Australia’s prehistoric ‘swamp king’: revision of the Plio-Pleistocene crocodylian genusPallimnarchusde Vis, 1886

Abstract

The crocodylian fossil record from the Cenozoic of Australasia is notable for its rich taxonomic diversity, and is primarily represented by members of the clade Mekosuchinae. Reports of crocodylian fossils from Australia date back to the late nineteenth century. In 1886, Charles Walter de Vis proposed the name Pallimnarchus pollens for crocodylian fossils from southeast Queensland—the first binomen given to an extinct crocodylian taxon from Australia. Pallimnarchus has come to be regarded as a large, broad-snouted crocodylian from Australia’s Plio-Pleistocene, and numerous specimens, few of which are sufficiently complete, have been assigned to it by several authors throughout the twentieth century. In the late 1990s, the genus was expanded to include a second species, Pallimnarchus gracilis. Unfortunately, the original syntype series described as Pallimnarchus pollens is very fragmentary and derives from more than one taxon, while a large part of the subsequently selected lectotype specimen is missing. Because descriptions and illustrations of the complete lectotype do not reveal any autapomorphic features, we propose that Pallimnarchus pollens should be regarded as a nomen dubium. Following this decision, the fossil material previously referred to Pallimnarchus is of uncertain taxonomic placement. A partial skull, formerly assigned to Pallimnarchus pollens and known as ‘Geoff Vincent’s specimen’, possesses many features of diagnostic value and is therefore used as basis to erect a new genus and species—Paludirex vincenti gen. et sp. nov. A comprehensive description is given for the osteology of ‘Geoff Vincent’s specimen’ as well as aspects of its palaeoneurology, the latter being a first for an extinct Australian crocodyliform. The newly named genus is characterized by a unique combination of premaxillary features such as a distinctive arching of the anterior alveolar processes of the premaxillae, a peculiar arrangement of the first two premaxillary alveoli and a large size disparity between the 3rd and 4th premaxillary alveoli. These features presently allow formal recognition of two species within the genus, Paludirex vincenti and Paludirex gracilis comb. nov., with the former having comparatively more robust rostral proportions than the latter. The Paludirex vincenti holotype comes from the Pliocene Chinchilla Sand of the Darling Downs, south-eastern Queensland, whereas the material assigned to Paludirex gracilis is from the Pleistocene of Terrace Site Local Fauna, Riversleigh, northwest Queensland. Phylogenetic analyses recover Paludirex vincenti as a mekosuchine, although further cladistic assessments are needed to better understand the relationships within the clade.

First record of a tomistomine crocodylian from Australia

“…Based on our results (Fig. 4 ; also, Ristevski et al 7 , 74 ) Harpacochampsa is neither a mekosuchine nor a tomistomine, and seems to be a representative of a different crocodylian lineage that inhabited Australia at some point in the past 66 Ma. Members of Mekosuchinae are still the most common crocodylians from Australia’s Cenozoic.…”

“…Another peculiarity of the phylogenetic results concerns Harpacochampsa camfieldensis , a Miocene crocodylian with enigmatic phylogenetic affinities. The phylogenetic placement of H. camfieldensis has been contentious 5 , 67 , with some studies recovering it as a possible mekosuchine 3 , 44 , 45 , 68 – 73 while others in various positions within Crocodyloidea 2 , 7 , 23 , 74 . A couple of recent studies 6 , 24 proposed gavialoid affinities for H. camfieldensis , and some of the analyses by Lee and Yates 24 recovered topologies where H. camfieldensis clustered with tomistomines.…”

“…If the above-elaborated hypothesis holds true, then G. maunala would be the first slender longirostrine crocodylian recognized from the Darling Downs. Alongside Gunggamarandu , there are two accepted crocodylian genera from the Pliocene and/or Pleistocene of south-eastern Queensland—the broad-snouted non-ziphodont Paludirex , and the altirostral (tall-snouted) ziphodont Quinkana 7 . Different snout and dental morphologies enable crocodyliforms to employ different methods of prey acquisition and feeding strategies, and even have different prey type preferences 60 , 101 – 103 .…”

“…Crocodylian evolution from the Australian Cenozoic is mainly characterized by the taxonomically rich and morphologically diverse crocodylian clade Mekosuchinae 1 – 7 . As currently understood, Australia’s mekosuchine fossil record spans from the Eocene 1 , 2 , 8 , 9 to the Pleistocene 7 , 10 – 13 , an inference based on remains collected across the continent, primarily the eastern half 2 – 4 , 14 . No mekosuchines are known to have survived in Australia after the late Pleistocene, thus leaving two species of Crocodylus — C. johnstoni (the sole extant crocodylian endemic to Australia) and C. porosus —as the only surviving crocodylians on the continent.…”

“…However, there are indications that the higher-clade crocodylian diversity in Australia was greater than previously perceived. Several studies 2 , 6 , 7 , 23 , 24 suggest that the Miocene Australian taxon Harpacochampsa camfieldensis is probably not a mekosuchine, although its precise phylogenetic affinities are uncertain.…”