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American localities in the Appalachian Basin
(Scheckler 1986b; Cressler 2006) as well as Alberta
(Scheckler 1978) and Arctic Canada (Andrews et al.
1965); South American localities in Venezuela
(Berry & Edwards 1996); Eurasian localities in
Great Britain and Ireland (Chaloner et al. 1977),
Belgium (Kenrick & Fairon-Demaret 1991), Sval-
bard (Nathorst 1900, 1902), Eastern Europe and
Russia (Snigirevskaya 1988, 1995), Siberia (Petros-
yan 1968) and China (Cai 1981, 1989; Cai et al.
1987); African localities in Morocco (Gaultier
et al. 1996; Meyer-Berthaud et al. 1997) and
South Africa (Anderson et al. 1995); Australian
localities (White 1986); and possibly Antarctica
(Retallack 1997).
Archaeopteris, and to some extent Rhacophyton,
are worldwide floral biomarkers for the Late Devo-
nian. When considered with other floral elements,
the plant assemblage at Red Hill most closely
resembles coeval assemblages elsewhere in the
Appalachian Basin, especially Elkins, West Virgi-
nia (Scheckler 1986c). They share many elements,
including a variety of Archaeopteris species, Rhaco-
phyton, Gillespiea, Barinophyton sibericum, arbor-
escent lycopsids and spermatophytes. The Elkins
locality is more diverse, preserving both sphenop-
sids and a cladoxylalean. Elkins is interpreted as a
deltaic shoreline deposit (Scheckler 1986c) in con-
trast to the alluvial plain interpretation for Red
Hill. Perhaps a more important factor in their simi-
larity is their geographic and temporal proximity.
The localities of the Evieux Formation in Belgium
are also of coeval palynozones, and have the most
similar plant assemblages to their North American
counterparts (Kenrick & Fairon-Demaret 1991).
Dispersal between these sites would have occurred
over a single landmass during the Late Devonian.
Such general and qualitative biogeographic assess-
ments need to be followed by quantitative analyses
of floral assemblage similarity to test further
hypotheses of biogeographic origin and dispersal.
Wilson et al. (2005) recognized Late Devonian
biogeographic continuity in archipolypodan milli-
pedes from the Euramerican landmass, including
Orsadesmus rubecollus from Red Hill. The Appala-
chian vertebrate fauna has biogeographic affinities
to Famennian sites from both the Euramerican and
Gondwanan landmasses. These similarities are
particularly striking with groenlandaspidid and
phyllolepid placoderms, gyracanthid acanthodians,
the chondrichthyan Ageleodus pectinatus and the
large tristichopterid sarcopterygian, Hyneria
lindae, which is closely related to Eusthenodon
spp., a taxon with a global distribution in the Famen-
nian. This cosmopolitan Famennian fish fauna is in
contrast to Frasnian faunas in which the Eurameri-
can and Gondwanan landmasses do not share sig-
nificant
endemism and Famennian cosmopolitanism is pre-
sumably a reflection of tectonic processes bringing
Euramerican and Gondwanan landmasses into
close enough contact to allow dispersal of organ-
isms that were unable to cross marine barriers.
The use of ecological models to explain the
origins of tetrapods
This palaeoecological profile of the Red Hill site
provides a view of the status of terrestrialization
towards the end of the Late Devonian. The range
of depositional settings at the site and the penecon-
temporaneous nature of the deposits provide a diver-
sity of fossil evidence for the interpretation of a
relatively in situ ecosystem. As seen here and in
other Late Devonian deposits, plants had established
complex communities by this time and inverte-
brates had a well-established terrestrial foothold.
Even although many morphological characteristics
important for terrestrial life had evolved among tet-
rapodomorphs, all vertebrates were still essentially
aquatic. The conditions at Red Hill can more confi-
dently be said to reflect selective pressures among
tetrapodomorphs for life in shallow, obstructed
and fluctuating waters rather than for full terrestrial-
ity. Multiple lines of evidence, as provided here, can
help in the construction of palaeoecological models
of the physical and biotic interactions in which early
tetrapods
evolved and diversified,
eventually
becoming fully terrestrial.
By the Late Devonian, the extensively vegetated
alluvial floodplains provided enhanced landscape
stabilization by means of deeper rooting depth,
habitat amelioration through shading, nutrient
enrichment of adjacent waters and increased com-
plexity of shallow water habitats through plant
debris accumulation. The avulsion cycles created
floodplain geomorphologic regimes that provided
a dynamically shifting range of habitats, accom-
panied by an annual wet-and-dry seasonality that
altered access to shallow water habitats and
resources in the shorter term. This range of habitats
includes shallow channel and wetland interfluve
settings that supported productive ecosystems.
Access to shallow water habitats could have pro-
vided a refuge for the earliest tetrapods to escape
predation from larger (and perhaps faster swim-
ming) sarcopterygians. The resources in these
habitats may have been out of reach of most large-
bodied sarcopterygian predators, except for those
that could navigate with appendages capable of
support and locomotion across the shallow water
substrates. Other morphological changes along the
tetrapodomorph lineage, such as loss of scale
cover and median fins and development of a neck,
may also have been related to locomotion and
elements. This
pattern
of Frasnian
