Gilbert J. Price1, Jonathan Cramb1, Julien Louys2, Yue-Xing Feng1. Affiliation: 1School of ... 1960s, with over 200 fossil sites now known (Archer et al.,. Figure 1.
Palaeontology Of Northeastern Australian Caves Gilbert J. Price1, Jonathan Cramb1, Julien Louys2, Yue-Xing Feng1 Affiliation:
School of Earth and Environmental Sciences, The University of Queensland, St Lucia, Queensland 4072, Australia; Department of Archaeology and Natural History, School of Culture, History, and Languages, ANU College of Asia and the Pacific, The Australian National University, Canberra, Australian Capital Territory 2601, Australia
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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. The 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’ (Thylacoleonidae) 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 (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 readily 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. After 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., feathers, fur, teeth and bones) in the form of pellets that fall to the floor of the cave. Subsequent breakdown and burial of those 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 of the Australian continent (Hocknull et al., 2007; Price et al., 2009a, b; Reed and Bourne, 2009; Prideaux et al., 2010). These caves have been critical in piecing together the story of the evolution of Australia’s terrestrial vertebrates. Among the most important caves are those of northeastern Australia, a region that contains not only the oldest record of fossilbearing caves, but also those that span the longest temporal sequence for any one place on the continent. The aim of this
Figure 1.
Map of Australia showing key sites of interest
paper is to provide an overview of the vertebrate fossil record of northeast Australia’s caves.
2. Riversleigh The oldest and longest record of fossil-bearing caves in Australia are those from Riversleigh of northwest Queensland (Figure 1). Fossils were first reported from the region in the 1960s, with over 200 fossil sites now known (Archer et al., Proceedings of the 17th International Congress of Speleology
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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. The 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 karstification, an extensive series of caves formed within the limestone that later functioned as natural pitfalls, effectively trapping a wide variety of terrestrial vertebrates (Arena et al., 2014). Other vertebratebearing cave deposits are thought to have accumulated at least in part by predators, such as ghost bats. The majority of Riversleigh’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 destroyed as a result of natural geological processes, they are still discernible on the modern landscape and have been the focus of targeted excavations since the mid-1970s. The brecciated deposits are remarkable for the variety of vertebrates that they contain, with the palaeo-diversity comparable to that of a modern rainforest in Borneo (Woodhead et al., 2016). Similarly, the vast majority of Riversleigh’s fossil vertebrates are considered to have been rainforest specialists, especially those from the early and middle Miocene. Amongst the extinct bestiary are the earliest ancestors of all the modern major groups of marsupials, such as Notoryctomorphia (marsupial ‘moles’), Peramelemorphia (bandicoots and bilbies), Dasyuromorphia (Tasmanian devils, Tasmanian ‘tigers’, and dunnarts among others), Vombatomorphia (wombats), Phascolarctomorphia (koalas), Phalangeroidea (possums) and Macropodoidea (kangaroos and allies) (Archer et al., 2006; Louys et al., 2007; Black et al., 2014; Butler et al., 2016). Many of Riversleigh’s fossil vertebrates share a similar body plan to modern Australian vertebrates, but with some bizarre exceptions and groups of organisms that are now extinct. For example, the yalkiparidons, informally known as ‘thingodonts’, were a group of small-bodied marsupials that possessed a bony ‘beak’ and may have been ecologically convergent with woodpeckers (Beck, 2009). Riversleigh also bore witness to the earliest members of the now-extinct Thylacoleonidae (marsupial ‘lions’) and Diprotodontoidea (marsupial ‘tapirs’ and ‘rhinos’; Figure 2A), two groups that eventually came to dominate the top mammalian predator and herbivore niches, respectively, of mid-late Cenozoic Australia (Wroe et al., 1999; Price and Piper, 2009). Riversleigh shows remarkable transitions of ecosystems through time, with the open forests of the late Oligocene overtaken by extensive rainforests for much of the Miocene. During the late Miocene, a trend towards drier conditions saw 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 deposits in the region dating to the early Pleistocene (ca. 2 million years ago) (Woodhead et al., 2016). The story of the evolution of northeastern Australia’s terrestrial vertebrates then takes up with cave deposits that date from the middlelate Pleistocene to Holocene (ca. last 500 thousand years).
A) Lower jaw of a diprotodontoid (wombat-like marsupial) encased in rock from Riversleigh; B) Holotype of landdwelling crocodile, Quinkana fortirostrum, from Chillagoe (Australian Museum Fossil 57844). C) Occlusal view of lower jaw of a sthenurine (giant short-faced) kangaroo from Robert Broom Cave, Broken River; D) Fossils of ‘microfauna’ from Colosseum Chamber, an owl roost deposit from the Capricorn Caves, Mt Etna region.
3. Chillagoe Extensive tower karst occurs in the Chillagoe area of northeastern Australia (Figure 1). The limestone here is late Silurian – early Devonian (ca. 400 million years old) and contains numerous fossil molluscs, brachiopods and corals (De Keyser and Wolff, 1964). Bone breccias are common in the caves, many of which are topographically higher than the modern land surface in the region. Unlike Riversleigh, the majority of caves in the region are still intact, but have not been explored by palaeontologists anywhere near as extensively. The majority of fossil vertebrates from Chillagoe are small-bodied and include bilbies (Macrotis) and pig-footed bandicoots (Chaeropus), species that are found in the historic period only in Australia’s central arid core (Muirhead and Godthelp, 1995). Their occurrence at Chillagoe suggests a more open habitat and drier climate in the area in the past. Several species of ‘megafauna’ have also been recorded from around Chillagoe including a species of land crocodile (Quinkana fortirostrum; Figure 2B), giant carnivorous kangaroo (Propleopus chillagoensis), and marsupial ‘tapir’ (Palorchestes azael). Chillagoe is also the type locality for the former two species (Archer et al., 1978; Molnar, 1981). Radiometric dating of the marsupial ‘tapir’ suggests that it occurred locally during the penultimate glacial cycle (ca. 137–200 thousand years ago) (Price et al., 2013).
4. Broken River The Broken River karst is situated approximately 60 km south of the small township of Greenvale in northeastern Queensland (Figure 1). Numerous caves occur within the SilurianDevonian limestones, the majority of which also contain vertebrate fossils (Withnall and Lang, 1993). Work in the region is preliminary but ongoing. The majority of vertebrate remains appear to have accumulated in the caves from the feeding activities of owls. Most species recovered
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and identified to-date are thus unsurprisingly from smallbodied species, but include extremely diverse assemblages of marsupials and rodents, species of which suggest an opentype savannah and thus similar to the palaeoenvironmental signal from Chillagoe. One of the most exciting deposits is that from Robert Broom Cave (a system named by the Chillagoe Caving Club in honour of the palaeoanthropologist famous for discovering the early hominid, ‘Mrs Ples’, the most complete skull of Australopithecus africanus). Here the taphonomic signature differs to that of other caves at Broken River, with the contained fossil species being those of large-sized taxa and includes the largest-ever marsupial ‘lion’, Thylacoleo carnifex, as well as the largest-ever marsupial overall, the enormous wombatlike Diprotodon. It is clear that this system acted as a pitfall trap. The cave also contains abundant fossils of macropods, the majority of which are those of the modern dominant kangaroo, Macropus, although fossils of sthenurines (giant short-faced kangaroos) have also recently been collected (Figure 2C). Previously, Pleistocene species of sthenurines were known only from southern Australia, thus, their occurrence at Broken River marks a ca. 1200 km geographic range extension for the time period. This finding highlights just how poorly explored the fossil record of northern Australia is in comparison to the south. An extensive, but mostly hitherto unpublished radiometric dating program of Broken River fossil deposits demonstrates that the majority are younger than ca. 350 thousand years old. The palaeoecological signature from fossil vertebrates suggests largely an open habitat through time, a finding that is in contrast with the record from the Mt Etna region, approximately 700 km to the south.
5. Mt Etna region The Mt Etna region includes both Mt Etna itself and the adjacent Limestone Ridge, thus encompassing both National Park and private land (Figure 1). The marine limestones here are Devonian and preserve abundant fossils of corals, brachiopods and molluscs (Sprent, 1970). Numerous caves occur within the limestone karst. Mt Etna was subjected to extensive quarrying activities from the 1970s to the 2000s. Serendipitously, while the quarrying destroyed numerous caves and roosting sites for local bats, many fossil deposits were exposed and made accessible for palaeontological assessment. The fossil faunas collected here, and on the adjacent Limestone Ridge, are particularly diverse and include a range of both small- and large-bodied taxa including molluscs, frogs, lizards, snakes, marsupials and rodents (Hocknull, 2005). The story of the Mt Etna region is somewhat similar to that of Riversleigh. The oldest fossil faunas recovered include many species that appear to have been rainforest specialists, some of which share close evolutionary links to those found today in 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. However, by ca. 200 thousand years ago, few rainforest-adapted taxa are known, the majority of which appear to have been ecologically replaced by open, arid-adapted forms including species such as bilbies (Hocknull et al., 2007). This purely cli-
matically driven extinction event occurred at least 150 thousand years before the arrival of humans on the continent. Several megafaunal species are recorded in the region, the geologically youngest of which is a variety of giant goanna (species uncertain, but likely referable to either Megalania, Varanus priscus, or Komodo dragon, V. komodoensis). Previously thought to have suffered extinction prior to human arrival (Price et al., 2011; Wroe et al., 2013), recent dating of a specimen from Colosseum Chamber (Capricorn Caves; Figure 2D) shows that it is slightly younger than 50 thousand years old, placing it within the timeframe of the earliest humans on the continent (Price et al., 2015). A local archaeological signature is absent for that time, thus it is unknown whether humans and giant lizards actually ever directly encountered each other.
6. Summary Caves of northeastern Australia record a drying continent over the last 25 million years, although certain areas appear to have maintained more humid conditions until geologically recent periods. The unique palaeoenvironmental signals from the region’s caves are critical in helping us understand faunal change in response to past climatic and environmental perturbations, which of course has important implications for understanding the effects of modern climate change. Although fossils have been known from northeastern Australian caves for well over a century, the region has been somewhat neglected in the past in terms of palaeontological investigation. Despite many advances, we are still scratching the surface of this most important palaeontological resource. 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. Palaeontologists will continue to rely on the co-operation and assistance of the speleological community for these discoveries.
Acknowledgements We are greatly indebted to Douglas Irvin and family; Paul Osborne and the Chillagoe Caving Club; Noel Sands and family; Ann Augustyn and staff from the Capricorn Caves; Michael Archer, Sue Hand, Henk Godthelp and other colleagues of the University of New South Wales; Scott Hocknull and staff of the Queensland Museum; and the many untiring supporters and volunteers who have diligently worked with us on northeastern Australia’s fossil deposits. This research was funded in part by: an Australian Research Council (ARC) Discovery Grant (DP120101752); ARC Linkage Grant (LP0989969) with key support from industry partners including Xstrata Copper, Queensland Museum, Outback at Isa, and Mt. Isa City Council; ARC Discovery Early Career Researcher Award (DE120101533); Australian Institute of Nuclear Science and Engineering grant (AINGRA09002); and an Ian Potter Foundation grant.
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