Geoscience Reference
In-Depth Information
Terrestrialization in the Late Devonian: a palaeoecological
overview of the Red Hill site, Pennsylvania, USA
WALTER L. CRESSLER III
1
*, EDWARD B. DAESCHLER
2
,
RUDY SLINGERLAND
3
& DANIEL A. PETERSON
3
1
Francis Harvey Green Library, 25 West Rosedale Avenue, West Chester University,
West Chester, PA 19383, USA
2
Vertebrate Paleontology, Academy of Natural Sciences, 1900 Benjamin Franklin
Parkway, Philadelphia, PA 19103, USA
3
Department of Geosciences, The Pennsylvania State University, University Park,
PA 16802, USA
*Corresponding author (e-mail: wcressler@wcupa.edu)
Abstract: Alluvial floodplains were a critical setting during the Late Devonian for the evolution
of terrestriality among plants, invertebrate and vertebrates. The Red Hill site in Pennsylvania,
US, provides a range of information about the physical and biotic setting of a floodplain ecosystem
along the southern margin of the Euramerican landmass during the late Famennian age. An avul-
sion model for floodplain sedimentation is favoured in which a variety of inter-channel
depositional settings formed a wide range of aquatic and terrestrial habitats. The Red Hill flora
demonstrates ecological partitioning of the floodplain landscape at a high taxonomic level. In
addition to progymnosperm forests, lycopsid wetlands and zygopterid fern glades, the flora
includes patches of early spermatophytes occupying sites disturbed by fires. The Red Hill fauna
illustrates the development of a diverse penecontemporaneous community including terrestrial
invertebrates and a wide range of vertebrates that were living within aquatic habitats. Among
the vertebrates are several limbed tetrapodomorphs that inhabited the burgeoning shallow water
habitats on the floodplain.
Although the process was already well underway by
the Silurian (Edwards & Wellman 2001; Shear &
Selden 2001), the Late Devonian was a time of
key evolutionary innovations that made possible
the further terrestrialization of life. For example, it
was during the Late Devonian that seed repro-
duction fully evolved in plants and the fin-to-limb
transition occurred in vertebrates (Rothwell &
Scheckler 1988; Clack 2002). Each of these evol-
utionary events occurred in association with the
aquatic ecological context of their ancestral con-
ditions. The appearance of novel features can be
seen in hindsight to have predisposed these lineages
to additional physiological and morphological
changes that promoted terrestrialization. As life
expanded over the landscape new ecological guilds
emerged, the trophic structure of continental ecosys-
tems became more complex (DiMichele et al. 1992)
and the resulting transformations in the transfer of
matter and energy changed the dynamics of biogeo-
chemical cycles in the sea and atmosphere as well as
on land (Algeo et al. 2001).
Significant aspects of the early stages of this
global transition can be documented through obs-
ervation and analysis of the physical and biotic
conditions present on Late Devonian alluvial
plains. The sedimentary sequence at the Red Hill
site in Clinton County, Pennsylvania (Fig. 1) was
deposited during the late Famennian age within
the alluvial plain of the Catskill Delta Complex
along the southern margin of the Euramerican (Lar-
ussian) landmass. The site preserves a rich sample of
plants and animals that lived penecontempora-
neously in floodplain habitats. Red Hill therefore
provides a comprehensive glimpse of a continental
ecosystem at this important stage in the terrestriali-
zation of life.
Background
Evolutionary and ecological events on
Devonian continents
Early Devonian land-plant communities were
characterized by a patchwork landscape of low-
stature plants growing in monotypic clonal stands
along watercourses and coastal zones (Griffing
et al. 2000; Hotton et al. 2001). During the Mid
Devonian, the competition for light and spore dispe-
rsal led several plant lineages to develop secondary
