In 1987 paleontologist Peter Wellnhofer named a new pterosaur, Tropeognathus mesembrinus, from the Lower Cretaceous Romualdo Formation of Ceará, Brazil. The original specimen is an almost entirely complete and three-dimensionally preserved skull and mandible. Another mandible of this species was described by André Jacques Veldmeijer in 2002, and a new specimen, including postcranial material, was described in 2013 by Alexander Kellner and colleagues. The name Tropeognathus comes from Greek, and translates to, “keeled jaw.” The species name comes from the Greek word for “southern,” referring to the southern hemisphere.
The skull is 63 cm (25 inches) long with a long and tapering rostrum before the eyes. At the end of the rostrum, there is a large semi-circular crest that is 23 cm (9 inches) long and 10.5 cm (4 inches) tall. The mandible bears a similar, but less prominent crest with a length of 13 cm (5 inches) and a depth of 5 cm (2 inches). When seen from above, the end of the rostrum doesn’t appear to have any lateral spooning, unlike many of its kin.
The upper jaw has 13 pairs of teeth or empty tooth sockets and the lower jaw has 11 pairs. The teeth are entirely in the front half of the jaws and are concentrated near the end of the jaws. There are only three upper teeth preserved in the original skull, but the lower jaw has ten preserved. The teeth are stout cones and slightly recurved.
The newest specimen described by Kellner and colleagues comes from a very large adult individual. It includes portions of the skull and mandible, vertebrae from the neck, torso, and sacrum, the sternum, the right shoulder and humerus, and pelvis. The skull of this individual was estimated to be nearly 1 m (39 inches) long, and differs from the original specimen by having a much steeper rear margin of the crest. Its wingspan was estimated to be over 8 m (26 feet), making it one of the largest pterosaurs known.
Tropeognathus lived about 110 million years ago in a rift valley environment formed by South America’s separation from Africa. The valley was filled with large numbers of lakes, and was periodically inundated by the incipient Atlantic Ocean. Tropeognathus was an aerial fisher, and hunted in the lakes and marine waters. Wellnhofer hypothesized that the semi-circular crests may have helped stabilize the snouts when dipping into the water, like the keel or prow of a boat.
Tropeognathus was recognized by Wellnhofer as quite similar to Ornithocheirus simus, known from roughly contemporaneous rocks in England. They are so similar in fact, several author have sunk T. mesembrinus into Ornithocheirus, as Ornithocheirus mesembrinus, although this is not universally accepted. Tropeognathus mesembrinus is differentiated from Ornithocheirus simus by its much thinner crest.
In phylogenetic analyses Tropeognathus is usually found to be a close relative of Coloborhynchus and Anhanguera, but has not always been found to be closely related to Ornithocheirus. In a 2018 Nick Longrich and colleagues found Tropeognathus to be most closely related to Ornithocheirus, Coloborhynchus, Uktenadactylus, and Siroccopteryx; and more distantly related to Anhanguera. In contrast, a 2019 analysis performed by Borja Holgado and colleagues found Tropeognathus in an unresolved trichotomy with the coloborhinchines and anhanguerines, and Ornithocheirus was found to be in a more basal position in the ornithocheiroid lineage.