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. History of Discovery. "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 Argentinia, 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. Description. "Mourasuchus" is well known for its strange anatomy, its skull exceptionally dorsoventrally flattened and 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 in length, with the holotype skull of "M. pattersoni" being 106 cm long while the type specimen of "M. arendsi" is 91 cm long. The largest skull belongs to "M. amazonensis" with 111.4  cm. In a 2020 study, Cidade "et al." attempted to determine the body sizes of the 4 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 6.30 meters for "M. atopus", the smallest species, and upwards of 9.47 meters and 4.3 tons for "M. amazonensis". However, these results are to be taken with caution as the exact proportions are still unknown. Phylogeny. 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). The following tree is based on the results recovered by  Cidade "et al." (2017), excluding the North American "Orthogenysuchus" and including the then newly named "Mourasuchus pattersoni" while also following the synonymy of "M. nativus" with "M. arendsi". Like Bona "et al." before them, the authors recover "Mourasuchus" and "Purussaurus as closely related clades, this time as sister genera with "Centenariosuchus" just outside this shared clade. Paleobiology. Diet. Much like with the unrelated but morphologically similar Stomatosuchidae, the feeding ecology of "Mourasuchus" is enigmatic and poorly understood, with a variety of hypothesis having been proposed to make sense of its specialised anatomy. With slender and short mandibular ramni and perfectly  occluding, slender and almost homodont teeth, Mourasuchus was not built to capture and hold prey like modern crocodilians. In his 1965 publication on the crocodilians of Cenozoic Columbia, Langston proposed three ideas on how "Mourasuchus" may have fed. He suggests that it may have waited with opened jaws for approaching prey, fed on aquatic and floating plant material or searched for prey by sifting through substrate at the bottom of bodies of water. While his other hypothesis have gone without much attention, the later of the three has been discussed by later publications, referring to this ecology as either "filter  feeding", "straining technique" or "gulp feeding". Cidade "et al." follow Langston's proposed feeding hypothesis, speculating that "Mourasuchus" may have fed on small invertebrates, molluscs, crustaceans and small fish, specialising in trying to ingest as much prey at once as possible. This they suggest explains the platyrostral-broad skull morphology, which differs from the usual crocodilian skull morphology that typically either favours longirostrine forms providing speediness or robust-snouted forms with increased bite force. Langston also proposed a throat pouch on the basis of comments Franz Nopsca made regarding "Stomatosuchus", however there is no evidence for such a structure in  Mourasuchus nor is its presence certain in stomatosuchids. As the prey "Mourasuchus" would have fed on in accordance with the straining technique hypothesis would likely be found either in free water or substrate, the animal would have also captured inedible material during feeding. For this a type of filtering mechanism or behavior would be beneficial, however nothing indicating as much has been found so far. Due to the absence of evidence for actual filtering, Cidade "et al." instead favor the term "gulp feeding" for "Mourasuchus" hypothetical hunting behavior. In a 2020 study Cidade "et al." aimed to determine "Mourasuchus" ability  to perform the death roll, a crucial behavior observed in modern crocodiles and alligators that is commonly observed during feeding, when crocodiles rapidly spin to rip pieces of flesh from their prey. The study calculated a very low "death roll" capability indicator (DRCI) similar to that seen in the Slender Snouted Crocodile, Indian Gharial and False Gharial, which suggests that "Mourasuchus" was very unlikely to be able to perform the death roll. Paleoenvironment. "Mourasuchus" lived during an era of great crocodilian diversity in South America, being found in environments shared with genera inhabiting a diverse set of nisches. In the  Peruvian Pebas Formation "M. atopus" was found alongside the large bodied "Purussaurus neivensis", a medium sized species of "Gryposuchus", "G. pachakamue" and 3 distinct taxa of durophagous caimans. The later species "M. arendsi" coexisted with an even greater hoast of contemporary crocodilians in the Urumaco Formation, sharing its habitat with the larger "Purussaurus mirandai", several large-bodied gharials, multiple durophagous caimans and the true crocodile "Charactosuchus". In some areas such as the Urumaco and Solimões Formation multiple species of "Mourasuchus" are known, with the former being home to both "M. pattersoni" and "M. arendsi" while the later has yielded "M.  arendsi" and "M. amazonensis". The great diversity of crocodylomorphs in these Miocene-age (Tortonian stage, 8 million years ago) wetlands suggests that niche partitioning was efficient, which would have limited interspecific competition.