The early evolution of the Neopterygii
Systematics and Morphology (Zoology)
Final Report Abstract
Neopterygii, including more than 50 % of modern vertebrate biodiversity, is an ancient clade of vertebrates that has evolved through the Mesozoic and Cenozoic. It constitutes a very interesting case of an evolutionary radiation because of the tremendous asymmetry in the amount of living representatives of its main lineages: Holostei (Halecomorphi and Ginglymodi; 8 species), versus Teleostei (> 32,000 species). During the Triassic-Jurassic however, the diversity of holosteans largely exceeds that of teleosts. In recent decades, there has been a significant focus on molecular studies in actinopterygian systematics, which led to new insights, but also to a considerable neglect of anatomical studies. Due to the paucity of detailed descriptions, the morphological variability of fossil neopterygians is rather poorly known and, thus, with a few exceptions, it has had little influence in systematics. Moreover, a few paradigms in neopterygian systematics have biased the study of fossils towards the search of a limited number of anatomical features that have been assumed to be apomorphic of a given clade, and the monophyly of the main neopterygean clades has never been thoroughly tested. Consequently, the origin and early evolution of neopterygians is still poorly understood, as are the interrelationships of their main lineages. This one-year project has been carried out within the framework of a long term research program aimed to complete the anatomical knowledge and clarify hypotheses of primary homologies in basal neopterygians, testing them in a large-scale phylogenetic analysis thus clarifying the interrelationships of the major lineages of the Neopterygii. Three new fossil taxa have been described and their phylogenetic relationships have been studied through cladistic analyses. First, the previously poorly known and incorrectly understood †Archaeosemionotus connectens Deecke from the Triassic from Perledo (Monte San Giorgio at Varenna, Italy) was redescribed. †Archaeosemionotus is the sister taxon of †Robustichthys from the Anisian of China, and they together form a clade with †Ophiopsis (= †Furo), which is known from several localities ranging from the Early to the Late Jurassic. Ionoscopiforms have a long stratigraphic range, though their fossil record is rather patchy. Other ionoscopiforms are so far known from the Kimmeridgian to the Albian. Second, a new callipurbeckiid genus (Ginglymodi: Semionotiformes) from the Tithonian (Late Jurassic) of Canjuers, France, was erected and described. After the incorporation of the new and recently described taxa and the re-evaluation and addition of morphological characters, the new cladistic analysis recovered a somewhat different pattern of relationships compared with previous phylogenetic hypotheses for Ginglymodi. Third, a new basal holostean genus (Actinopterygii: Neopterygii), including two new species, was described from the Middle Triassic of the Monte San Giorgio (Canton Ticino, Switzerland). The new taxon shows a mosaic of halecomorph and ginglymodian anatomical characters and might represent a basal holosteans, the morphology of which challenges the monophyly of both Halecomorphi and Ginglymodi. The study of the phylogenetic relationships of this new genus requires a largescale cladistic analysis and is still in progress. The taxonomic study led to very interesting results concerning patterns of intraspecific variation, ontogenetic changes and morphological variation over time, and evidence for habitat partitioning and different adaptability to palaeonevironmental changes. In addition to these studies, the analysis of the phylogenetic relationships of †Luisiella feruglioi from the Jurassic of Patagonia was completed and revealed several significant aspects on the early evolution of Teleostei.
Publications
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2014. Phylogenetic relationships of the Triassic Archaeosemionotus Deecke (Halecomorphi, Ionoscopiformes) from the 'Perledo fauna'. PLoS ONE 9(10): e108665
López-Arbarello, A., Stockar, R. and Bürgin, T.
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2015. Phylogenetic relationships of †Luisiella feruglioi (Bordas) and the recognition of a new clade of freshwater teleosts from the Jurassic of Gondwana. BMC Evolutionary Biology 15: 268
Sferco, E., López-Arbarello, A., and Báez, A.M.
