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.