Diversity and Evolution of Semionotid Fishes
Final Report Abstract
With more than 27000 species, including almost all living ray-finned fishes, the Neopterygii is the largest clade of Recent vertebrates. However, its early history and the phylogenetic relationships among basal neopterygians, and thus the origin of the teleosts, are still poorly understood. One of the most important and controversial groups of basal neopterygians has been the so-called “semionotids”. Although “semionotids” are among the most common basal neopterygians in Mesozoic sediments, and some of them were among the first fossil fishes to be described, their phylogenetic relationships and even their alpha taxonomy were very problematic at the time this project began. Therefore, this project was aimed to solve the taxonomy of the main “semionotid” taxa, to explore the phylogenetic relationships of these fishes and to define a monophyletic Semionotiformes. With these objectives, five new species (Scheenstia zappi, Tlayuamichin itzli, Sangiorgioichthys sui, Lepidoites pankowskii, and Macrosemimimus fegerti) and three new genera (Scheenstia, Macrosemimimus, and Callipurbeckia) were established. The monophyly of the most common “semionotid” genera Semionotus and Lepidotes was established and the two taxa were redefined. The phylogenetic relationships between these and other "semionotiform" genera, five genera of gars and three macrosemiid genera were analysed through a cladistic analysis, based on 90 morphological characters and 37 taxa. The results of the analysis showed that the “semionotiforms”, the gars and the macrosemiids form a monophylgetic clade, the Ginglymodi, which includes two main lineages: the Lepisosteiformes and the Semionotiformes. The genera Pliodetes, Araripelepidotes, Lepidotes, Scheenstia, and Isanichthys are lepisosteiforms, and not semionotiforms, as previously thought. Thus, these taxa extend the stratigraphic range of the lineage leading to the gars by c. 70 Ma reaching back up to the Early Jurassic. A monophyletic Lepidotes is restricted to the Early Jurassic species, whereas the strongly tritoral species previously referred to Lepidotes are referred to Scheenstia. Other species previously referred to Lepidotes represent other genera or new taxa. The macrosemiids are well nested within semionotiforms, together with a monophyletic Semionotidae restricted to the genus Semionotus, and a new family including Callipurbeckia, Macrosemimimus, Tlayuamichin, Paralepidotus, and Semiolepis. To summarize, at the time when the project was applied for, the “semionotids”, were considered monophyletic when including Semionotus and Lepidotes only, but the monophyly of the group was doubtful if the other “semionotiform” genera were considered. Also, the systematic position of these fishes within the Neopterygii was uncertain. The situation after the project is radically different: Semionotus and Lepidotes are not sister groups, but belong to different lineages, and the Semionotiformes, excluding thus Lepidotes and other “semionotiform” genera, and including the macrosemiids, are one of the two lineages of the major neopterygian clade Ginglymodi. The other ginglymodian lineage is the Lepisosteiformes, which includes Lepidotes and Scheenstia, Isanichthys, Pliodetes, and Araripelepidotes, the other genera excluded from the monophyletic Semionotiformes. Also, the project resulted in the clarification of the taxonomic status of Lepidotes, which had been a "wastebasket" taxon for “semionotid”-like fishes. Thus, a countless number of fossils from Early Jurassic to latest Cretaceous rocks have been referred to this genus, spanning about 130 million years. The project led to the definition of a monophyletic Lepidotes, which is restricted to several Early Jurassic species from central Europe. Other well-known species previously referred to Lepidotes are now referred to Scheenstia or Callipurbeckia. Several questions are still open, such as the taxonomoy of numerous Triassic species referred to Semionotus or the possible monophyly of the family Lepidotidae including Lepidotes and Scheenstia, but this project produced the necessary phylogenetic framework to explore and solve this pending issues in the future. Not only that, the project lead to a much better understanding of the Lepisosteiformes and the Ginglymodi as a whole, which now allows a broader analysis to explore the systematic position of the Ginglymodi among neopterygians. The study of Mesozoic ginglymodians led to confirm Patterson’s observation that these fishes show morphological affinities with both halecomorphs and teleosts. Therefore, further studies on Mesozoic ginglymodians are essential to test the hypotheses of the Halecostomi vs. the Holostei, which is one of the major topics in the evolution of Mesozoic vertebrates and the origin of modern fish faunas at the moment.
Publications
- 2007. Semionotids (Neopterygii, Semionotiformes) from the Lower Cretaceous Lagarcito Formation, San Luis Province, Argentina. Journal of Vertebrate Paleontology 27(4): 811-826
López-Arbarello, A. & Codorniú, L.
- 2008. Jurassic fishes of Gondwana. Revista de la Asociación Geológica Argentina 63(4): 586–612
López-Arbarello, A., Rauhut, O.W.M. and Moser, K.
- 2008. Revision of Semionotus bergeri Agassiz, 1833 (Upper Triassic, Germany), with comments on the taxonomic status of Semionotus (Actinopterygii, Semionotiformes). Palaeontologische Zeitschrift 82(1): 40-54
López-Arbarello, A.
- 2010. The Triassic fish fauna of the Cuyana Basin, Western Argentina. Palaeontology 53(2): 249–276
López-Arbarello, A., Rauhut, O.W.M. and Cerdeño, E.
- 2011. A new specis of Lepidotes (Actinopterygii: Semionotiformes) from the Cenomanian (Upper Cretaceous) of Morocco. Palaeontologia electronica 14.1.7A
Forey, P. L., López-Arbarello, A. and MacLeod, N.
- 2011. New semionotiform (Actinopterygii: Neopterygii) from the Late Jurassic of southern Germany. Journal of Systematic Palaeontology 9(2): 197-215
López-Arbarello, A. and Sferco, E.
- 2011. New semionotiform (Neopterygii) from the Tlayúa Quarry (Early Cretaceous, Albian), Mexico. Zootaxa 2749: 1–24
López-Arbarello, A. and Alvarado-Ortega, J.
- 2011. New Species of Sangiorgioichthys Tintori and Lombardo, 2007 (Neopterygii, Semionotiformes) from the Anisian of Luoping (Yunnan Province, South China). Zootaxa 2749: 25–39
López-Arbarello, A., Sun, Z.-Y., Sferco, E., Tintori, A., Xu, G.-H., Sun, Y.-L., Wu, F.X., and Jiang, D.-Y.
- 2012. Macrosemimimus, gen. nov. (Actinopterygii, Semionotiformes), from the Late Jurassic of Germany, England, and France. Journal of Vertebrate Paleontology 32(3): 512-529
Schröder, K. M., López-Arbarello, A. and Ebert, M.
(See online at https://doi.org/10.1080/02724634.2012.649626) - 2012. Phylogenetic Interrelationships of ginglymodian fishes (Actinopterygii: Neopterygii). PLoS ONE 7(7): e39370
López-Arbarello, A.
(See online at https://doi.org/10.1371/journal.pone.0039370)
