At the crossroads: early Miocene marine fishes of the proto-Mediterranean Sea

Connectivity and climate control fish distribution today as well as in the geological past. We present here the Aquitanian (early Miocene) marine fish of the Mesohellenic Basin, a restricted basin at the border between the protoMediterranean and Paratethyan seas. Based on fish otoliths, we were able to identify 19 species from 17 genera, including two new species: Ariosoma mesohellenica and Gnathophis elongatus. This fish assemblage, in conjunction with the accompanying molluscan assemblage, indicates a variable shelf paleoenvironment with easy access to the open ocean. Although available data on the Indo-Pacific fishes of the early Miocene are very limited, the fish fauna of the Mesohellenic Basin has many elements in common with the North Sea, the NE Atlantic, and the Paratethys.


Introduction
Connectivity between marine realms largely controls the fate of fish populations and biodiversity (Worm and Tittensor, 2018).Early Miocene proto-Mediterranean biodiversity is particularly interesting because, at that time, the basin's connection with the Indo-Pacific realm ceased to exist (Rögl, 1998;Bialik et al., 2019;Torfstein and Steinberg, 2020;Sun et al., 2021), while the marine biodiversity hotspot was shifting toward the southeast (Renema et al., 2008) and the climate was cooling (Cornacchia et al., 2018), marking the onset of a subtropical gradually oligotrophic regime in the proto-Mediterranean Sea.However, the early Miocene fossil fish record from the Mediterranean area is particularly sparse.Moreover, very few studies have focused on the fish otolith record (Steurbaut, 1979(Steurbaut, , 1982(Steurbaut, , 1984;;Reichen-bacher and Cappetta, 1999;Reichenbacher, 2004;Hoedemakers and Batllori, 2005), despite its importance for revealing the evolution of fish faunas and fish biogeography (Agiadi et al., 2011(Agiadi et al., , 2017(Agiadi et al., , 2018)).At the crossroads between the proto-Mediterranean Sea, the Atlantic Ocean, the North Sea, the Paratethys, and the Indo-Pacific realm, the Mesohellenic Basin (MHB) during the early Miocene, a molassic basin at the northern part of the proto-Mediterranean, directly at the intersection with the Paratethys epicontinental sea, offers a unique opportunity to explore the effects of interbasinal connectivity on fish faunas.
In this study, we identify the fossil fish otoliths found in the Aquitanian (lower Miocene) sediments of the Felli section (Grevena Prefecture, NW Greece) that were deposited in the MHB.The reconstructed assemblage is the first record of fishes from the eastern part of the Aquitanian (∼ 23-20 Ma) proto-Mediterranean region.Furthermore, we discuss the paleobiogeographic range of the identified fish species in order to draw a conclusion regarding the factors that may have determined the subsequent evolution of the Mediterranean marine fish fauna.

Geological setting
The study area is located in Grevena Prefecture, NW Greece, where the molassic sedimentary sequence of the MHB is exposed (Fig. 1; Zelilidis et al., 2002;Ferrière et al., 2004).The MHB was a synorogenic molasse-type basin (Kilias et al., 2015), a separate paleogeographic area from the late Eocene until the middle Miocene, between the internal and external Hellenides, ENE and WSW, respectively.It was a narrow, 300 km long trench, filled with up to 4.5 km of sedi-Published by Copernicus Publications on behalf of the Museum für Naturkunde Berlin.ments, which have been separated into the Krania, Eptachori, Pentalofos, Tsotyli, and Ondria formations (Brunn, 1956;Wielandt-Schuster et al., 2004;Ferrière et al., 2013;Kilias et al., 2015), deposited parallel to each other (except for the Krania Formation) along the eastern margin of the trough (Kilias et al., 2015).The studied sediments belong to the Pentalofos Formation, consisting in general of upper Chattianlower Miocene sandstones to silty marlstones (Mavridis et al., 1985;Ferrière et al., 2004;Wielandt-Schuster et al., 2004;Kilias et al., 2015).The Felli section, which is our study section, includes only the uppermost part of the Pentalofos Formation corresponding to the Aquitanian according to Mavridis et al. (1985).The Pentalofos Formation overlies the Eptachori Formation and is overlain, conformably in the Grevena area, by the Tsotyli Formation (Fermeli and Ioakim, 1992).

Material and methods
We sampled a small section by the Aliakmon River banks, located east of Felli village, SE of Grevena City (40 • 01 4.55 N, 21 • 33 34.37 E).The section is mainly composed of conglomerates and sands, and the grain size decreases toward the top, where we find marls and then clays.We obtained ∼ 20-25 kg of sediment from each of two marlclay beds, F11 and F12, following the numbering scheme of Thivaiou et al. (2019).The samples were water-sieved using a 250 µm sieve, and the residues were dried in an oven.The otoliths were handpicked from the residues and identified under the microscope.We adopted specific descriptive characters for the otoliths of Congridae (Schwarzhans, 2019) and Pleuronectiformes (Schwarzhans, 1999).We followed the fish classification scheme of Nelson et al. (2016).We made the following measurements on the identified otoliths (reported in mm; Fig. 2): OL, otolith length; OH, otolith height; AL, antirostrum length; RL, rostrum length; CL, colliculum length; OCL, ostial colliculum length; CCL, caudal colliculum length; OSL, ostium length; CaL, caudal length.In addition, in Congridae, it proved significant to measure the sulcus angle (α), as the angle formed between the sulcus direction and the otolith length direction (Fig. 2).All data can be found in the Supplement.

Systematic paleontology
In this section, we describe and explain the species identification of the fossil otoliths from the Felli section.The measurements for all identified otoliths are available in the Supplement.The studied material has been deposited at the Athens Museum of Palaeontology and Geology -Vertebrates Collection (AMPG(V)).

Etymology
After the MHB, where the type section sediments were deposited.

Description
Elongated, thick spindle-shaped otoliths with an elongated central sulcus that opens toward the dorsal rim through an intricate ostial channel.The ostial channel is placed almost at the proximal edge of the sulcus and is perpendicular to it.The dorsal depression is well expressed.The dorsal rim is curved in the distal part.

Distribution
Aquitanian of the western proto-Mediterranean and the MHB.

Remarks
These otoliths have a more elongated sulcus than Ariosoma balearicum otoliths, which almost reaches the posterior edge (Fig. 3E-F; Agiadi and Albano, 2020).Moreover, the dorsal extension is located more frontally, as it is in Ariosoma  4) Tsotyli Formation (middle Miocene), and (5) Quaternary.(C) Lithostratigraphic column of the Felli section with the sampled levels (modified after Zelilidis et al., 2002, andThivaiou et al., 2019).moravica (Sulc 1932) (Nolf, 2013).The otoliths assigned to A. balearicum from the lower-middle Miocene of Catalonia by Hoedemakers and Batllori (2005) also show a frontal positioning of the dorsal extension.In contrast through to A. moravica, the sulcus is more elongated, indicating that these specimens also should be placed in Ariosoma mesohellenica sp.nov.Furthermore, the specimens assigned to Ariosoma aff.balearicum by Steurbaut (1984) should not be placed within this species because their sulcus is much shorter and does not extend close to the distal rim.Similarly, the otoliths

Etymology
Due to the distinct elongated shape of its otoliths.

Diagnosis
Elongated otoliths with OL/OH of 1.60-2.06.The angle of the sulcus relative to the otolith length plane is 10-20 • .

Description
These otoliths are elongated with a long sulcus that forms an angle to the otolith length direction, which opens posterodor-sally through a wavy ostial channel.The posterior edge is slightly protruding dorsally.The ventral rim forms an angle.

Distribution
Aquitanian of the proto-Mediterranean Basin.

Remarks
The otoliths of Gnathophis elongatus are elongated and more similar to Conger, unlike Pliocene-Holocene specimens of Gnathophis mystax that are more quadrate (Tuset et al., 2008;Agiadi et al., 2020Agiadi et al., , 2019Agiadi et al., , 2018)).However, the orientation of the sulcus forms an angle with the otolith length direction, as in Gnathophis species.The otolith figured as Gnathophis catalinensis (Wade) by Steurbaut (1984), now accepted as a synonym of Gnathophis cinctus (Garman) (Grove and Lavenberg, 1997), also displays the same elongate shape, distinct sulcus-to-otolith length angle as those described here.However, the sulcus is much narrower in G. elongatus, making it distinguishable from G. cinctus.Compared to G. saubriguensis (Steurbaut), G. elongatus has a much more elongated shape, the posterodorsal edge is protruding, and the distal-ventral angle is not pointed.

Material
Fifteen specimens from level 11 and three specimens from level 12.

Remarks
The orientation of the sulcus relative to the posterior-anterior axis is angular, with an 11-24 • angle.The anterior edge shows the same opening feature as the specimens from the late Burdigalian of Piedmont, Italy (Nolf and Brzobohaty, 2004).The distal-ventral angle is pointed, as in the specimens figured from the late Oligocene-lower Miocene of France (Steurbaut, 1984).Vinciguerria sp.

Material
Two otoliths from level 11 and one otolith from level 12.

Remarks
These otoliths resemble Vinciguerria lucetia (Garman) in the shape and angle of the ventral rim.However, they are too eroded for confident specific identification.

Material
Five otoliths from level 11.

Remarks
The state of preservation of these otoliths does not permit their identification to species level.

Fig. 4A
Material One otolith from level 11.

Remarks
Compared to Lobianchia dofleinoides Steurbaut, the otolith found here shows neither an anterodorsal curvature of the rim nor a strong dorsal depression.Moreover, the ostium is separated from the cauda with a clearly angular boundary, not one that is perpendicular to the posterodistal direction, as is the case for L. dofleinoides.Nevertheless, the sulcus, especially the ostium part, is quite wide, resembling more L. dofleinoides than L. dofleini (Zugmayer).Thus, it was not possible to assign this specimen to either species, despite similarities, and we therefore left it in open nomenclature.

Material
One otolith from level 11 and one otolith from level 12.

Remarks
Myripristis verus otoliths are not as high as those of Myripristis planodorsalis Schwarzhans, 1994.Compared to the "genus Myripristinarum" banaticus Weiler (Steurbaut, 1979) and the "genus Myripristinarum" sp.figured by Steurbaut (1984), the inner face of M. verus is more triangular, and its sulcus is more strongly bending ventrally.

Material
Two otoliths from level 11.

Remarks
These otoliths are elongate with a higher anterior than posterior part and a prominent anterodorsal high.The sulcus is divided into a large, oval ostium that almost reaches the anterior rim, and a smaller, also oval-shaped cauda that is situated slightly dorsally.The specimens examined here resemble very much those figured by Steurbaut (1984)

Material
One otolith from level 11.

Remarks
This specimen is significantly shorter than those assigned to Apogon vigneauxi (Steurbaut, 1984), and it shows the same dorsal depression and relative size of the ostium to the cauda (OSL : CaL = 1.128) as A. moyesi from the Oligocene-Miocene of Aquitaine Basin (Steurbaut, 1982(Steurbaut, , 1984)).Compared to the modern species Apogon imberbis from the Pliocene-Holocene of the eastern Mediterranean Sea (Agiadi et al., 2019(Agiadi et al., , 2020;;Agiadi and Albano, 2020), this specimen is shorter (OL : OH = 1.33 compared to a mean of 1.42 in A. imberbis).

Fig. 4G
Material One otolith from level 11.

Remarks
This otolith is triangular in shape with a sulcus divided into an oval-shaped ostium and a thinner but approximately equal in length oval cauda that slightly turns towards the dorsal side.These characteristics make this specimen similar to "Blennida" bicipitis Steurbaut from the Rupelian of the Aquitaine Basin (Nolf, 2013).However, the specimen examined here has a square posterior rim.

Remarks
This small otolith is rectangular in shape, with rounded angles.The sulcus is sloping posteroventrally and approximately straight, divided into a long ostium and a rectangular, short cauda.In contrast to the specimen figured by Nolf and Brzobohaty (2004), the otolith found in Felli shows a more regular rectangular shape.Our specimen is quite similar to the modern species Arnoglossus rueppellii (Cocco), but its inner and outer face have the same convexity, whereas A. rueppellii has a flatter inner face (Agiadi and Albano, 2020).

Material
Two otoliths from level 11 and one otolith from level 12.

Distribution
Lower Miocene of the Aquitanian Basin (Steurbaut, 1984) and the MHB.

Remarks
These otoliths have a long sulcus, comprised of a small circular ostium and a long cauda, which turns toward the ventral area and opens in the posterior end.The dorsal rim is straight.Compared to the modern species Mullus surmuletus Linnaeus and Mullus barbatus Linnaeus (Hoedemakers and Batllori, 2005;Tuset et al., 2008;Agiadi et al., 2019), the otoliths of M. elognatus are not as high, but more elongated (OL : OH = 1.76-1.82 in M. elongatus but 1.23 in M. barbatus and 1.52 in M. surmulatus, both values from the Pleistocene of the eastern Mediterranean; Agiadi et al., 2019), more so than Mullus bifurcatus Strashimirov reported from the lower-middle Miocene of the Paratethys (Bratishko et al., 2015).

Remarks
These otoliths are boat-shaped and have open, divided sulcus.
The ostium is oval-rectangular and has an oval colliculum, whereas the cauda is oval-shaped, with an oval colliculum, and placed more dorsally than the ostium.There is a pronounced ventral fissure.These specimens have distinct posterior and posterodorsal angles that only appear on the otoliths of C. yrieuensis previously reported from the Rupelian of France (Steurbaut, 1984).Compared to C. macrophthalma, these otoliths have an oval, rather than circular cauda, which is placed dorsally.Cepola aff.macrophthalma (Linnaeus) from the middle Miocene of Austria (Nolf, 2013) also has an oval-shaped cauda but lacks a posterodorsal angle.On the contrary, this angle is present in C. robusta Nolf from the Ypresian of France (Nolf, 2013), but only in the young specimens.

Material
Six otoliths from level 11.

Remarks
These otoliths are oval-shaped, with a small circular-oval ostium and a long straight cauda.The ventral area is distinctly larger than the dorsal area, and its rim is rounded.

Material
One otolith from level 11 and one otolith from level 12.

Remarks
These otoliths are fusiform in shape with a well-curved ventral rim and a sulcus divided into a triangular ostium and a long straight cauda, which is curved at the posterior end.

Fig. 4O
Material One otolith from level 11.

Remarks
This otolith is characterized by a pronounced ventral angle.
The sulcus is divided into a triangular ostium and a long cauda, which curves at its posterior end.The dorsal area forms a clear angle in the anterior part.There is some resemblance to Pagrus sp. 1 figured by Steurbaut (1984).

Material
Nine otoliths from level 11 and seven otoliths from level 12.

Remarks
The general shape of the otoliths is fusiform.They have a straight sulcus and a triangular ostium, which is characteristic of all Spicara spp., but these specimens are particularly angular in shape and have a strongly delineated sulcus.The rostrum is more robust in these specimens than in the modern species Spicara flexuosa, S. smaris, and S. maena (Agiadi and Albano, 2020), and S. gossei from the Oligocene-lower Miocene of France (Steurbaut, 1984).In addition, the posterior end of the cauda is very pointed in the examined specimens.
5 Results and discussion

Aquitanian fish biogeography
The fish fauna of the Aquitanian MHB is quite diverse.We identified 19 species from 17 genera, including 2 new species: Ariosoma mesohellenica and Gnathophis elongatus.
Overall, the identified fish fauna comprises several species of congrid eels, flatfishes, and sparids, along with lanternfishes, reflecting a shelf environment with easy access to the open ocean.The great diversification of congrids especially reflects a variety of microhabitats along this shelf.
The geological events that led to the formation of the proto-Mediterranean have shaped the distribution of marine taxa (Hou and Li, 2018;Worm and Tittensor, 2018).In the area of the Western Tethys, the late Eocene-middle Miocene MHB (Fig. 5) constituted a marine corridor at the border between the proto-Mediterranean and the Paratethys (Rögl, 1998); the proto-Mediterranean Sea developed due to the closure of the western part of the Tethys Ocean (Bialik et al., 2019;Torfstein and Steinberg, 2020).At this time, the proto-Mediterranean Sea, including the MHB, transited from a global biodiversity hotspot to a subtropical marine sea (Renema et al., 2008).Therefore, any faunal exchanges between the proto-Mediterranean, the Paratethys, the North Sea, the Atlantic, and the Indo-Pacific realms would be expressed in this study area.
Possibly due to the warm Aquitanian climate (Zachos et al., 2001;Westerhold et al., 2020), the distribution ranges of North Sea and NE Atlantic fish species included the MHB in the south.Indeed, this is supported by the distribution of the identified fish species (Fig. 5).Most fish species in the MHB were also present in the NE Atlantic, and, in fact, Echiodon heinzelini was also present in the North Sea.Moreover, the flatfishes Arnoglossus holleri and Microchirus latior were also reported in the Paratethyan realm, which is understandable considering the close proximity between these areas.We do not find any exclusively Indo-Pacific taxa in our fossil assemblage.However, we must note that data on the Miocene fishes of the Indo-Pacific realm are scarce; data outside the Mediterranean and the Atlantic exist mostly from New Zealand (Grenfell, 1984;Schwarzhans et al., 2017).This knowledge gap hampers any further evaluation of the paleobiogeographic distribution of the identified species.

Comparison with the paleobiogeography of the accompanying molluscan fauna
In the early Neogene, the Mediterranean-Iranian biogeographical province of mollusks (Harzhauser et al., 2002) covered part of the Western Tethys region.The molluscan fauna in this province had a similar taxonomic composi-tion with those of the North Atlantic of France and the Paratethys (Harzhauser et al., 2002;Lozouet, 2014), as in the case of fishes.Shallow marine faunas from the Indo-West Pacific show high degrees of endemism in the early Miocene (Harzhauser et al., 2007(Harzhauser et al., , 2017)), whereas a turnover in the composition of nearshore mollusks is evident in the Oligocene to early Miocene in the Indian Ocean (Harzhauser et al., 2009).
The molluscan fauna of the Felli section is most similar to shallow marine and coastal assemblages from the NE Atlantic and the Paratethys (Thivaiou et al., 2019).However, in contrast to fishes (for which we do not have an Indo-Pacific fossil record to compare with), the molluscan fauna of the Felli section also includes genera that are characteristic of subtropical to tropical environments (Houbrick, 1991;Janssen et al., 2011), such as the widespread coastal mudflat Terebralia (Thivaiou et al., 2021), currently inhabiting only the Indo-West Pacific, and Finella, which has recently re-entered the Mediterranean Sea from the Red Sea after the opening of the Suez Canal (Albano et al., 2021).This observation might suggest that mollusk and fish distribution ranges may not be similarly affected by paleoenvironmental change, although such a conclusion is in both cases heavily affected by sampling and study bias.

Conclusions
The fish fauna of the early Miocene MHB was in general influenced by the connectivity of the proto-Mediterranean to its surrounding marine basins.The early Miocene fish fauna of the proto-Mediterranean, as revealed by the otolith assemblage of the Felli section, was a mixture of Atlantic, Paratethyan, North Sea, and Mediterranean endemic elements.Unfortunately, the fossil record so far for this interhttps://doi.org/10.5194/fr-24-233-2021Foss. Rec., 24, 233-246, 2021 val has been scarce.Nevertheless, the present findings offer some insight into the early Miocene evolution of the Mediterranean marine fish fauna.Future research should aim to fill in the considerable gaps in the record for the early-middle Miocene, especially in the Indo-Pacific realm.
Data availability.The supplementary data are available openaccess with this article.
Author contributions.KA was responsible for data curation, formal analysis, investigation, methodology, resources, and writing; DT for conceptualization, funding acquisition, investigation, methodology, resources, and writing; and EK for investigation, resources, and writing.
Competing interests.The authors declare that they have no conflict of interest.
Disclaimer.Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figure 5 .
Figure 5. Paleogeographic map of the proto-Mediterranean, showing its connections to the Paratethys, Atlantic, and Indo-Pacific realms, and the location of the Mesohellenic Basin (MHB) and the Felli section (modified after early Burdigalian scheme of Popov et al., 2004), and list of the species identified at Felli color-coded to indicate their paleobiogeographic distribution.