Late-surviving megafauna in Tasmania, Australia, implicate human involvement in their extinction

Abstract

Establishing the cause of past extinctions is critical if we are to understand better what might trigger future occurrences and how to prevent them. The mechanisms of continental late Pleistocene megafaunal extinction, however, are still fiercely contested. Potential factors contributing to their demise include climatic change, human impact, or some combination. On the Australian mainland, 90% of the megafauna became extinct by ≈46 thousand years (ka) ago, soon after the first archaeological evidence for human colonization of the continent. Yet, on the neighboring island of Tasmania (which was connected to the mainland when sea levels were lower), megafaunal extinction appears to have taken place before the initial human arrival between 43 and 40 ka, which would seem to exonerate people as a contributing factor in the extirpation of the island megafauna. Age estimates for the last megafauna, however, are poorly constrained. Here, we show, by direct dating of fossil remains and their associated sediments, that some Tasmanian megafauna survived until at least 41 ka (i.e., after their extinction on the Australian mainland) and thus overlapped with humans. Furthermore, a vegetation record for Tasmania spanning the last 130 ka shows that no significant regional climatic or environmental change occurred between 43 and 37 ka, when a land bridge existed between Tasmania and the mainland. Our results are consistent with a model of human-induced extinction for the Tasmanian megafauna, most probably driven by hunting, and they reaffirm the value of islands adjacent to continental landmasses as tests of competing hypotheses for late Quaternary megafaunal extinctions.

• Paleoclimate
• Pleistocene
• sea level change
• island colonization
• human hunting

Full text

Late-surviving megafauna in Tasmania, Australia,
implicate human involvement in their extinction
Chris S. M. Turney
a,b,c
, Timothy F. Flannery
c,d
, Richard G. Roberts
a,c
, Craig Reid
e
, L. Keith Fifield
f
, Tom F. G. Higham
g
,
Zenobia Jacobs
a
, Noel Kemp
h
, Eric A. Colhoun
i
, Robert M. Kalin
j
, and Neil Ogle
k
aGeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Wollongong NSW 2522, Australia; dDivision of
Environmental and Life Sciences, Macquarie University, North Ryde NSW 2109, Australia; eQueen Victoria Museum and Art Gallery, Launceston, Tasmania
7250, Australia; fResearch School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia; gOxford Radiocarbon
Accelerator Unit, Research Laboratory for Archaeology and the History of Art, University of Oxford, Oxford OX1 3QY, United Kingdom; hTasmanian
Museum and Art Gallery, Hobart, Tasmania 7001, Australia; iSchool of Environmental and Life Sciences, University of Newcastle, Newcastle NSW 2308,
Australia; jDavid Livingstone Center for Sustainability, Graham Hills Building, University of Strathclyde, Glasgow G1 1XN, Scotland; and kEnvironmental
Engineering Research Centre, School of Planning, Architecture and Civil Engineering, Queen’s University, Belfast BT9 5AG, United Kingdom
Edited by Jared M. Diamond, University of California, Los Angeles, CA, and approved June 6, 2008 (received for review February 11, 2008)
Establishing the cause of past extinctions is critical if we are to
understand better what might trigger future occurrences and how
to prevent them. The mechanisms of continental late Pleistocene
megafaunal extinction, however, are still fiercely contested. Po-
tential factors contributing to their demise include climatic change,
human impact, or some combination. On the Australian mainland,
90% of the megafauna became extinct by ⬇46 thousand years (ka)
ago, soon after the first archaeological evidence for human colo-
nization of the continent. Yet, on the neighboring island of
Tasmania (which was connected to the mainland when sea levels
were lower), megafaunal extinction appears to have taken place
before the initial human arrival between 43 and 40 ka, which
would seem to exonerate people as a contributing factor in the
extirpation of the island megafauna. Age estimates for the last
megafauna, however, are poorly constrained. Here, we show, by
direct dating of fossil remains and their associated sediments, that
some Tasmanian megafauna survived until at least 41 ka (i.e., after
their extinction on the Australian mainland) and thus overlapped
with humans. Furthermore, a vegetation record for Tasmania
spanning the last 130 ka shows that no significant regional climatic
or environmental change occurred between 43 and 37 ka, when a
land bridge existed between Tasmania and the mainland. Our
results are consistent with a model of human-induced extinction
for the Tasmanian megafauna, most probably driven by hunting,
and they reaffirm the value of islands adjacent to continental
landmasses as tests of competing hypotheses for late Quaternary
megafaunal extinctions.
Paleoclimate 兩Pleistocene 兩sea level change 兩island colonization 兩
human hunting
Since Charles Darwin’s discover y of giant ground sloth re-
mains in South America, debate about the cause of global
late Quaternary extinctions of megafauna (animals ⬎40 kg) has
been intense (1, 2). Potential mechanisms invoked for
megafauna demise include climatic change, human and extra-
terrestrial impacts, or some combination. In Australia, initial
human colonization of the mainland occurred by 50 ka
l
(3–5) and
has been linked to vegetation reorganization (6) and increased
burning (7) in the landscape, driving the megafauna to extinction
on the mainland by 46 ka (6, 8, 9). Some workers have argued
that most megafauna were extinct before human arrival, citing
the apparent disappearance of megafauna in Tasmania before
people reached this island as support for extinction driven by
climatic and/or environmental change (10, 11).
Currently an island of 68,300 km
2
, Tasmania lies 240 km from
the mainland across Bass Strait (Fig. 1) with a climate dominated
by cyclonic activity associated with prevailing westerly airf low.
During the Pleistocene, Tasmania was periodically connected to
mainland Australia (12), allowing faunal migrations. The island
was isolated for much of the interval 135– 43 ka and continuously
from 14 ka to the present day, with the first sustained land bridge
of the last glacial cycle occurring between 43 and 37 ka (12).
Compared with mainland occupation, human colonization was
relatively late, with the earliest radiocarbon (
14
C) ages for arrival
being obtained from rock shelters in southwest Tasmania: these
indicate a human presence before the oldest radiocarbon-dated
horizon of 34,790 ⫾510 BP at Warreen Cave (13) (Fig. 1),
equivalent to a calendar age of ⬇40 ka (14); stone artifacts were
present in deeper deposits (13), suggesting that humans arrived
in Tasmania between 43 ka (when the land bridge first emerged)
and 40 ka.
The Tasmanian megafauna consists of seven species (six
marsupials and one monotreme), which also occurred on main-
land Australia. There is some evidence of subspecific endemism:
Tasmanian Simosthenurus occidentalis have different dental
proportions from those of the adjacent mainland (15), and
Tasmanian Thylacoleo also exhibit distinctive dental features.
The island has yielded abundant remains, with at least eight
locations in the northwest and south, producing thousands of
bones (16, 17) (see Table 1). No Tasmanian megafaunal remains
have hitherto been reliably dated. Early attempts at dating
suggested that megafauna may have survived until relatively
recently. For example, charcoal associated with megafaunal
remains in Titans Shelter (17) was radiocarbon dated to 14,310
⫹2,970
/
⫺2,160
BP. Subsequent direct dating of bone from this site
by using amino acid racemization (AAR) and electron spin
resonance (ESR) gave ages of 40–27 ka, suggesting that the
dated charcoal was intrusive (18). No megafaunal remains have
been found in human occupation sites (19), but only 0.6% of the
material examined dates to the first 4 ka of human occupation,
so any temporal overlap of humans and megafauna may have
been missed (20).
Results and Discussion
We undertook dating and paleontological investigations of ar-
chived fossil material from four reported megafauna sites in
Author contributions: C.S.M.T., T.F.F., and R.G.R. designed research; C.S.M.T., T.F.F., R.G.R.,
C.R., L.K.F., T.F.G.H., Z.J., N.K., E.A.C., R.M.K., and N.O. performed research; C.S.M.T., T.F.F.,
R.G.R., C.R., L.K.F., T.F.G.H., Z.J., N.K., E.A.C., R.M.K., and N.O. analyzed data; and C.S.M.T.,
T.F.F., and R.G.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
bTo whom correspondence should be sent at the present address:School of Geography,
Archaeology and Earth Resources, Department of Geography, University of Exeter, Exeter,
EX4 4RJ, United Kingdom. E-mail: c.turney@exeter.ac.uk.
cC.S.M.T., T.F.F., and R.G.R. contributed equally to this work.
lIn this paper, calibrated radiocarbon ages and those derived from alternative dating
methods are reported as ka (thousands of calendar years ago). Uncalibrated radiocarbon
ages are reported as BP (before present; relative to 1950 AD).
This article contains supporting information online at www.pnas.org/cgi/content/full/
0801360105/DCSupplemental.
© 2008 by The National Academy of Sciences of the USA
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Tasmania [Fig. 1 and supporting information (SI) Tables S1–S3]:
Pleisto Scene Cave, Scotchtown, Titans Shelter, and Un-named
Cave. In addition, new fossil finds were investigated from Mount
Cripps (chambers CP118 and CP213). To avoid concerns about
intrusive charcoal and other associated carbon (18), we applied
14
C dating to bone collagen, by using either demineralization
followed by gelatin extraction and step-combustion (C-SC) or
gelatin isolation with ultrafiltration (C-AF) (21). The
14
C con-
tent was measured by accelerator mass spectrometry, and the
ages were calibrated following ref. 14. The fossiliferous cave
sediments were dated by optically stimulated luminescence
(OSL), which indicates the time elapsed since the sediment
grains were last exposed to sunlight (22). In sequences with
articulated megafaunal remains in primary context, OSL dating
gives the time of fossil burial; where no articulation is observed,
minimum estimates of burial age are obtained. Full
14
C and OSL
dating details are given in SI Dating Methods and Results.
The Tasmanian megafauna dated in this work provide a
record encompassing much of the late Pleistocene, from ⬎127
to 41 ka (see Table 1 and Tables S1–S3). The deposits in Pleisto
Scene Cave contained skeletal elements of megafauna in articula-
tion and were dated by OSL to marine isotope stage (MIS) 5e or
earlier (⬎120 and ⬎127 ka, the infinite ages arising from dose
saturation of these samples). The Un-named Cave deposits gave an
OSL age of 103 ⫾9 ka, which is indistinguishable from the 97-ka
age obtained on the nonarticulated remains with ESR (18). Evi-
dence of articulation was also not reported for the rich megafaunal
site of Scotchtown Cave, which was interpreted as a carnivore lair
(16). Because articulated remains are not expected in such contexts,
it is not unreasonable to assume that the OSL age of 56 ⫾4ka,
obtained for sediment grains in a mixed soil– bone sample from the
original excavation, approximates that of the entombed fauna. The
samples from all of these sites had negligible collagen content and
were unsuitable for
14
C dating.
Younger ages for Tasmanian megafauna were obtained from
Titans Shelter and Mount Cripps. A Protemnodon anak specimen
was obtained from Titans Shelter and dated by
14
C (C-SC) to
39,980 ⫾610 BP, equivalent to ⬇44 ka (Table S1). At Mount Cripps
(chamber CP213), faunal remains were discovered lying on the
chamber floor and wedged in rock crevices; their state of preser-
vation was excellent. An entire cranium was recovered and most of
Titans Shelter ,
Un-named &
Beginners Luck
caves
Mowbray,
Scotchtown & Pulbeena
Mt Cripps
Warreen
Cave
Hobart
Melbourne
Lake Selina
Main &
Pleisto Scene
caves
Launceston
Fig. 1. Locations of megafaunal, environmental, and archaeological sites in
Tasmania. Megafaunal sites are marked with solid circles, Lake Selina with a
closed triangle, and Warreen Cave with a solid square.
Table 1. Ages and faunal assemblages of principal Tasmanian megafaunal sites
Event
Age of unit(s),
ka
Megalibgwilia
sp.
Thylacoleo
sp.
Palorchestes
azael
Zygomaturus
trilobus
Protemnodon
anak
Simosthenurus
occidentalis
Metasthenurus
newtonae
Human arrival in
Tasmania
43–40
Mount Cripps,
CP213
42.9–40.9 X
Mount Cripps,
CP118
X
Titans Shelter, Site
G
44 ? X X
Mowbray Swamp ⬎38 X
Main Cave
(MU201)
X
Pulbeena
Limeworks
⬎54 X
Scotchtown Cave 56 ⫾4X XX X X X X
Beginner’s Luck
Cave M (JF-79)
80–63 X X
Un-named Cave
(JF-155)
103 ⫾9X XX
Pleisto Scene Cave B
(MU203–205; 206)
⬎127 ⫾28
⬎120 ⫾21
XXX ? X X
See Supporting Information for further details. Note that human arrival in Tasmania is contemporaneous with the most recently dated megafaunal remains
from Mount Cripps, demonstrating that there was some overlap (albeit brief) between humans and megafauna. That the megafauna had survived on Tasmania
for millions of years prior to human arrival suggests that the sudden disappearance of megafauna was caused by humans.
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a second specimen. No skeletons were articulated, but associated
elements were found in close proximity. The only megafaunal
species present in the deposit was P.anak, of which the remains of
three individuals are represented: a large subadult
(QVM:2001:GFV 5, body mass estimate ⬇60 kg) (23), a pouch
young (QVM:2001:GFV 9), and a young at foot (QVM:2000:GFV
10, body mass estimate ⬇30 kg) (see Tables S4 and S5). OSL dating
of sediment wedged in the nasal cavity of the subadult yielded an
age of 36 ⫾3ka(Table S3 and Fig. S1), which we view as a
minimum age for the skull because the sediment grains were
emplaced postmortem. Radiocarbon dating of two individuals from
Mount Cripps, with C-SC pretreatment, produced ages of 32,780 ⫾
370 BP and 30,400 ⫾270 BP, but significantly older ages of 36,200 ⫾
300 BP and 37,920 ⫾340 BP were obtained from duplicate samples
by using the C-AF procedure (Table S1); the latter is thought to
yield more accurate ages for ancient bone (21). We attribute the
younger C-SC
14
C ages to the incomplete removal of low-molecular
weight contaminants from the gelatin. The C-AF
14
C ages are
therefore considered more reliable. When these ages are calibrated
and considered in conjunction with the OSL age from Mount
Cripps, they indicate that Tasmanian megafauna survived to at least
42.9–40.9 ka (at 1 SD).
The Scotchtown and Pleisto Scene Cave deposits differ in age
by ⬎70 ka (from ⬇56 to ⬎127 ka) but contain essentially the
same suite of megafauna (Table S4), the only exception being
Metasthenurus newtonae, which is represented by three isolated
molars at Scotchtown (and known also from an undated context
at Mount Cripps, chamber CP118). If our assumption is correct
that the OSL age of the sediments at Scotchtown approximates
that of the faunal remains, then Tasmania experienced no
significant change in its faunal diversity between the last inter-
glacial (MIS 5e) and the succeeding glacial stages. The survival
of all seven species of Tasmanian megafauna until at least 56 ka
argues against proposals for a staggered series of climate-
induced extinctions before human arrival (10), similar to findings
on the Australian mainland (9, 20, 24, 25).
As an independent test of possible climatic and/or environ-
mental influences (including anthropogenic burning) on
megafaunal extinction (6, 10), we investigated the late Quater-
nary vegetation and charcoal records preserved in Lake Selina
(Fig. 1), which provides a crucial southern midlatitude perspec-
tive on paleoecological changes over the past 130 ka (26). The
pollen record indicates that cool temperate rainforest was
present during MIS 5e and the Holocene (MIS 1), with grass-
land, herbs, and heath vegetation dominating throughout the
intervening period (Fig. 2). There is no evidence of a significant
increase in burning at the time of human arrival or any associated
shift in vegetation. The bulk sediments exhibit a relatively
narrow range of stable carbon isotope values (
␦
13
C), reflecting
the local C
3
vegetation, with the small f luctuations probably
representing changes in lake productivity. No major change
occurred at 41 ka.
The overall vegetation trend at Lake Selina mirrors other
regional proxies of environmental and climatic conditions (27),
including winter sea-ice cover in the Southern Ocean, as inferred
from the sea-salt sodium flux in Antarctica (28) (Fig. 2).
Elevated levels of sea-ice persisted from ⬇65 to 17 ka, with no
significant deviation at 41 ka. Furthermore, we note that moa
extinction did not take place ‘‘downwind’’ in New Zealand at this
time (29). We consider it highly unlikely, therefore, that climatic
or environmental change could have played a significant role in
the demise of the megafauna on Tasmania.
Although our dataset is small, the results show that megafauna
persisted in Tasmania until at least 41 ka, which is similar to, or
slightly later than, the time of extinction on the Australian
mainland (48.9–43.6 ka at 1 SD) (9, 30). Importantly, the 41-ka
age postdates the establishment of a land bridge across the Bass
Strait and is statistically concordant (
␹
2
test T⫽3.39; 5% ⫽3.84)
with the minimum age for the earliest evidence of humans in
Alpine/subalpine
Herbs
Other trees/
small trees/shrubs
Rainforest
Pollen %
250
500
750
1000
1250
-32
-30
-28
-26
-24
-22
δ13C, ‰ ssNa flux, µg/m2/yr
100
75
50
25
0 10 20 30 40 50 60 70 80 90 100 110 120 130
0
1 2 3 4 5a 5b 5c 5d 5e
MIS
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Land bridge
Human arrival
Megafaunal ages
800
600
400
200
0
Charcoal particles/cm3
Calibrated age (ka)
Calibrated age (ka)
Fig. 2. Environmental and climatic changes in Tas-
mania and the Southern Ocean. Solid gray boxes de-
note when a land bridge was established between the
Australian mainland and Tasmania (12). For human
arrival, solid (black) arrow indicates minimum age for
earliest known occupation of Tasmania (13); open
(white) arrows delimit 1 SD age range for earliest
occupation of mainland Australia (3–5). Ages associ-
ated with megafaunal remains dated in this work are
shown as 1 SD ranges. Dashed line denotes (minimum)
overlap between most recent megafaunal remains and
earliest human arrival in Tasmania. Summary plots of
vegetation change (26), charcoal concentration (26),
and
␦
13C of bulk sediments (this work) are from Lake
Selina. Sea-salt sodium (ssNa) flux is a measure of win-
ter sea-ice cover in the Southern Ocean, as recorded in
the EPICA ice core (28); note that the ssNa flux scale is
inverted.
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Tasmania [Warreen Cave (13)]. This introduces the possibility,
hitherto discounted (10, 11), that humans caused the extinction
of the Tasmanian megafauna. Because we find no evidence for
a major change in vegetation and/or fire dynamics [in contrast to
the mainland (6, 7)] and because the megafauna had relatively
low reproductive rates, we favor the hypothesis that human
hunting led to a rapid population collapse (31). A brief coex-
istence of humans and megafauna is consistent with the absence
of megafaunal remains at human occupation sites (19). To
elucidate the precise mechanism of extinction will require im-
proved age estimates for both the first human arrivals and the
last-surviving megafauna in Tasmania.
Materials and Methods
Samples for 14C dating were obtained from museum specimens held in col-
lections at Launceston and Hobart. Fragments of bone were hand-picked, and
the collagen was extracted in two ways. At the University of Wollongong (C-SC
pretreatment), bone fragments were washed in distilled water to remove
surface contaminants, then they were broken into small pieces and decalcified
with dilute hydrochloric acid. The gelatin was isolated and step-combusted to
remove any labile contaminants. The CO2evolved at 850°C was collected,
purified, and graphitized for accelerator mass spectrometry (AMS) radiocar-
bon measurement at the Australian National University (ANU). At the Univer-
sity of Oxford (C-AF pretreatment), coarsely ground bone powder was given
a series of acid, base, and acid washes, rinsing with ultrapure water between
each reagent. Crude collagen was gelatinized, and the ⬎30-kDa gelatin
fraction was isolated by ultrafiltration for AMS radiocarbon measurement at
Oxford (21). All 14C ages were calibrated following ref. 14. For stable isotope
analysis, samples were combusted in an elemental analyzer interfaced to a
continuous-flow isotope-ratio mass spectrometer at ANU (extracted gelatin)
and Queen’s University Belfast (Lake Selina). Bulk sediments from Lake Selina
were treated before measurement with dilute hydrochloric acid to remove
carbonates, and the CO2evolved was analyzed for isotopic abundance by gas
chromatography.
␦
13C values are reported as per mille (‰) relative to the PDB
scale; analytical precision is ⫾0.15 ‰ at 1 SD. Sediment samples for OSL dating
were obtained from the Launceston and Hobart museum collections, and
analyzed at the University of Wollongong. Quartz grains of 90- to 250-
␮
m
diameter were extracted from the light-safe portion of each sample and
purified by using standard procedures, including etching with hydrofluoric
acid to remove the external
␣
-dosed layer. The burial ages were calculated
from the equivalent dose, estimated by using a single-aliquot regenerative-
dose procedure (22), divided by the total dose rate arising from ionizing
radiation.
ACKNOWLEDGMENTS. We thank members of the Savage River Caving Club,
the Tasmanian Museum and Art Gallery, Sue Wood (Australian National
University) who ran the University of Wollongong bone samples for
␦
13C and
C:N, and Richard Miller (University of Wollongong) who helped prepare Fig.
1. This work was supported by Royal Society Research Grant 24210 and
Australian Research Council (ARC) Grants DP0451152 and F00103154. C.S.M.T.
held an ARC Queen Elizabeth II Fellowship and R.G.R. an ARC Senior Research
Fellowship.
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