Evidence" by Wegener and His Followers

DRIFTING CONTINENTS AND LATE PALEOZOIC GEOGRAPHY* BY P. J. DARLINGTON, JR. MUSEUM OF COMPARATIVE ZOOLOGY, HARVARD UNIVERSITY Communicated August 20, 1964 As a result of 19 months spent in Tasmania and eastern Australia and 7 weeks at the southern tip of South America, followed by a year of reading and writing on the biogeography and history of the southern end of the world, I have gradually become convinced that continental drift is not just an intellectual exercise but a probable reality. I was not easily convinced of this. I am a conservative, sixty-year-old biologist and also a professional biogeographer (if there is such a thing), and I have been and still am repelled by the exaggerations and unintentional misrepre- sentations of biogeographic "evidence" by Wegener and his followers. Neverthe- less I am now a Wegenerian, of sorts. I think that some continents probably have drifted, although not in quite the usual Wegenerian pattern. I give this fragment of my own history to justify reviewing a subject which, if there is any reality in it, is of great interest and importance to many persons. Convincing evidence-evidence that has convinced me-of movement of continents comes from the matching shapes of Africa and South America and their relation to the Mid-Atlantic Ridge, from new evidence of convection currents in the earth's mantle that might move continents, from the distribution of glaciation and of floras in the late Paleozoic, and from the new (and still very incomplete) record of paleomagnetism. Two questions in the general hypothesis of continental movement are critical and can perhaps be answered. First, when did Africa and South America separate, if they were united? And second, what were the positions of the southern continents when they were glaciated late in the Paleozoic, 200 million or more years ago? Time of Separation of Africa and South America.-The fitting of edges of Africa and South America' is virtual proof that these continents were united, if the hypothesis of continental displacement is accepted at all. Wilson2 suggests as a "highly speculative" hypothesis that convection in the earth's mantle, rising along the line of the Mid-Atlantic Ridge, has separated the continents and is pushing them apart, and that the rate of movement calculated from the ages of volcanic islands, which increase in age with distance from the Ridge, indicates that separation occurred about 120 million years ago, about the end of the Jurassic. How- ever, a suboceanic ridge exists in the Pacific too, apparently similar to the Mid- Atlantic Ridge and similarly bordered by islands of graded ages.2 3 Convection currents may be pushing South America from both sides, and both currents (if they exist) may be diving downward under South America rather than moving the continent. Therefore, rate of movement of South America and time of separation from Africa cannot safely be calculated from Wilson's data, although the data may indicate the latest time that separation can have occurred. Paleomagnetism does not show longitudes (see page 1089) and therefore does not show how far apart Africa and South America were in the past. However, if basic assumptions are correct, paleomagnetism does show rotations of continents. If Africa and South America were united, they have not only separated but have also 1084 Downloaded by guest on September 27, 2021 VOL. 52, 1964 GEOLOGY: P. J. DARLINGTON, JR. 1085 rotated so that once-parallel coast lines now diverge southward at an angle of about 450, and paleomagnetic data seem to show that the divergence was already about 220 early in the Jurassic.4 This suggests that the continents separated much earlier than supposed by Wilson. Biogeographic evidence of time of separation of Africa and South America might have been decisive, but is not. For example, angiosperms (the existing dominant land plants) might have shown a uniquely close relationship between the floras of Africa and South America if these continents were joined when the angiosperms dispersed, which was primarily during the Cretaceous. However, angiosperms do not show this. As many plant taxa, most of them angiosperms, are shared by the Indo-Malaysian region and tropical America as by Africa and tropical America.' The angiosperms have an additional significance. 1\Jost families of trees that now dominate tropical rain forests are angiosperms, and if the rain-forest trees dispersed without benefit of contact betweeen Africa and South America, the insects and other animals in the rain forests may have done so too. This is just one example, but an important one, from a very large, largely controversial body of biogeographic evidence. In sum, this evidence seems to me to show no definite time of separation of Africa and South America and no clear indication that the continents were ever united at all. If these continents were united, as I think they probably were, they apparently separated too long ago to leave any recognizable traces of the union in the distributions of existing plants and animals. In summary of time of separation of Africa and South America (if they were united): the latest time allowed by geologic dating (ages of Atlantic islands) is probably late Jurassic; paleomagnetism suggests that the continents had rotated independently before the Jurassic; and other evidence (not all reviewed here) seems consistent with separation in the Triassic, or earlier. Southern Supercontinents?-Wegenerians have been possessed by a fixed idea: that, before drift, the continents must have formed a great supercontinent, an all- embracing Pangaea or at least a southern Gondwanaland. This idea seems to find support in the apparent former union of Africa and South America. How- ever, union of these continents (if they were united) need not require that others were united too. Actual evidence of union should be looked for in each case. Cri- tical evidence comes especially from late Paleozoic time, from the late Carboni- ferous and early Permian periods (the Permo-Carboniferous) when the southern continents were heavily glaciated. The rest of the present paper will be concerned chiefly with the situation in the southern hemisphere during this small segment of geologic time. Supposed evidence of union of southern continents has been derived from fitting of the continents' shapes, from continuity of geosynclines and other geologic struc- tures, from the distribution and directions of movement of Permo-Carboniferous ice, from the distribution of ancient floras, and from paleomagnetism. Much of this evidence does strongly suggest that most continents lay farther south on the world than they do now, but no evidence clearly indicates a general union of southern continents. A hypothetical grouping of the continents without union, which seems to satisfy all requirements now known, is suggested in Figure 1. Evidence of Shapes. The southern continents, excepting Africa and South Amer- ica, do not fit together well. This fact is emphasized by the many different ways Downloaded by guest on September 27, 2021 1086 GEOLOGY: P. J. IARLINGTON, JR. PROC. N. A. S. FIG. 1.-Suggested arrangement of southern continents inl late Paleozoic time. Only Australia is placed, inl relation to the pole, by paleomagnetic data for the time in question (see text, p. 1089). West Antarctica, which is a relatively recent, geologically active area, is shown as an archipelago, as it may still be, under the ice cap. Wegenerians have tried to make the continents fit, unsuccessfully. The contrast between the precise fit of Africa and South America and the poor fit of other southern continents strongly suggests that the others were not united or that, if they were, they separated long before Africa and South America did. Geologic Evidencee.- Wegenerians have stressed the supposed continuity of a synclinal system, the "Samfrau Geosyncline," connecting southeastern South America, the southern tip of Africa, western Antarctica, and eastern Australia, and supposedly indicating that these pieces of land were once aligned and in contact.6 Two serious criticisms of this argument can be made. First, part of the supposed geosyncline may not exist. Harrington7 shows no geosyncline where the hypothesis requires it in South America. And second, continuity of geosyndlines does not require continuity of land. A geosynclinal, mountain-forming system crosses the ocean gap between South America and western Antarctica, for example. However, this system does suggest a long-standing relationship between the tip of South America and the Antarctic Peninsula which should be a guide to orientation of the continents in the past. Wilson2 suggests that secondary (not mid-oceanic) ridges on the ocean bottom may connect former contact-points of continents, and he offers a mid-Mesozoic "supercontinent" assembled according to these guides. The results disqualify Downloaded by guest on September 27, 2021 VOL. 52, 1964 GEOLOGY: P. J. DARLINGTON, JR. 1087 the method, I think, so far as the southern continents are concerned. The continents are brought together in an unnatural way, with the Antarctic Peninsula rotated out of its probable relation to South America, and with the southern quarter of South America overlapping the Precambrian shield of Antarctica! In the space available I can consider only small samples of the nonglacial geologic evidence (above), and by way of summary can say only that, so far as I can judge, no such evidence indicates an actual union of southern continents, except of Africa and South America. Glacial Evidence.-Late in the Paleozoic, in late Carboniferous or early Permian time, ice sheets were widespread on southern Africa and southern Madagascar, southern South America, and southern Australia including Tasmania. Ice occurred north into what is now the southern edge of the tropics on all these continents. Antarctica was glaciated too, at least in part, and so was India, which now lies mainly in the tropics north of the Equator.

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