Tropeognathus

Tropeognathus (meaning "keel jaw") is a genus of large pterosaurs from the late Early Cretaceous of South America. This genus is considered to be a member of the family Anhangueridae, however, several studies have also recovered it within another family called Ornithocheiridae. Both of these families are diverse groups of pterosaurs known for their keel-tipped snouts and large size. "Tropeognathus" is regarded as the largest pterosaur found in the Southern Hemisphere, only rivaled by the huge azhdarchids. The type and only species is "Tropeognathus mesembrinus". Fossil remains of "Tropeognathus" have been recovered from the Romualdo Formation, which is a Lagerstätte located in the Santana Group of the Araripe Basin in northeastern Brazil.
Discovery and naming.
In the 1980s the German paleontology museum "Bayerische Staatssammlung für Paläontologie und historische Geologie" in Munich acquired a pterosaur skull from Brazilian fossil dealers that had probably been found in Ceará, in the geological group called the Santana Group, which is located in the Araripe Basin (Chapada do Araripe) of Brazil. In 1987, it was named and described as the type species "Tropeognathus mesembrinus" by Peter Wellnhofer. The generic name is derived from Greek τρόπις, "tropis", "keel", and γνάθος, "gnathos", "jaw". The specific name is derived from Koine "mesembrinos", "of the noontide", simplied as "southern", in reference to the provenance from the Southern hemisphere.
The holotype, BSP 1987 I 46, was discovered in a layer of the Romualdo Formation within the Santana Group, dating to the latest Aptian and earliest Albian stages. Along with the holotype, several other pterosaur specimens were found in the fossil site, these specimens however, were referred to genera such as "Anhanguera" and "Cearadactylus". The uncovered holotype consists of a skull with lower jaws. A second specimen was referred by André Jacques Veldmeijer in 2002: SMNS 56994, which consists of a partial mandible. In 2013, Brazilian paleontologist Alexander Kellner referred a third, larger, specimen: MN 6594-1, a skeleton with skull, with extensive elements of all body parts, except the tail and the lower hindlimbs.
After "Tropeognathus mesembrinus" was named by Peter Wellnhofer in 1987, other researchers tended to consider it part of several other genera, leading to an enormous taxonomic confusion. It was considered an "Anhanguera mesembrinus" by Alexander Kellner in 1989, a "Criorhynchus mesembrinus" by Veldmeijer in 1998 and a "Coloborhynchus mesembrinus" by Michael Fastnacht in 2001. Later the same year, David Unwin referred the "Tropeognathus" material to "Ornithocheirus simus", making "Tropeognathus mesembrinus" a junior synonym, though he had reinstated a "Ornithocheirus mesembrinus" in 2003. In 2006, Veldmeijer accepted that "Tropeognathus" and "Ornithocheirus" were cogeneric, but rejected "O. simus" as the type species of "Ornithocheirus" in favor of "O. compressirostris", which was named as "Lonchodectes" by Unwin due to an analysis by English paleontologist Reginald Walter Hooley in 1914. This made Veldmeijer use the names "Criorhynchus simus" and "Criorhynchus mesembrinus" instead. In 2013 however, Taissa Rodrigues and Alexander Kellner concluded "Tropeognathus" to be valid, and containing only "T. mesembrinus", the type species.
Back in 1987, Wellnhofer had named a second species called "Tropeognathus robustus", based on specimen BSP 1987 I 47, which is a more robust lower jaw. In 2013 however, "T. robustus" was considered as a species of "Anhanguera", resulting in an "Anhanguera robustus".
Description.
Size.
"Tropeognathus" is known to have reached wingspans of about , as can be inferred from the impressive size of the specimen MN 6594-1. The maximum wingspan estimate for "Tropeognathus" reaches , making it slightly larger than the average estimate, though much larger than other close relatives such as "Ornithocheirus" and "Coloborhynchus", which are typically estimated to be between . A skull unearthed belonging to the related "Coloborhynchus" likely measured , which led to a wingspan estimate of , respectively larger than the average estimates for this genus, but still shorter than that of "Tropeognathus". This specimen however, was concluded to belong to the another genera, though in several studies, some paleontologists consider it under the species "Coloborhynchus capito", which was originally called "Ornithocheirus capito" by the British paleontologist Harry Govier Seeley back in 1870.
Skull and crests.
The skull of "Tropeognathus" bore distinctive convex "keeled" crests on its snout and underside of the lower jaws, and this was prominent, well-developed, and relatively large in "Tropeognathus", specially in males, however, compared to other relatives such as "Ornithocheirus", they were relatively thin rather than thick. The upper crests arosed from the snout tip and extended back to the "fenestra nasoantorbitalis", the large opening in the skull side. An additional, smaller crest projected down from the lower jaws at their symphysis ("chin" area). The similar anhanguerid "Anhanguera" possessed jaws that were tapered in width, but expanded into a broad, spoon-shaped rosette at the tip, which differed from "Tropeognathus" for having a narrower appearance. The jaws can be distinguished from its relatives by a few differences in the crest: unlike its close relatives "Coloborhynchus" and "Ornithocheirus", the crest on the upper jaw of "Tropeognathus" was more prominent and much larger, and therefore resulting in a broader skull.
Vertebrae.
The first five dorsal vertebrae of "Tropeognathus" are fused into a notarium, with five sacral vertebrae fused into a synsacrum, and the third and fourth sacral vertebrae are keeled within. The front blade of the ilium is strongly directed upwards, resulting in a narrow structure.
Classification.
In 1987, Wellnhofer assigned "Tropeognathus" to a Tropeognathidae. This concept was not adopted by other workers; several researchers place "Tropeognathus mesembrinus" in the Anhangueridae, along with "Anhanguera", while other cladistic analyses place "Tropeognathus" within the Ornithocheiridae as a basal member, meaning that it was more closely related to "Ornithocheirus" than "Anhanguera". This concept is mostly used by the European colleagues, who prefer to use the Ornithocheiridae as the most inclusive group rather than the Anhangueridae. A topology made by Andres and Myers in 2013 placed "Tropeognathus" within the family Ornithocheiridae in a more basal position than "Ornithocheirus", and the family itself is placed within the more inclusive clade Ornithocheirae. However, many subsequent analyses made in 2019 and 2020 have recovered "Tropeognathus" within the family Anhangueridae, with a specific one by Borja Holgado and Rodrigo Pêgas in 2020, placing "Tropeognathus" more specifically within the subfamily Tropeognathinae, sister taxon to "Siroccopteryx".
Topology 1: Andres & Myers (2013).
Topology 2: Holgado & Pêgas (2020).
In popular culture.
"Tropeognathus mesembrinus" was the subject of an entire episode of the award-winning BBC television program "Walking with Dinosaurs" (which used the first name of its cousin "Ornithocheirus" but was incorrectly named as a species of it, as "Ornithocheirus mesembrinus"). In "Walking with Dinosaurs: A Natural History", a companion book to the series, it was claimed that several large bone fragments from the Santana Group (known as Santana Formation in the book) of Brazil had indicated that "O. mesembrinus" may have had a wingspan reaching almost and a weight of , making it one of the largest known pterosaurs. However, the largest definite "Ornithocheirus mesembrinus" specimens described at the time measured , in terms of wingspan. The specimens which the producers of the program used to justify such a large size estimate were described in 2012, and were under study by Dave Martill and David Unwin at the time of "Walking With Dinosaurs"' production. The final description of the remains found a maximum estimated wingspan of for this large specimen. Unwin stated that he did not believe the higher estimate used by the BBC was likely, and that the producers likely chose the highest possible estimate because it was more "spectacular." Nevertheless, specimen MN 6594-V in 2013 was, at its degree of completeness, the largest known pterosaur individual.