MCCLAIN, AMY M. Department of Botany, University of Florida, Gainesville, FL 32611-7800. - Acer section Palmata in the leaf fossil record.
Acer section Palmata, the group of maple trees that
includes the horticulturally important Japanese maple, Acer
palmatum, has usually been considered to be among the most
primitive sections of Acer. Species in this section have few
bud scales and terminal inflorescences, which place it close to
Dipteronia, the presumed sister group of the genus. Recent
molecular studies using a limited number of taxa also seem to support
the basal placement of section Palmata in Acer. However,
the stratigraphic record does not fully support this hypothesis. All
modern species in the section Palmata occur in eastern Asia,
except the single western North American species Acer
circinatum. Acer section Palmata can be divided into
three series: Palmata, Sinensia, and Penninervia.
The latter contains species with entire margined, unlobed leaves,
which are not known from the fossil record. Series Palmata has
distinctive many-lobed leaves with serrate margins. Leaves of this
type are apparently lacking in the fossil records of North America and
Europe, but are found in Asia beginning in the Miocene. Acer
series Palmata has been suggested to be a derived subset of the
paraphyletic series Sinensia. Species in the series
Sinensia have 3- to 7-lobed leaves with serrate or entire
margins. However, no fossils of this series have been reported. It is
possible that leaves of species in series Sinensia may have
been misidentified, since they are similar to the leaves of species in
many other Acer sections. There are species in other sections
of Acer that date back to the Eocene and perhaps the Paleocene
in both North America and Asia. Therefore, the fossil record suggests
that the section Palmata may be a relatively recent, derived
group rather than one of the basal Acer sections, and that the
dispersal of the section between North America and Asia occurred in
the late Tertiary.
Key words: Acer, Asia, North America, paleobotany, Palmata, Tertiary