First Record of a Coelacanth Fish from the Middle Triassic Meride Limestone of Monte San Giorgio (Canton Ticino, Switzerland)
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Rivista Italiana di Paleontologia e Stratigrafia (Research in Paleontology and Stratigraphy) vol. 124(3): 639-653. November 2018 FIRST RECORD OF A COELACANTH FISH FROM THE MIDDLE TRIASSIC MERIDE LIMESTONE OF MONTE SAN GIORGIO (CANTON TICINO, SWITZERLAND) SILVIO RENESTO1 & RUDOLF STOCKAR2 1Dipartimento di Scienze Teoriche ed Applicate, Università degli Studi dell’Insubria, Via Dunant 3 I 41100 Varese, Italy. E-mail: [email protected] 2Repubblica e Cantone Ticino, Dipartimento del territorio, Museo cantonale di storia naturale, Viale Carlo Cattaneo 4, CH-6900 Lugano, Switzerland. To cite this article: Renesto S. & Stockar R. (2018) - First record of a coelacanth fish from the Middle Triassic Meride Limestone of Monte San Giorgio (Canton Ticino, Switzerland). Riv. It. Paleontol. Strat., 124(3): 639-653. Keywords: Actinistia; Middle Triassic (Ladinian); Meride Limestone; New species; Description. Abstract. A new specimen of coelacanth based on a new specimen from the Meride Limestone Formation of the UNESCO World Heritage area of Monte San Giorgio is described. It represents the first occurrence of an actinistian in this formation. The newly discovered specimen shares many characters with the poorly known Heptanema paradoxum Bellotti, 1857 from the Ladinian Perledo Formation of Northern Italy. A comparison with the holotype and only existing specimen of H. paradoxum supports the assignment of the new specimen to the genus Heptanema. Some anatomical differences between the two specimens are most probably due to different ontogenetic stages, while few may support the erection of a new species; since the specimen is a juvenile it is preferred not to erect a new species, but to classify the specimen as Heptanema sp. New available data from both the holotype of H. paradoxum and form the new specimen allows an attempt to assess the phylogenetic relationships of Heptanema. INTRODUCTION have recently been made in the in levels of Prosan- to Formation coeval with the Lower Meride Lime- Coelacanths (Actinistia) are a clade of sarcop- stone, which yielded large specimens of Ticinepomis terygian fishes that are first known from the Early cf T. peyeri (Cavin et al. 2013) and the highly derived Devonian (Johanson et al. 2006). They peaked in Foreya maxkuhni (Cavin et al. 2017; a reappraisal of diversity in the Early-Middle Triassic being most- the Middle Triassic coelacanths diversity from Swit- ly known from Europe (see Cavin et al. 2013 and zerland has been recently published by Ferrante et Ferrante et al. 2017 for a reappraisal of European al. 2017). and American taxa) and China (Tong et al. 2006; Here we describe a new coelacanthiform Wen et al. 2013), their diversity declined in the Cre- specimen, from the Sceltrich beds at the base of the taceous and are absent from the Cenozoic fossil Upper Meride Limestone (Ladinian). It was discov- record. They were considered extinct until a liv- ered in 2016 and represents the first record of this ing specimen of Latimeria was discovered in 1938 clade in over 160 years of excavations in the Meride (Smith 1939). Coelacanths are known in the Middle Limestone Formation Triassic of Monte San Giorgio, across the Italian Swiss boundary, with nearly complete specimens of Ticinepomis peyeri (Rieppel 1980), and fragments GEOLOGICAL SETTING of a larger coelacanth tentatively referred to cf. The Middle Triassic carbonate succession Holophagus by Rieppel (1985), both from the Ani- of Monte San Giorgio (Switzerland-Italy; Figs 1, sian-Ladinian, Besano Formation. Further findings 2), belonging to the western part of the Southern Alps, has been inscribed in the UNESCO World Received: August 27, 2018; accepted: October 29, 2018 Heritage List (WHL) because of its unique paleon- 640 Renesto S. & Stockar R. Fig. 1 - Map of the Monte San Gior- gio area (Ticino, Southern Switzerland), showing the carbonate Anisian-Ladinian sequence together with the locality. tological value. It is, in particular, world-famous for Furrer 1995; Röhl et al. 2001). The Besano Forma- the exceptionally well-preserved fossil fishes and tion (“Grenzbitumenzone”; Frauenfelder 1916) marine reptiles (e.g. Rieber 1973; Kuhn-Schnyder directly overlies the Lower Salvatore Dolomite and 1974; Bürgin et al. 1989; Etter 2002). In Middle is composed of a 16 m thick alternation of black Triassic times, the South-Alpine domain was situ- shale and laminated dolostone. Its uppermost part ated at a northern intertropical latitude of about includes the Anisian-Ladinian boundary (Brack & 15-18° (Muttoni et al. 2004) and was strongly in- Rieber 1993; Brack et al. 2005). Most of the spec- fluenced by monsoonal circulation (Preto et al. tacular vertebrate fossils together with important 2010). This passive continental margin open to the index invertebrate fossils come from this forma- western Neo-Tethys was progressively submerged tion, which also yielded the only fossil coelacanths by a long-term transgression from the east. The so far described from Monte San Giorgio (Riep- marginal location of the Monte San Giorgio Ba- pel 1980; Rieppel 1985). The Besano Formation sin resulted in a peculiar sedimentary succession grades upwards into the San Giorgio Dolomite and in temporarily dysoxic to anoxic bottom water and the Meride Limestone, together constituting a conditions (e.g. Bernasconi 1994; Röhl et al. 2001; 614-m thick sequence in total (Stockar et al. 2012). Etter 2002; Stockar 2010; Stockar et al. 2013). The Recent studies (Stockar 2012; Stockar et al. 2013) Middle Triassic succession (Fig. 2) starts with flu- showed that the San Giorgio Dolomite results vio-deltaic deposits (Bellano Formation, Illyrian; from early and late diagenetic dolomitization, the Sommaruga et al. 1997), unconformably overlying latter cutting across stratification and affecting the Lower Triassic transitional clastic deposits (Servi- original limestone in an irregular pattern up to a no, Induan-Olenekian; Frauenfelder 1916; Sciun- major volcaniclastic bed (“Val Serrata tuff ”). The nach et al. 2015), in turn onlapping an erosional Lower Meride Limestone consists of well-bed- unconformity at the top of a Lower Permian vol- ded micritic limestone, laminated limestone and canic basement. The following upper Anisian sed- volcaniclastic layers. Three fossiliferous inter- iments indicate the progressive transgression of a vals, informally known as “Cava inferiore beds”, shallow epicontinental sea and the related expan- “Cava superiore beds” and “Cassina beds”, mainly sion of carbonate platforms (San Salvatore Dolo- consist of finely laminated limestone and yielded mite; Zorn 1971) north of an emerged land area, different vertebrate fossil assemblages (e.g. Peyer which is nowadays covered by the Po Plain (Brusca 1931, 1939a); Sander 1989; Furrer 1995; Stockar et al. 1981; Picotti et al. 2007). During the latest 2010; Stockar & Renesto 2011). The top of the Anisian and the Ladinian, although shallow-water Lower Meride Limestone is defined by a very dis- sedimentation continued in the north, an intraplat- continuous dolostone horizon (“Dolomitband”; form basin opened in the area of the Monte San Frauenfelder 1916) resulting from late diagenetic Giorgio, which led to the deposition of the Besa- dolomitization cutting across the stratification of no Formation, the San Giorgio Dolomite, and the the Meride Limestone (Stockar 2012; Stockar et Meride Limestone (Rieber 1973; Bernasconi 1994; al. 2013). The overlying Upper Meride Limestone Coelacanth fish from Meride Limestone of Monte San Giorgio (Canton Ticino, Switzerland) 641 Rieber 1993) of the Ladinian Stage (Stockar et al. 2012). After a first exploration in 2010 yielding the first fossils from this horizon (Stockar 2012), two small bed by bed excavations on a surface of around 6 and 10 square meters respectively were started in 2012 by the Museo cantonale di storia naturale (MCSN, Lugano) under the direction of the second author. The site is located on the northern bank of a small creek (Valle di Sceltrich; Swiss National Coordinates: 716’910/84’370; WGS8 coordinates 8.4503/45.90084; Fig. 1), northwest of the village of Meride. The fossilif- erous interval consists of a 30 cm thick sequence of prevailing organic-rich laminated limestone (up to 3.1% TOC) intercalated between thick-bedded marly limestone. In the Valle di Sceltrich area, this fossiliferous horizon lies around 2.5 m above the “Dolomitband”. At places, the laminated lime- stone shows storm-generated concentrations of platform-derived skeletal grains and thin-shelled bivalve pavements (Stockar 2012; Stockar et al. 2013). Benthic microbial activity accounts for the microfabrics observed in the laminated limestone, including clotted-peloidal micrite and amorphous organic matter showing EPS (extracellular poly- meric substance) -like structures as well as for the geochemical signature being characterized by high Fig. 2 - Stratigraphic column of the Monte San Giorgio area; coe- lacanth shapes indicate the levels that yielded coelacanth hydrogen indices and prevailing Type-II (Type material. I) kerogen (Stockar et al. 2013). Preservation of such a labile lipoid-rich organic material requires is a sequence of alternating well-bedded micrit- anoxic/lower dysoxic bottom-water conditions ic limestone and marlstone. The uppermost part (Stockar et al. 2013 and references therein). Low- comprises the 120 m thick “Kalkschieferzone” er dysoxic conditions were able to allow episodic (Senn 1924), made up of thinly-bedded, mostly seafloor colonization by thin-shelled opportunis- laminated,