The “lower” Hamamelidae sensu Endress (1989a) comprises seven families: Trochodendraceae, Tetracentraceae, Cercidiphyllaceae, Myrothamnaceae,
Eupteleaceae, Platanaceae and Hamamelidaceae. In the present paper, the systematic position, modern distribution pattern and fossil history of each family are analyzed, and the origin and dispersal of them are discussed according to the principle of the unity between the phylogeny and distribution of plants. The paper consists of three parts. The conclusions are as follows:
1. The center of distribution
According to Takhtajan's (1986) regionalization of the world flora, there are
13 distribution types in the “lower” Hamamelidae (Table 1 ). Eastern Asiatic Region, with five families, 19 genera and 73 species, ranks the first based on the
numbers of species, genera and families. Four families: Trochodendraceae,
Tetracentraceae, Cercidiphyllaceae and Eupteleaceae which were considered as more
primitive in the “lower” Hamamelidae and three genera: Disanthus,
Exbucklandia and Rhodoleia, primitive in the Hamamelidaceae, are all found in
Eastern Asiatic Region. In addition, the groups at different evolutionary stages in
the “lower” Hamamelidae survive in this region. Indochinese Region, with two
families, 15 genera and 32 species, ranks the second. It was shown that southern
Eastern Asiatic region and northern Indochinese Region are the distribution center
of the “lower”Hamamelidae based on further analysis (see Table 2).
2. The place and time of the origin
The fossil records of the “lower” Hamamelidae are abundant in angiosperms.
Nordenskioldia, supposed as the extinct ancestral group of Trochodendraceae and
Tetracentraceae, was widely distributed during the latest Cretaceous and the early
Tertiary in the Northern Hemisphere; Trochodendroides appeared during the
Cretaceous in North America, former USSR and Japan; the ancestral group of
Cercidiphyllaceae, the Joffrea-Nyssidum complex, also occurred during the
Cretaceous in the middle and higher latitude area of the Norhern Hemisphere. In
addition, the earliest fossil records of the Eupteleaceae, Platanaceae and
Hamamelidaceae appeared in North America, Europe and Asia of the Northern
Hemisphere respectively. Therefore, the Laurasian origin of the “lower”
Hamamelidae is supported by fossil evidence. On the other hand, the fossil data
are still insufficient to determine the place of the origin, especially because the fossil records are rather poor in Asia. For this reason, the analyses of birthplace
should combine with the information from the distribution of the primitive groups
or outgroup of the “lower” Hamamelidae.
Based on the statistics of distribution types, there are four primitive families in
the “lower” Hamamelidae and three primitive genera in the Hamamelidaceae in
southern Eastern Asiatic Region and northern Indochinese Region. Platanus
kerrii Gagnep. of the Platanaceae, distributed in northern Vietnam, is considered
as one of the most primitive species which has survived in modern times in this
family because of its pistillate inflorescence comprising 10-12 heads. The
Magnoliaceae was selected as an outgroup in our other paper “A phylogenetic
analysis of families in the Hamamelidae” (Lu et al. 1991 ). All its 13 genera and
most species occur from East to Southeast Asia, but in North America only three
genera are found. Takhtajan (1969) considered that it was plants of the
Magnoliaceae that were dispersed from East Asia to North America. Because the
primitive groups of the “lower” Hamamelidae and its outgroup almost occur in
the same area, their ancestor also appeared most probably in this area according
to the principle of common origin. It was inferred that the area from southern
Eastern Asiatic Region to northern Indochinese Region is the birthplace of
the “lower"” Hamamelidae.
The differentiation of the “lower” Hamamelidae took place rather early in
angiosperms. The origin of them may be traced at least back to the Barremian of
the early Cretaceous according to pollen fossil records. From more unequivocal fossil evidence, Platanoid plants appeared during the late Albian of the early
Cretaceous, and the Trochodendraceae, Tetracentraceae, Cercidiphyllaceae and
Hamamelidaceae diverged from their ancestral groups respectively no later than the
late Cretaceous (Fig. 6).
3. The causes for the formation of the modern distribution pattern
The “lower” Hamamelidae is a. rather old group. It is one of the most
abundant and widespread components of fossil floras in the Northern Hemisphere
during the late Cretaceous-middle Tertiary, the interval, when the global temperature was warm, although the extant Trochodendraceae, Tetracentraceae,
Cercidiphyllaceae and Eupteleaceae which are now confined to East Asia are
monotypical or oligotypical families. This distribution pattern indicates that most
plants became extinct in Europe, northern Asia and North America because of the
climatic changes during the late Tertiary, and especially the Quaternary glaciation,
but East Asia, usually called “plant refuge”during the glacial period, became the
survival place of many plants. From the viewpoint of evolution, these four families might be “living fossil plants” preserved from the Tertiary.
The distribution of Hamamelidaceae is disjunct, but the causes leading to this
pattern are not the same in different genera. The disjunction among Europe,
North America, Australia and southern Africa is due to the tectonic movements of
the earth; , and that between southeastern Europe-northern West Asia and
southeastern Asia is developed as a result of the Quaternary glaciation.
Fothergilla found from Carolina to Alabama in the United States and
Hamamelis disjunct between East Asia and North America were widely distributed
during the Tertiary in the Northern Hemisphere (Hu & Chaney 1940). The formation of their distribution patterns is a synthetic process owing to the tectonic
movements and the Quaternary glaciation.
Parrotia and Parrotiopsis, endemic to Iran and the West
Himalayas respectively, are very similar in morphology. They might have a common ancestor, and the latter is more primitive than the former. It seems that several groups in the Hamamelidaceae were dispersed from east to west in Eurasia.
Of the five genera in the Southern Hemisphere, Dicoryphe and Trichocladus
are Madagascarian and southern African, and Ostrearia, Neostrearia and
Noahdendron occur in northeastern Australia. They are usually considered as rather isolated groups, but Hufford and Endress (1989) found that they are closely related. The African genera might be dispersed from Asia via India, Sri Lanka and
Lemuria continent; the Australian Hamamelidaceae also from Asia, but via the islands distributed in the Pacific Ocean.
The Myrothamnaceae, comprising 2 species distributed in Madagascar and
southern Africa, is closely related to the Hamamelidaceae. Based on morphological
analyses, an evolutionary series exists among Myrothamnus, Dicoryphe and
Trichocladus in which the distribution patterns are the same, and Myrothamnus
is more specialized than the two genera of the Hamamelidaceae. Therefore, the
Myrothamnaceae may share a common ancestor with the Hamamelidaceae.
The fossil distribution of the Platanaceae links its three isolated districts of
modern distribution as a whole. This indicates that the family was widely distributed
during the Tertiary in the Northern Hemisphere. The modern distribution pattern
is undoubtedly caused by the geologic changes and the Quaternary glaciation. Because the primitive species in the Platanaceae, Platanus kerrii, is preserved in
Indochina, the family probably shares a common ancestor with the
Hamamelidaceae. Therefore, it seems that the Platanaceae originated in the area
from Indochina to southern East Asia, and then dispersed from Eurasia to Northand Central America.
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