Temporal overlap of humans and giant lizards (Varanidae; Squamata) in Pleistocene Australia
Introduction
Monitor lizards comprise more than 70 species within the genus Varanus (Varanidae, Squamata). They have a natural geographic distribution spanning Africa, Asia and Australia, and include the largest-bodied lizards living today. The biggest extant species, Varanus komodoensis, (Komodo dragon) has been recorded in association with hominins (Homo erectus, Homo floresiensis, Homo sapiens, and an unknown tool-making hominin in Flores) spanning back at least 900 ka in Southeast Asia (van den Bergh et al., 2009, Dennell et al., 2014).
Australia was home to three very large-bodied monitor lizards during the Pleistocene, including the Komodo dragon, an even larger (but presently undescribed) taxon from the Lake Eyre Basin of central Australia (Fig. 1), as well as the largest lizard known to have ever existed on Earth, the giant Megalania (Varanus priscus) (Molnar, 2004, Hocknull et al., 2009). Those giant monitor lizards are commonly included amongst a bestiary of prehistoric ‘megafauna’ of Pleistocene Australia, thought by some to have been driven extinct as a result of anthropogenic factors, such as overhunting, landscape modification, and/or other human-induced pressures (Flannery, 1990, Flannery, 1994, Roberts et al., 2001, Johnson, 2006, Roberts and Brook, 2010). A critical prerequisite for testing such a hypothesis is first demonstrating that the earliest human inhabitants, who arrived at least by ca. 50 ka (Roberts et al., 1994, Thorne et al., 1999, Clarkson et al., 2015), and the giant monitor lizards indeed overlapped temporally. Strikingly however, despite strong assertions that humans were directly implicated in the extinction of large-bodied taxa such as Megalania, firm evidence of a temporal overlap has yet to be demonstrated. Although some fossil deposits, such as Cuddie Springs (Fig. 1), have produced records of Megalania that may be younger than 50 ka (Field et al., 2008), the stratigraphic integrity of those fossil-bearing units has been questioned and it is possible that the remains have been reworked from older deposits (Grün et al., 2010). Other areas, such as at Wyandotte (Fig. 1), have produced Megalania fossils in strata dated as young as ca. 30 ka (McNamara, 1990), but the dates have since been considered to be unreliable for a variety of technical reasons (Gillespie et al., 2006). Excluding such records, the youngest reliably dated giant varanids with firm stratigraphic control (principally Megalania) are from the Darling Downs of southeastern Queensland (Price and Sobbe, 2005, Price and Webb, 2006, Sobbe et al., 2013), and are 83 ± 10 ka (Price et al., 2011), thus, pre-dating the earliest appearance of humans by several tens of millennia.
Here we report the results of new cave excavations of Colosseum Chamber, Mt Etna region, central eastern Queensland (Fig. 1), which bear on this matter. Although the deposit appears to have been accumulated largely by owls, and thus, contains predominantly the remains of small-bodied species such as rodents (Cramb and Hocknull, 2010), fossils of larger-bodied animals, including giant monitor lizard reported here, have been recovered. The aim of this study is to date the deposit using an integrated geochronological approach, and to discuss the temporal significance of the new lizard record.
Section snippets
Geological and geographical settings
Colosseum Chamber forms part of Olsen's Caves (Fig. 2), a cave system consisting of sixteen interconnected caverns within the Capricorn Caves Tourist Park, Mt Etna region (Shannon, 1970). The caves occur within an isolated allochthonous Devonian limestone block of the Mount Alma Formation (Yarrol Project Team, 1997). The caverns within Olsen's Caves, including Colosseum Chamber, are joint-controlled and vary in dimensions (Shannon, 1970). Colosseum Chamber itself is a single chamber,
Excavation
The floor of Colosseum Chamber was excavated to a depth of approximately 2 m below the modern cave floor surface. As there is little well-defined stratigraphy within the deposit, systematic spits of every 5 cm in thickness were excavated from the modern surface to the former limestone basement using trowels and brushes. The absolute depth of the deposit, including the associated spits, were measured from a horizontal datum line that stretched over the top of the deposit, and was fixed directly
Identification of the lizard fossil
The new record of the giant monitor is represented by an osteoderm (Queensland Museum Fossil (QMF) 58597), a type of dermal bone commonly associated with scales (Fig. 4). The specimen is elongate and round in section with no branching, slightly curved over its length leading to a vermiform shape, with maximum dimensions of 9.6 mm × 2.3 mm. Non-branched osteoderms, such as QMF58597, are typically associated with parts of the body with low bone density (Erickson et al., 2003). The osteoderm is
Age of the new fossil record
The dating results clearly demonstrate that the Colosseum Chamber deposit accumulated over the late Pleistocene and most likely through much of the Holocene (Fig. 5). Each of the U–Th dated straw stalactites from the deposit are within error, or older, than the calibrated 14C dates for the charcoals. On the assumption that the deposit has remained stratigraphically intact since the time of its initial deposition (there is no evidence to suggest that it was disturbed prior to excavation,
Final remarks
At best, we suspect that future collecting and dating may produce both significantly younger and older fossil records of Australian giant monitor lizards, but predicting how young or old those records might be, is not possible. Only new empirical data will help. Thus, at this time, we caution that while the new geochronological data we present adds valuable new information on the temporal and spatial range of giant lizards in Australia, by themselves, they cannot be used to unequivocally
Acknowledgements
We thank Ann Augustyn and staff from the Capricorn Caves for support and access to Colosseum Chamber. Kristen Spring, kindly facilitated access to the Colosseum Chamber fossil collection in the Queensland Museum. We acknowledge the untiring support of countless supporters and volunteers who have worked with us on fossil deposits in the Mt Etna region including Noel Sands and family, Leanne Phillipson, Craig Edwards, Kenny Travouillon, Nicholas Wiggins, Kyle Ferguson, Kaylene Butler, and many
References (60)
- et al.
The half-lives of uranium-234 and thorium-230
Chem. Geol.
(2000) - et al.
Testing the precision and accuracy of the U–Th chronometer for dating coral mortality events in the last 100 years
Quat. Geochronol.
(2014) - et al.
Spatial variability of initial 230Th/232Th in modern Porites from the inshore region of the great barrier reef
Geochim. Cosmochim. Acta
(2012) - et al.
Discerning the timing and cause of historical mortality events in modern Porites from the great barrier reef
Geochim. Cosmochim. Acta
(2014) - et al.
The archaeology, chronology and stratigraphy of Madjedbebe (Malakunanja II): a site in northern Australia with early occupation
J. Hum. Evol.
(2015) - et al.
The origins and persistence of Homo floresiensis on Flores: biogeographical and ecological perspectives
Quat. Sci. Rev.
(2014) - et al.
Chronological overlap between humans and megafauna in Sahul (Pleistocene Australia-New Guinea): a review of the evidence
Earth Sci. Rev.
(2008) - et al.
ESR and U-series analyses of faunal material from Cuddie Springs, NSW, Australia: implications for the timing of the extinction of the Australian megafauna
Quat. Sci. Rev.
(2010) U–Th dating of speleothems with high initial 230Th using stratigraphical constraint
Quat. Geochronol.
(2006)- et al.
Responses of Quaternary rainforest vertebrates to climate change in Australia
Earth Planet. Sci. Lett.
(2007)
Island differences in population size structure and catch per unit effort and their conservation implications for Komodo dragons
Biol. Conserv.
AMS at ANTARES – the first 10 years
Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms
Application of sedimentary and chronological analyses to refine the depositional context of a Late Pleistocene vertebrate deposit, Naracoorte, South Australia
Quat. Sci. Rev.
Direct U–Th dating of vertebrate fossils with minimum sampling destruction and application to museum specimens
Quat. Geochronol.
Dating megafaunal extinction on the Pleistocene Darling Downs, eastern Australia: the promise and pitfalls of dating as a test of extinction hypotheses
Quat. Sci. Rev.
New U/Th ages for Pleistocene megafauna deposits of southeastern Queensland, Australia
J. Asian Earth Sci.
Turning back the clock on the extinction of megafauna in Australia
Quat. Sci. Rev.
The human colonisation of Australia: optical dates of 53,000 and 60,000 years bracket human arrival at Deaf Adder Gorge, Northern Territory
Quat. Sci. Rev.
Inferring extinction from a sighting record
Math. Biosci.
U-series dating of soda straw stalactites from excavated deposits: method development and application to Blanche Cave, Naracoorte, South Australia
J. Archaeol. Sci.
Expanding the utility of Uranium-series dating of speleothems for archaeological and palaeontological applications
J. Archaeol. Sci.
Preliminary U-series and thermoluminescence dating of excavated deposits in Liang Bua sub-chamber, Flores, Indonesia
J. Archaeol. Sci.
Extinction chronology of the cave lion Panthera spelaea
Quat. Sci. Rev.
Australia's oldest human remains: age of the Lake Mungo 3 skeleton
J. Hum. Evol.
The Liang Bua faunal remains: a 95 k.yr. sequence from Flores, East Indonesia
J. Hum. Evol.
Recent massive coral mortality events in the South China Sea: was global warming and ENSO variability responsible?
Chem. Geol.
High-precision 238U–234U–230Th disequilibrium dating of the recent past: a review
Quat. Geochronol.
Inferring threat from scientific collections
Conserv. Biol.
Conservation genetics
sExtinct: Calculates the Historic Date of Extinction Given a Series of Sighting Events
Cited by (19)
Timing of Neanderthal occupations in the southeastern margins of the Massif Central (France): A multi-method approach
2021, Quaternary Science ReviewsCitation Excerpt :The chemical procedure for the extraction and purification of uranium and thorium isotopes and the isotopic analysis using MC-ICP-MS were performed according to the procedure described above and in SOM for the flowstone from Ranc-Pointu 2. Bones and teeth (n = 9) were sampled for U-series dating (Table 2) by extracting powder material using a micro-drill and measured by multi-collector inductively-coupled plasma mass spectrometry at the Radiogenic Isotope Laboratory, University of Queensland, following the protocol described in Price et al. (2015). Separation and purification of U and Th isotopes was conducted using standard ion-exchange methods (Edwards et al., 1987).
New ages of the world's largest-ever marsupial: Diprotodon optatum from Pleistocene Australia
2021, Quaternary InternationalCitation Excerpt :This makes testing current hypotheses for why these species became extinct particularly difficult, if not impossible. Some efforts to model extinction times have not only produced inconsistent results between methods, but have also suggested that some clearly extinct species, such as giant monitor lizards (e.g., Varanus priscus), may still be extant (e.g. Price et al., 2015). This reflects problems not so much with the models themselves, but more so the paucity of geochronological information with which to feed into them (Price et al., 2015, 2018).
Simultaneous extinction of Madagascar's megaherbivores correlates with late Holocene human-caused landscape transformation
2021, Quaternary Science ReviewsMegabeasts under the microscope: a closer look at Quaternary extinctions in the Asia-Pacific
2020, Quaternary InternationalCitation Excerpt :U-series and coupled ESR/U-series dating have been applied to date the first record of Sahulian tropical rainforest taxa that became extinct due to increased aridity in the Middle Pleistocene (Hocknull et al., 2007, 2020), as well as the first evidence of migratory behaviour in marsupials, based on the largest marsupial known to have existed on Earth, Diprotodon optatum (Price et al., 2017). ESR applications include dating of multiple Diprotodon-bearing sites (Grün et al., 2008), multiple Sthenurine kangaroos (Fraser and Wells, 2006) as well as the largest lizard known to have lived: the monstrous ‘megalania’ Varanus priscus, that co-existed with Homo sapiens (Price et al., 2015). In terms of environmental context, this area proves that mycological fungal spores are not restricted to the arctic.
Fossils from Quaternary fluvial archives: Sources of biostratigraphical, biogeographical and palaeoclimatic evidence
2017, Quaternary Science ReviewsCitation Excerpt :In Australia, Quaternary palaeoenvironmental research based on fluvial archives has a long pedigree in two important regions of the continent: the arid interior of the Lake Eyre Basin, where fluvial, lacustrine and aeolian sequences representing the last ∼300 ka are preserved (recently reviewed in detail by Habeck-Fardy and Nanson, 2014), and the extensive meandering river systems of southeastern Australia, particularly the well-dated terraces of the Lachlan and Macquarie rivers in the Murray-Darling Basin (Kemp and Spooner, 2007; Yonge and Hesse, 2009; Kemp and Rhodes, 2010). Faunal and floral responses to Pleistocene climate change in these regions remain less well understood, primarily due to the rarity of stratified fossil assemblages and significant issues with directly dating fossil material (Price et al., 2015; Westaway et al., 2017). As is the case in North America, much research in Australia has been focussed on the extinction of megafaunal species during the Late Pleistocene; approximately 96% of the large mammal fauna was extinct by ∼45 ka, a period broadly concurrent with human colonization, although the extent to which these extinctions can be directly related to human activity remains a source of considerable debate (e.g. Koch and Barnosky, 2006; Prideaux et al., 2007, 2010; Price et al., 2015; Dortch et al., 2016; Johnson et al., 2016; Johnson, 2017; Westaway et al., 2017, Fig. 11).
- 1
Current address: Department of Archaeology and Natural History, School of Culture, History, and Languages, The Australian National University, Canberra 0200, Australia.