Continental movements, “Wallace’s Line” and Indomalayan-Australasian dispersal of land plants: Some eclectic concepts

Summary

An attempt is made to bring into concordance some of the known facts of zoogeography, phytogeography, and geology with respect toextant geographical dispersal of certain land plant groups (Hepaticae, Coniferales, Angiospermae). Such a synthesis faces innumerable obstacles, some of which have only recently been removed by proliferation of paleomagnetic and other pertinent geological data suggesting that the Australian bloc was contiguous to Antarctica until at least the start of the Tertiary and has rapidly moved northward. As a consequence, concepts of a single “center” of origin of the angiosperms — or any other group — which encompasses a region including all or part ofboth Southeast Asia (and the adjoining islands that are historically part of the Eurasian bloc) and Australasia, seem untenable since these two regions have only recently been juxtaposed. It is assumed that this recent approximation offers the most valid analysis of the significance of “Wallace’s Line” — various versions of which have been almost uniformly accepted by three generations of zoogeographers.

In the general area “between Assam and Fiji” and between Japan and Tasmania-New Zealand — the Pacific region both Smith (1967, 1970) and Takhtajan (1969) postulate as the “center of origin” of the angiosperms or the “cradle of the angiosperms” — we have a similar “center of diversity” for other groups, including the Jungermanniae (leafy Hepaticae), Musci, Ferns, and Conifers. The richness of the fauna of this area has been amply documented. The thesis is presented that the reason for this extraordinary luxuriousness in types and numbers, in the entire biota, is not necessarily owing to theorigin of the angiosperms or of any other group in such a raonolithically conceived region, but tojuxtaposition of elements of two rich biotas — Laurasian-derived and Gondwanaland-derived. Both elements, furthermore, having probably developed in areas peripheral to the central land masses of Laurasia and Gondwanaland — in other words in oceanic loci where climatic parameters are more “favorable” — are assumed to have been intrinsically richer than “continental biotas.” Thus the extreme diversity which has been noted is presumed to have arisen by juxtaposition and partial fusion of already diversified, numerically and taxonomically rich biotas. It is assumed that the varying reconstructions of land and ocean areas, as they are thought to have existed about 150–180 m.y. ago (see, e.g., the reconstruction of Pangaea by Wilson, 1963, or the reconstruction of Dietz & Holden, 1970), are essentially correct, although all are probably incorrect in some details. If such a concept of an early Mesozoic configuration juxtaposing the continents in one, or two, major land masses is correct, then the regions that involved most of Africa, all of eastern North and South America, and most of Europe were once strongly “continental” in character, prior to the formation of the Atlantic Ocean. In contrast, the entire region from Australasia to northern India, the “belly” of Asia, and east Asia has been consistently peripheral to what has become the Pacific Ocean — hence has been highly oceanic. It is assumed, thus, that climatic conditions have been consistently more “fit” for the evolution of a mesophytic and mesophyllous flora. The perhaps insupportable assumption is made that, since virtually allextant primitive Angiospermae are large leaved and very few are truly microphyllous and sclerophyllous, at least the immediate common ancestral types probably had a similar morphology — and, hence, similar ecological requirements and restrictions.

In addition, it is postulated that not only was there a Tertiary “rafting” northward of elements of a Gondwanaland flora,via the Australian bloc, but there had been a prior such “rafting”via the Indian bloc during Jurassic-Triassic times. The “Indian raft” was presumably of major significance in enriching the Laurasian flora, during the first half of the Mesozoic, by moving northward a host of taxa. The fossil Coniferales studied in detail by Florin (1963) demonstrate nearly conclusively that such “rafting” of various taxa north-ward on the Indian bloc took place. The presence today in the flora of the Himalayan Uplands of a number of highly disjunct, ancient taxa of Hepaticae such asApotreubia Hatt. et al.,Lophochaete Schust.,Takakia Hatt. & Inoue,Haplomitrium Nees, and others —taxa typical of highly oceanic sites — suggests that, at least in the case of the Bryophyta, such ancient links have not been wholly obliterated. The role of the “Indian raft” in the dispersal of ancient angiosperms or pre-angiosperms is conjectural at best, but the timing may have been too early.If the existence of Triassic, simple angiosperms can be demonstrated, however, it is not impossible that,if these existed in Gondwanaland, some of them may have been dispersed to Laurasia in the manner postulated for the dispersal of certain conifers and Hepaticae.Since the existence of a variety of angiosperms by early Cretaceous times almost requires extrapolation backwards in time to the Jurassic or Triassic for theorigin of the earliest Angiospermae, the role of the Indian migration in the possible infusion of Gondwanaland angiosperms or pre-angiosperms into Laurasia cannot be rejected at this time. Furthermore,if recurrent reports of pre-Cretaceous angiosperms can be confirmed,then the horizon on which the angiosperms appeared is pushed back far enough in time so that the Indian migration becomes relevant to the early dispersal of the angiosperms.

Sanmiguelia, cited (by Axelrod, 1970, and others) as a Triassic angiosperm, is considered by many other botanists to belong to some other major group of plants. Supposedly pre-Cretaceous plants referred toPalmoxylon (Tidwell et al., 1970) have been a source of controversy and are now widely believed to be of much later age, although Axelrod (loc. cit., p. 279) claims the fossils occur “insitu, in rocks certainly dated as Jurassic....” Axelrod, indeed, gives an optimistic evaluation of the evidence for pre-Cretaceous angiosperms, stating (p. 312) that they “were definitely contributing to the fossil record as far back as the middle Jurassic, if not in the late Triassic....” This evaluation is at variance with that of Scott, Barghorn & Leopold (1960). However,by extrapolation one must almost assume an origin of the angiosperms no later than just before the dawn of the Cretaceous.

It is concluded that existing evidence does not allow us to infer anything definite as to the locus of origin of the angiosperms. There is no positive evidence that Gondwanaland was — or was not —involved. However, the virtual restriction of a number of primitive, vesselless angiosperm taxa (Winteraceae, Amborellaceae) to Southern Hemisphere localities that were part of a former Gondwanaland is suggestive. The presence of primitive Hamamelidae (Trochoden-draceae, Tetracentraceae, Cercidiphyllaceae — the first two vesselless, the last with fairly primitive wood structure) in Southeastern Asia is equally suggestive. The possibility must be entertained that some dispersal of the earliest angiosperms took place from Laurasia to Gondwanaland, orvice versa, shortly after origin of the group. Since large land blocs (the Indian bloc early in the Mesozoic; the Australian bloc in the Tertiary) are now known to have made rapid migrations northward, leaving their former Gondwanaland attachment, there is an obvious and simple mechanism by which massive transport of whole floras and faunas was possible. As this flow was of necessity unidirectional, it is likely that the rich diversification of some groups in the Northern Hemisphere was derived in part by the infusion of elements from the Gondwanaland flora. The luxuriousness, thus, of the southeast Asiatic biota may have had a triple origin, with partial origin (in the case of organisms originating prior to the Cretaceous) in the “rafting” north of the Indian bloc, while a second infusion probably late in the Tertiary occurred when the Australian bloc became approximated to the Asiatic bloc; subsequent expansion in ranges resulted in diffusion of these two elements into the endemic Laurasian-derived stock. The, at times, seemingly close links between the Laurasian-derived flora of the Indomalayan-East Asiatic region and the Gondwanaland-derived Australasian floras which are seen in the zone from New Guinea to Taiwan and the Philippines are probably at least in large part derived by transgression across “Wallace’s Line” (or, earlier, in the Tertiary, across the wider but rapidly narrowing straits which existed prior to the present narrow channel constituting “Wallace’s Line”). Because there were two massive transfers of large land areas since the start of the Mesozoic (the Indian bloc in the early Mesozoic; the Australasian bloc in early and mid-Tertiary times), there must have been, equally, massive transfers of foreign floristic elements — the infusion of genera and families from Gondwanaland regions to Laurasia. Such mass transfers, at two times and two loci, if this can be demonstrated to have taken place, so complicates the disentanglement of the early history of several plant groups — including that of the angiosperms — that it may well prove impossible to ever definitively place the point oforigin of the angiosperms. If the above analysis is even remotely correct, we must assume that by the start of the Cretaceous primitive angiosperm units existed in both Laurasia and Gondwanaland; this assumption is not too difficult to accept in view of the demonstrated occurrence by early Cretaceous times of angiosperms with several distinct pollen types (hence presumably already somewhat diversified) in areas as remote from the Pacific Basin (and from Gondwanaland) as present-day Maryland and New Jersey.

The assumption that the massive Indian bloc rafted northward, not as a denuded bloc but as a large, physiographically and biotically complex unit, carries with it one dividend: it explains a paradox which, to this author, has been long irresolvable, namely the basically “oceanic” and extremely rich and mesophytic nature of much of the Himalayan flora — a flora with repeated close affinities to that of oceanic regions of the western Pacific. We can find a simple explanation in the assumption that the “leading edge” of the Indian bloc, folded by resistance to its migration, consisted of a series of mountains which were a fit environment for the survival (or evolution) of basically oceanic and mesophytic plants whose floristic affinities in many cases seem to be with those of other sections of Gondwanaland.