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.[3] Most, but not all members of the groups had jaws with round and flattened teeth,[4] well adapted to crush food items (
durophagy), such as echinoderms, crustaceans and molluscs.[3] 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 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.[3]
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.[3] 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.[7]
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.[3]
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).[3]
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 Pycnomicrodon Hay 1916 non Hibbard & Graffham, 1941
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.[13]
As a means to avoid potential competition, the families of Pycnodontiforms evolved different body and jaw shapes, resulting in a highly diverse group.[3]Pycnodontidae were the most advanced group, being the largest family, comprising 26 known described genera.[14]
^Cooper, Samuel L.A.; Martill, David M. (August 2020). "A diverse assemblage of pycnodont fishes (Actinopterygii, Pycnodontiformes) from the mid-Cretaceous, continental Kem Kem Group of south-east Morocco". Cretaceous Research. 112: 104456.
doi:
10.1016/j.cretres.2020.104456.
^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
^
abPoyato-Ariza, Francisco (2015). "Studies on Pycnodontid fishes (I): Evaluation of their phylogenetic position among actinopterygians". Rivista Italiana di Paleontologia e Stratigrafia. 121 (3): 329–343.
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.