Alors que la présence de poissons dans le Tortonien de Gavdos était déjà connue depuis plus d'un siècle [34], leur découverte dans les diatomites messiniennes de cette île est beaucoup plus récente. Le Messinien préévaporitique de Gavdos, qui affleure au nord de l'île, au sommet de la coupe Metochia C (Fig. 1), est principalement constitué par une alternance de diatomites argileuses, de marnes diatomitiques et de diatomites à structure laminaire. Son épaisseur égale 14,5 m.

Compte tenu des résultats d'études antérieures [1,6,9,15,17,18], il est possible de préciser les points suivants : ●

le dépôt de la base de la séquence diatomitique est postérieur à la dernière occurrence de Globorotalia nicolae, un événement biostratigraphique qui est daté à 6,7 Ma ;

Whereas the presence of fossil fishes in the Tortonian of Gavdos was already known since more than one century [34], the occurrence of Messinian fossil fishes at Gavdos was observed rather recently in the pre-evaporitic diatomitic sequence outcropping at the top of Metochia C section (Fig. 1). There, a rhythmically layered succession of hemipelagic marls and sapropels is overlain in stratigraphic continuity by cyclically bedded diatomites of Messinian age.

The diatomitic sequence of Metochia, which has a thickness of 14.5 m, is characterized by alternations of clayey diatomites, white diatomites and diatomaceous laminated marls, locally associated with marly limestones. At the top of the section, grey limestones are interbedded with clayey diatomites and diatomaceous marls. Although Perez-Folgado et al. [24] used the term ‘sapropel’ instead of ‘diatomaceous laminated marls’, these layers cannot be considered as true sapropels, since they are really thin, faintly laminated reddish layers that mark the transition from clayey diatomites to genuine diatomites.

The age of the diatomites was first determined by Triantaphyllou et al. [35], who studied the calcareous nannofossils of the Metochia diatomitic sequence. They concluded that the contemporaneous presence of simple primitive ceratolithids (Amaurolithus delicatus, A. primus, A. amplificus) and Discoaster quinqueramus, D. pentaradiatus, and of the planktonic foraminifer Globorotalia conomiozea, together with the absence of Reticulofenestra rotaria, allows the correlation of the diatomites with the CN9b biozone [23] and with the upper part of NN11 [20].

The biostratigraphical events used for dating the diatomitic succession of the Metochia section were first reported by Hilgen and Krijgsman [15], and later refined and expanded by Drinia et al. [9].

The Tortonian–Messinian boundary was identified in the Metochia C section; it is marked by the first occurrence of Globorotalia miotumida group at cycle M73 dated at about 7.2 Ma [1] and [18]. The beginning of the deposition of the diatomitic series is dated, using the astronomically calibrated time scale for the Messinian, at 6.6 Ma. The first diatomitic layer was deposited after the last occurrence of Globorotalia nicolae, a biostratigraphical event which is dated at 6.7 Ma. A number of other first and second-order bioevents provide additional information: (1) the first abundant occurrence of Globigerina obesa (about 6.6 Ma), (2) the last occurrence of Globorotalia conomiozea (6.5 Ma), (3) the first occurrence of Turborotalita multiloba (6.4 Ma), (4) dextral to sinistral coiling change of Neogloboquadrina acostaensis (6.3 Ma), (5) influx of Globorotalia scitula (6.2 Ma), (6) dominance sinistral forms interval of Neogloboquadrina acostaensis, and (7) the last influx of sinistral N. acostaensis. All demonstrate that the diatomitic sequence where fish fauna is abundant ends at about 6.0 Ma. This chronostratigraphical framework relies on ages proposed for the bioevents identified by Blanc-Valleron et al. [6], Hilgen and Krijgsman [15] and Krijgsman et al. [17] in different Messinian sections of the Mediterranean.

The present study relies on about 75 more or less complete specimens of fossil fishes which were collected in the diatomites.

Family Clupeidae

Genus Alosa Linck, 1790

Alosa sp.

Five isolated scales, the surface of which is divided by parallel furrows and punctuated in its central region, indicate the presence of the genus Alosa Linck in the Messinian of Gavdos.

Family Sternoptychidae

Genus Maurolicus Cocco, 1838

Maurolicus muelleri (Gmelin, 1789)

Two isolated heads and four body fragments exhibit characters that are observed in better preserved representatives of Maurolicus muelleri (Gmelin). However, it is impossible to give of them any documented anatomical description.

Family Paralepididae

Genre Paralepis Cuvier, 1817

Paralepis albyi (Sauvage, 1880)

(Fig. 2, 1)

Four incomplete specimens (i.e. 5.3% of the collected material) demonstrate the occurrence of a Paralepidid in the Messinian of Gavdos. The largest one had a standard length of about 600 mm, as suggested by a slightly disarticulated large head, the length of which equals 160 mm. In this head, it is possible to see the typical long and narrow skull roof, the maximum width of which is about a quarter of its length.

Family Myctophidae

Genus Myctophum Rafinesque, 1810 (s. l.)

Myctophum licatae (Sauvage, 1870)

(Fig. 2, 2)

With 40 collected specimens (i.e. 53.3% of the fish material), this species is by far the most abundant one found in the Messinian diatomites of Gavdos. The standard length of these fishes ranges from 28 to 55 mm, with a maximum frequency between 35 and 40 mm. The body is rather slender, its maximum height being generally included 3.5 to 5.4 times in standard length. The vertebral column includes about 36 vertebrae, 19 or 20 being postabdominal. Twelve pterygiophores are present in the endoskeleton of the dorsal fin. The body was covered by rather thin scales which are generally poorly preserved, so that it is not possible to observe any light organ.

Myctophum dorsale (Sauvage, 1870)

With nine more or less complete specimens (i.e. 12% of the collected fish material), this species is the second most abundant found in the Messinian diatomites of Gavdos. This species differs from the former one by its thicker scales which may bear rather large light organs, especially along the ventral edge of the body.

Family Bregmacerotidae

Genus Bregmaceros Thompson, 1840

Bregmaceros albyi (Sauvage, 1880)

(Fig. 2, 3)

Whereas it is abundant in the Tortonian of Gavdos, this species is rather scarce in the Messinian diatomites, in which only seven skeletons were found (i.e. 9.3% of the fish material). Their standard length ranges from 28.5 to 46.5 mm, which is in good agreement with the sizes observed in the Tortonian of Gavdos.

Family Centriscidae

Genus Amphisile Cuvier, 1816

Amphisile sp.

(Fig. 2, 4)

A small isolated head bears witness of the occurrence, in the diatomitic Messinian of Gavdos island, of a Centriscid which was probably belonging to the genus Amphisile Cuvier.

Family Syngnathidae

Genus Syngnathus Linné, 1758

Syngnathus albyi Sauvage, 1870

Six isolated opercula and some dermal scutes demonstrate the occurrence of pipe fishes in the diatomitic Messinian of Gavdos. They are exactly similar to the corresponding remains found in the Tortonian and also in many Messinian localities of the Mediterranean.

Family Trichiuridae

Genus Lepidopus Gouan, 1770

Lepidopus sp.

A dentary and an incomplete premaxillary bearing a strong fang indicate the occurrence of the genus Lepidopus Gouan in the diatomitic Messinian of Gavdos Island.

The fish fauna from the Messinian diatomites of Gavdos is strongly dominated by the Myctophids which build up 65.3% of the collected material (among them, Myctophum (s.l.) licatae (Sauvage) reaches 53.3%). Another meso- to bathypelagic species, Paralepis albyi (Sauvage) is present (5.3% of the fish fauna). Bregmaceros albyi (Sauvage) (9.3% of the collected material) is a mesopelagic to epipelagic fish that belongs to a recent genus which is mainly distributed through the upper 300 m of the water column [2]. Additionally, Maurolicus muelleri (Gmelin) (8% of the collected material) is a species which is generally living at depths ranging from 100 to 400 m. All these fishes are found together with isolated opercula and dermal scutes of Syngnathids that are preserved in the same sediments. This demonstrates that the deposition of the diatomites cannot have taken place far from the sea-shore because the living representatives of the recent genus Syngnathus L. are mainly living in the sea grass meadows, from where the opercula and scutes were probably transported.

Although it exhibits a relatively low taxonomical diversity, as its species number is limited to about ten species, the fish fauna of the diatomitic Messinian of Gavdos Island exhibits a great similarity with that of several well-known Messinian localities of the pre-evaporitic Messinian of the Mediterranean, as the relative abundance of its main components does not significantly differ from that of specimens found in Sicily, at Licata [3], and Masseria il Salto, near Caltagirone [12], at Senigallia (Italy) [10]), and at Columbares (Spain) [11] (Table 1). All these localities are interpreted as generated at moderate depths, rather near the sea-shore, in areas having a direct connexion to a deep open sea in which upwelling phenomena were temporarily active.

It should be noted that some pteropods were also collected at Gavdos, together with the actinopterygian fishes, in the middle part of the diatomitic sequence: Cavolinia gypsorum (Bellardi), Cavolinia sp. et Diacria cf. trispinosa (de Blainville) [identification made by Irene Zorn, Wien]. The recent species of Cavolinia Abildgaard and Diacra Gray are oceanic molluscs living in surface waters under tropical to subtropical climates.

The examination of several samples collected in the middle to upper part of the diatomitic sequence (between 3 and 8 m under the calcareous bed overlying the diatomites) has shown that: ●

(1) the diatomitic flora includes at least 36 species of Centrics belonging to 13 genera and 19 species from 10 genera of Pennates;

Consequently, the Messinian diatomitic flora of Gavdos Island characterizes a marine coastal environment that was periodically influenced by upwelling phenomena, as suggested by the abundance of Thalassionema nitzschioides in two of the studied samples [4], [27] and [28], and that of a species which is indicative of rather cold waters (Actinocyclus curvatulus) in another one [5].

Although they are not abundant, benthic foraminifera are always present in the pre-evaporitic succession of the Metochia section. Wholly, 4536 benthic foraminifera specimens, belonging to 83 different benthic species, were identified from 35 samples of the pre-evaporitic sequence of the Metochia section.

The most representative species of the benthic foraminiferal fauna are: Bolivina plicatella, Bolivina spathulata group (including B. spathulata, B. dilatata, B. tortuosa), Bulimina aculeata group (including B. aculeata, B. elongata, B. lappa), Elphidium spp., Asterigerinata planorbis and Gyroidinoides neosoldanii. Additional and significant species are Anomalinoides sp., Cibicidoides spp., Melonis sp., Uvigerina spp. and Valvulineria bradyana.

Bolivinidae and Buliminidae dominate throughout the succession, strongly fluctuating, respectively in the ranges of 0.5–79% and 1–98%. Among the Bolivinidae, B. plicatella and B. spathulata groups are the most common. Bolivina plicatella is relatively abundant in the basal part of the section, displaying peak occurrences at 0.08 m and 3.9 m (76.5% and 64.6% respectively). Bulimina aculeata group shows a fluctuating pattern throughout the succession, reaching its highest frequency at 1.4 m (98%). Gyroidinoides neosoldanii is only present in the lower part of the record, displaying its highest occurrence at 0.7 m (68%). Asterigerinata planorbis and Elphidium spp. occur from the base to the top of the section with their highest frequencies in the middle of the succession, at 7.25 and 6.94 m (78% and 46% respectively). Cibicidoides kullenbergii, which is the main representative of the Cibicidoides spp. group, is abundant in the middle part of the succession, from 5.1 to 6.94 m, and then shows a fluctuating but diminishing abundance pattern up to the top of the section. Among the other species, Melonis sp., Uvigerina spp., Anomalinoides sp. and Valvulineria bradyana are present along the succession with fluctuating patterns and comparatively low-frequency values.

The statistical analysis of the benthic foraminiferal fauna [9] reveals two distinctly different assemblages dominating throughout the succession: a high diversity assemblage (A. planorbis assemblage including Asterigerinata planorbis, Elphidium spp., H. boueana, Cibicidoides spp., Melonis sp. and Valvulineria bradyana) indicating an oligotrophic, stress-free marine environment, and a low-diversity assemblage (B. aculeata assemblage including only species belonging to the B. aculeata group), which can be explained by the combined effect of increased food availability, oxygen depletion and unstable bottom conditions [8]. In addition, a third assemblage (B. plicatella assemblage including B. plicatella with G. neosoldanii) is recorded in the lower part of the succession, indicating a temporal increase in salinity [16] and [29], whereas the B. spathulata group assemblage (B. spathulata group, Uvigerina spp. and Anomalinoides sp.), dominating mainly in the middle part of the section, illustrates a low-oxygen environment with high productivity related to local upwelling episodes [7], [13], [25] and [31].

For estimating the depositional depth of the studied sediments, we used published data on the recent bathymetric distribution of the most important species. The benthic taxa identified in the section with significant frequency percentages are commonly distributed in the modern Mediterranean areas in depth range of 100–600 m [21], [22] and [32]). The studied succession is mainly dominated by the Bulimina aculeata group, indicating a deposition at outer shelf to middle bathyal (100–700 m [26], [30] and [33]), and Bolivina spathulata group which has been reported to be widespread in moderately sheltered to exposed, fully marine, inner to outer shelf depths (e.g. Hayward et al. [14]).

High percentages of transported benthic foraminifera mainly of the genus Elphidium, normally indicate shallower water depths [19], [21] and [22]. These tests are probably related to sea-level stillstand conditions allowing the development of a coastal environment.