Taxonomy and phylogeny of the genus Eosemionotus (Neopterygii, Holostei): insights into the evolutionary history of modern fishes
Final Report Abstract
Immediately after the largest event of mass extinction at the end of the Palaeozoic, the Triassic is a very important period in the evolution of life. The biotas in general recovered rather rapidly and the Middle Triassic outcrops of the Alps preserve evidence of well-established ecosystems only 10 Myr after the Permo-Triassic mass extinction. In particular, ray finned fishes diversified very rapidly in the shallow marine and brackish environments, producing a new variety of fishes with very distinct feeding specializations and habitat preferences. Among them, the small fishes of the genus Eosemionotus have been enigmatic. The first species was discovered by Frisch in 1906 based on a single specimen from the Middle Muschelkalk (late Anisian) of Förderstedt bei Bernsburg (Germany), which Stolley distinguished as Eosemionotus vogeli in an independent genus in 1920. Although the genus was reported from several localities, E. vogeli was the only named species of the genus over 80 years until 2004 when Bürgin published a second species, E. ceresiensis, from the upper Besano Formation (earliest Ladinian) of Monte San Giorgio, Switzerland, and announced the presence of additional new species of this genus in this famous UNESCO World Heritage Site. New and excellently preserved specimens recovered from the Cassina and Sceltrich beds of the Meride Limestone (Ladinian) of Monte San Giorgio triggered the study framed within this project. As a result, the taxonomy of the genus has been revised and three new species have been established. Differential diagnoses are proposed for the five species, which differ in body proportions, relative position of the fins, shape and other features of several skull bones, squamation pattern, and some meristic characters, including the number of premaxillary teeth, and branchiostegal rays. The new material further provided valuable morphological information and it was possible to perform a cladistic analysis and solve the phylogenetic relationships of the genus and between its five species. The genus Eosemionotus is retrieved well-nested in the order Semionotiformes (Ginglymodi) as the sister-group of the four included macrosemiid genera. Therefore, the genus is classified within the family Macrosemiidae. The monophyly of the genus and its sister-group relationships with the other macrosemiids are among the best supported nodes with high Bremer, jacknife and bootstrap values, and numerous synapomorphies. The pattern of phylogenetic relationships between the five species of Eosemionotus indicates that the Muschelkalk species E. vogeli originated through dispersal from the Tethys into the Germanic Basin, most probably across the Silesian-Moravian or the East Carpathian gates before the late Anisian. This very specious genus, which is so far almost restricted to the c. 10 Myr of the Middle Triassic, has also been reported from localities in Eastern Switzerland, The Netherlands, Spain, Slovenia, and China. Pending the taxonomic and phylogenetic study of these other material of Eosemionotus, the origin of the genus in the Western or Eastern Tethys cannot be established with certainty. However, current evidence suggests that the clade Macrodemiidae probably originated and flourished rather rapidly during the Anisian and Ladinian (c. 10 Myr) within the protected environments developed in the very rich marine platforms of the Tethys and dispersed from there into the Germanic Basin. This idea is further supported by the inferred palaeoecological requirements of these fishes, which most probably fed on algae, detritus, and/or small mollusks and crustaceans, and needed refuge from predators, conditions that were certainly fulfilled in the tropical and subtropical marine platforms of the Tethys.
Publications
- Three new species of middle Triassic Eosemionotus stolley, 1920 (Actinopterygii: Neopterygii) from Monte San Giorgio. XVI Annual Meeting of the European Association of Vertebrate Palaeontologists in Caparica, Portugal (26.06–01.07.2018), Abstract book: 95
López-Arbarello, A., Bürgin, T., Furrer, H., and Stockar, R.
