Biology Reference
In-Depth Information
For example, on the North Slope of Alaska the MAT based on paleobio-
logical evidence is estimated at 13°C for the Coniacian circa 89 Ma, with
a seasonal cold month temperature of approximately 5°C-6°C (i.e., above
freezing), declining to an MAT of around 8°C by the Maestrichtian circa
66 Ma, probably with occasional freezing during the dark winter months.
Moderate seasonality in temperature was augmented by distinct seasonality
in light regimes, contributing to the development and imparting advantage
to those gymnosperms and angiosperms that were deciduous (i.e., could
lose their leaves and physiologically hibernate or “chill out” in the dark and
in winter).
Across the continent in western Greenland, Cretaceous fl oras also re-
veal conditions markedly different from those of the present. At 71°N in
the Early Cretaceous, the fl oras include an extinct group of plants called
the Bennettitales (fi g. 5.1; Boyd 1998a). They are morphologically similar
to modern Cycadales (cycads) in having long, thick, evergreen, pinnately
divided leaves, and together with the cycadophytes they constituted a prin-
cipal food source for the dinosaurs (Butler et al. 2008). The closest relatives
presently grow in warm-temperate regions of the world. The angiosperm
fossils include thin-textured, entire-margined, heart-shaped leaves similar
to the Nymphaeales (order of water lilies), the fl oating aquatic
Ceratophyl-
lum
,
Cercidiphyllum
(a deciduous tree currently of eastern Asia),
Platano-
phyllum
(a sycamore-like leaf 5.5 inches across), and
Dicotylophyllum
of
unknown affi nities but with leaves nearly 8.5 inches wide (Boyd, 1998b).
The leaf morphology of these genera suggests warm climates, and all were
growing above the Arctic Circle (66
°
33'N) in the Cretaceous.
From later in the Cretaceous, the Lower Atanikerluk fl ora of Greenland
is a relatively diverse terrestrial assemblage with deciduous gymnosperms
(e.g.,
Metasequoia
, dawn redwood), and fi fty-six angiosperm species of
which roughly 77 percent have entire margined leaves (tropical) and a
smaller but substantial number, about 23 percent, have toothed or lobed
leaves (temperate). Identifi cations and the relationships with modern taxa
are uncertain, but leaf types named
Proteoides
,
Dermatophyllites
, and
Chon-
drophyllum
in the Late Cretaceous at 71°N are also known from the south-
eastern United States at 32°N, a distance of around 4350 km (2700 mi).
In the far northwest, a palynofl ora from the Campanian to the Mae-
strichtian Tongue River Formation on the Arctic Slope of Alaska includes
a diverse plant microfossil fl ora of 110 different types of spores and pollen
(Frederiksen 1989). Many represent herbaceous understory vegetation rare
or absent in the macrofossil record (bryophytes, ferns and allied groups,
herbaceous and aquatic angiosperms). The plant microfossils are distinct
