A new family of gymnodont fish (Tetraodontiformes) from the earliest Eocene of the Peri-Tethys (Kabardino-Balkaria, northern Caucasus, Russia)

The environmental changes that occurred during the Paleocene-Eocene transition are crucial for the interpretation of the patterns and processes of diversification of vertebrate clades. A prominent increase of the number of vertebrate families occurred between the late Paleocene and early Eocene, resulting in the appearance of many in the earliest representatives of extant lineages, including a number of marine fish groups. Tetraodontiforms are a monophyletic group of derived teleost fishes encompassing a variety of bizarre morphologies. Even though the earliest members of this order appeared in the Late Cretaceous, most of the crown lineages date back to the Eocene. One of the crown tetraodontiform groups that appeared in the fossil record during the Eocene are the gymnodonts (pufferfishes, porcupinefishes, ocean sunfishes and their allies), which include a variety of species characterized by highly modified teeth incorporated into beak-like jaws and scales usually modified into prickly spines. Herein, we describe dagger Balkaria histiopterygia gen. et sp. nov., a gymnodont fish characterized by a strikingly peculiar morphology. The single available specimen in part and counterpart documented herein was recovered from the sapropelitic deposits that originated in the northern Peri-Tethys during the transition between the Paleocene and Eocene. Today, these deposits are exposed near the village of Gerpegezh, Republic of Kabardino-Balkaria, Russia. The skeletal structure reveals an extreme mosaicism of primitive and derived characters that result in a very bizarre and unexpected morphology. dagger Balkaria histiopterygia gen. et sp. nov. is unique among the extant and other fossil gymnodont fishes by, among many other features, the huge size of itsspiny-dorsal fin and the position of these spines on the top of the head. dagger Balkaria histiopterygia gen. et sp. nov. is the earliest unequivocal gymnodont fish, representing the sole member of the new family dagger Balkariidae. More particularly, dagger Balkaria histiopterygia gen. et sp. nov. is shown herein to be the oldest and arguably the most informative fossil of the gymnodont suborder Tetraodontoidei. The phylogenetic placement of this new taxon has been assessed based on both morphology alone and on a combination of morphological and molecular data that strongly supports the close relationship of dagger Balkaria gen. nov. to the herein restricted Tetraodontoidei. However, its position within Tetraodontoidei is unstable depending on the type of method of phylogenetic inference. Significantly younger ages, during the Late Cretaceous, are estimated for the diversification of Tetraodontiformes than in previous tip-dating analyses (Jurassic and Early Cretaceous) using the fossilized birth-death process; these new age estimates are in better agreement with the tetraodontiform fossil record.http://zoobank.org/urn:lsid:zoobank.org:pub:41764800-B0D8-4CA4-A111-5F4C4A281C37