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
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Current address: Department of Archaeology and Natural History, School of Culture, History, and Languages, The Australian National University, Canberra 0200, Australia.