Pycnodontiformes Explained

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.[1]

They were small to middle-sized fish, generally with laterally-compressed deep bodies, some with almost circular outlines,[2] adapted for manuverability 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,[3] well adapted to crush food items (durophagy), such as echinoderms, crustaceans and molluscs.[4] Some pyncodontiformes developed piranha like teeth used for eating flesh.[5] [6] 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

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.[16]

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.[17]

References

Notes and References

  1. Web site: Pycnodontiformes. Palaeos vertebrates. 27 July 2009. dead. https://web.archive.org/web/20081025044133/http://www.palaeos.com/Vertebrates/Units/100Neopterygii/100.600.html#Pycnodontiformes. 25 October 2008.
  2. Web site: Pycnodontid fishes from the Kansas Cretaceous. Oceans of Kansas. 16 April 2009. 29 July 2009.
  3. Book: 2009. Paleotorus: The Laws of Morphogenetic Evolution. Meanma Press. McMenamin, M. A. S.. 978-1-893882-18-8.
  4. Cawley . John J. . Marramà . Giuseppe . Carnevale . Giorgio . Villafaña . Jaime A. . López‐Romero . Faviel A. . Kriwet . Jürgen . February 2021 . Rise and fall of †Pycnodontiformes: Diversity, competition and extinction of a successful fish clade . Ecology and Evolution . en . 11 . 4 . 1769–1796 . 10.1002/ece3.7168 . 2045-7758 . 7882952 . 33614003.
  5. Kölbl-Ebert . Martina . Ebert . Martin . Bellwood . David R. . Schulbert . Christian . 2018-11-05 . A Piranha-like Pycnodontiform Fish from the Late Jurassic . Current Biology . en . 28 . 21 . 3516–3521.e2 . 10.1016/j.cub.2018.09.013 . 30344113 . 53045425 . 0960-9822. free .
  6. Vullo . Romain . Cavin . Lionel . Khalloufi . Bouziane . Amaghzaz . Mbarek . Bardet . Nathalie . Jalil . Nour-Eddine . Jourani . Essaid . Khaldoune . Fatima . Gheerbrant . Emmanuel . 2017-07-28 . A unique Cretaceous–Paleogene lineage of piranha-jawed pycnodont fishes . Scientific Reports . en . 7 . 1 . 6802 . 10.1038/s41598-017-06792-x . 28754956 . 5533729 . 2045-2322.
  7. Book: Nelson. Joseph S.. Grande. Terry C.. Wilson. Mark V. H.. 2016. Fishes of the World. 5th. John Wiley & Sons. 9781118342336.
  8. van der Laan. Richard. 2016. Family-group names of fossil fishes.
  9. Cooper . Samuel L. A. . Martill . David M. . 2020-08-01 . A diverse assemblage of pycnodont fishes (Actinopterygii, Pycnodontiformes) from the mid-Cretaceous, continental Kem Kem Group of south-east Morocco . Cretaceous Research . 112 . 104456 . 2020CrRes.11204456C . 10.1016/j.cretres.2020.104456 . 0195-6671.
  10. Cooper, S.L.A. and Martill, D.M. (2020). "Pycnodont fishes (Actinopterygii: Pycnodontiformes) from the Upper Cretaceous (lower Turonian) Akrabou Formation of Asfla, Morocco" Cretaceous Research 116, 104607
  11. Cawley . John . Lehnmann . Jens . Wiese . Frank . Kriwet . Jürgen . 2020 . Njoerdichthys dyckerhoffi gen. et sp. nov. (Pycnodontiformes, lower Turonian) northward migration caused by the Cretaceous Thermal Maximum . . 116.
  12. Özdikmen . H. . 2009-01-06 . Texasensis nom. nov., a new name for the preoccupied fossil fish genus Callodus Thurmond, 1974 (Osteichthyes: Pycnodontiformes) . Texasensis Nom. Nov., A New Name for the Preoccupied Fossil Fish Genus Callodus Thurmond, 1974 (Osteichthyes: Pycnodontiformes) . en . 4 . 2 . 616 . 1306-3022.
  13. Vullo . Romain . Cavin . Lionel . Khalloufi . Bouziane . Amaghzaz . Mbarek . Bardet . Nathalie . Jalil . Nour-Eddine . Jourani . Essaid . Khaldoune . Fatima . Gheerbrant . Emmanuel . 2017-07-28 . A unique Cretaceous-Paleogene lineage of piranha-jawed pycnodont fishes . Scientific Reports . 7 . 1 . 6802 . 10.1038/s41598-017-06792-x . 2045-2322 . 5533729 . 28754956.
  14. L. Taverne . L. Capasso . 2014 . Ostéologie et phylogénie des Coccodontidae, une famille remarquable de poissons Pycnodontiformes du Crétacé supérieur marin du Liban, avec la description de deux nouveaux genres . dead . Palaeontos . 25 . https://web.archive.org/web/20170705013931/http://www.palaeontos.be/25/det25.htm . 2017-07-05 . 2016-07-05.
  15. Taverne . Louis . Capasso . Luigi . 2013-10-02 . Gladiopycnodontidae, a new family of pycnodontiform fishes from the Late Cretaceous of Lebanon, with the description of three genera . European Journal of Taxonomy . en . 57 . 10.5852/ejt.2013.57 . 2118-9773.
  16. Poyato-Ariza . Francisco . 2015 . Studies on Pycnodontid fishes (I): Evaluation of their phylogenetic position among actinopterygians . . 121 . 3 . 329–343.
  17. Ebert . Martin . 2020 . A new genus of Pycnodontidae (Actinopterygii) from the Upper Jurassic of France and Germany, included in a phylogeny of Pycnodontiformes . . 188 . 2 . 434–454.

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