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.

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.

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.

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.

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.

In 1982 Australian paleontologist Ralph Molnar proposed that the "Lansdowne Snout" actually belonged to a saltwater crocodile,[2] even going as far as to consider the idea that Pallimnarchus was actually a species of Crocodylus.

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.

Beyond the type material, which consisted of a premaxilla found in association with a dentary fragment, they also assigned both the "Lansdowne snout" and the "Mirani Shire skull" to this new species while placing "Geoff Vincent's specimen" in Pallimnarchus pollens.

This was not helped by the fact that Molnar's lectotype was lost following his initial redescription, with Jorgo Ristevski and colleagues suspecting that the material disappeared sometime during the late 90s or early 2000s.

The fate and current whereabouts of the lectotype are unknown and even a thorough search of the collection of the Queensland Museum in 2004 only yielded a singular, non-diagnostic fragment of said specimen with the remainder of the mandible nowhere to be found.

[3] By then it had become clear that Pallimnarchus had once again entered taxonomic limbo in spite of the previous efforts by Willis and Molnar, as the attempts at redefining the genus were insufficient to differentiate it from the surge of new mekosuchines and the lectotype itself disappeared.

Ristevski et al. published a third and final revision of the genus in 2020, declaring it dubious based on the fact that no distinguishing features could be found in the small piece of the lectotype that still remained.

[3] While this decision finally established a well preserved holotype and provided a detailed diagnosis, it also meant that the vast quantity of material previously assigned to Pallimnarchus had to be reevaluated, with mixed results.

A premaxilla and maxilla found near the Condamine River near Warra were attributued to Paludirex vincenti and the "Mirani Shire skull" could at least be tentatively assigned to the species.

However, both of these possible scenarios hinge on the lectotype being rediscovered, it preserving previously unrecognized diagnostic features and finally for there to be sufficient overlapping material of other crocodilians to compare it to.

[5] Paludirex was a large-bodied mekosuchine that bears all the hallmarks of a semi-aquatic ambush predator, possessing a platyrostral (flattened) skull as well as nostrils that were directed upwards like in many modern crocodilians.

The external nares of Paludirex are approximately circular in shape and almost entirely encased by the premaxilla, with only a small section at the posterior margin being composed of the nasal.

The transition from premaxilla to maxilla is marked by deep notches on each side which accommodate the large dentary teeth when the jaws are closed, constricting the skull in top view.

[3][5] Although no complete mandibles are known yet, the fossils of the upper jaws indicate that Paludirex had an overbite akin to an alligator, with the exception of the enlarged fourth tooth that would have slid neatly into the notch between the premaxilla and maxilla.

The teeth themselves were conical and showed no grooves in the tooth enamel like in Crocodylus species, while the cutting edges (carinae) had minor crenulations that were likely not true denticles.

[12] Although Pallimnarchus, as the earliest discovered fossil crocodilian of Australia, was crucial in establishing the presence of the "Australian tertiary radiation" (later known as Mekosuchinae), its precise position within this group remained unclear for a long time.

[3] Willis and Molnar go into more detail, comparing the anatomy of the "Mirani Shire skull" and the "Lansdowne snout", both now recognized as Paludirex vincenti, to the modern mugger crocodile and the extinct Purussaurus neivensis back in 1997.

Though little information was inferred from Purussaurus, they conclude from the similarities to mugger crocodiles that Paludirex may have preyed on a variety of animals, including but not limited to fish, birds, turtles and moderately sized mammals.

Like the extant species, Paludirex may have inhabited a wide variety of environments ranging from lakes and swamps to rivers and marshes,[8] although some researchers suggest that these animals may have avoided coastal waters.

Proportional differences do show that the bone couldn't have belonged to a saltwater crocodile, although later work emphasized that the Bluff Downs Crocodylus likely represents an as of yet unnamed species anyways, and Quinkana is disregarded by Mackness and Sutton due to its inferred terrestrial habits and its size.

The bone is thought to have belonged to a large male based on its size, which would match well with the fights that occur between members of this sex during the mating season in modern crocodiles.

While P. gracilis was more gracile in respects to P. vincenti, it still displayed a broad snout suited for a more generalist lifestyle, whereas the slender jaws of freshwater crocodiles are better adapted to catching small prey such as fish.

Ristevski and colleagues argue that saltwater crocodiles only arrived in Australia comparatively recently, showing no confirmed overlap in their distribution with the last individuals of Paludirex.

Hocknull et al. suggest that species of Crocodylus may have avoided the brunt of these events by retreating into more coastal waters, whereas the native mekosuchines may have been more dependent on freshwater.