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suggested by Warren & Wakefield (1972). The
makers of the Genoa River trackways are unknown.
The Metaxygnathus lower jaw comes from the
Cloghnan Shale at Jemalong quarry, central New
South Wales, very often cited as Famennian
(Ahlberg & Clack 1998; Clack 2006; Young 2006).
Associated with Metaxygnathus is a fish assem-
blage which includes Soederberghia groenlandica,
Bothriolepis, Remigolepis, Groenlandaspis, phyllo-
lepids and possibly holoptychiid scales. This fits
very well with the lower part of the fifth assemblage
of Blom et al. (2007), that is, the Remigolepis series
which also yielded Acanthostega and Ichthyostega.
This is dated as Famennian because of palynological
data from above and below the formations of this
series (Marshall et al. 1999; Blom et al. 2007, fig. 2).
However, new geological mapping and lithostra-
tigraphical correlations as well as comparison of the
Jemalong fish-tetrapod fauna with the Canowindra
fish assemblage (about 80 km east of Jemalong in
New South Wales) led Young (2006) to suggest
that the age of the Jemalong assemblage is not
very different from that at Canowindra, and is also
likely to be Frasnian. Both the Jemalong and Cano-
windra fish assemblages have been grouped by
Young (1993) under the Macrovertebrate Fauna
MAV13, originally dated as Famennian (Young
1993, fig. 9.2) and then considered as Frasnian
(e.g. Young & Turner 2000, figs 2 & 3). We
remain unconvinced by the Frasnian age of the
Jemalong fauna which biostratigraphically corre-
lates rather well with the fifth assemblage of Blom
et al. (2007) and is more likely to be Famennian,
and therefore consider the age of Metaxygnathus
as unresolved (Fig. 1; Blieck et al. 2007).
One of the consequences of the supposed Fras-
nian age of the Canowindra-Jemalong fauna is that
it constitutes one of the arguments used by Young
(2003) to indicate that several groups of placoderms
(including bothriolepids and phyllolepids) dispersed
from eastern Gondwana to Euramerica (Laurussia);
they would indeed be older (Frasnian) in Gondwana
than in Euramerica (Famennian). However, if older
ages for some of the Australian faunas are not con-
firmed, this contradicts Young's (2003) scenario.
The third SE Australian locality which is
assumed to have yielded Devonian tetrapod
remains (a trackway) is the courtyard of Glenisla
Homestead in the Grampians Mountains, western
Victoria (Warren et al. 1986). The source of the
courtyard flagstones and their age have recently
been resolved: they come from the Major Mitchell
Sandtone of the Grampians Group, underlying the
Silverband Formation (Gouramanis et al. 2003;
Young 2006) which yielded turiniid thelodont
scales and poracanthodid acanthodian scales and
tooth whorls suggestive of a Late Silurian to Loch-
kovian (Early Devonian) age (Turner 1986; Young
& Turner 2000, fig. 2 and p. 459: Microvertebrate
Zone MV1; Burrow 2003). The Glenisla trackway
is therefore either Late Silurian or earliest Devonian
at the youngest (Young 2006, p. 418) and would be
the oldest tetrapod remains known.
The only problem with the above theory is that it
may not be a tetrapod. For Clack (1997), the tetra-
pod interpretation is very doubtful due to the lack
of symmetry of the trackway and the absence of
clear alternation in its assumed manus and pes
tracks. For Gouramanis et al. (2003), it is attribu-
table to a Diplichnites species made by an arthro-
pod. Doubt therefore remains on what type of
animal may have formed this trackway (Young
2006). The proper identification of the Glenisla
Homestead track maker will have a huge impact
on interpretations of the origin of tetrapods as
discussed below.
Earliest diversification of tetrapods
The results of Figure 1 are used to plot the taxa
included in the most recent phylogenetic (cladistic)
analysis by Ahlberg et al. (2008). They are shown
on Figure 2.
Comments on the origin of tetrapods
As already commented by various authors, a con-
sensus arose on the age of origin of tetrapods
which, based upon presently available evidence,
would be late Givetian to early Frasnian. The ear-
liest known fossil bony elements of tetrapods are
Frasnian in age, and the elpistostegid (pander-
ichthyid) sarcopterygians (now considered as para-
phyletic, but including Tiktaalik รพ Elpistostege,
the sister-group of tetrapods: (Daeschler et al.
2006, Ahlberg et al. 2008) are late Givetian at the
oldest (Clack 2002, 2007; Schultze 2004; Blieck
et al. 2007).
However, the recent paper of Young (2006) has
highlighted the unresolved trackway from Glenisla,
Victoria (Australia), originally described by Warren
et al. (1986) and thought to be Early Devonian or
even Silurian in age. After new geological and
palaeontological information published, the age of
this trackway is confirmed as being either Late
Silurian, or earliest Devonian at the youngest
(Young 2006, p. 418). If confirmed as a tetrapod
trackway, this would immediately introduce a long
ghost range of ca. 30 Ma for the elpistostegids,
from the Silurian - Devonian boundary to the late
Givetian. Discussing uncertainties regarding the
phylogenetic relationships of sarcopterygian fishes
(including the rhizodontids, the osteolepiforms, the
actinistians and basal taxa from the Lower Devonian
of China), Young (2006) suggests the possibility
