Gladiopycnodus is an extinct genus of pycnodont fish from what is now Lebanon. Fossils of the genus have been found at the Haqel and Hjoula localities of the Sannine Formation, both of which have been dated to the late Cenomanian. One of the more notable features distinguishing the fish from other members of Gladiopycnodidae is the two pairs of large backwards-pointing spines on the ventral area of the fish. These spines are made from scales, often referred to as ventral scutes, and are extremely large compared to other pycnodonts. The larger of the two being positioned near the pelvic fin pointing backwards and spanning past the caudal fin. Gladiopycnodus has been suggested to have had demersal ecology though it has also been suggested that the animal could have had a niche similar to the modern snipefish and bellowsfish. These fish would have lived in a warm marine ecosystem throughout a continental shelf. Throughout this shelf, small but deep basins would have been present with them being surrounded by rudist reefs. There are two species currently recognized: G. karami and G. byrnei.
History and classification
The type species of Gladiopycnodus, G. karami, was described by Louis Taverne and Luigi Capasso in 2013 based on a single specimen (CLC S-393) that was a part of the Luigi Capasso collection. Three other specimens of the species are known from private collections, which were able to show anatomical features not well preserved in the holotype. The holotype was originally collected from Lebanon at the Haqel locality of the Sannine Formation, a formation known for its diverse assemblage of bony fish which includes a very large number of other pycnodont genera. A second species, G. byrnei, would later be named in 2016 by Giuseppe Marramà et al. based on single specimen (MGPTPU130335) also found within the Sannine Formation, although at a different site, the Hjoula locality. Along with the description of the new species, this publication would also reinterpret certain features of the genus' anatomy. The name Gladiopycnodus derives from the Latin word "gladius" which translates to sword, in reference to shape of the fin spines of the pectoral and anal fins, along with the word "pycnodus" which is a common suffix for pycnodonts. The species name of the type genus "karami" is named after Youssef Bey Karam, a hero in the history of Lebanon that lead the rebellion against the Ottoman Empire during the late 1860s. The species name of the second species on the other hand "byrnei" is named after the American musician and composer David Byrne.
Classification
Gladiopycnodus is the type genus of the family Gladiopycnodidae, which was described in the same publication as the genus. Fish in this family have long rostrums that are formed by the prefrontal bones, and which are much longer than the lower jaw. Though the loss of pectoral fins was initially thought to be a trait shared by all members of the family, Gladiopycnodus byrnei is an exception to this. This family is placed within the superfamily Coccodontoidea, another group named in the publication, which are a group of what have been referred to as armored pycnodonts. These fish, like some other more derived pycnodonts, have a reduced operculum with a much larger preoperculum. Along with this, members of the family have been considered to have what has been compared to a cephalothorax, due to fusion between the pectoral girdle and skull elements. Like in Gladiopycnodidae, small fins are seen in all members of the Cocodontoidea. Due to a number of unknown characters in comparison to other groups of pycnodonts, the family is generally not included in phylogenic trees of the order. The family Coccodontidae, however, has been included with it being considered an early branch of Pycnodontoidea in a 2002 paper by Francisco José Poyato-Ariza and Sylvie Wenz.
Within the family itself, Gladiopycnodus, is considered to be a more derived member of the family with it being placed near other similar members of the family such as Stenoprotome and Rostropycnodus. This placement is based on the 2015 publication by Taverne and Capasso which used a total of 88 characters from both previous publications by the authors along with the description of the second species of Gladiopycnodus by Marramà et al. Below is the phylogenic tree from this publication.
Description
Skull and pectoral girdle
Gladiopycnodus was a small fish with neither species having a length of over 13 centimetres (5.1 in). The most noticeable feature of the skull of the genus is the presence of a very long, sword-shaped rostrum that made up at least half of the skull length. This rostrum is only made up of the prefrontal, mesethmoid, parasphenoid, and premaxillae bones along with potentially the vomer. The prefrontal is the only one of these bones that forms the tip of the rostrum. The toothless premaxilla, similar to the prefrontal, was very long and broad and would have been connected to the underside of the prefrontal with the suture. The lower jaw is much shorter that the upper jaw though is much deeper and triangular in shape and processes two incisiform prehensile teeth on the dentary along with molariform teeth on the prearticular bone. This jaw is in articulation with the quadrate and symplectic bones with the quadrate being overall strange for the group due to the fact that the bone lacks a ossified quadratic process. The skull roof of the fish is made up of a number of bones including the frontals which are long and trapezoid-shaped in the genus. One difference between the two species is the presence of partial fusion in the skull of G. byrnei, which is in contrast to the type species in which these bones are unfused.
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