Paleobiology, 36(4), 2010, pp. 658–671
An Australian land mammal age biochronological scheme
Dirk Megirian, Gavin J. Prideaux, Peter F. Murray, and Neil Smit
Abstract.—Constrained seriation of a species-locality matrix of the Australian Cenozoic mammal
record resolves a preliminary sixfold succession of land mammal ages apparently spanning the late
Oligocene to the present. The applied conditions of local chronostratigraphic succession and
inferences of relative stage-of-evolution biochronology lead to the expression of a continental
geological timescale consisting of, from the base, the Etadunnan, Wipajirian, Camfieldian, Waitean,
Tirarian, and Naracoortean land mammal ages. Approximately 99% of the 360 fossil assemblages
analyzed are classifiable using this method. Each is characterized by a diagnostic suite of species. An
interval of age magnitude may eventually be shown to lie between the Camfieldian and Waitean, but
is currently insufficiently represented by fossils to diagnose. Development of a land mammal age
framework marks a progressive step in Australian vertebrate biochronology, previously expressed
only in terms of local faunas. Overall, however, the record remains poorly calibrated to the Standard
Chronostratigraphic Scale. Codifying the empirical record as a land mammal age sequence provides
an objective basis for expressing faunal succession without resort to standard chronostratigraphic
terms with the attendant (and hitherto commonly taken) risks of miscorrelating poorly dated
Australian events to well-dated global events.
Dirk Megirian* and Peter F. Murray. Museum of Central Australia, Alice Springs, Northern Territory 0871,
Australia
Gavin J. Prideaux. School of Biological Sciences, Flinders University, Bedford Park, South Australia 5042,
Australia. E-mail: [email protected]
Neil Smit. Marine Biodiversity Group, Department of Natural Resources, Environment and the Arts,
Casuarina, Northern Territory 0811, Australia
* Dirk Megirian passed away while digging at Alcoota on 27 July 2009.
Accepted:
16 March 2010
Introduction
The Australian terrestrial mammal record
is sparse compared to those of all other
continents except Antarctica (Rich 1991). The
comparatively few localities are patchily
distributed and mostly stratigraphically discontinuous. Consequently, faunal succession
cannot effectively be expressed in conventional biostratigraphic terms at a continental
or even regional scale. However, assemblages
that locally occur in stratigraphic sequence
are unequivocally oriented in time by superposition, and temporal relationships of isolated assemblages may be inferred on the basis
of hypotheses of evolutionary succession
within mammal lineages. A body of rock
producing taxa at apparently more advanced
stages of evolution than closely related forms
in another body of rock is most likely to be the
younger of the two. A mammal stage-ofevolution biochronology combines available
empirical evidence of faunal succession from
’ 2010 The Paleontological Society. All rights reserved.
chronostratigraphic observations with inferences of relative ages based on evolutionary
hypotheses. Once calibrated, stage-of-evolution biochronologies constitute geological
timescales comparable to those derived purely from chronostratigraphic data (Fig. 1).
Geological timescales based on land mammals have been formalized for North America, South America, Europe, and Asia (see Fig.
S1 in online supplemental material at http://
dx.doi.org/10.1666/06028.s1). Each has its
own history of development reflecting the
uniqueness of the records and the extent to
which faunal succession has been resolved.
These timescales are variously expressed in
terms of conventional mammal biostratigraphic zones or as land mammal ages
(LMAs), defined by Lindsay (2003: p. 222) as
‘‘relatively short interval[s] of geological time
that can be recognized and distinguished
from earlier and later such units (in a given
region or province) by a characterizing
assemblage of mammals.’’
0094-8373/10/3604–0008/$1.00
AUSTRALIAN LAND MAMMAL BIOCHRONOLOGY
659
FIGURE 1. A, Conceptualization of a conventional geological timescale as a merger of a chronometric scale and a
chronostratigraphic scale (after Gradstein et al. 2005: Fig. 1.1). B, Conceptualization of a geological timescale based on
land mammals, where stage-of-evolution biochronology has been used to order stratigraphically isolated, fossiliferous
strata in time. The situation depicted is from the Lake Eyre Basin, where Ektopodon stage-of-evolution biochronology
has been used to order the undated Ulta Limestone in time, relative to the unconformable succession of the Wipajiri
Formation over the Etadunna Formation (Megirian et al. 2004).
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DIRK MEGIRIAN ET AL.
The main impediment to formalizing an
Australian timescale based on mammals has
been the lack of sufficient data and a
comprehensive, objective method of differentiating time intervals. Australian land mammal biochronology was pioneered by Ruben
A. Stirton and associates. Three seminal
papers laid the foundation for the current
Australian framework (Fig. 2A). Stirton et al.
(1961) briefly articulated a philosophical basis
for it, drawing on North American precedents, although their main focus was describing the chronostratigraphic succession of
vertebrate assemblages in the Lake Eyre
Basin, northern South Australia. Observations
on the wider applicability of stage-of-evolution biochronology were of a general nature,
in anticipation of the publication of systematic studies of the Diprotodontidae, an extinct
family of quadrupedal marsupial herbivores
(Stirton and Woodburne 1967). At the core of
a synthesis presented by Stirton et al. (1967)
were zygomaturine diprotodontids found in
chronostratigraphic succession in deposits of
the eastern Lake Eyre Basin. Together, this
work established a reference standard for a
continent-wide biochronological framework
(Stirton et al. 1968).
The present Australian framework, as last
comprehensively reviewed by Woodburne et
al. (1985), consists of local faunas (LFs)
organized temporally on the basis of local
superposition, biocorrelation, and relative
stage of evolution (Fig. 2A). As the most basic
unit of land mammal biochronology, a local
fauna is defined as an assemblage of fossils
that may contain as few as one taxon and that
may be drawn from a single site or only a few
geographically adjacent, temporally equivalent sites. In effect, it represents an instant in
geological time at a particular locality (Tedford 1970; Woodburne et al. 1985; Megirian
1994). The only notable elaboration beyond
the local fauna framework is the informal
zonal scheme corresponding to local fauna
succession in the Etadunna Formation presented by Woodburne et al. (1994).
Over the past two decades considerably
more taxonomic and occurrence data have
accumulated, new localities have been discovered, updated faunal lists for key assem-
blages have been produced, parts of the
framework have been recalibrated, and new
or elaborated stage-of-evolution biochronologies have been set out. Here we demonstrate
that the available species-level occurrence
data provide an objective basis for resolving
an Australian LMA succession.
Materials and Methods
Database.—A fossil vertebrate database covering the Australian Cenozoic was compiled
in Microsoft ExcelH by D.M. and G.J.P. from
published records. The subset covering mammals identified formally or informally to
species level was abstracted and a specieslocality matrix (species as rows, localities as
columns) generated. Because Excel cannot
generate matrices in excess of 255 columns,
the large data set was divided into two
overlapping parts. One includes records of
notionally Eocene to late Miocene age, and
the other covers the Pliocene to Recent (Figs.
S12, S13 in online supplementary material
at http://dx.doi.org/10.1666/06028.s2). Both
parts were then combined into a tabdelimited text file for analysis. See Table S1
for a list of species occurrences and the
reference for each occurrence.
Nomenclature.—Informal zoological taxonomy varies in the literature and some conventions of biochronological expression have
changed over time (Megirian 1994), so some
standardization was applied. Species identifications are presented in the form of Linnean
binomials or as unique, informal identifiers,
either as applied by the original source or
standardized or modified as necessary or
convenient for the purpose of analysis.
Changes of this type were necessary to avoid
synonymizing different forms from different
localities that would otherwise result in
miscorrelation or to explicitly indicate that a
new species had been identified rather than
merely an indeterminate species of a particular genus. Some authors recognized a new
species but may have labeled it in their
reports merely as a ‘‘Genus sp.’’ rather than
more explicitly as a ‘‘Genus sp. nov.’’ Assemblages are identified either by a formalized
local fauna name (LF) or by only a site name
(Megirian 1994). Site assemblages from the
AUSTRALIAN LAND MAMMAL BIOCHRONOLOGY
661
FIGURE 2. A, Steps in the calibration of the Australian land mammal biochronological framework, as represented by
key reference local faunas (LFs) of historical significance. This work encapsulates developments post-Woodburne et al.
(1985), particularly with regard to grounds for a recalibration of the base of the scheme based on correlative data sets,
refinements to Lake Eyre Basin paleontology, stratigraphy and geochronology, and elaboration of zygomaturine stageof-evolution biochronology (Wells and Callen 1986; Tedford and Wells 1990; Tedford et al. 1992; Megirian 1992, 1994;
Woodburne et al. 1994; Murray et al. 2000; Megirian et al. 2004). On the right-hand side is faunal succession expressed
in terms of Australian LMAs. B, Changes in the calibration of the Standard Global Chronostratigraphic Scale, 1937–2004
(after Gradstein et al. 2005: Fig. 1.7).
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DIRK MEGIRIAN ET AL.
Carl Creek Limestone at Riversleigh, Queensland, are prefixed by RIV.
Seriation.—Seriation is a type of ordination,
a process of organizing a matrix so that
presences are concentrated in an optimal
way along a diagonal as an aid to pattern
recognition (Hammer et al. 2001). Archaeologists have long applied seriation (‘‘the concentration principle’’) to sort artifacts into a
one-dimensional series representative of time
or cultural development (Brower and Kile
1988). The process is achieved by reordering
rows and columns and may be done manually or by using mathematical routines. In
unconstrained seriation, both rows (herein,
species) and columns (herein, localities) may
be freely moved. In constrained seriation,
rows (or columns) are immovable, requiring
additional evidence to establish the direction
of a sequence (e.g., observed stratigraphic
level or inferred temporal or faunal gradient)
and only the columns (or rows) may be
moved (see Figs. S2, S3).
Seriation was preferred to the Appearance
Event Ordination method (AEO) of Alroy
(1994). It provides a means of analyzing the
full Australian data set without having to
meet the specific statistical requirements of
AEO compelling the exclusion of a large
number of sites (e.g., sites with less than 4
taxonomic records). This would substantially
reduce the ability to recognize any differentiating time intervals based on otherwise
unique faunal assemblages. AEO also requires broadly overlapping, temporally continuous suites and a substantial proportion of
absolutely dated sites to establish first and
last appearance events for taxa. By comparison with the Northern Hemisphere continents
in particular the Australian data set is unique
with respect to its biostratigraphic and paleozoogeographic isolation. It is further constrained by the large number of sites that
have low species diversity, a majority of
records lacking absolute age estimations and
many records of species that occur in nonoverlapping suites with significant gaps of
unknown duration. Due to a paucity of
absolute dates stage-of-evolution-based relative chronology is critical to the success of the
analysis. Seriation allows the use of relative
dating, hence the preference for an arguably
less sophisticated analytical method. Seriation
also allows direct comparison with previous
Australian studies in which the method has
been used (Travouillon et al. 2006).
The program PAST, version 1.94b (http://folk.
uio.no/ohammer/past/download.html; Hammer et
al. 2001) was used to carry out unconstrained
seriation analysis on the Australian specieslocality matrix (Figs. S2, S3). PAST seriates
matrices by using an iterative absencepresence
algorithm
(for
details
see
Wilkinson 1974; Brower and Kile 1988;
Travouillon et al. 2006). Given an ordering
of columns, the routine optimizes the
movable row data (taxa) into the most
elegant configuration. PAST runs a Monte
Carlo simulation of the resulting seriation for
comparison with the original matrix to
determine if it is more informative than a
random one (Puolamäki et al. 2006). For
constrained
seriation,
two
temporal
conditions were imposed on the matrix. (1)
A small number of LFs and site assemblages
occur in nature in physical superposition
(Fig. 3). Superposed assemblages, presented
as columns in the matrix, could not be
seriated into a relative temporal position
contravening observed chronostratigraphic
succession. (2) A number of marsupial
lineages have stage-of-evolution biochronological utility. Species (rows in the matrix)
that have been interpreted as being
phyletically successional (Fig. 4) could not
be seriated out of evolutionary sequence
relative to each other. A few localities that
do not correlate at the species level, but for
which chronometric data exist, were ordered
into a position consistent with other dated
assemblages.
Constrained seriation was carried out in
search of an optimal result in which the
maximum possible number of assemblages
and biochronologically informative species
are constrained to discrete intervals of time
(LMAs). Quadrilateral borders capturing the
maximum number of species occurrences
without temporal overlap were imposed on
the matrix: matrix spaces encompassed by
these boundaries are conceptual representations of LMAs (Figs. 5, S5–S10, S12, S13),
AUSTRALIAN LAND MAMMAL BIOCHRONOLOGY
663
FIGURE 3. Key localities where fossil assemblages occur in chronostratigraphic succession. Temporal condition 1 states
that a younger assemblage may not be moved into a position underlying an older assemblage. The extant fauna is
included as a temporal marker for the present (based on van Dyck and Strahan 2008) and includes taxa that became
extinct in historical times. Tight Entrance Cave beds succession follows Ayliffe et al. (2008); ages derived from 14C, U/
Th and optical dating. Madura Cave succession follows Lundelius and Turnbull (1989); ages derived from 14C dating.
Naracoorte Cathedral Cave succession follows Prideaux et al. (2007); ages derived from optical dating. Waite
Formation succession follows Megirian et al. (1996). Lake Eyre Basin succession follows Tedford et al. (1992) and
Woodburne et al. (1985, 1994); age estimates derived from magnetostratigraphy for Etadunna and Tirari Formations
(adjusted here to the recalibrated polarity timescale, see Table 5.2 in Gradstein et al. 2005), and thermoluminescence
dating for Katipiri and Kutjitara Formations (Nanson et al. 2008).
definable in faunal terms. Faunas consist of
marker species that have stage-of-evolution
biochronological and / or interregional biocorrelative utility (Table 1). Species (rows)
with stage-of-evolution significance were
used both to order assemblages in time and,
using two or more species within any
particular lineage, to characterize a span of
time. Row data thus control positions of the
upper (younger) and lower (older) temporal
bounds. Species with interregional biocorrelative utility produce a biocorrelation framework linking assemblages (columns) across
space. The spatial attribute is encompassed
within the biochronological concept of a
fauna (Tedford 1970; Megirian 1994; Lindsay
2003), which, along with temporal attributes
also expressed in columns by chronostratigraphic and chronometric values, controls
position of the left (younger) and right (older)
temporal boundaries.
In keeping with the objective of obtaining an
optimal result, the matrix was seriated under
constraint by giving greater weight to the
possibility that those species that could not be
included within age boundaries ranged across
an age boundary, rather than that they represented ‘‘incongruous’’ occurrences. Apparently incongruous species occurrences are those
where the association of species in a particular
assemblage contains one or more occurrences
that appear to be anomalous with respect to a
stronger, constrained pattern of species distribution. Incongruous associations suggest the
possibility of diachronous (mixed) assemblages. The constrained seriation method cannot
discriminate which of the alternative possibilities is correct: they simply stand as alternative
hypotheses to be considered and further
investigated. Where species occurrences are
evenly balanced between occurrences in two
successional LMAs, the species is presented as
one that apparently spans the age boundary.
Where the majority of occurrences fall into one
LMA and only a small number of occurrences
(typically one or two) fall into another; the
minority occurrences are identified as being the
apparently incongruous ones.
Resolution of Land Mammal Ages
Constrained seriation resolves a clear pattern of faunal succession (Figs. 5, S12, S13).
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DIRK MEGIRIAN ET AL.
FIGURE 4. Key marsupial lineages in inferred evolutionary succession based on relative stage of evolution. Temporal
condition 2 states that species (rows of the matrix) within a lineage may not be moved out of position relative to each
other. Younging applies only within a lineage, not across lineages. Zygomaturinae stage of evolution after Murray et al.
(2000); Palorchestidae after Murray (1990) and Black (1997); Ilariidae after Murray and Megirian (2006);
Thylacoleonidae (Wakaleo) after Murray and Megirian (1990); Pseudocheiridae (Marlu, Pildra) after Woodburne et al.
(1987); Ektopodontidae after Megirian et al. (2004); Sthenurinae after Kear (2002), Kirkham (2004), and Prideaux and
Warburton (2010).
Six discrete intervals spanning the Late
Paleogene through Neogene are recognizable,
each typified by a distinctive mammal fauna
(Table 1) to which we attach, from the base
upward, the following land mammal age
names: Etadunnan, Wipajirian, Camfieldian,
Waitean, Tirarian, Naracoortean (Figs. 2, 4, 5).
For convenience, localities producing fossil
species considered older than Etadunnan but
which are not yet unifiable into LMA, are
grouped as ‘‘pre-Etadunnan’’ assemblages
(Figs. 2, 4, 5, S4, S5). At present, these span
a large time frame with long intervals in
between that are currently unrepresented by
fossils. For example, the Tingamarra LF is
likely of early Eocene age (Godthelp et al.
1992; Sigé et al. 2009) whereas the Pwerte
Marnte Marnte LF may be late Oligocene, but
older than Etadunnan (Murray and Megirian
2006). The geographic distributions of the
localities of different LMAs are plotted in
Figure S4.
AUSTRALIAN LAND MAMMAL BIOCHRONOLOGY
665
FIGURE 5. A screen snapshot of the combined species-locality matrix for the Australian vertebrate record following
constrained seriation and imposition of temporal conditions 1 and 2. Imposed quadrilateral borders capture the
maximum number of species occurrences without temporal overlap and conceptually represent land mammal ages.
Species and locality names have been removed. A, Overall (interregional) pattern excluding Riversleigh. B, Riversleighspecific pattern highlighting numerous incongruous occurrences and species apparently ranging locally across the
Wipajirian/Camfieldian boundary. Key to symbols: black rectangles, species occurrences falling within land mammal
ages; open circles, species that ranged across land mammal age boundaries; black triangles, incongruous species
occurrences; black stars, unassignable assemblages.
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DIRK MEGIRIAN ET AL.
TABLE 1. Species with stage-of-evolution significance and/or interregional biocorrelative utility that characterize the
six recognized Australian land mammal ages. Species not yet the subject of published descriptions are excluded, with
the exception of Neohelos sp. C, which, although unnamed, has been described in detail (Murray et al. 2000).
Etadunnan fauna
Ilaria lawsoni
Ilaria illumidens
Marlu praecursor
Pildra antiquus
Pildra secundus
Waitean fauna
Hadronomas puckridgi
Kolopsis torus
Palorchestes painei
Wakaleo alcootaensis
Zygomaturus gilli 5 Kolopsis yperus
Wipajirian fauna
Dasylurinja kokuminola
Ektopodon litolophus
Ektopodon serratus
Ektopodon stirtoni
Ektopodon ulta
Kuterintja ngama
Litokoala kutjamarpensis
Marlu kutjamarpensis
Nambaroo saltavus
Neohelos tirarensis
Ngapakaldia bonythoni
Ngapakaldia tedfordi
Paljara tirarensae
Pildra magnus
Pildra tertius
Propalorchestes ponticulus
Rhizophascolonus crowcrofti
Wakaleo oldfieldi
Wakiewakie lawsoni
Tirarian fauna
Dasyurus dunmalli
Euowenia grata
Jackmahoneya toxoniensis
Macropus dryas
Macropus pan
Macropus pavana
Macropus woodsi
Palorchestes parvus
Perameles allinghamensis
Perameles bowensis
Prionotemnus palankarinnicus
Protemnodon chinchillaensis
Protemnodon snewini
Thylacoleo crassidentatus
Zygomaturus keanei
Camfieldian fauna
Mutpuracinus archibaldi
Neohelos sp. C
Neohelos stirtoni
Palorchestes anulus
Rhizosthenurus flanneryi
Wakaleo vanderleueri
Wanburoo hilarus
Naracoortean fauna
Bettongia pusilla
Congruus kitcheneri
Diprotodon optatum
Ektopodon paucicristata
Glaucodon ballaratensis
Macropus ferragus
Macropus siva
Macropus titan
Megalibgwilia ramsayi
Metasthenurus newtonae
Nototherium inerme
Out of a total of 360 assemblages analyzed,
350 cluster into one of the six defined LMAs or
are pre-Etadunnan. Of the balance, three are
unassignable (Billeroo Creek, RIV Microsite,
RIV VIP) because they are represented only by
unique occurrences of species that have no
recognized biochronological value and are also
undated (Fig. S12). The remaining seven
assemblages, all from the Riversleigh Carl
Creek Limestone, are classifiable only as either
Wipajirian or Camfieldian on the basis of
species that apparently ranged locally across
the Wipajirian/Camfieldian boundary (Table
S5). Including them means that 99% (357/360)
of site assemblages are LMA classifiable.
Incongruous Species Associations
Some assemblages apparently contain incongruous species associations, including
Naracoortean fauna (cont.)
Palorchestes azael
Palorchestes pickeringi
Phascolarctos stirtoni
Phascolonus gigas
Procoptodon goliah
Procoptodon pusio
Procoptodon rapha
‘‘Procoptodon’’ browneorum
‘‘Procoptodon’’ gilli
‘‘Procoptodon’’ oreas
‘‘Procoptodon’’ williamsi
Propleopus oscillans
Protemnodon anak
Protemnodon brehus
Protemnodon roechus
Ramsayia magna
Sarcophilus laniarius
Simosthenurus maddocki
Simosthenurus occidentalis
‘‘Simosthenurus’’ baileyi
‘‘Simosthenurus’’ pales
Sthenurus atlas
Sthenurus stirlingi
Sthenurus tindalei
Thylacoleo carnifex
Thylacoleo hilli
Vombatus hacketti
Warendja wakefieldi
Zygomaturus trilobus
+modern species
‘‘Wellington Caves NSW (site not specified),’’
‘‘Wellington Caves Phosphate Mine,’’ ‘‘Eastern Darling Downs LF M&K’’ (M&K 5
Molnar and Kurz 1997), and ‘‘Chinchilla LF’’
(Fig. S13). The first two align most strongly
with Naracoortean assemblages but have the
odd incongruous Tirarian element, whereas
the reverse is true for ‘‘Chinchilla LF.’’ By
contrast, the ‘‘Big Sink LF’’ and ‘‘Mitchell
Cave (5Breccia Cave)’’ assemblages of the
Wellington Caves are resolved unambiguously as Tirarian and Naracoortean, respectively
(Fig. S13). Therefore, although natural assemblages of significantly different ages do occur
within the Wellington Caves complex (Dawson 1985; Dawson et al. 1999), at least two
‘‘assemblages’’ are artificial groupings accommodating records lacking specific locality
data. The incongruous associations reflect
667
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TABLE 2. Geochronometric data imposing constraints on land mammal age boundaries, and whether or not species
range across boundaries (details discussed in text).
Land mammal
age
Oldest
date (Ma)
Youngest
date (Ma)
Naracoortean
3.03–2.58
0.0
---------------------------------------------------------------------------------------------------------------------------------------------------------Tirarian
4.46
3.6
------------------------------------------------------------------------------------------------------------------------------------------------------Waitean
—
.5.84
---------------------------------------------------------------------------------------------------------------------------------------------------------Camfieldian
—
—
---------------------------------------------------------------------------------------------------------------------------------------------------------Wipajirian
24.9–24.6
?17.6
---------------------------------------------------------------------------------------------------------------------------------------------------------Etadunnan
25.3–24.9
24.9–24.6
---------------------------------------------------------------------------------------------------------------------------------------------------------‘‘Pre-Etadunnan’’
—
—
the mixing of material from different deposits
as a result of poor collection practices
(Dawson 1985). Similarly, much of the earlier-collected material from the Darling Downs
(southeastern Queensland) assigned to the
‘‘Chinchilla LF’’ (western Darling Downs) or
to the ‘‘Eastern Darling Downs LF M&K’’ has
inadequate collection data, and these localities have long been considered diachronous
(e.g., Molnar and Kurz 1997; Mackness and
Godthelp 2001). As with the Wellington
Caves, a few discrete Darling Downs assemblages for which good locality information
exists were included in the analysis and all
resolved as Naracoortean (Fig. S13).
The remaining incongruous associations
are from the Carl Creek Limestone at Riversleigh, northwestern Queensland (Table S5,
Figs. 5, S12). Species recorded in the Carl
Creek Limestone that lack ascribed stage-ofevolution significance are ordered into the
lower part of Figure 5. Species of interregional significance present in some Riversleigh
assemblages, however, impose temporal limitations on these assemblages; the majority of
assemblages containing no such species are
tied into the interregional framework by local
biocorrelation. The Carl Creek Limestone
produces distinctly Wipajirian and distinctly
Camfieldian assemblages plus a suite of
species that apparently ranged locally across
the Wipajirian/Camfieldian boundary, and
then five site assemblages (‘‘RIV Keith’s
Chocky Block,’’ ‘‘RIV Henk’s Hollow,’’ ‘‘RIV
Boundary
estimates (Ma)
Species range
across boundaries
3
Yes
5
No
12
No
17
Yes
25
Yes
30
No
Gag,’’ ‘‘RIV Jim’s Carousel,’’ ‘‘RIV Sticky
Beak’’) containing incongruous associations
of Camfieldian and Wipajirian species. The
presence of apparently diachronous Carl
Creek Limestone assemblages has been previously noted (Megirian 1994; Murray et al.
2000; Megirian et al. 2004).
Land Mammal Age Calibration
The oldest age estimate for an Etadunnan
assemblage is derived from the magnetic
polarity of Etadunna Formation faunal zone
A at Lake Palankarinna, northern South
Australia. This has been referred to magnetic
polarity chrons 7An and 7Ar (Woodburne et
al. 1994), which has an age of 25.3–24.9 Ma
(Gradstein et al. 2005, Table 5.2). The Etadunnan/Wipajirian boundary, the only LMA
boundary with a stratigraphic context that
could be pegged as a datum event, corresponds to the boundary between Etadunna
Formation faunal zone B, which is Etadunnan, and faunal zone C, which is Wipajirian
(Table 2, Figs. 2, 3). Both faunal zones have
been referred to magnetic polarity chron 7r
(Woodburne et al. 1994), which has an age of
24.9–24.6 Ma (Gradstein et al. 2005). The next
youngest available age estimate is for the
Fossil Bluff Sandstone from Wynyard, Tasmania, which corresponds to planktonic
foraminiferal zone M2 (5N5) (Macphail
1996), currently falling within an age bracket
of 21.0–17.6 Ma (Gradstein et al. 2005). The
Wynyard date is of limited applicability
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DIRK MEGIRIAN ET AL.
because it calibrates only a unique species
occurrence, that of Wynyardia bassiana, type
species of the Wynyardiidae. Probable wynyardiids are known from mainland Wipajirian, Etadunnan, and pre-Etadunnan assemblages, though W. bassiana and the Wynyard
LF can only tentatively be regarded as
Wipajirian. No dates at all are available for
the Camfieldian. Consequently there is no
restriction on where the Wipajirian/Camfieldian boundary might lie (Table 2), other than
that it is possibly #17.6 Ma as indicated by
the Wynyard LF. A suite of species apparently ranged locally across the Wipajirian/
Camfieldian boundary at Riversleigh (Table
S5, Figs. 5, S12). Improved resolution of Carl
Creek Limestone biostratigraphy has the
potential to provide a stratigraphic context
for this datum event. The observation that
only one genus and no species range from the
Camfieldian into the Waitean (Table 2, Fig. 5)
suggests that a significant unrepresented
interval, probably of LMA magnitude, lies
between them. The intermediate stages-ofevolution expressed by Rhizosthenurus flanneryi and Palorchestes anulus, which are both
from the Riversleigh Encore Site, and Neohelos
sp. C from Riversleigh Jaw Junction Site, may
eventually be recognized as marker species of
another LMA. Encore Site has previously
been deemed representative of the youngest
known depositional phase within the Carl
Creek Limestone (Arena 2004; Travouillon et
al. 2006). At present, however, the number of
species in the Encore LF that occur in
unequivocally Camfieldian sites (e.g., Wakaleo
vanderleueri, Wanburoo hilarus), and more
critically in both Camfieldian and Wipajirian
sites (e.g., Burramys brutyi, Ekaltadeta ima),
suggests that formalizing an ‘‘Encorean’’
LMA would be premature. The taxonomy of
these apparently long-ranging species may
require reexamination, as may the potential of
this assemblage for being of mixed age.
Chronometric control for the Waitean is
afforded by the mean 87Sr/86Sr age of 5.84 Ma
(Table 2) for the base of the Black Rock
Sandstone at Beaumaris, Victoria (Wallace et
al. 2005), which overlies a phosphate nodule
bed containing reworked remains of diprotodontid taxa conspecific or biocorrelated with
species of the Waite Formation, southern
Northern Territory. Although there is no
species-level continuity from the Waitean to
Tirarian, the existence of a major time gap
between them is unlikely; some undescribed
taxa that we are currently studying from the
Ongeva LF (Waitean) and Curramulka LF
(Tirarian) share close phyletic ties. The oldest
age for the Tirarian is for the bone-bearing
paleosol beneath a basalt 40K/40Ar-dated
to 4.46 6 0.01 Ma near Hamilton, Victoria
(Table 2). The paleosol contains burnt remains of trees in growth position, indicating
that the basalt flow terminated its formation
(Whitelaw 1991). Therefore, the age of the
basalt can be taken to closely approximate the
age of the paleosol. Whitelaw (1991) matched
the normal magnetic polarity of the paleosol
with subchron C3n.3n of the early Gilbert
chron, but subsequent recalibration of the
polarity timescale would suggest that the
paleosol and basalt actually accumulated
during subchron C3n.2n, which has an age
of 4.63–4.49 Ma (Gradstein et al. 2005). The
youngest available age estimate for the
Tirarian is for the Pompapillina Member of
the Tirari Formation, which Tedford et al.
(1992) placed near the top of the Gilbert
chron, now calibrated to 3.60 Ma (Table 2,
Fig. 3) (Gradstein et al. 2005).
The oldest Naracoortean assemblage is the
Fisherman’s Cliff LF, which Whitelaw (1991)
dated with magnetic polarity to subchron
2An.1n (3.03–2.58 Ma) within the Gauss
normal chron. The upper boundary is defined
as the present and is represented by the
extant fauna (Table 2, Fig. 3). Although the
constrained seriation method resolves a clear
pattern of faunal succession for the continent,
LMA boundary (faunal turnover) events can
be only tentatively calibrated to the Standard
Chronostratigraphic Scale (Figs. 2A, S1).
The Cenozoic portion of the Standard
Chronostratigraphic Scale has been recalibrated numerous times since 1937 (Fig. 2B)
(Gradstein et al. 2005). At the same time,
there have been some significant changes in
the calibration of the Australian land mammal biochronological framework, most notably with respect to the putative age of the
Etadunna and Wipajiri Formations near the
AUSTRALIAN LAND MAMMAL BIOCHRONOLOGY
base (Fig. 2A). Recalibrations after Stirton et
al. (1968) by Woodburne and colleagues
(1985, 1994) have significantly influenced
ideas of suggested ages of key reference local
faunas (see also Megirian 1992; Megirian et al.
2004). This has been particularly so for the
Bullock Creek LF, whose possible age in
terms of the Standard Chronostratigraphic
Scale has never been more than a suggestion
based solely on interpolation between the
Etadunna/Wipajiri assemblages and the next
available calibration point (Beaumaris LF). In
an endeavor to convey the degree of uncertainty in calibration of the mammal record, it
has become established practice to apply
informal qualifiers to standard epoch names,
e.g., ‘‘middle (rather than Middle) Miocene,’’
‘‘mid Miocene,’’ ‘‘medial Miocene,’’ and as
finer distinctions of relative ages are sought,
‘‘late middle Miocene,’’ and so forth. Unfortunately, these are all too easy to equate or
conflate with the rigorously defined divisions
of the Standard Global Chronostratigraphic
Scale and in turn, to correlate poorly dated
Australian events to well-dated global events.
Conclusions
Constrained seriation resolves a preliminary succession of six Australian LMAs for
the late Oligocene to present, plus a currently
undiagnosable interval of probable age magnitude. This is broadly comparable to the six
North American land mammal ages spanning
the late Paleogene and Neogene originally
identified by Wood et al. (1941). From modest
beginnings, the North American Land Mammal scheme has been greatly elaborated and
refined (Woodburne 2004). The empirical
basis underlying Australia’s first Cenozoic
LMA-based biochronological framework will
allow it to be tested and honed with each new
species-occurrence record, refined taxonomic
determination, and new or refined stage-ofevolution hypothesis. New records may extend temporal ranges, such that marker
species may become reclassified as species
that apparently range across age boundaries.
The Australian LMA system allows the
rapidly expanding knowledge of the continent’s terrestrial faunal record to be expressed in terms of faunal succession without
669
resort in the first instance to standard
chronostratigraphic terms. Ultimately, as on
other continents, improved chronometric dating of localities will refine Australian LMA
boundaries, but its inception at least provides
terrestrial paleontologists, stratigraphers, and
evolutionary biologists with the opportunity
to add and analyze data within the confines of
one empirical biochronologic framework.
Acknowledgments
We thank E. Lundelius, P. Marianelli, and
an anonymous reviewer for their constructive
comments on the manuscript.
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