Quinkana
Fig 1. An artist’s interpretation of Varanus priscus and Quinkana. Painting used with permission from the artist Hodari Nundu.
Taxonomy
The precise affinities of the crocodile Quinkana have been debated since its first discovery, though they are now mostly settled. The first description of Quinkana-remains was in 1977 (Molnar, R. 1977), though the genus was not named until 1982, with the type-species Quinkana fortirostrum (10). On account of its unique, ziphodont dentition (discussed beneath), the animal was from its earliest description compared with the likewise ziphodont Pristichampsines and Sebecosuchians, both of whom were conjectured to form an interrelated clade of supposedly terrestrial, ziphodont crocodilians. Today, it is clear that no such relationship exists, with the taxonomic validity of the Pristichampsines in question (8), the Sebecosuchians classified as non-Eusuchian survivors of the Mesozoic clade Notosuchia (15), and Quinkana related to neither (14).
Today, Quinkana is classified confidently within the Mekosuchinae, a now-totally extinct clade of Crocodilians, aligned to the crocodile-gharial branch of the order (as opposed to the alligators) (14). At the time of the End-Pleistocene extinctions, the Mekosuchines were still quite diverse in Oceania, numbering alongside Quinkana at least 3 other taxa, from the large, semi-aquatic Paludirex of mainland Australia, to the peculiar and diminutive Mekosuchus inexpectatus of New Caledonia (13) (2).
Distribution
The distribution of Quinkana was centred in the northeast of Sahul, with remains known from sites as far apart as New South Wales and New Guinea (10). Though known from terrestrial locations, the species evidently did not inhabit the barren, desert interior of the continent, and seems instead to have had a range centred on the savannas of northern Australia and the now-flooded Arafura Shelf. Owing to the sparse nature of our fossil remains, however, we cannot confidently state that the known sites accurately circumscribe the entire range.
Fig. 2. The estimated range of Late Pleistocene Quinkana, based on fossil sites. Based upon data from PaleoDB (17).
Morphology and ecology
For many years now, debate has persisted regarding the morphological and, by extension, ecological nature of Quinkana, as well as other Mekosuchines (2) (3). Upon the initial discovery of Quinkana-remains, and even before the naming of the taxa, a predominantly terrestrial lifestyle was inferred. The grounds for this was the presence of ziphodont dentition—teeth which are serrated and laterally compressed. Such teeth, suitable for cutting and tearing (7) (6), are found also in presumably terrestrial extinct Crocodilians such as ‘Pristichampsus’ and the Sebecosuchians, hence the inference (1). For many years, the nature of Quinkana as a ‘terrestrial crocodile’ was taken for granted in the literature, until the early 2000s. In an iconoclastic paper, Stephen W. Wroe re-evaluated the evidence for the taxa’s terrestriality and found it severely lacking (2). He noted that no actual direct evidence existed for a terrestrial lifestyle in Quinkana, and that four of the five cranial attributes typically said to indicate as much in fossil crocodiles are in fact found in the extant South American Caimans (Paleosuchus), including laterally compressed teeth and a head raised high. This counted not merely for Quinkana, but also for Pristichampsus, about which he also asserted that its hoof-like claws, typically regarded as evidence for terrestriality, were in fact preservation-artifacts (2). In more recent times, ‘Pristichampsus’ as a genus has been entirely dismantled, now considered a nomen dubium with its material referred to the planocraniid crocodile Boverisuchus (8). Things, it might be surmised, did not look good for the terrestrial Quinkana.
Fig 3. The cranium of Quinkana, not including the King Creek specimen. The areas in white are preserved and scaled to lectotype, the areas in grey speculative. Black bar represents 10 cm. Skeletal by August Helmke, used with permission.
Wroe was writing in 2002, however, and much time has passed. One of Wroe’s central assertions, that there existed no strong evidence for terrestriality in Mekosuchines outside possibly the insular Mekosuchus inexpectatus, is now false (3) (4). Furthermore, his assertion that the hoof-like claws of Pristichampsus (now Boverisuchus) were a mere artifact was never correct (8). While he downplayed the importance of serration in Quinkana, focusing on the other similarities with Paleosuchus, this feature is not irrelevant—as noted previously, ziphodont dentition is strongly associated with cutting and tearing, and the one other group of Cenzoic crocodilians in which it occurs is the unambiguously terrestrial and cursorial Sebecosuchians (7). Wroe notes that serrated dentition is not unique prehistoric crocodilians, occurring also in Komodo dragons (Varanus komodoensis) and even great white sharks (Carcharodon carcharias) (2). This seems, however, an odd point to raise in opposition to the hypothesis of a terrestrial lifestyle, since the one thing both large varanids and great whites do have in common, beyond their mere dentition, is an active, pursuit-predator lifestyle. This too they share with the theropod dinosaurs (6), among which ziphodontism was ubiquitous, and nearly all of which were cursorial predators.
Returning to the new evidence for terrestriality in Mekosuchines, it comes from two analyses of post-cranial remains across multiple taxa (3) (4). Studying respectively the forelimbs and pectoral and pelvic girdles of taxa from the Eocene to the Miocene, they find evidence for pillar-like locomotion in several genera (3), and even, in some genera, adaptations to rough terrain (4). Furthermore, this trait appears to have arisen independently of the cursorial adaptations in Sebecosuchians, since the earliest known Mekosuchines show pelvic anatomy more similar to extant gharials and alligators (3). It must be noted here that neither of these studies directly involve Quinkana, since no definite post-cranial remains exist from any species in the genus (10) (11) (12) (9). What they uncover is the evident development of two separate morphological lineages within the Mekosuchinae, one tending towards terrestriality, the other retaining a more semi-aquatic lifestyle (3). Whether Quinkana is referable to this more terrestrial lineage is therefore not confirmable, but we may consider several points of evidence. For one, ziphodont dentition, which as prior noted is strongly associated with a pursuit-predator lifestyle, does not occur in all Mekosuchines—the contemporary Late Pleistocene Paludirex, considered a more clearly semi-aquatic taxa, is not ziphodont (13). One possible reason proposed for the original split in the Mekosuchines between terrestrial and semi-aquatic forms is that of niche-partitioning, allowing the various members of the group to exploit different or even the same prey-resources without direct competition (3). In the Late Pleistocene Sahulian context, we see on the semi-aquatic side the presence not only of the aforementioned Paludirex, a large, related taxa, but also the similarly massive saltwater crocodile (Crocodylus porosus) and the Freshwater crocodile (Crocodylus johnstoni). It would make sense, in this setting, for Quinkana to adopt a lifestyle as independent as possible of the already strongly competed waterways. Indeed, this expectation seems at least strengthened by the presence of Quinkana-remains in clearly terrestrial assemblages, though the genus is known also from water-adjacent settings (2) (10). We may note, in synergising all of this, that it would seem quite a remarkable coincidence for a genus, inferred from its very discovery to be terrestrial, bearing traits known only from other evidently terrestrial taxa, to be found to belong to one of the few groups of Cenozoic crocodilians in which terrestriality is strongly implied, only for it not to belong to the terrestrial lineage of this group.
Fig 4. The cranium of Quinkana. The areas in white are preserved and scaled to lectotype, the areas in grey speculative. The light-grey outline in the back is the estimated size based on the King Creek specimen (tooth). Skeletal by August Helmke, used with permission.
The exact size of Quinkana is difficult to determine, on grounds of our poor remains. Some estimates lie at around 200 kg (16), but factoring in the King Creek specimen—a single tooth—nearly doubles the predicted size of the skull. Bodily length cannot be deduced with any certainty in the absence of post-cranial remains. In lieu of more certain morphological and biomechanical data regarding Quinkana, it is difficult to say too much with certainty regarding its ecology. Evidently it was a large predator—likely more than twice the weight of Thylacoleo, the largest carnivorous mammals, and thus the third-heaviest predator on the continent. Only by Megalania (Varanus priscus) and the fellow Mekosuchine Paludirex exceed it (16). The latter being semi-aquatic, this means that if indeed terrestrial, it would make Quinkana the second-largest land-living predator in Australia. As stated prior, fossils of Quinkana are known from both terrestrial and water-adjacent sites, making it difficult to state anything confident about habitat-preference. It might be noted, however, that taphonomic (preservational) factors being as they are, remains are almost inevitably going to be biased towards wetland-sites. Furthermore, it seems intuitively easier to explain the occurrence of a terrestrial animal by a watery location than the inverse. Regardless of its habitat-preference or terrestriality, it is quite certain that Quinkana’s diet would have consisted primarily of large, mammalian megafauna. With its north-easterly range, this would have included taxa such as the large Diprotodon optatum and smaller Maokopia ronaldi, the supposed ‘marsupial tapir’ Zygomaturus trilobus and a range of kangaroos, extinct and extant.
Citations
1. Molnar, R. (1977). Crocodile with Laterally Compressed Snout: First Find in Australia. Science. 197(4298). 62-64. DOI: 10.1126/science.197.4298.62
2. Wroe, S. (2002). A review of terrestrial mammalian and reptilian carnivore ecology in Australian fossil faunas, and factors influencing their diversity: the myth of reptilian domination and its broader ramifications. Australian Journal of Zoology. 50(1). 1-24
3. Stein, M. D., Yates, A., Hand, S. J., Archer, M. (2017). Variation in the pelvic and pectoral girdles of Australian Oligo–Miocene mekosuchine crocodiles with implications for locomotion and habitus. PeerJ. 5:e3501 https://doi.org/10.7717/peerj.3501
4. Stein, M., Hand, S., Archer, M., Wroe, S. and Wilson, L. (2020). Quantitatively assessing mekosuchine crocodile locomotion by geometric morphometric and finite element analysis of the forelimb. PeerJ. 8. p.e9349. DOI: 10.7717/peerj.9349
5. Sobbe, I. H., Price, G. J., Knezour, R. A. (2013). A ziphodont crocodile from the late Pleistocene King Creek catchment, Darling Downs, Queensland. Memoirs of the Queensland Museum. 56(2). 601-606
6. D'Amore, D. (2009). A Functional Explanation for Denticulation in Theropod Dinosaur Teeth. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology. 292. 1297-1314.
7. Legasa, O., Buscalioni, A. D., Gasparini, Z. (1994). The serrated teeth of Sebecus and the Iberoccitanian crocodile, a morphological and ultrastructural comparison. Stvdia Geologica Salamanticensia, XXIX. 127-144.
8. Brochu, C. (2012). Phylogenetic relationships of Palaeogene ziphodont eusuchians and the status of Pristichampsus Gervais, 1853. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 103. 3-4. DOI: 10.1017/S1755691013000200
9. Mackness, B & Sutton, R. (1998). Possible evidence for intraspecific aggression in a Pliocene crocodile from north Queensland. Alcheringa: An Australasian Journal of Palaeontology. 55-62. DOI: https://doi.org/10.1080/03115510008619523
10. R. E. Molnar (1982). Pleistocene ziphodont crocodilians of Queensland. Records of the Australian Museum. 33(19): 803–834.
11. Megirian, D. (1994). A new species of Qujnkana molnar (Eusuchia: Crocodylidae) from the Miocene Camfield beds of Northern Australia. The Beagle: Records of the Museums and Art Galleries of the Northern Territory. 11. 145-166.
12. Willis, P. M. A. & Mackness, B. S. (1995). Quinkana Barbarra, a New Species of Ziphodont Mekosuchine Crocodile from the Early Pliocene Bluff Downs Local Fauna, Northern Australia with a Revision of the Genus. School of Biological Sciences. 116. 143-151.
13. Ristevski, J., Yates, A., Price, G., Molnar, R., Weisbecker, V. and Salisbury, S. (2020). Australia’s prehistoric ‘swamp king’: revision of the Plio-Pleistocene crocodylian genus Pallimnarchus de Vis, 1886. PeerJ, 8, p.e10466. DOI: 10.7717/peerj.10466
14. Lee, M. S. Y. & Yates, A. M. (2018). Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil record. 285. 1881. DOI: https://doi.org/10.1098/rspb.2018.1071
15. Pol, D. & Powell, J. E. (2012). A new sebecid mesoeucrocodylian from the Rio Loro Formation (Palaeocene) of north-western Argentina. Zoological Journal of the Linnean Society. 163. 1. 7-36. DOI: htt ps://doi.org/10.1111/j.1096-3642.2011.00714.x
16. Sobbe, I.H., Price, G.J. & Knezour, R.A. (2013). A ziphodont crocodile from the late Pleistocene King Creek catchment, Queensland. Memoirs of the Queensland Museum – Nature. 56(2). 601–606.
17. Palaeobiology Database. (2021). Quinkana. https://paleobiodb.org