Introduction

Today, with about 37,000 species been described, fishes (e.g. Cyclostomata, Chondrichthyes and piscine Osteichthyes) represent more than half of all known extant vertebrates (Fricke et al., 2023). Their origin lies in the lower Cambrian (Shu et al., 2003). In the Ordovician and the Silurian, they radiated in a large number of different higher level taxa (Andreev et al., 2022; Friedman, 2022; Nelson et al., 2016; Zhou et al., 2022). The Devonian is often ascribed as the “age of fishes”, whereas the Mesozoic is informally called the “age of reptiles”. It is well known that during their life on earth, fishes represent the most important part of marine biotas. This holds also for the Triassic. After the massive end-Permian extinction event, the recovery of the marine fish fauna seems to have been considerably faster than previously thought (Benton et al., 2013; Chen & Benton, 2012; Hu et al., 2010; Romano, 2021; Tintori et al., 2014). This is shown by newly discovered Early Triassic faunas from North America and China (Brayard et al., 2017; Dai et al., 2023).

The various Middle Triassic fossil sites on the Swiss side of Monte San Giorgio (village of Meride, today part of the community of Mendrisio, Canton Ticino), who have been exploited by researchers from the University of Zurich since 1924, got their worldwide fame largely due to the large number of often well preserved fossils of marine reptiles (Peyer, 1944). Taxa such as Ceresiosaurus or Serpianosaurus ven reflect their geographic origin. Until 1980, the fish fossils clearly stood in the shadow of these emblematic reptiles (Kuhn-Schnyder, 1974). This changed during the following decades. Today the Middle Triassic fish fossils from Monte San Giorgio have clearly shown their scientific importance (Etter, 2002; Felber, 2005; Felber et al., 1997; Rieppel, 2019). They have additionally stimulated ongoing research in the contemporaneous Prosanto Formation in Canton Graubünden, Eastern Switzerland (Bürgin et al., 1991; Furrer, 2019).

Palaeoichthyological research before 1924

The Middle Triassic fossil bearing strata of Monte San Giorgio (Canton Ticino, Southern Switzerland) are exposed also on the Italian side of the border, near the village of Besano. The bituminous shales there have been exploited in open quarries and galleries since 1830, first to produce oil and gas for the city of Milano and later for pharmaceutical components and products (Saurolo). Italian palaeontologists were aware of the fossils there and have collected material as early as the middle of the nineteenth century (Bassani, 1886; Curioni, 1847, 1863). At the turn of the century, the industrial exploitation of the bituminous shales started on Swiss territory, too. The Società Anonima Miniere Scisti bituminosi di Meride e Besano began to work in 1907. The first mining site named Cava Tre Fontane is near Serpiano on the western slope of Monte San Giorgio. By 1917, a second site with the name of Val Porina was exploited (Furrer, 2023). Most of the fossil material found at these two sites was sent to the Museo Civico di Storia Naturale in Milano (Repossi, 1909). In his voluminous study, Giuglio de Alessandri (1869–1921), curator of geology and palaeontology at the Museo Civico di Storia Naturale in Milano, listed besides the Italian material six species originating from Cava Tre Fontane (de Alessandri, 1910). These include his Belonorhynchus stoppanii Bassani 1886, Colobodus? triasicus Bassani 1886, Colobodus bassanii de Alessandri 1910, Ptycholepis barboi Bassani 1886, Ophiopsis bellotti Bassani 1886, Pholidophorus meridensis de Alessandri 1910 and Pholidopleurus typus Bronn, 1858. An allied air raid in August 1943 damaged greater parts of the museum in Milano and almost all of de Alessandri’s fish material was destroyed (Visconti, 1988) (Fig. 1).

Fig. 1
figure 1

Title paper of the publication by Erik Andersson (1916) on some Triassic fish remains from Monte San Giorgio

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Due to the initiative of Carl Johann Josef Ernst Wiman (1867–1944), the Geological Institute of the University of Uppsala achieved between 1908 and 1916 a small collection of vertebrate fossils from Cava Tre Fontane. The ichthyosaur material was described by Wiman himself (Wiman, 1912). In 1916, Erik Andersson (later known as Erik Helge Osvald Stensiö, 1891–1984) published a paper on the fossil fishes of this collection (Andersson, 1916). Within the introduction he writes: “Wenigstens in den Schiefern von der Cava Tre Fontane sind die Fossilien sehr häufig, und es scheint mir ein wenig rätselhaft, dass solch ein Fundort interessanter Triasvertebraten verhältnismässig so unbeachtet geblieben ist” (Andersson, 1916, p. 13). Additionally, to the species mentioned by de Alessandri (1910), he added the coelacanth Undina sp. and the actinopterygians Urolepis sp., Belonorhynchus robustus Bellotti, Heterolepidotus? Belotti de Alessandri and Peltopleurus splendens Kner. For Pholidophorus meridensis Bassani he coined the new genus Meridensia (Anderson, 1916).

The first fossils from Monte San Giorgio were purchased by the University of Zurich 1916 in connection with a dissertation on the geology of this region (Frauenfelder, 1916). This collection included some fish and reptile material, a total of eight specimens from Cava Tre Fontane. In 1919, inspired by the German palaeontologist Ferdinand Broili (1874–1946), Bernhard Peyer (1885–1963), then lecturer at the University of Zurich, visited the fabrication site Spinirolo near Meride during a meeting in Lugano. Here, on a pile of bituminous shales, he found some fossil fishes and the remains of an ichthyosaur paddle and recognized the great potential of the fossil sites on Swiss territory. But it was only in 1924, when he was able to start the first scientific digs there (Furrer, 2024).

Palaeoichthyological research after 1924

By the aid of a grant from the Georges and Antoine Claraz foundation, Bernhard Peyer and his team started the first scientific excavations in Cava Tre Fontane and in Val Porina in spring 1924 (Peyer, 1925, 1934). They have been followed until 1938 with the new sites Acqua del Ghiffo (1927 & 1928), Val Serrata (1930), Cassina (1933 & 1937), Acqua Ferruginosa (1937) and Cassinello (1938).

In 1931, the German palaeontologist Hermann Aldinger (1902–1993) described some fragmentary material from the Grenzbitumenzone of Cava Tre Fontane, which he ascribed to the genus Birgeria Stensiö, 1916. Based on that material, he erected the new species B. stensioei, honouring the prolific work of Erik Stensiö (Aldinger, 1931).

1935 the Trustees of the British Museum of Natural History (today Natural History Museum) in London purchased a collection of 180 fossil fishes mainly from Besano from the German private collector and fossil dealer Karl Bender (1888–1940). They have been collected between 1929 and 1933. Most of these fossils have been studied by the British palaeontologist James Brough (?1904–1988). He published his studies in a book titled “The Triassic Fishes of Besano, Lombardy” (Brough, 1939). Among the material studied, there are some specimens originating from the Swiss side of Monte San Giorgio. These include Meridensia meridensis BMNH P. 19440 and P. 23808 from Valporina, Peltopleurus lissocephalus BMNH P. 19296 and Luganoia lepidosteoides, the holotype BMNH P. 19316 and P. 19327 (counterpart) labelled from Monte San Giorgio, Switzerland (Brough, 1939). In his introduction, Brough mentioned ongoing work by Bernhard Peyer on the genus Colobodus. Therefore, he omitted this species in his book, as well as the material of Saurichthys and some semionotiform fossils of minor quality (Brough, 1939). All these specimens are stored as “Bender collection” in the Department of Palaeontology and have been studied by in 1990 during my research on small- and medium-sized actinopterygians from Monte San Giorgio (Bürgin, 1991) (Fig. 2).

Fig. 2
figure 2

Cover page of James Brough’s book on the Triassic fishes of Besano, 1939

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From 1950 to 1968 Peyer’s successor as professor of palaeontology and first director of the newly founded Palaeontological Institute and Museum, University of Zurich (PIMUZ), Emil Kuhn-Schnyder (1905–1994), opened the largest excavation at Point 902 near Mirigiolo (Kuhn-Schnyder, 1974). Another excavation followed from 1970 to 1975 at the Cassina site. In 1983 and 1984 Hans Rieber, the successor of Kuhn-Schnyder, exploited a site in Valle Stelle. From 1994 to 2004, further excavations at different localities were led by Heinz Furrer, curator at the PIMUZ. Between 1996 and 2003, on the invitation by Markus Felber—then curator at the Museum Cantonale di Storia Naturale in Lugano—a team of the University of Milano lead by Andrea Tintori, investigated the Kalkschieferzone at Vecchi Mulini, close to Meride (Tintori, 2017; Tintori & Felber, 2015; Tintori et al., 1998). Since 2006, the Museo Cantonale di Storia Naturale in Lugano organized yearly diggings under the lead of Rudolf Stockar at different sites, too (Furrer, 2024). Among the material collected there are many fish specimens that have been and need to be further studied (Bürgin, 1998).

Chondrichthyan research

Presently, there are five genera and species of chondrichthyan fishes recognized from the Middle Triassic of Monte San Giorgio (Rieppel, 2019). The first shark remains, an isolated tooth of Hybodus cf. plicatilis, was found in 1924 during the first field season at Cava Tre Fontane. In 1944, Peyer mentioned further chondrichthyan material collected during his excavations at Monte San Giorgio. He ascribed it as belonging to the genera Acrodus and Hybodus (Peyer, 1944, p. 81). The first description of chondrichthyan fossils from the Swiss part of Monte San Giorgio was published by Emil Kuhn (Kuhn, 1946a). He ascribed the four specimens as belonging to the genus Acrodus. They had been collected between 1929 and 1933 from the Grenzbitumenzone in Val Porina and in Cava Tre Fontane/Valle Stelle. The material consisted of associated skull and skeletal remains with teeth and fin spine (Kuhn, 1946a: Fig. 1), a specimen with articulated teeth, an isolated fin spine and an unprepared specimen (Fig. 3).

Fig. 3
figure 3

Emil Kuhn’s figure 1 from Kuhn (1946a), showing the large Acrodus fossil from Monte San Giorgio

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Olivier Rieppel described the larger part of the chondrichthyan material anew and ascribed it to the genera Hybodus, Acrodus, Asteracanthus and Palaeobates (Rieppel, 1981). A year later, based on a small, nearly complete specimen, he erected the new genus and species Acronemus tuberculatus (Rieppel, 1981). In 1998, Raoul Mutter described the tooth variability in the dentition of Acrodus sp. (Mutter, 1997, 1998b) and described a new species, Acrodus georgii (Mutter, 1998a). Acrodus georgii, with a length between 2 and 3 m, was a large species. Acronemus tuberculatus was considerably smaller with a total length of 30 to 35 cm. The systematic position of the later is still debated (Maisey, 2011). The four other species are representatives of the family Hybodontiformes (Maisey, 1982). Further anatomical und taxonomic details can be found in the book of Olivier Rieppel on the fossils from Monte San Giorgio (Rieppel, 2019).

Osteichthyan research

Osteichthyan fishes are divided into the class Actinopterygii, the ray-finned fishes—and into the class Sarcopterygii, the lobe-finned fishes including tetrapods. Whereas the former taxon represents almost all of the 36,000 extant species, the lobe-finned fishes without the Tetrapodomorpha is represented by only 6 living species: 4 species of lungfishes and two coelacanths (Nelson et al., 2016).

Ray-finned fishes—Actinopterygians

Despite the wealth of material collected by Bernhard Peyer and Emil Kuhn-Schnyder, the ray-finned fishes did not receive any greater attention during their lifetimes. The dissertation of Sigmund Egil Guttormsen (1909–1999) was done under the supervision of Bernhard Peyer. He studied the jaws of some ganoid fishes and included material of the genus Colobodus from the Tessin collection (Guttormsen, 1937). In 1964, Kuhn-Schnyder published a list with the names of identified actinopterygians from Monte San Giorgio (Kuhn-Schnyder, 1964, p. 399) (Fig. 4).

Fig. 4
figure 4

List of actinopterygian fishes from Monte San Giorgio compiled by Emil Kuhn-Schnyder (1964)

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The first profound investigation of actinopterygian material from the collection of the PIMUZ was done by Walter Schwarz (1941–2022) on the genus Birgeria. Schwarz worked mainly on a single, well preserved specimen, already mentioned by Kuhn (1946a, 1946b), which formed the basis for his detailed monograph (Schwarz, 1970). One reason for the restriction on a single specimen lay in the capacity of preparation work, which was during this time mainly used on the obviously more spectacular aquatic reptiles. The remaining material (67 specimens) was much later studied by Carlo Romano in a Ph. D. thesis and helped to clarify some important details in the anatomy of this large-sized predatory fish (Romano, 2007; Romano & Brinkmann, 2009).

In 1980, John Griffith (?1933–?2003), then at the Department of Zoology at Westfield College, University of London, started to study some of the small actinopterygians from Monte San Giorgio in the Tessin collection of the PIMUZ. His focus was mainly on the perleidiform, peltopleuriform and luganoiiform material. Previously he had studied a Carnian marine fish fauna from Polzberg, Austria, and discussed the interrelationships of several actinopterygian taxa found there (Griffith, 1977). During his stay in Zurich, he studied and reclassified the actinopterygian fish material from Monte San Giorgio in the PIMUZ collection and prepared a comparative list with his and the identification by Kuhn-Schnyder from 1964 set aside. In April 1980, he loaned 73 specimens, mainly from the excavation Point 902 (Grenzbitumenzone). Later in August 1981, he loaned six additional specimens. He started to prepare a preliminary description of a new peltopleuriform species, but due to a change in position and health problems, he did not finish his manuscript. All the specimens he loaned, have been given back in September 1987. In June 1989, he handed over to me some of the original drawings and photographs of his new species.

The next actinopterygian taxa studied in detail was the genus Saurichthys, a medium- to large-sized predatory fish with a pointed, elongated snout. Olivier Rieppel studied 17 specimens from the Tessin collection of the PIMUZ (Rieppel, 1985a) and described the following species: Saurichthys curionii (Bellotti, 1857), S. macrocephalus (Deecke 1886) and his new species S. costasquamosus. In this study he recognized some peculiar anal fin structures, as well as a cone-shaped, presumably intromittent organ. He, for the first time speculated on the possibility of viviparity in this genus (Bürgin, 1990; Maxwell et al., 2018; Rieppel, 1985a). Additional, smaller specimens of Saurichthys from the Tessin collection were described in a subsequent paper (Rieppel, 1992). Besides the description of some juvenile specimens of Saurichthys curionii and S. costasquamosus, he erected the new species S. paucitrichus. Furthermore, he presented a phylogenetic analysis of the genus Saurichthys and discussed its ecology. In 2015, Erin Maxwell, Carlo Romano, Feixiang Wu and Heinz Furrer described two new species auf Saurichthys from the Middle Triassic Besano Formation of Monte San Giorgio: S. breviabdominalis and S. rieppeli (Maxwell et al., 2015). Sue Beardmore and Heinz Furrer studied the interesting taphonomy of these frequent fossils (Beardmore & Furrer, 2015, 2019). In 2021 Silvio Renesto, Fabio Magnani and Rudolf Stocker described the new species S. sceltrichensis from the Sceltrich beds (Meride Limestone) (Renesto et al., 2021). Additional studies on morphology, biology, systematics, taphonomy and palaeogeography of the genus Saurichthys complemented the picture of this taxon (Argyriou et al., 2018; Beardmore & Furrer, 2015; Maxwell et al., 2013, 2021; Renesto & Stockar, 2009, 2015; Romano et al., 2012; Scheyer et al., 2014; Schmid & Sanchez-Villagra, 2010; Tintori, 2019).

From October 1987 to May 1990 I, with the support of a Swiss National Fund project (No. 3.535.0.86.), studied the small- and medium-sized actinopterygians in the Tessin collection of the PIMUZ. I was able to use some of the previous work done by John Griffith (see above). During this project a total of 730 specimens have been carefully measured and studied in detail. The result of this project was a publication entitled “Basal Ray-finned Fishes from the Middle Triassic of Monte San Giorgio”, which listed 18 different genera, 3 of them new, and 30 species, 16 of them new (Bürgin, 1992). The species described in detail are Gyrolepis sp., Ptycholepis barboi Bassani 1886, P. priscus Bürgin 1992, P. schaefferi Bürgin 1992, P. magnus Bürgin 1992, Bobasatrania ceresiensis Bürgin 1992, Pholidopleurus ticinensis Bürgin 1992, Gracilignathichthys microlepis Bürgin 1992, Platysiagum minus Egerton 1872, Perleidus sp., Meridensia meridensis Andersson (= Stensiö)1916, Aetheodontus besanensis Brough 1939, Ctenognathichthys bellottii (Bürgin 1992), Peltoperleidus bellipinnis Bürgin 1992, P. elongignathus Bürgin 1992, P. macrodontus Bürgin 1992, P. triseries Bürgin 1992, Dipteronotus ornatus Bürgin 1992, Luganoia lepidosteiformes Brough 1939, Peltopleurus lissocephalus Brough 1939, P. rugosus Brough 1939, P. nothocephalus Bürgin 1992, Peltopleurus sp., Nannolepis sp., Peripeltopleurus vexillipinnis Bürgin 1992, P. besanensis Bürgin 1992, P. hypsisomus Bürgin 1992, Cephaloxenus macropterus Brough 1939, C. squamiserratus Bürgin 1992, Habroichthys minimus Brough 1939, H. griffithi Bürgin 1992 and Habroichthys sp. (Fig. 5).

Fig. 5
figure 5

Various actinopterygian fishes from Monte San Giorgio, A, large-sized fishes (30–100 cm), B medium-sized fishes (10–29 cm), C small sized fishes (2–9 cm); reconstructions by the author. 1—Birgeria stensiöi, 2—Saurichthys curionii, 3—Colobodus bassanii, 4—Ptycholepis barboi, 5—Bobasatrania ceresiensis, 6—Pholidopleurus ticinensis, 7—Ticinolepis crassidens, 8—Ctenognathichthys bellotti, 9—Meridensia meridensis, 10—Peltopleurus lissocephalus, 11—Aetheodontus besanensis, 12—Placopleurus minus, 13—Habroichthys minimus, 14—Luganoia lepidosteoides, 15—Peltoperleidus macrodontus, 16—Ophiopsidae indet. Scale bars A and B = 10 cm, C = 1 cm

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The neopterygian genera and species identified during the studies of 1987 to 1990 have been listed in an overview of Middle Triassic marine fish faunas from Switzerland (Bürgin, 1999a, Appendix 1). However, due to a change of job and occupation, the neopterygian material has only partly been published. The unpublished material will be used in subsequent and collaborative work.

One of the is the genus Placopleurus Brough 1939. There are 98 specimens being studied by myself. These include Placopleurus primus Brough, 1939 and P. besanensis Brough, 1939, as well as two probably new species. The genus itself shows close anatomical similarities to the Chinese genus Venusichthys (Xu & Zhao, 2016) (Fig. 6).

Fig. 6
figure 6

Placopleurus besanensis Brough 1939, reconstruction by the author based mainly on PIMUZ T 1234

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The genus Colobodus, due to its larger size, was not included in my studies, as well as the specimens of the genera Saurichthys (studied by Olivier Rieppel, see above) and Birgeria (studied by Carlo Romano, see above). From 1998 to 2002, Raoul Mutter in his Ph.D. thesis studied the complete material of the genus Colobodus in the collection of the PIMUZ (Mutter, 2001a, b, 2002, 2004). He revised the family Colobodontidae sensu Andersson 1916 and discriminated on anatomical and meristic characters several taxa, some of them probably new. Unfortunately, due to a change of occupation, all the proposed new species have not yet been published (Mutter, 2002). The valid species recognized in his thesis are: Crenilepis sandbergeri Dames 1888, Colobodus bassanii De Alessandri 1910, C. maximus (Quenstedt, 1867) and C. koenigi Stolley 1920. Due to the absence of a holotype, he designated a neotype for C. bassanii (Mutter, 2003).

In 1995, I described new material of actinopterygian fishes from the middle Kalkschieferzone (uppermost Ladinian), excavated by the team of Heinz Furrer (PIMUZ) in the Val Mara near the village of Meride (Bürgin, 1995; Furrer, 1995a). The taxa described therein are Gyrolepis sp., Perleidus altolepis (Deecke, 1889), two probably new species of Peltopleurus, Archaeosemionotus sp., Ophiopsis cf. lepturus and Prohalecites porroi (Bellotti, 1857). The later species was described in detail by Andrea Tintori based on abundant material from the Italian locality Ca’del Frate (Tintori, 1990a). Because of the small sample size and the lack of distinct discriminating characters I refrained from the designation of two new species of Peltopleurus. Archaeosemionotus sp. was later transferred into the new genus Ticinolepis (see López-Arbarello et al., 2016).

In 2002, Cristina Lombardo described the new genus Caelatichthys, with the new species C. nitens from late Ladinian Kalkschieferzone of Ca’ del Frate in Northern Italy, and the new species C. meridensis from the Kalkschieferzone near Meride in Canton Ticino (Lombardo, 2002). The genus shows a mixture of primitive and advanced characters. She placed it tentatively as a palaeoniscid grade actinopterygian. Andrew Neuman und Raoul Mutter grouped both species in the family Platysiagidae (Neuman & Mutter, 2005).

In 2003, Annette Herzog, as a result of her thesis on the Middle Triassic actinopterygians from the Prosanto Formation in Graubünden, published a new description of Eoeugnathus megalepis BROUGH 1939 (Herzog, 2003b). This species was originally established by James Brough in his study on the Triassic fishes of Besano (Brough, 1939). The species is well represented in the fish fauna of Monte San Giorgio, too, where 46 specimens are in the collection of the PIMUZ. They have been studied in detail by myself and recently by Adriana López-Arbarello (pers. com.) The species shows close anatomical similarities to the ionoscopiform genus Subortichthys from the Middle Triassic of Yunnan, China (Ma & Xu, 2017). Contrary, Zuoyu Sun and co-authors placed Eoeugnathus in their newly erected order Panxianichthyformes, the basal sister-group to the Ionoscopiformes (Sun et al., 2016) (Fig. 7).

Fig. 7
figure 7

Eoeugnathus megalepis BROUGH 1939, composite reconstruction by the author based mainly on PIMUZ T 1356

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The genus Eosemionotus STOLLEY, 1920 was subject of a further study by myself. I described the new species Eosemionotus ceresiensis based on well preserved material from the upper Besano Formation (Bürgin, 2004). Additional material from the Meride Limestone (Cava inferiore, Cava superiore, Cassina and Sceltrich beds) resulted in a taxonomic and systematic revision of the genus Eosemionotus by Adriana López-Arbarello, Heinz Furrer, Rudolf Stockar and myself. Described have been three new species: Eosemionotus diskosomus, E. sceltrichensis, and E. minutus (López-Arbarello et al., 2019). The cladistic analysis retrieved Eosemionotus as the oldest member of the family Macrosemiidae within the order Semionotiformes (Ginglymodi).

In 2005, Annette Herzog and myself described well preserved material of the genus Besania Brough 1939 from the contemporaneous Prosanto Formation in Graubünden. Brough (1939) proposed a close relationship with the genus Luganoia. We could show however, that both genera are distinctly different from each other and Besania to be close to the basal Halecostomi (Herzog & Bürgin, 2005). The genus Besania is well represented in the Middle Triassic of Monte San Giorgio, too (Fig. 8).

Fig.8
figure 8

Besania sp., reconstruction by the author based mainly on PIMUZ T 3927

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In 2007, Andrea Tintori and Cristina Lombardo described the new semionotiform genus and species Sangiorgioichthys aldae from the lower Kalkschieferzone (upper Meride Limestone, uppermost Ladinian) of Val Mara (near Meride) and Ca’ del Frate (Viggiu, Italy) (Tintori & Lombardo, 2007). Five years later, a second species, Sangiorgioichthys valmarensis, was described from the same locality by Cristina Lombardo, Andrea Tintori and Daniele Tona (Lombardo et al., 2012). The genus was also shown to be present in the Middle Anisian Luoping fauna of China (López-Arbarello et al., 2011).

In 2007, Gloria Arratia and Annette Herzog described a new halecomorph fish from the Middle Triassic Prosanto Formation of Graubünden (Arratia & Herzog, 2007). Initially listed as Teleostei indet. (Herzog, 2003a, 2003b), the new genus and species Prosantichthys buergini, seems closer to the halecostome family Parasemionotidae. The same species is present at Monte San Giorgio too, where it was tentatively ascribed to the family Pholidophoridae (Bürgin, 1999a, 1999b) (Fig. 9).

Fig. 9
figure 9

Prosantichthys (= Pholidophoridae gen. et sp. indet. by Bürgin, 1999a, 1999b), plate and counterplate, PIMUZ T 3928

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In 2013, Cristina Lombardo published a paper on an enigmatic basal actinopterygian fish from the Late Ladinian Kalkschieferzone of Monte San Giorgio. The single specimen shows anatomical characters, which are representative for fishes of the family Redfieldiiformes (Lombardo, 2013). This family was previously known from freshwater deposits of North America, Africa and Australia (Schaeffer, 1984). The presence of abundant freshwater crustaceans in the Kalkschieferzone indicates a temporary influx of freshwater. A similar species, referred as Redfieldiiformes indet., is described in a fish fauna of the Erfurt-Formation of the German Lettenkeuper (Böttcher, 2015).

In 2014, Adriana López-Arbarello, Rudolf Stockar and myself clarified the difficult phylogenetic relationships of the genus Archaeosemionotus Deecke, 1889 from Perledo (López-Arbarello et al., 2014) and described subsequently together with Heinz Furrer the new genus Ticinolepis with its two new species T. crassidens and T. longaeva from the Middle Triassic of Monte San Giorgio (López-Arbarello et al., 2016) (Fig. 10).

Fig. 10
figure 10

Ticinolepis crassidens, reconstruction by the author based mainly on PIMUZ T 273

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Since many years Gloria Arratia studies basal Teleostomorpha (e.g. Arratia, 1997a, 1997b, 2000a, 2000b, 2001, 2004, 2013, 2015; Arratia & Schultze, 2024). In 2022, she published a paper on the outstanding small suction-feeder Marcopoloichthys furreri from the Middle Triassic Prosanto Formation of Graubünden (Arratia, 2022). The genus itself was erected by Italian and Chinese scientists (Tintori et al., 2007) on fossils from the Early Ladinian of the Cunardo Formation (Lombardy, Northern Italy) and the Middle Anisian of the Guanling Formation (Yunnan Province, South China). The same genus is present at Monte San Giorgio, too. It was listed as Neopterygii incertae sedis in the list of Bürgin (1999a, 1999b: Appendix 1). Meanwhile it is described as a new species of genus Marcopoloichthys (Arratia et al., subm.). It shows the same unique feeding apparatus.

Lobe-finned fishes—Sarcopterygians

The first remains of lobe-finned fishes (Sarcopterygii) on the Swiss side of Monte San Giorgio have been found at Cava Tre Fontane (Andersson, 1916). They have been assigned to the actinistian genus Undina. Some material collected by the teams of Bernhard Peyer and Emil Kuhn-Schnyder at Monte San Giorgio was first studied in detail by Olivier Rieppel (Rieppel, 1980). Based on that material he erected the new genus and species Ticinepomis peyeri. It is a small species of about 12 cm total length. Fragmentary remains have been attributed to cf. Undina picenus (Rieppel, 1985b).

In 2013, Lionel Cavin, Heinz Furrer and Christian Obrist described new coelacanth material from the Middle Triassic Prosanto Formation in Graubünden (Cavin et al., 2013). They restudied the material of Ticinepomis peyeri Rieppel 1980 and emended the diagnosis of this genus and species.

In 2017, Christophe Ferrrante, Rossana Martini, Heinz Furrer and Lionel Cavin published a paper on the Middle Triassic coelacanths from Switzerland (Ferrante et al., 2017). Therein they mentioned an unknown species from the middle Besano Formation of Monte San Giorgio. It was described in a subsequent paper as Rieppelia heinzfurreri (Ferrante & Cavin, 2023). Like Foreyia maxkuhni from the Prosanto Formation of Graubünden, it shows a highly derived anatomy, with a relatively large head and a short and compact body (Ferrante & Cavin, 2023, Fig. 4).

In 2018, Silvio Renesto and Rudolf Stocker described a new, small coelacanth specimen from the Cassina beds, Lower Meride Limestone (Renesto & Stockar, 2018). Based on its anatomy they classified it as Heptanema sp., with close affinities to the poorly known Heptanema paradoxum Bellotti, 1857 from the Ladinian Perledo Formation of Northern Italy.

In 2023 Christophe Ferrrante, Heinz Furrer, Rossana Martini and Lionel Cavin revised the material of Ticinepomis Rieppel 1980 and erected the new species Ticinepomis ducanensis, which is present both in the Besano Formation of Monte San Giorgio as well as in the Prosanto Formation of Graubünden (Ferrante et al., 2023). Additionally, they included some palaeobiological and palaeoecological considerations on the known coelacanths from the Middle Triassic of Switzerland.

Discussion and conclusion

One hundred years after the start of the scientific investigations by Bernhard Peyer from the University of Zurich, the research on Middle Triassic fishes from the numerous sites of Monte San Giorgio no longer stands in the shadow of the aquatic reptiles. Especially the last four decades have shown a wealth of new investigations and many taxa new to science. The restart of fish investigations in the years around 1980 has shown the great potential of the previously collected material by the PIMUZ, as well as new material collected from the Kalkschieferzone by researchers of the PIMUZ, the University of Milano and the Museo Cantonale di Storia Naturale in Lugano. Since 2012, a nice selection of fishes is also exposed in the Museum of Fossils from Monte San Giorgio in Meride (www.museodeifossili.ch).

My SNF project on small- and medium-sized actinopterygians initiated systematic excavations in the contemporaneous Prosanto Formation (Canton Graubünden, Eastern Switzerland), which resulted in another great number of often exquisitely preserved fish fossils (Bürgin et al., 1991; Bürgin & Herzog, 2002; Cavin et al., 2017; Furrer, 1995b, 2019; Herzog, 2003a, 2003b; Mutter & Herzog, 2004; Herzog & Bürgin, 2005). One of them is the remarkable stem teleostean Marcopoloichthys furreri, a very small species with suction feeding abilities (Arratia, 2022).

The Middle Triassic fishes of Monte San Giorgio already known come from six different fossil bearing beds. The oldest is the Besano Formation at the Anisian–Ladinian boundary (Furrer, 2003, 2024). It is the most species-rich horizon. Somewhat younger (Early Ladinian) are the Cava inferiore, the Cava superiore and the Cassina beds of the Lower Meride Limestone (Early Ladinian). At the base of the Upper Meride Limestone lie the Sceltrich beds, which has been studied between 2010 and 2021. The youngest horizon is the Kalkschieferzone (Late Ladinian). All these beds show a clearly reduced taxic diversity.

The fossil fishes from the Middle Triassic of Monte San Giorgio offer a stimulating reference to other contemporaneous fish faunas worldwide. These include the research on the Italian side by Andrea Tintori, Cristina Lombardo and co-workers from the University of Milano (Lombardo, 1999; Lombardo & Tintori, 2004; Tintori, 2013, 1990b, 1990c; Tintori et al., 2007), the Middle Triassic sites of the Kamnik-Savinja Alps in Slovenia (Hitji et al., 2010; Miklavc et al., 2016), the Muschelkalk in Germany (Arratia & Schultze, 2024; Schultze & Kriwet, 2021; Schultze & Möller, 1986) and the Netherlands (Haarhuis & Diependaal, 2019; Oosterink, 1986), the Muschelkalk of Spain (Beltan, 1972, 1984, 2019; Cartanya, 1999; Cartanya et al., 2015) and especially the prolific outcrops in the Middle Triassic of China, with the Luoping and the Panxian fauna (Lin et al., 2011; Lombardo et al., 2011; Ren & Xu, 2011; Sun et al., 2008, 2009, 2012; Tintori et al., 2010a, 2010b; Wu et al., 2018; Xu, 2019) and the Xingyi fauna (Tintori et al., 2011, 2015).

Most of the work up to now has been focused primarily on the alpha-level taxonomy. There are still many open questions concerning the higher-level systematics of most of the actinopterygian taxa being studied. Nevertheless, all the present studies open new research opportunities on global dispersal, evolution, faunal composition and ecology of early Mesozoic chondrichthyan and osteichthyan fishes. Although the main goal of the present paper is to present the research history of the fossil fishes from Monte San Giorgio, it is to hope, that it will stimulate new studies and further cooperation in the field.