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Sinostega is from North China and appears to be
older than originally published (Fig. 1). If it also
appears to be a very basal taxon (unsolved on
Fig. 2), this could of course be used to sustain the
Asian origin. Nevertheless, we should not forget
that Elginerpeton, a limbed tetrapod, is already in
Scotland in the Frasnian. We consider here that
the presently known fossil evidence is in favour
of the out-of-Euramerica hypothesis for tetrapods
in the globally distributed exchange of fish-tetrapod
faunas (now known as the Great Devonian Inter-
change) between the major Late Devonian land-
masses (Young 2006, p. 423).
from that interval, compared to more abundant
Frasnian
and
late
Famennian
units
(E.
B.
Daeschler, pers. comm., 2007).
† Most reviews of Late Devonian tetrapods usually
include only those taxa named after preserved
bony elements (e.g. Clack 2006, 2007; Ahlberg
et al. 2008) because no phylogenetic analysis
can be based upon tracks. Since the review of
Clack (1997) it appears that a series of trackways
are known from the Late Devonian and are still
to be attributed to actual animals, making the
fossil record (Figs 1 & 2) largely underestimated.
Comments on the global (climatic) context
of earliest diversification of tetrapods
Comments on the biodiversity of Late
Devonian tetrapods
Several attempts at correlating the origin and early
diversification of tetrapods to global events have
been made (e.g. Clack 2002, 2006, 2007; Long &
Gordon 2004; Carroll et al. 2005; Ward et al.
2006). One recent tendency is to try to link this to
climatic changes in relation to plate tectonic/oro-
genic global events. Late Devonian tetrapods
occurred during a period of intense tectonic activity
(Acadian - Ligerian orogeny) due to the collision of
major landmasses, reduction of most oceanic
domains and building of a pre-Pangaean configur-
ation of the Earth (Averbuch et al. 2005; Blieck
et al. 2007).
At a global scale, the transition from fish to tetra-
pod occurred during the Givetian through Frasnian
time slice which corresponds to a low level of
atmospheric oxygen, rather high temperature
(remember that the Givetian - Frasnian period corre-
sponds to the maximum development of marine
reefs in the fossil record) and a major time of
plant diversification (increase in penetration
depths of vascular plant roots in the soils and
increase in the main axis diameter of land plants)
(Ward et al. 2006; Algeo et al. 2007; Clack 2007).
The earliest diversification of tetrapods is bracketed
by the two first radiations of terrestrial arthropods:
the first in the Early Devonian and the second in
the Middle and Late Mississippian (Ward et al.
2006). However, putting such different events on a
single diagram does not mean that there are causal
relations between those events. For instance, when
Ward et al. (2006, fig. 1) plot the earliest phases of
diversification of terrestrial arthropods and of tetra-
pods together, they simply show that both phases
apparently follow a period of low atmospheric
oxygen rate which might be the trigger for the devel-
opment of respiratory organs in air; it does not imply
a trophic relation between terrestrial arthropods and
earliest tetrapods which are now interpreted as
aquatic animals (e.g. Clack 2002).
From data included in Figure 2, it seems that we are
facing two episodes of diversification (one in the
Frasnian and one in the late and latest Famennian)
with a single record in between that is, Jakubsonia.
All genera are monospecific except Ichthyostega
(three species: Blom 2005), so that we would have
four species for the Frasnian, one for the lower
Famennian and ten for the upper-uppermost Famen-
nian (Fig. 2). Even when removing the taxa which
are not included in Ahlberg et al.'s (2008) phyloge-
netic analysis, two episodes of radiation appear:
from the Frasnian and the late-latest Famennian.
However, several critical remarks must be made:
† As long as the phylogenetic relationships of
several taxa are unknown, this scheme is incom-
plete. For example, Obruchevichthys is out from
Ahlberg et al.'s (2008) scheme, but was con-
sidered as the sister-group of Elginerpeton in
some earlier analyses (e.g. Ahlberg 1995;
Schultze 2004) where it was an element of the
most basal group of tetrapods and hence charac-
terized their very first diversification. It would
also be interesting to know what groups Sinostega
(Zhu et al. 2002) and the ichthyostegid of Strud
(Belgium: Cl´ment et al. 2004) are related to.
† We know that the fossil record of early tetrapods
is incomplete. Undescribed Late Devonian taxa
are awaiting description (e.g. Clack et al. 2004,
in press, pers. comm., 2007) and the ca. 15 Ma
long interval with almost no tetrapod remains
in the Lower Carboniferous (Romer's Gap)
may simply be due to a lack of fossil collection,
related to the lack of suitable continental (strati-
graphical) sequences for that period (Clack
2007, p. 11). This is perhaps also the case for
the Lower Famennian. The apparent rarity of
Early Famennian tetrapods might be a bias
caused by lack of an extensive rock record
