Pycnodontiformes

Pycnodontiformes is an extinct order of primarily marine bony fish. The group first appeared during the Late Triassic and disappeared during the Eocene. The group has been found in rock formations in Africa, Asia, Europe, North and South America.

They were small to middle-sized fish, generally with laterally-compressed deep bodies, some with almost circular outlines, adapted for manoeuvrability in reef-like environments, though the group was morphologically diverse. Most, but not all members of the groups had jaws with round and flattened teeth, well adapted to crush food items (durophagy), such as echinoderms, crustaceans and molluscs. Some pycnodontiformes developed piranha like teeth used for eating flesh. Most species inhabited shallow marine reef environments, while a handful of species lived in freshwater or brackish conditions.

While rare during the Triassic and Early-Middle Jurassic, Pycnodontiformes became abundant and diverse during the Late Jurassic, exhibiting a high but relatively static diversity during the Early Cretaceous. At the beginning of the Late Cretaceous they reached their apex of morphological and species diversity (much of this due to fossils found in the Sannine Formation of Lebanon, such as Gebrayelichthyidae and Ichthyoceros), after which they began to gradually decline, with a more sudden decline at the end of the Cretaceous due to the collapse of reef ecosystems, finally becoming extinct during the Eocene. They are considered to belong to the Neopterygii, but their relationship to other members of that group is uncertain.

Evolution and diversity

Pycnodontiforms first appeared in the Late Triassic, alongside other successful groups of early neopterygians such as dapediiforms. A contributor for this early radiation of neopterygians was their effectiveness at adapting to different diets. Pycnodonts from the Late Triassic Zorzino Limestone in Italy had short and stout jaws with big crushing teeth for eating hard-shelled prey, while other actinopterygians like saurichthyids and birgeriids mainly occupied top predator piscivorous niches. They originated from marine habitats, specializing for manoeuverability in reef environments, but developed a variety of adaptations during the Mesozoic that enable them to pursue new diets and habitats, such as estuaries and freshwaters. Only a few species adapted for open waters, like those of the family Gyrodontidae. In the Western Tethys, pycnodonts have always had a high species diversity. This stable environment, alongside its favorable climate conditions, supported the dispersal patterns within basal pycnodontids.

They evolved such different jaw structures to avoid potential competition with other groups of durophagous neopterygians, such as the Dapediiformes and the Ginglymodi. Furthermore, their improved jaw performance also differentiated the pycnodonts from these neopterygians in that they allowed them to feed on tougher prey, while their increase in size allowed them to prey on larger items or specialise on a few species. Their families also differentiated amongst themselves in body and jaw shape, implying that they were more diverse in diet and habitat than previously thought.

The fossil record of pycnodonts spans 175 million years, from the Triassic to the Eocene, existing longer than non-avian dinosaurs. Their early record is incomplete, having only three genera from the Late Triassic, all with complete specimens. Whereas from the Early to Mid-Jurassic there are only isolated teeth and jaws, and rarely a few exceptions for better, but still incomplete, fossils. By the Late Jurassic Pycnodontiforms became more common in the fossil record, a fact that relates to the presence of Lagerstätten, providing articulated fossils. This growth continues and, by the Late Cretaceous, they experience a peak in diversity during the Cenomanian. They were severely struck by the K/Pg Extinction and afterwards their diversity shrunk, having never achieved pre-extinction levels of diversity, eventually going extinct by the Late Eocene (Priabonian).

Taxonomy

• Order Pycnodontiformes (Berg, 1937) Genus ?Acrorhinichthys Taverne & Capasso, 2015 Genus ?Archaeopycnodon Sanchez & Benedetto, 1980 Genus Arduafrons Frickhinger, 1991 Genus ?Athrodon le Sauvage 1880 non Osborn, 1887 Genus ?Callodus Thurmond, 1974 Genus ?Ellipsodus Cornuel, 1877 Genus ?Grypodon Hay, 1899 [Ancistrodon Dames, 1883 non De Beauvois, 1799 non Roemer, 1852 non Wagler, 1830] Genus ?Mercediella Koerber, 2012 [Camposichthys Figueiredo & Silva-Santos, 1991 non Travassos, 1946 non Whitley, 1953] Genus Paramesturus Taverne, 1981 Genus ?Piranhamesodon Kölbl-Ebert et al., 2018 Genus ?Pseudopycnodus Taverne, 2003 Genus ?Tergestinia Capasso, 2000 Genus ?Thurmondella Thurmond, 1974 non [Paramicrodon Thurmond, 1974 non de Meijere, 1913] Genus ?Uranoplosus le Sauvage, 1879 Genus ?Woodthropea Swinnerton, 1925 Family Brembodontidae Tintori, 1981 [Brembodidae; Gibbodontidae Tintori, 1981] Genus Brembodus Tintori, 1981 Genus Eomesodon Woodward, 1918 Genus Gibbodon Tintori, 1981 Family ?Hadrodontidae Thurmond & Jones, 1981 Genus Hadrodus Leidy, 1858 [Propenser Applegate, 1970] Family Gyrodontidae Berg, 1940 Genus Gyrodus Agassiz, 1833 Family Macromesodontidae Genus Macromesodon Blake 1905 non Lehman, 1966 [Mesodon Wagner, 1851 non Rafinesque, 1821; Gyronchus Agassiz, 1839; Apomesodon Poyato-Ariza & Wenz, 2002] Family Mesturidae Nursall, 1996 Genus Mesturus Wagner, 1862 Family Pycnodontidae Agassiz, 1833 corrig. Bonaparte, 1845 [Nursalliidae Bloy, 1987; Sphaerodontidae Giebel, 1846; Palaeobalistidae Blot, 1987; Proscinetidae Gistel, 1848; Gyronchidae]Fossil of Neoproscinetes penalvai Genus Abdobalistum Poyato-Ariza & Wenz, 2002 Genus Acrotemnus Agassiz, 1843 (=Macropycnodon Shimada, Williamson & Sealey, 2010) Genus ?Agassizilia Cooper & Martill, 2020 Genus Agoultpycnodus Taverne & Capasso, 2021 Genus Akromystax Poyato-Ariza & Wenz, 2005 Genus Anomiophthalmus Costa, 1856 Genus Anomoeodus Forir, 1887 Genus Apomesodon Poyato-Ariza & Wenz, 2002 Genus Athrodon Sauvage, 1880 Genus Brauccipycnodus Taverne & Capasso, 2021 Genus Coelodus Heckel, 1854 Genus Costapycnodus Taverne, Capasso & del Re, 2019 Genus Flagellipinna Cawley & Kriwet, 2019 Genus Gregoriopycnodus Taverne, Capasso & del Re, 2020 Genus Haqelpycnodus Taverne & Capasso, 2018 Genus Iemanja Wenz, 1989 Genus Libanopycnodus Taverne & Capasso, 2018 Genus Micropycnodon Hibbard & Graffham, 1945 Genus ?Neomesturus Cooper & Martill 2020 Genus Neoproscinetes De Figueiredo & Silva Santos, 1990 Genus Njoerdichthys Cawley, Lehmann, Wiese & Kriwet, 2020 Genus Nonaphalagodus Thurmond, 1974 Genus Nursallia Blot, 1987 Genus Ocloedus Poyato-Ariza & Wenz, 2002 Genus Omphalodus von Meyer, 1847 Genus Oropycnodus Poyato-Ariza & Wenz, 2002 Genus Palaeobalistum Taverne et al., 2015 Genus Paranursallia Taverne et al., 2015 ?Genus Phacodus Dixon, 1850 Genus Polazzodus Poyoto-Ariza, 2010 Genus Polypsephis Hay, 1899 (=Microdon Agassiz 1833 (preoccupied)) Genus Potiguara Machado & Brito, 2006 Genus Proscinetes Gistl, 1848 [Microdon Agassiz, 1833 non Meigen, 1803 non Fritsch, 1876 non Conrad, 1842 non Gistl, 1848 non Dixon, 1850; Polypsephis Hay, 1899] Genus Pycnodus Agassiz, 1833 Genus Pycnomicrodon Hay 1916 non Hibbard & Graffham, 1941 Genus Rhinopycnodus Taverne & Capasso, 2013 Genus Scalacurvichthys Cawley & Kriwet, 2017 Genus Sigmapycnodus Taverne & Capasso, 2018 Genus Sphaerodus Agassiz, 1833 Genus Sphaeronchus Stinton & Torrens, 1967 Genus Stenamara Poyato-Ariza & Wenz, 2000 Genus Stemmatias Hay, 1899 [Stemmatodus St. John & Worthen, 1875 non Heckel, 1854 non] Genus Stemmatodus Heckel, 1854 non St. John & Worthen, 1875 non Genus Sylvienodus Poyato-Ariza & Wenz, 2013Life Reconstruction of Thiollierepycnodus wagneri Genus Tepexichthys Applegate, 1992 Genus Tergestinia Capasso, 2000 Genus Texasensis Özdikmen, 2009 (=Callodus Thurmond, 1974 (preoccupied)) Genus Thiollierepycnodus Ebert, 2020 Genus Tibetodus Young & Liu, 1954 Genus Turbomesodon Poyato-Ariza & Wenz, 2004 [Macromesodon Lehman, 1966 non Blake, 1905] Genus Typodus Quenstedt, 1858 Family Serrasalmimidae Vullo et al, 2017 Genus Eoserrasalmimus Vullo et al, 2017 Genus Damergouia Vullo et al, 2017 Genus Polygyrodus White, 1927 Genus Serrasalmimus Vullo et al, 2017 Superfamily Coccodontoidea Taverne & Capasso, 2013 Genus Congopycnodus Taverne, 2019 Family Stanhopellidae Capasso, 2023 Genus Stanhopella Capasso, 2023 Family Coccodontidae Berg, 1940 Genus Coccodus Pictet, 1850 Genus Corusichthys Taverne & Capasso, 2014 Genus ?Cosmodus le Sauvage, 1879 [Glossodus Costa, 1851 non Agassiz, 1828 ex Spix & Agassiz, 1829 non McCoy, 1848] Genus Hensodon Kriwet, 2004 Genus Ichthyoceros Gayet, 1984 Genus Paracoccodus Taverne & Capasso, 2014 Genus Trewavasia White & Moy-Thomas, 1941 [Xenopholis Davis, 1887 non Peters, 1869; Xenopholoides Fowler, 1958] Family Gebrayelichthyidae Nursall & Capasso, 2004 Genus Gebrayelichthys Nursall & Capasso, 2004 Genus Maraldichthys Taverne & Capasso, 2014 Family Gladiopycnodontidae Taverne & Capasso, 2013 Genus Ducrotayichthys Taverne & Capasso, 2015 Genus Gladiopycnodus Taverne & Capasso, 2013 Genus Hayolperichthys Taverne & Capasso, 2015 Genus Joinvillichthys Taverne & Capasso, 2014 Genus Monocerichthys Taverne & Capasso, 2013 Genus Pankowskichthys Taverne & Capasso, 2014 Genus Rostropycnodus Taverne & Capasso, 2013 Genus Stenoprotome Hay, 1903 Genus Tricerichthys Taverne & Capasso, 2015

Phylogeny

The phylogenetic relation between pycnodonts and other actinopterygians is uncertain. The difficulty of placing them on a phylogenetic tree arises from the fact that they are a clade defined by a high number of autapomorphies (characteristics shared by a single taxon), which makes them easy to identify, but also makes the study of their relations with other actinopterygians difficult, since characteristics shared by other groups might be obfuscated by the immense amount of features and diversity of pycnodonts.

Previously, Pycnodontiformes where proposed to be a sister group of Teleostei or Teleosteomorpha, but in a 2015 analysis by Poyato-Ariza, they turned up as the most basal Neopterygii among the others of the group included, those being Lepisosteiformes, Semionotiformes, Macrosemiiformes, Halecomorphi and Teleostei.

As a means to avoid potential competition, the families of Pycnodontiforms evolved different body and jaw shapes, resulting in a highly diverse group. Pycnodontidae were the most advanced group, being the largest family, comprising 26 known described genera.

• Capasso, Luigi (2021). "Pycnodonts: An overview and new insights in the Pycnodontomorpha Nursall, 2010". Occasional Paper of the University Museum of Chieti, Monographic Publication, 1: 1–223.

Capasso, Luigi (2023). "Atlas of Pycnodonts: A pictorial guide to the Pycnodontomorpha (Pisces, Actinopterygii)". èDicola Publisher, Castellana, Italy; pp. 1-240.

• Sepkoski, Jack (2002). . Bulletins of American Paleontology. 364: 560.