Paludirex

Paludirex (meaning "swamp king") is an extinct genus of mekosuchine crocodylian from the Pliocene and Pleistocene of Australia. A large and robust semi-aquatic ambush hunter capable of attaining lengths of up to 5 m (16 ft), it was likely the top predator of Australia's waterways prior to the appearance of modern saltwater crocodiles. Two species are known, the smaller Paludirex gracilis and the larger Paludirex vincenti. A third as of yet unnamed species may have also existed.

The history of Paludirex is long and complicated, largely due to its connection with the historic genus Pallimnarchus. While the name Pallimnarchus was coined in 1886, making it the first fossil crocodile named from Australia, this was done so merely out of convenience, and this initial description did not come with a proper diagnosis. Despite the lacking definition, subsequent authors referred more and more material to this genus, leading to several attempts at redefining it during the late 20th century. However, the loss of the lectotype material and a sudden surge in mekosuchine research meant that Pallimnarchus was insufficiently differentiated from other crocodilians, leading to the name being declared a nomen dubium in 2020. The better preserved material, including several skulls, were used to erect the new genus Paludirex. A side effect of this was that much of the material once assigned to Pallimnarchus is now of uncertain affinities, meaning that it is unknown whether or not it belongs to Paludirex or some other as of yet unrecognized crocodilian.

Though roughly the size of a modern saltwater crocodile, Paludirex vincenti was notably more robust, with much deeper and wider jaws. The jaws of Paludirex gracilis were shallower but nonetheless wide. This, combined with the upwards facing nostrils and eyes, indicates that Paludirex was a semi-aquatic ambush predator, likely preying on a wide range of prey animals from fish to mammalian megafauna. This would also set them apart ecologically from other contemporary crocodilians like the terrestrial Quinkana and the narrow-snouted freshwater crocodiles that appeared around the same time as P. gracilis.

Little is known about the extinction of Paludirex. It was among the last mekosuchines still found in Australia, surviving until at least 50,000 years ago, and likely disappeared alongside much of the continent's megafauna as part of the Late Quaternary extinction event. This disappearance is generally attributed to climate change, which would have led to widespread aridification and the destruction of the freshwater systems these crocodilians inhabited, though the extinction of the terrestrial Australian megafauna (which may have been human caused) may have been a contributing factor. Competition with saltwater crocodiles has also been proposed and is under investigation.

Although only named in 2020, Paludirex has a long and rich history thanks to its ties to the historic genus Pallimnarchus. Pallimnarchus pollens was informally described in 1886 by Charles Walter De Vis, who based the genus on skull remains and osteoderms discovered around 1860 that was mineralised by apatite. De Vis' collection lacked cohesion and not only consisted of material that belonged to different individuals, but even to different species, as one bone was later proven to have belonged to a species of Quinkana. Their exact origin is likewise not well recorded by De Vis, with the type locality of the remains being unknown beyond the fact that they stem from the Darling Downs region of Queensland. While this marked the first fossil crocodile ever described from Australia, De Vis, by his own admittance, was generally unfamiliar with the fossil record of Cenozoic crocodilians. Unable to guarantee that it could not be placed in any of the genera known at the time, De Vis coined the cabinet name Pallimnarchus "out of convenience". Despite the informal nature of this early research, and the fact that De Vis did not establish a holotype specimen, subsequent researchers generally accepted the name which came to see widespread use throughout the following 150 years.

During this timespan more and more crocodilian material came to be found across Australia, which was oftentimes simply assigned to Pallimnarchus based on superficial similarities. Among the more significant specimens referred to Pallimnarchus over the years was the so-called "Lansdowne Snout", which would go on to change classification multiple times before most recently being included within Paludirex. The name Pallimnarchus remained unchanged all the while, with the exception of one instance in 1968 when it was erroneously referred to as Crocodylus pallimnarchus by W. D. Sill. While the nomenclature used by Sill was incorrect, this would not be the only time a link between Pallimnarchus and the genus Crocodylus was made. In 1982 Australian paleontologist Ralph Molnar proposed that the "Lansdowne Snout" actually belonged to a saltwater crocodile, even going as far as to consider the idea that Pallimnarchus was actually a species of Crocodylus.

Molnar however did not follow through on that, instead publishing a formal redescription of Pallimnarchus later that same year. Molnar tentatively accepted the validity of Pallimnarchus as a genus but recognized the highly flawed nature of De Vis' work. Given the fact that De Vis' material consisted of fragmentary remains of multiple individuals, Molnar established a lectotype, the anterior portion of a lower jaw (specimen QMF1149) that was part of De Vis' original collection. Beyond being labeled as the holotype in collections already, this immature specimen was chosen as it was significantly more complete than the other material the genus had previously been based on. However, Molnar's description was still limited in comparison, with only four other Australasian crocodilians being known at the time: the saltwater crocodile, freshwater crocodile, New Guinea crocodile and Quinkana.

Not long after this first attempt at revising Pallimnarchus, even more material came out of Queensland, most notably the "Mirani Shire skull" and the "Dalby specimen", also known as "Geoff Vincent's specimen". The former was discovered prior to 1986 by Jack Williams Jr. in what may have been the Pioneer Valley near Mackay and represents a partial skull of which the entire dorsal surface is encased by concrete. The latter on the other hand was discovered sometime between 1984 and 1990 by Geoff Vincent near the Chinchilla Rifle Range in the western Darling Downs region and consists of a series of associated skull fragments that were eventually loaned to the Queensland Museum.