid species, such as Praeradiolites cf. P. ßeuriausus and Sauvagesia sharpei, in the interval 1. This colonization is interrupted several times, as recorded in the dolostones of the interval 1, the bioclastic high bottom energy limestones of the interval 2, and in the dolostones and bioclastic limestones of the interval 3. Thus, the above cited radiolitid species are able to colonize the substrate in the scattered way, only in correspon-dence with the favourable environmental conditions, probably linked to restricted rather than very restricted settings. In the intervals 3 and 4, the specimens of Chondrodonta joannae begin colonizing the substrate with their lamellar shells. The succession of several generations of shells of Chondrodonta joannae is recorded in the thick build-up of the interval 4 and testifies their builders' activity. This action is again interrupted in the interval 5, in correspondence with the dolomitic limestones of very restricted environment. In the interval 6, several radiolitid species (Radiolites carsicus n. sp., Radiolites peroni, Praeradiolites acutilamellosus n. sp., Praeradiolites cf P. ßeuriausus, Sauvagesia sharpei, oxiá Durania acuticostata n. sp.), Chondrodonta joannae, and rare specimens of Neithea ßeuriausiana occur probably in relation to a change from very restricted to restricted environment. Such molluscs take advantage of this situation and begin again colonizing the substrate with specimens of Chondrodonta joannae and radiolitid elevators (PI. 11, fig. 1; sensu Skelton, 1978Skelton, ,1985. As soon as these forms have prepared a sufficiently coherent substrate, successive radiolitids, i.e. the encrusters (PI. 11, fig. 2; sensu Skelton, 1978Skelton, , 1985, appear and settle on the previous shells.
Among the radiolitids, the elevators are more frequent with respect to the encrusters. The elevators are able to aggregate in monospecific clusters mainly consisting oi Radiolites carsicus n. sp. or Praeradiolites acutilamellosus n. sp. These clusters consist of up to 10 specimens, which present thick, well ornamented, cylindric-conical, straight, more or less equal in size shells, with neighbouring megacycles. This shell morphology corresponds to an increased sedimentation and a decreased bottom energy. Other elevators present an individual development; they are characterized by very flattened ornamentations, such the megacycles in Praeradiolites acutilamellosus n. sp., or by conical shapes with broad basis in correspondence with the commissure, as present in Sauvagesia sharpei. These ornamentations and shapes represent adaptive strategies to avoid the burial. The encrusters always show an individual development and are represented by small specimens from 29 to 40 mm in size.
The following intervals show a probably local decline of the radiolitid fauna, probably due to an increased bottom energy (limestones of the intervals 7, and 9), with an interbedded period of very restricted conditions (dolomitic limestones of the interval 8).
In conclusion, the molluscs found in the Visogliano sequence are represented by a number of species, which is the highest of all the outcrops hitherto examined within the Zolla Member. Among them, three radiolitid species are described as new (Praeradiolites acutilamellosus, Radiolites carsicus, and Durania acuticostata.
It is also possible to underline the role of both Chondrodonta joannae and the elevators in preparing the substrate for the life of successive molluscs and for their builders' activity. In addition, among the radiolitids, it is also possible to consider Praeradiolites cf. P. ßeuriausus and Sauvagesia sharpei, which are able to tolerate a continuous shifting from restricted to very restricted environment (interval 1), as pionier species in colonizing the environment. Thus, they can be considered as opportunistic species. Other radiolitids species (Radiolites carsicus n. sp., Radiolites peroni, Praeradiolites acutilamellosus n. sp., and Durania acuticostata n. sp.) appear successively and probably indicate stabler environmental conditions with respect to the previous ones. With respect to the tolerance of environmental changes, this can suggest a possible hierarchy of radiolitids from the most tolerant (Praeradiolites cf. P. ßeuriausus and Sauvagesia sharpei) to the least tolerant (Radiolites carsicus n. Radiolites peroni, Praeradiolites acutilamellosus n. sp., and Durania acuticostata). This hierarchy wiU have to be tested and, if verified, enlarged in further studies on rudist communities of the Karst area.