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Wikipedia / Pycnodontiformes

Pycnodontiformes (sometimes included inside a larger superorder, Pycnodontomorpha) 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. The oldest pycnodont known from articulated skeletons is Valgarzaella antiquissima (lower Norian Dolomie Zonate Formation, Garza Valley, Brescia, northern Italy). A contributor for this early radiation of neopterygians was their effectiveness at adapting to different diets. Pycnodonts from the Upper 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

OPTION A

  • Superorder †Pycnodontomorpha Nursall, 2010

Order †Gyrodontiformes Nursall, 2010

Family †Valgarzaellidae Capasso et al., 2026

Family †Gyrodontidae Berg, 1940

Family †Mesturidae Nursall, 1996

Order †Pycnodontiformes Berg, 1937

OPTION B (The currently most accepted)

Order Pycnodontiformes (Berg, 1937)

Family Brembodontidae Tintori, 1981 [Brembodidae; Gibbodontidae Tintori, 1981]

Family †Valgarzaellidae Capasso et al., 2026

Family †Gyrodontidae Berg, 1940

Family †Mesturidae Nursall, 1996

Family ?Hadrodontidae Thurmond & Jones, 1981

Family Macromesodontidae

Family Pycnodontidae Agassiz, 1833 corrig. Bonaparte, 1845 [Nursalliidae Bloy, 1987; Sphaerodontidae Giebel, 1846; Palaeobalistidae Blot, 1987; Proscinetidae Gistel, 1848; Gyronchidae]

Family Serrasalmimidae Vullo et al, 2017

Superfamily Coccodontoidea Taverne & Capasso, 2013

Family Stanhopellidae Capasso, 2023

Family Coccodontidae Berg, 1940

Family Gebrayelichthyidae Nursall & Capasso, 2004

Family Gladiopycnodontidae Taverne & Capasso, 2013

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.

References

  • 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.