Pallimnarchus Lectotype

''Paludirex'' (meaning "swamp king") is an extinct genus of mekosuchine crocodylian from the Pliocene of Australia. A large and robust semi-aquatic ambush hunter capable of attaining lengths of up to , 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.

Discovery and naming

Early history of ''Pallimnarchus''

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''.

Revisions and second species

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.

The 90s saw a marked increase regarding the known extinct crocodilian fauna of Australia, with a multitude of taxa being named from 1990 onward. Although at first only referred to as the "Australian tertiary radiation", the group eventually became known as the Mekosuchinae, with ''Pallimnarchus'' serving as one of its earliest recognized members. A second attempt at refining ''Pallimnarchus'' was published some years later in 1997, with Molnar this time working alongside crocodilian expert Paul Willis. Willis and Molnar worked on multiple specimens that were referred to ''Pallimnarchus'' at the time, which resulted in them recognizing a second species that they dubbed ''Pallimnarchus gracilis''. 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''.

''Paludirex''

Things once again fell silent around ''Pallimnarchus'' during the 2000s, with publications on this genus only appearing occasionally. 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. This means that the only source for information on the lectotype are the illustrations and photographs provided by Molnar in the 1982 paper. In 2008, "Geoff Vincent's specimen" was returned to Dot Vincent, the wife of the late Geoff Vincent, who subsequently donated it to the Chinchilla Museum. In the process one skull fragment was accidentally left behind in Brisbane, which led to different parts of the skull now having different specimen numbers.

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. The team, which included Ralph Molnar, instead established a new name for the material previously assigned to ''Pallimnarchus''. The result of their work was ''Paludirex vincenti'', a taxon not based on mandibular remains as ''Pallimnarchus'' was but based on the skull fragments that compose "Geoff Vincent's specimen".

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. Some specimens, notably those that preserve elements of the cranium, could confidently be assigned to the newly erected genus. 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. Additionally, the "Lansdowne Snout" was referred to P. vincenti three years later. Furthermore, the validity of the material previously dubbed ''Pallimnarchus gracilis'' was confirmed, creating the new combination ''Paludirex gracilis''. However, ''Paludirex gracilis'' was now restricted to the type material, the premaxilla and the associated dentary fragment.

The switch to "Geoff Vincent's specimen" as the holotype did have one big drawback, which is that the specimen has no associated mandibular remains. This means that the only lower jaw remains currently referable to ''Paludirex'' are those directly associated with the premaxilla of ''P. gracilis'', meaning that the validity of many of the mandibles previously referred to Palimnarchus is up in the air. Although it is deemed likely that some, although not all, of these lower jaws did belong to ''Paludirex'', they cannot be confidently assigned to the new genus until fossils are found that preserve both the upper and lower jaws in association with one another. Until such a fossil is found, these remains can only be identified as Crocodilia indet. and nothing more specific. Ristevski and his team further highlight various possible scenarios for the future of ''Pallimnarchus''. Should the lectotype be rediscovered, there is the possibility that ''Paludirex'' may eventually become a junior synonym of ''Pallimnarchus''. At the same time, it is just as likely that future research finds the two to be different animals altogether, as the presence of multiple broad-snouted crocodilians in the Pliocene of Australia is not only possible but very likely. 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. Until then, ''Pallimnarchus'' is treated as a nomen dubium and ''Paludirex'' as valid and distinct taxon.

Etymology

''Paludirex'' is derived from the Latin words "paludis" and "rex", translating to "swamp king". This etymology was deliberately chosen to maintain a connection with its predecessor, as it serves as a rough equivalent to the etymology of ''Pallimnarchus'', whose name translated to "ruler of all swamps".

Species

*''Paludirex gracilis''
:''P. gracilis'' is the smaller of the two species and its fossils are exclusively known from the Late Pleistocene deposits of the Terrace Site within the Riversleigh World Heritage Area, making it one of the last remaining mekosuchines in Australia. Originally described as ''Pallimnarchus gracilis'', this species managed to maintain its validity even after the 2020 revision of ''Pallimnarchus''. The name is derived from the fact that the fossil material was notably more gracile than that of ''Pallimnarchus pollens'' and ''Paludirex vincenti''.

*''Paludirex vincenti''
:The larger and more robust of the two species, ''P. vincenti'' is also the older of the two, although the age records are muddy. The type specimen is confirmed to have been collected from Pliocene

HOME


Content is Copyleft
Website design, code, and AI is Copyrighted (c) 2014-2017 by Stephen Payne


Consider donating to Wikimedia



As an Amazon Associate I earn from qualifying purchases