Palaeontology of Northeastern Australian Caves Gilbert J

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Palaeontology Of Northeastern Australian Caves Gilbert J. Price1, Jonathan Cramb1, Julien Louys2, Yue-Xing Feng1 Afliation: 1School of Earth and Environmental Sciences, Te University of Queensland, St Lucia, Queensland 4072, Australia; 2Department of Archaeology and Natural History, School of Culture, History, and Languages, ANU College of Asia and the Pacifc, Te Australian National University, Canberra, Australian Capital Territory 2601, Australia

Abstract Numerous Australian caves have produced fossil records that have been critical in piecing together the story of the evolution of the continent’s ecosystems through time. Among the most important are those of the Riversleigh World Heritage Area of northeastern Australia. Although the majority of caves in the region have since been destroyed as a result of natural collapses and erosive activities, now-exposed breccias are remarkably abundant in fossils. Te fossil faunas include the distant ancestors of the majority of modern Australian vertebrates, as well as extinct lineages such as those of the marsupial ‘lions’ (Tylacoleonidae) and the diprotodontoids, a group of oversized wombat-like marsupials. Recent radiometric dating in the region, coupled with biochronology, demonstrates that the majority of Riversleigh’s fossil assemblages are Miocene in age (ca. 10-18 million years old). Notably, the younger end of the fossil record is poorly represented at Riversleigh, but other cave systems across northeastern Australia provide the necessary temporal extension that can document the emergence of the modern faunas and ecosystems. Karst-dominated regions such as Chillagoe have produced Quaternary-aged (<2.6 million years) fossils, including species found nowhere else on the continent such as Propelopus chillagoensis (‘Chillagoe’s carnivorous kangaroo’). Chillagoe is also home to the enigmatic extinct land-dwelling crocodile, Quinkana fortirostrum. Recent work in the region has also extended slightly southward to the Broken River karst system. Te fossil assemblages in the Broken River caves mostly represent owl-roost accumulations, although deposits thought to be collected by ghost bats and natural pit-fall traps have also been recognised. Te oldest radiometrically dated fossil deposits are around 350 ka (thousand years old) and include a combination of both extinct and extant faunas. Quaternary geographic range extensions have been recognised for several species including pig-footed bandicoots (Chaeropus spp.), as well as the now-extinct giant short-faced kangaroos (sthenurines), a group previously thought to only occur in southern Australia during the Quaternary. Further to the south, the Mt Etna region provides evidence of signifcant faunal and ecosystem change through the late Quaternary. Rainforest-adapted vertebrates occur in abundance in cave deposits >280 thousand years old, but give way to more open-adapted faunas by around 200 thousand years ago.

Keywords: Caves, owls, pitfall traps, vertebrates, megafauna, extinction

1. Introduction Caves play a seemingly unlikely, but remarkable role in documenting the evolution of terrestrial ecosystems through time (Lundelius, 2006). Unwary land animals typically fall into caves, and unable to make their way back out, die underground, with their tough and durable skeletons read- ily becoming incorporated into the fossil record. Such caves are commonly called ‘natural pitfall traps’. Airborne predators such as owls also play a key role in accumulating terrestrial vertebrates in cave settings. Owls will commonly hunt at night targeting a wide range of food resources including small-bodied vertebrates. Afer securing a kill, they will tear their victim apart with their beak and talons, usually swallowing the entire carcass in ‘bite-sized’ pieces. Upon returning to the roosting cave, they will regurgitate any undigested remains (e.g., feath- ers, fur, teeth and bones) in the form of pellets that fall to the foor of the cave. Subsequent breakdown and burial of those Figure 1. Map of Australia showing key sites of interest remains may then make their way into the fossil record. Numerous fossil-yielding caves have been reported from the extensive limestone deposits that fringe the modern margin paper is to provide an overview of the vertebrate fossil record of the Australian continent (Hocknull et al., 2007; Price et al., of northeast Australia’s caves. 2009a, b; Reed and Bourne, 2009; Prideaux et al., 2010). Tese caves have been critical in piecing together the story 2. Riversleigh of the evolution of Australia’s terrestrial vertebrates. Among Te oldest and longest record of fossil-bearing caves in Aus- the most important caves are those of northeastern Australia, tralia are those from Riversleigh of northwest Queensland a region that contains not only the oldest record of fossil- (Figure 1). Fossils were frst reported from the region in the bearing caves, but also those that span the longest temporal 1960s, with over 200 fossil sites now known (Archer et al., sequence for any one place on the continent. Te aim of this

Proceedings of the 17th International Congress of Speleology 25 2006). So important are the fossil deposits of Riversleigh that the region was declared a World Heritage Area in 1994, an honour shared with the Pleistocene-aged Naracoorte Caves of southeastern South Australia. Te basement rocks at Riversleigh are Cambrian in age. During the late Oligocene (ca. 24 million years ago), numerous freshwater limestones developed, some of which are rich in fossil vertebrates. Following karstifcation, an extensive series of caves formed within the limestone that later func- tioned as natural pitfalls, efectively trapping a wide variety of terrestrial vertebrates (Arena et al., 2014). Other vertebrate- bearing cave deposits are thought to have accumulated at least in part by predators, such as ghost bats. Te majority of River- sleigh’s cave deposits are Miocene, the oldest dating to ca. 18 million years in age (Woodhead et al., 2016).

Although the majority of Riversleigh’s caves have since been A) Lower jaw of a diprotodontoid (wombat-like marsupial) destroyed as a result of natural geological processes, they are encased in rock from Riversleigh; B) Holotype of land- still discernible on the modern landscape and have been the dwelling crocodile, Quinkana fortirostrum, from Chillagoe focus of targeted excavations since the mid-1970s. Te brecci- (Australian Museum Fossil 57844). C) Occlusal view of lower ated deposits are remarkable for the variety of vertebrates that jaw of a sthenurine (giant short-faced) kangaroo from Robert they contain, with the palaeo-diversity comparable to that of Broom Cave, Broken River; D) Fossils of ‘microfauna’ from a modern rainforest in Borneo (Woodhead et al., 2016). Simi- Colosseum Chamber, an owl roost deposit from the Capri- larly, the vast majority of Riversleigh’s fossil vertebrates are corn Caves, Mt Etna region. considered to have been rainforest specialists, especially those from the early and middle Miocene. Amongst the extinct bes- 3. Chillagoe tiary are the earliest ancestors of all the modern major groups Extensive tower karst occurs in the Chillagoe area of north- of marsupials, such as Notoryctomorphia (marsupial ‘moles’), eastern Australia (Figure 1). Te limestone here is late Silu- Peramelemorphia (bandicoots and bilbies), Dasyuromorphia rian – early Devonian (ca. 400 million years old) and contains (Tasmanian devils, Tasmanian ‘tigers’, and dunnarts among numerous fossil molluscs, brachiopods and corals (De Keyser others), Vombatomorphia (wombats), Phascolarctomorphia and Wolf, 1964). Bone breccias are common in the caves, (koalas), Phalangeroidea (possums) and Macropodoidea many of which are topographically higher than the modern (kangaroos and allies) (Archer et al., 2006; Louys et al., 2007; land surface in the region. Black et al., 2014; Butler et al., 2016). Unlike Riversleigh, the majority of caves in the region are still Many of Riversleigh’s fossil vertebrates share a similar body intact, but have not been explored by palaeontologists any- plan to modern Australian vertebrates, but with some bizarre where near as extensively. Te majority of fossil vertebrates exceptions and groups of organisms that are now extinct. For from Chillagoe are small-bodied and include bilbies (Macro- example, the yalkiparidons, informally known as ‘thingo- tis) and pig-footed bandicoots (Chaeropus), species that are donts’, were a group of small-bodied marsupials that possessed found in the historic period only in Australia’s central arid a bony ‘beak’ and may have been ecologically convergent with core (Muirhead and Godthelp, 1995). Teir occurrence at woodpeckers (Beck, 2009). Riversleigh also bore witness to Chillagoe suggests a more open habitat and drier climate in the earliest members of the now-extinct Tylacoleonidae the area in the past. (marsupial ‘lions’) and Diprotodontoidea (marsupial ‘tapirs’ and ‘rhinos’; Figure 2A), two groups that eventually came to Several species of ‘megafauna’ have also been recorded from dominate the top mammalian predator and herbivore niches, around Chillagoe including a species of land crocodile (Quin- respectively, of mid-late Cenozoic Australia (Wroe et al., kana fortirostrum; Figure 2B), giant carnivorous kangaroo 1999; Price and Piper, 2009). (Propleopus chillagoensis), and marsupial ‘tapir’ (Palorchestes azael). Chillagoe is also the type locality for the former two Riversleigh shows remarkable transitions of ecosystems species (Archer et al., 1978; Molnar, 1981). Radiometric through time, with the open forests of the late Oligocene dating of the marsupial ‘tapir’ suggests that it occurred locally overtaken by extensive rainforests for much of the Miocene. during the penultimate glacial cycle (ca. 137–200 thousand During the late Miocene, a trend towards drier conditions saw years ago) (Price et al., 2013). many of Australia’s terrestrial ecosystems become patchier and more open. A Pliocene (2.6–5.3 million years ago) fossil record is absent from Riversleigh, with the youngest cave 4. Broken River deposits in the region dating to the early Pleistocene (ca. 2 Te Broken River karst is situated approximately 60 km south million years ago) (Woodhead et al., 2016). Te story of the of the small township of Greenvale in northeastern Queens- evolution of northeastern Australia’s terrestrial vertebrates land (Figure 1). Numerous caves occur within the Silurian- then takes up with cave deposits that date from the middle- Devonian limestones, the majority of which also contain late Pleistocene to Holocene (ca. last 500 thousand years). vertebrate fossils (Withnall and Lang, 1993). Work in the region is preliminary but ongoing. Te majority of vertebrate remains appear to have accumulated in the caves from the feeding activities of owls. Most species recovered

26 Proceedings of the 17th International Congress of Speleology and identifed to-date are thus unsurprisingly from small- matically driven extinction event occurred at least 150 thou- bodied species, but include extremely diverse assemblages of sand years before the arrival of humans on the continent. marsupials and rodents, species of which suggest an open- type savannah and thus similar to the palaeoenvironmental Several megafaunal species are recorded in the region, the signal from Chillagoe. geologically youngest of which is a variety of giant goanna (species uncertain, but likely referable to either Megalania, One of the most exciting deposits is that from Robert Broom Varanus priscus, or Komodo dragon, V. komodoensis). Pre- Cave (a system named by the Chillagoe Caving Club in viously thought to have sufered extinction prior to human honour of the palaeoanthropologist famous for discovering arrival (Price et al., 2011; Wroe et al., 2013), recent dating the early hominid, ‘Mrs Ples’, the most complete skull of Aus- of a specimen from Colosseum Chamber (Capricorn Caves; tralopithecus africanus). Here the taphonomic signature dif- Figure 2D) shows that it is slightly younger than 50 thou- fers to that of other caves at Broken River, with the contained sand years old, placing it within the timeframe of the earliest fossil species being those of large-sized taxa and includes the humans on the continent (Price et al., 2015). A local archaeo- largest-ever marsupial ‘lion’, Tylacoleo carnifex, as well as logical signature is absent for that time, thus it is unknown the largest-ever marsupial overall, the enormous wombat- whether humans and giant lizards actually ever directly like Diprotodon. It is clear that this system acted as a pitfall encountered each other. trap. Te cave also contains abundant fossils of macropods, the majority of which are those of the modern dominant kangaroo, Macropus, although fossils of sthenurines (giant 6. Summary short-faced kangaroos) have also recently been collected Caves of northeastern Australia record a drying continent (Figure 2C). Previously, Pleistocene species of sthenurines over the last 25 million years, although certain areas appear were known only from southern Australia, thus, their occur- to have maintained more humid conditions until geologically rence at Broken River marks a ca. 1200 km geographic range recent periods. Te unique palaeoenvironmental signals from extension for the time period. Tis fnding highlights just how the region’s caves are critical in helping us understand faunal poorly explored the fossil record of northern Australia is in change in response to past climatic and environmental per- comparison to the south. turbations, which of course has important implications for understanding the efects of modern climate change. An extensive, but mostly hitherto unpublished radiometric dating program of Broken River fossil deposits demonstrates Although fossils have been known from northeastern Aus- that the majority are younger than ca. 350 thousand years old. tralian caves for well over a century, the region has been Te palaeoecological signature from fossil vertebrates sug- somewhat neglected in the past in terms of palaeontological gests largely an open habitat through time, a fnding that is investigation. Despite many advances, we are still scratching in contrast with the record from the Mt Etna region, approxi- the surface of this most important palaeontological resource. mately 700 km to the south. Exploration of new caves, and new caving regions, are almost certainly likely to result in discoveries that will challenge or rewrite existing paradigms in Australian palaeontology. Pal- 5. Mt Etna region aeontologists will continue to rely on the co-operation and Te Mt Etna region includes both Mt Etna itself and the assistance of the speleological community for these discover- adjacent Limestone Ridge, thus encompassing both National ies. Park and private land (Figure 1). Te marine limestones here are Devonian and preserve abundant fossils of corals, bra- Acknowledgements chiopods and molluscs (Sprent, 1970). Numerous caves occur within the limestone karst. We are greatly indebted to Douglas Irvin and family; Paul Osborne and the Chillagoe Caving Club; Noel Sands and Mt Etna was subjected to extensive quarrying activities from family; Ann Augustyn and staf from the Capricorn Caves; the 1970s to the 2000s. Serendipitously, while the quarrying Michael Archer, Sue Hand, Henk Godthelp and other col- destroyed numerous caves and roosting sites for local bats, leagues of the University of New South Wales; Scott Hocknull many fossil deposits were exposed and made accessible for and staf of the Queensland Museum; and the many untiring palaeontological assessment. Te fossil faunas collected supporters and volunteers who have diligently worked with here, and on the adjacent Limestone Ridge, are particularly us on northeastern Australia’s fossil deposits. Tis research diverse and include a range of both small- and large-bodied was funded in part by: an Australian Research Council taxa including molluscs, frogs, lizards, snakes, marsupials and (ARC) Discovery Grant (DP120101752); ARC Linkage Grant rodents (Hocknull, 2005). (LP0989969) with key support from industry partners including Xstrata Copper, Queensland Museum, Outback at Isa, and Te story of the Mt Etna region is somewhat similar to that of Mt. Isa City Council; ARC Discovery Early Career Researcher Riversleigh. Te oldest fossil faunas recovered include many Award (DE120101533); Australian Institute of Nuclear Sci- species that appear to have been rainforest specialists, some of ence and Engineering grant (AINGRA09002); and an Ian which share close evolutionary links to those found today in Potter Foundation grant. lowland rainforests of New Guinea today. Radiometric dating demonstrates that such deposits are >280 thousand years old, implying more humid conditions locally at that time. How- References ever, by ca. 200 thousand years ago, few rainforest-adapted Archer, M., Arena, D.A., Bassarova, M., Beck, R.M.D., Black, taxa are known, the majority of which appear to have been K., Boles, W.E., Brewer, P., Cooke, B.N., Crosby, K., Gillespie, ecologically replaced by open, arid-adapted forms including A., Godthelp, H., Hand, S.J., Kear, B.P., Louys, J., Morrell, A., species such as bilbies (Hocknull et al., 2007). Tis purely cli- Muirhead, J., Roberts, K.K., Scanlon, J.D., Travouillon, K., Wroe, S., 2006. Current status of species-level representation

Proceedings of the 17th International Congress of Speleology 27 in faunas from selected fossil localities in the Riversleigh Price, G.J., Louys, J., Cramb, J., Feng, Y.-x., Zhao, J.-x., World Heritage Area, northwestern Queensland. Alcheringa Hocknull, S.A., Webb, G.E., Nguyen, A.D., Joannes-Boyau, Special Issue, 1, 1-17. R., 2015. Temporal overlap of humans and giant lizards (Varanidae; Squamata) in Pleistocene Australia. Quaternary Archer, M., Bartholomai, A., Marshall, L.G., 1978. Propleo- Science Reviews, 125, 98-105. pus chillagoensis, a new north Queensland species of extinct giant rat-kangaroo (Macropodidae: Potoroinae). Memoirs of Price, G.J., Piper, K.J., 2009. Gigantism of the Australian the National Museum of Victoria, 39, 55-60. Diprotodon Owen 1838 (Marsupialia, Diprotodontoidea) through the Pleistocene. Journal of Quaternary Science, 24, Arena, D.A., Black, K.H., Archer, M., Hand, S.J., Godthelp, 1029-1038. H., Creaser, P., 2014. 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