The Origin and Dispersal of the “Lower” Hamamelidae

Abstract

Abstract:

The “lower” Hamamelidae sensu Endress (1989a) comprises seven fami-

lies: Trochodendraceae, Tetracentraceae, Cercidiphyllaceae, Myrothamnaceae,

Eupteleaceae, Platanaceae and Hamamelidaceae. In the present paper, the systema-

tic position, modern distribution pattern and fossil history of each family are ana-

lyzed, and the origin and dispersal of them are discussed according to the princi-

ple of the unity between the phylogeny and distribution of plants. The paper con-

sists 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 Re-

gion, 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 fos-

sil 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 fos-

sil 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 tempera-

ture 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 fami-

lies 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 forma-

tion 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 com-

mon ancestor, and the latter is more primitive than the former. It seems that seve-

ral 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 ra-

ther isolated groups, but Hufford and Endress (1989) found that they are closely re-

lated. The African genera might be dispersed from Asia via India, Sri Lanka and

Lemuria continent; the Australian Hamamelidaceae also from Asia, but via the is-

lands 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. Be-

cause 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 North

and Central America.

Key words: “lower” Hamamelidae, Modern distribution pattern, Fossil history, Origin and dispersal

Lu An-ming, Li Jian-qiang, Chen Zhi-duan. The Origin and Dispersal of the “Lower” Hamamelidae[J]. .

The Origin and Dispersal of the “Lower” Hamamelidae

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