Mourasuchus

Mourasuchus is an extinct genus of giant, aberrant caiman from the Miocene of South America. Its skull has been described as duck-like, being broad, flat, and very elongate, superficially resembling Stomatosuchus from the Late Cretaceous.

Mourasuchus was first described by Price in 1964 based on a strange and nearly complete skull from the Solimões Formation of Amazonian Brazil, calling it Mourasuchus amazoniensis. Unaware of Price's discovery, Langston described "Nettosuchus" atopus ("Absurd Duck Crocodile") only a year later based on fragmentary cranial, mandibular and postcranial remains from the middle Miocene La Venta Lagerstätte, a part of the Honda Group. Although he did recognize its similarities to caimans and alligators, Langston reasoned that its bizarre anatomy warranted its own monotypic family, naming it Nettosuchidae. After being informed about the existence of Mourasuchus by Mr. W.D. Sills, Langston wrote a follow-up publication acknowledging his taxon to be a junior synonym of Mourasuchus.

A third species was found in the Urumaco Formation of Venezuela in 1984 and named Mourasuchus arendsi by Bocquentin-Villanueva. Following this discovery, "Carandaisuchus" nativus was named in 1985 based on fossils of the Ituzaingó Formation of Argentina, as well as remains from Brazil and Bolivia. However, by 1990 Caraindaisuchus had been lumped into Mourasuchus and Scheyer & Delfino (2016) eventually concluded that M. nativus was merely a junior synonym of M. arendsi, bringing the number of species from four to three. It wasn't long until the genus returned to four species however, with M. pattersoni (named after Anglo-American paleontologist Bryan Patterson) being described by Cidade et al. the following year. Indeterminate Mourasuchus fossils were found in the Yecua Formation of Bolivia.

Mourasuchus is well known for its strange anatomy, its skull exceptionally dorsoventrally flattened, broad, and overall platyrostral in shape, differing greatly from any other caiman currently known, most closely resembling the enigmatic crocodyliform Stomatosuchus from the Late Cretaceous Bahariya Formation. The nares are elevated and so is the high postrostral cranium and short skull table. The orbital margins are likewise raised above the rostrum with thickened anteromedial margins formed by a knob-like swelling of the frontal and prefrontal bones. Overall, the orbits are smaller than the infratemporal fenestrae. The teeth were generally small and conical, numbering upwards of 40 teeth on each side of the upper and lower jaw and interlocking perfectly. The posterior teeth of the dentary and maxilla are laterally compressed. The premaxilla of Mourasuchus show noticeably large occlusal pits, especially noticeable in M. arendsi. However, such pits can be observed in a variety of extant and fossil taxa and are not considered to be diagnostic between species of Mourasuchus.

The mandible is U-shaped with a sharp transition between the first five teeth and the rest of the dentary teeth. The first to fifth tooth are located on the curved anterior portion of the mandible, while the bone is straight from the sixth onward. The mandibular symphysis is short and slender, only extending to the first posterior alveoli, meaning the animal most likely had a relatively weak bite. The osteoderms of Mourasuchus show conspicuous spines on their dorsal surface.

The humerus of Mourasuchus is long and slender and interpreted to possibly indicate weakened forelimbs, potentially corresponding with a predominantly aquatic lifestyle.

Due to the fragmentary remains of Mourasuchus the body size is difficult to determine. Mourasuchus skulls range up to a meter (3.3 ft) in length, with the holotype skull of M. pattersoni being 108.1 centimetres (42.6 in) long while that of M. arendsi could reach a length of 108.5 centimetres (42.7 in) long. The largest skull belongs to M. amazonensis at 113.5 centimetres (44.7 in). In a 2020 study, Cidade et al. attempted to determine the body sizes of the four recognized Mourasuchus species based on the head: body ratio of modern genera like Caiman latirostris and Alligator mississippiensis. Their results recovered a mean body size of 6.3 m (21 ft) and 1.2 metric tons (1.3 short tons) for M. atopus, the smallest species, and upwards of 9.5 m (31 ft) and 4.4 metric tons (4.9 short tons) for M. amazonensis. However, a later study contradicted these results. In 2022 Paiva and colleagues argued that the dorsal cranial length was a poor basis for size estimates compared to the width of the skull. Additionally, they found that prior studies commonly included juvenile animals in their data, muddying the results. This study calculated a length between 2.5–3.1 m (8 ft 2 in – 10 ft 2 in) for Mourasuchus atopus, 3.8–4.8 m (12–16 ft) for Mourasuchus arendsi, 4.3–5.5 m (14–18 ft) for Mourasuchus pattersoni and for Mourasuchus amazoniensis, the largest species, 4.7–5.98 m (15.4–19.6 ft). These results were achieved by specifically restricting the dataset to extant species of caimans, while calculations using measurements from all of Crocodilia generally rendered greater sizes but may be overestimates. Still, the authors note that the generally smaller stature of modern caimans may have influenced the results in a similar fashion.

Although Mourasuchus has been firmly placed within Caimaninae by authors, the exact relationship it has with other crocodilians of this family and even the relationship between the different species of the genus had long been unclear. Some studies have proposed that Mourasuchus was closely related to the Eocene Orthogenysuchus from North America, and more distantly to the giant caiman Purussaurus which it shared its habitat with. However, more recent papers indicate that ongoing preparation conducted on Orthogenysuchus significantly influences the scoring of this taxon's characters, leading to some authors removing the animal from analysis until further publications. The cladogram below shows the phylogenetic tree used by Bona et al. (2012).