Geoscience Reference
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feeding behaviour included herbivory on spores and
plant stems (Labandeira 2007). The Late Devonian
provides little evidence that the array of functional
feeding types among terrestrial arthropods diversi-
fied much beyond the few that originated in the
Late Silurian and Early Devonian. Despite the evol-
utionary appearance of true roots, leaves, wood and
seeds by the Late Devonian, these plant tissues do
not show evidence of extensive herbivory until the
Late Mississippian-Early Pennsylvanian boundary
(Labandeira 2007).
Early and Mid Devonian vertebrates are best
known from 'red bed' deposits formed in marine
and estuarine settings along continental margins.
During the Early Devonian these strata were domi-
nated by lineages of agnathans and acanthodians
that appeared during the Silurian (Janvier 1996).
Importantly, placoderms emerged in many Early
Devonian faunas, and sarcopterygians also became
more predominant. The dipnomorph clade first
appeared and includes durophagus lungfish and
predatory porolepiforms (Janvier 1996). The diver-
sity and abundance of gnathostomes continued to
increase during the Mid Devonian with placoderm
and acanthodian radiations. Early actinopterygians
appeared and tetrapodomorph sarcopterygians
diversified to fill a wide variety of predatory aquatic
niches. By the Late Devonian, many of these groups
were well established in non-marine habitats associ-
ated with vegetated floodplains.
It is during the Late Devonian that several groups
of placoderms, including phyllolepids, groenlandas-
pidids and bothriolepids, were common in continen-
tal and marginal settings, although these groups
disappeared by the end of the period. A diverse
array of sarcopterygians, including porolepiforms,
dipnoans, rhizodontids and 'osteolepiforms' were
also found in continental and marginal deposits.
Elpistostegalian sarcopterygians first appear in the
late Mid Devonian (Givetian). By the latter part of
the Late Devonian (Famennian), the fossil record
documents a variety of early limbed forms from a
range of fluvial and near-shore depositional settings
across theglobe (Blieck et al. 2007;Astin et al. 2010).
Some lineages of tetrapodomorph sarcoptery-
gians show morphological specializations in the
pectoral girdles and fins that reflects experimen-
tation in the use of the appendage for substrate
locomotion. Among the rhizodontids and some
'osteolepiforms', pectoral fins were used to push
off from the substrate (Davis et al. 2004). Within
the basal elpistostegalian lineage, pectoral fins
developed a limb-like endoskeletal configuration
and other specializations that may have allowed
these animals to move through very shallow waters
via substrate contact (Daeschler et al. 2006). It is
within the elpistostegalian lineage that this configur-
ation of fins with wrist, elbow and shoulder joints
leads to the development of limbs with digits and
therefore to the origin of tetrapods.
The Late Devonian witnessed early tetrapods
that were still linked closely to aquatic ecosystems
(Clack & Coates 1995; Clack 2002). As the record
of the fin-to-limb transition has improved, we have
gained a better understanding of the sequence of
anatomical changes with the goal of reconstructing
the acquisition of features that eventually allowed
terrestriality. Fully terrestrial vertebrates do not
appear in the fossil record until the Visean (352-
333 Ma). The period between the origin of limbs
(in the Late Devonian) and fully terrestrial habits
(in the Visean) has been called Romer's Gap.
More data are slowly emerging which will elucidate
details of this critical interval in tetrapod history
(e.g. Clack & Finney 2005).
According to recent models, the large increase in
plant biomass and corresponding increase in depth
of rooting and soil formation on Late Devonian
floodplains led to a significant alteration of biogeo-
chemical cycles. Enhanced weathering on the conti-
nents and the influx of plant detritus into fluvial
systems increased nutrient availability in aquatic
environments, and were possibly a causal factor
for periodic marine anoxia (Algeo & Scheckler
1998). Black shale deposition in the epicontinental
seaways record the anoxic episodes possibly result-
ing from these eutrophic conditions (Algeo et al.
2001). The Late Devonian black shale horizons
are global in extent and are associated with major
marine extinctions, particularly of stromatoporoid-
tabulate reef communities (McGhee 1996). The
decline of CO
2
levels in the atmosphere and sub-
sequent climate cooling have also been attributed
to this weathering and burial of organic carbon,
resulting in a brief glacial episode at the end of the
Devonian (Caputo 1985; Algeo et al. 2001).
Models of fluctuating atmospheric O
2
levels for
the Devonian and Carboniferous have been used
recently to invoke causal mechanisms for terrestrial
diversification patterns (Ward et al. 2006; Laban-
deira 2007). After their first major diversification
in the Late Silurian-Early Devonian, low oxygen
levels during the Mid-to-Late Devonian are postu-
lated as a cause for the suppression of further diver-
sification of terrestrial arthropods until the late
Mississippian (Labandeira 2007). The suppression
of evolutionary diversification by low oxygen
levels has also been invoked as an explanation for
Romer's Gap, the 15 Ma interval between the Late
Devonian and late Mississippian with few known
tetrapod fossils (Ward et al. 2006).
In contrast, Clack (2007) points to the diverse
Visean East Kirkton tetrapod fauna that demon-
strates significant evolutionary advancements
during the Romer's Gap interval which simply has
not been preserved or recovered from the fossil
