Palaeontology of Northeastern Australian Caves Gilbert J

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;

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