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After introductory comments on scale-model theory, descriptions and discussions of experimental models of continental drift and orogenesis are presented. The driving mechanism of the models is the spontaneous overturn in layered systems with gravitationally unstable density stratification. The pattern of the overturn movement is similar to thermal convection currents. A model Pangaea breaks in tension above the upwelling branches of the overturn cells in the model mantle, and the fragments are set adrift by the horizontal limbs of the current. Though buckling occurs along the leading edges of the wandering continental fragments, this fold structure is not of the kind encountered in natural orogens. The latter kind of structures — including basement upheaval, batholith intrusion, nappe formation etc. — is generated in models with no net horizontal shortening across the orogen.

Evaporites (gypsum, anhydrite, halite, etc.) of all ages, Proterozoic to the present, have been plotted on world maps. Without exception, 95–100% of the evaporites of each age, by volume and area, occupy areas which today are the earth's dry wind belts. This implies that the wind-circulation patterns of the lower astmosphere have not changed for 1,000 million years. This cannot be true unless the present position of the earth's rotational pole, ocean basins, and continents have also remained essentially unchanged since Middle Proterozoic time. Additional evidence corroborates this conclusion. 1. (1) The present horse-latitude (desert) belts of the Northern and Southern Hemispheres, as shown by the stratigraphic record, have occupied the same approximate positions during the same length of time. The widths of the two horse-latitude belts have fluctuated, but the mean positions have been constant. This suggests great latitudinal stability through time. 2. (2) The evaporate zones of the earth since Middle Proterozoic or earlier time have been symmetrical about the present equator and, therefore, symmetrical with respect to the present rotational axis. 3. (3) Since Late Silurian time, two coal zones, one north of the evaporite zone and the other south of it, have maintained a similar symmetry. 4. (4) In any reconstruction of Gondwanaland, Laurasia, or Pangaea, tillite deposits (e.g., Late Ordovician and Permo-Carboniferous tillites) are in the centers of the supercontinents. This is a physical impossibility because the central parts of the supercontinents are 3,000–4,000 km from the nearest ocean moisture source. Glaciers require abundant moisture (1,000 mm plus per year) for initiation and growth. In all current Laurasia-Gondwanaland reconstructions, not even one-tenth of this amount of moisture could have reached the supercontinents' interiors. 5. (5) The same argument applies to the extensive coal deposits of interior Gondwanaland and Laurasia, except that 1,500–2,000 mm of annual rainfall is required. 6. (6) An argument repeatedly raised in favor of drift is that large trees could not have grown in polar regions. Plots of coal deposits on widely accepted Gondwanaland-Laurasia reconstructions show that dense forests of Permian and Carboniferous ages grew at the Permo-Carboniferous pole, and that thick evaporite deposits (up to 2,000 m thick) were deposited within 15° of the Permian pole (as determined from paleomagnetic studies). We conclude that continental drift (by many mechanism) and polar wandering should be subjected to large-scale reevaluation. Our studies show that, since Middle Proterozoic time, the “new global tectonics” are not applicable and that they introduce major, unexplained inconsistencies into geology and geophysics.

Many genera and species of present-day aecial rusts on seed plants are derived directly or indirectly from a limited group of these pathogens on northern conifers, whereas conifer rusts are believed to have descended from their progenitors on ferns. It is worth noting that only the Pinaceae, Taxodiaceae and Cupressaceae serve as alternate hosts to an extensive rust flora on ferns and angiosperms which are distributed predominantly in the northern hemisphere. Numerous conifers in the southern hemisphere do not bear rusts, except a few endemic species on Araucariaceae in Malaysia and south central Chile. Such a disjunctive distribution of conifers and their specialized rusts is attributed now to extensive continental drift. Evidence indicates that the breakup of the primary land mass Pangaea into northern Laurasia and southern Gondwanaland, with successive splitting and drifting of these supercontinents, has also scattered ancient land faunas and floras.

Cambrian, Ordovician and Lower Devonian sediments from northern Argentina have been examined palaeomagnetically. Of the six Cambrian localities investigated, five are shown to have magnetically stable deposits. The Devonian samples, however, are demonstrated to hold two secondary magnetizations and no detectable primary remanence. The five Cambrian and one Ordovician palaeomagnetic pole positions South Tilcara 52N 27E (A 95 =23 Purmamarca vill 61N 67E (A 95 =26 North Tilcara 49N 24E (A 95 =23 Purmamarca 5N 39E (A 95 =14 Abra de Cajas 2N 28E (A 95 =50 Salta (Ordovician) 31N 13E (A 95 =40) are used in conjunction with three previously determined Lower Palaeozoic palaeomagnetic pole positions to suggest a new South American Lower Palaeozoic polar wander path, which with palaeomagnetic results from other southern continents forms a common Gondwanan Lower Palaeozoic polar wander path. The relationship of the Gondwanan curve with the North American, European and Asian Palaeozoic polar wander paths is discussed in terms of the welding of Pangaea.

We present a new continental drift reconstruction of the universal continent of Pangaea in the Permian plus a series of five world maps to depict the breakup and dispersion of continents with each subsequent geologic period, Triassic to Recent. Plate tectonics and sea-floor spreading are accepted as the guiding rationale. Also utilized are the morphologic fitting of continental margins and paleomagnetic pole positions. Rigor is imposed by the geometric requirements involved in presenting continental drift dispersion on maps in orderly time sequence and by following certain assumed rules of plate tectonics. The reconstructions were first made on a globe and then transferred to an Aitoff world projection. In the Permian, the Atlantic and Indian oceans were closed so that all the continents were configured into the universal landmass of Pangaea. The reconstruction is based largely on the morphologic best fit of continental margins to the 1000-fathom isobath, except for India, the east coast of which is placed against Antarctica, as dictated by plate tectonics. In the Triassic the breakup of Pangaea commenced. The southwest Indian Ocean rift was created, which split West Gondwana (South America and Africa) away from East Gondwana while a Y junction lifted India off Antarctica. An independent North Atlantic–Caribbean rift also formed, which lifted Laurasia (North America and Eurasia) off of South America and the bulge of Africa. In the Jurassic, northward and westward sea-floor spreading further opened the central North Atlantic and the Indian oceans. At the end of the period, a new rift incipiently split South America away from Africa. The Walvis mantle thermal center or ‘hot spot’ formed, which would subsequently provide an absolute geographic reference point for subsequent continental drift. In the Cretaceous, the motions already established continued. The North Atlantic rift grew northward, blocking out the Grand Banks and the western margin of Greenland. Spain rotated sinistrally, forming the Bay of Biscay. An offshoot rift split Madagascar from Africa, dropping off this subcontinent from Africa, which continued its northern flight. The northward trek of India continued, and Australia incipiently split away from Antarctica. During the Cenozoic, Antarctica rotated further westward. Australia experienced a remarkable flight northward, and New Zealand was split away from its east coast. The North and South Atlantic oceans continued to open; the rift that formerly passed west of Greenland now switched to the east and split Greenland away from northern Europe and extended through the Arctic Ocean. Africa moved slightly northward, continuing sinistral rotation. The Tethyan megashear became dextral for the first time, India collided with and underran Asia.

Summary Samples collected from three Jurassic igneous formations in South Morocco carry a stable component of magnetic remanence, giving a mean palaeomagnetic pole-position at 61° S, 71° E, A95= 14°. Two of the formations were situated in the stable Saharan Shield region, and one within the Central High Atlas fold-belt. There is no systematic difference between the pole-positions derived from these two regions of contrasting tectonic history, and furthermore, no significant difference between the mean Jurassic pole-position for Morocco and that for South-east Africa. It is concluded that no significant post-Jurassic relative motion has occurred between NW and SE Africa. Two of the formations studied were previously thought to be Upper Palaeozoic, but radiometric age determinations presented in this paper give consistent values of between 181 and 187 My. These results indicate the occurrence of a widespread episode of Jurassic igneous activity, producing structures which frequently parallel the coastline, and are possibly related to the break-up and dispersal of the fragments of Pangaea during the Mesozoic.

Eocambrian glacial deposits are restricted, in Australia, to a zone which is oriented approximately northwest and which crosses the continent. To the west and southwest, these beds were marginal to a land-mass. This distribution roughly coincides with that of the fossiliferous Cambrian. Lower Proterozoic banded iron formations and Archaean rocks occur in the western portion of Australia. When the Eocambrian glacial deposits, the Lower Proterozoic banded iron formations and the Archaean rocks are plotted on a globe with the continents reassembled according to Wegener's Pangaea, they form a grouping to which the occurrences of eastern Asia are exceptions. It is suggested that the different areas of eastern Asia fitted between Europe (north of the Alps) and Africa-India, during the Late Precambrian and Early Palaeozoic times.

The composition of aboriginal land mammal faunas in Australia and New Guinea (prototherians and metatherians), South America (metatherians and eutherians) and Madagascar (eutherians only) is reconsidered in light of continental drift reconstructions of Mesozoic-Tertiary world paleogeography It is proposed that these three faunas represent successively detached samples of the evolving world mammal fauna as it existed when each of these land masses became faunally isolated from the rest of the world as a result of the progressive fragmentation of Pangaea. Isolation of aboriginal prototherians and metatherians in Australia and New Guinea may date from the Upper JurassicLower Cretaceous; isolation of aboriginal metatherians and eutherians in South America may date from the Middle Cretaceous-Upper Cretaceous; isolation of aboriginal eutherians in Madagascar may date from the Paleocene-Eocene.

THE plate tectonics model1,2 has been successful in explaining the orogenic features of the ocean basins and continents associated with the dispersal of Pangaea in post-Triassic time. Dewey and Bird3 extended these concepts, suggesting that there is a unique relationship between orogenesis and plate-margin tectonism, and various authors have, accordingly, interpreted certain Precambrian and Palaeozoic orogenic zones or ‘mobile belts’ as plate sutures4–5. We argue here that plate-margin tectonism is only one of several possible conditions for the development of an orogenic belt. We suggest that the plate tectonic model of orogenesis needs to be extended and modified, and this suggestion is discussed for Gondwanide Australia. Recent attempts to impose a rift (aulacogene)6 or subduction7 model for orogenesis in central Australia are confounded by the observed geological and geophysical facts pertinent to this region.

South Africa and Australia were invaded and colonized by-European man less than 200 years ago. In both places the settlers found ample grasslands and savannah-woodlands and an equable Mediterranean type of climate, and they soon introduced all the kinds of domestic animals that they had known in Europe. The impact of infectious diseases on these herds differed greatly in the two continents. Australia had been separated biogeographically from the rest of the world ever since the initial breakup of Pangaea, some 180 million years ago, and the only grazing animals there were marsupials, most of which were small in size and few in number. The rickettsial infection, Q fever, is the only disease that Australians have recognized as having been transferred to cattle and sheep from a marsupial reservoir host, and it caused trivial disease in the domestic as in the native animals. South Africa, on the other hand, is at the southern tip of a continent on which dwell a greater variety and a larger number of indigenous grazing animals than are found on any other continent. Wave after wave of novel infections, of differing virulence but often quite severe in the introduced species, emerged from this reservoir to assail one or another of the local domestic animals.

Gondwanaland, that hypothetical supercontinent composed of what are now the continents of Africa, South America, Australia, and Antarctica, and the subcontinent of peninsular India, has been the subject of much lively discussion among geologists, paleontologists, and biologists for more than half a century. Likewise, the inferred phenomenon of continental drift, involving the rifting of Gondwanaland into several large fragments, which are the continents listed above, and the subsequent drifting of those continental blocks to the positions they now occupy, has likewise occupied the attention of many interested authorities, as well as other people, through the past six decades. (It might be added that there has also been a correlative concept of Laurasia, a superconti-nent of what are now the northern hemisphere land masses, with its rifting, and the drifting of its several parts to their present positions. Furthermore, many authorities have envisaged a still larger land mass, Pangaea, embracing both Gondwanaland and Laurasia.) According to most students of the problem, Gondwanaland and Laurasia (or Pangaea) are supposed to have existed during at least the latter part of the Paleozoic and the beginning of the Mesozoic eras, the rifting and drifting having taken place at some time subsequent to the middle or final stages of Triassic history.

The following paper shows that Wegener's attempt to fit the Americas against Euro-Africa leaves discrepancies of as much as 1,500 miles; that there is no fitting at all in the Central American region; that when Newfoundland is united with Ireland and the easternmost cape of Brazil fitted into the African Bight of Biafra, Central America is parted from South America by 1,200 miles, and Alaska from Siberia by 600 miles, leaving in the latter instance a deep ocean that is fatal to all inter-migration of marine and land life between these continents. It next explains that the tectonic structures and the faunal assemblages on either side of the Atlantic fit badly, that we have here only similarities and not identities, and that the faunas do not have more than 5 per cent of species in common instead of the 50-75 per cent called for on the basis of Pangaea. The detailed historical geology and stratigraphy of Newfoundland and Ireland, contrasted in adjacent columns, show that there are here no exact identities and few similarities. Finally, the writer discusses the small residuum of the Wegener hypothesis that has been becoming more and more apparent during the past fifteen years to all students of geosynclines and mountain structures, namely, that the continents appear to have moved horizontally and differently throughout geological time, but how much and in what directions are problems for the future. He also points out briefly how more harmony may be hoped for between geologists, paleontologists, and geophysicists working along these lines.

THE possibility that the continents of the Earth once formed part of a unified mass, Pangaea, has recently received much support1–3. The actual form of Pangaea is by no means agreed, however. Cook4 has figured a girdle of land encircling the globe, but most previous workers have sketched a roughly oval land mass surrounded by continuous ocean. Carey5, after repeated attempts to fit together accurate tracings of the main continental masses on a large plastic globe, commented finally “The assembly of Pangaea is not possible on the earth of the present radius”, and supported the hypothesis that the Earth's diameter has gradually increased. The history of the expanding Earth hypothesis has been recounted by Egyed6, prompted by my attempt7 to reconstruct the existing land masses of the Earth as a complete crust on a smaller globe.

THE Kranz syndrome is a group of related anatomical, cytological and physiological characters reported one way or another in about 10 families of angiospermous plants. The syndrome centres around the CO2 fixation aspect of photosynthesis, including synthesis of C4-dicarboxylic acids as the initial products of CO2 assimilation, utilization of most of the available CO2 producing a high 13C/12C ratio, having maximum efficiency at high temperatures, and all related to a sheath around the bundles in the leaf. All present evidence indicates that detection of any one of the aspects of the syndrome denotes the whole syndrome1. The syndrome is uncommon among plants including angiosperms and is therefore considered to be the evolved or derived condition. Although in Atriplex2 it seems to have evolved recently within the genus, in the Gramineae the evolution of the Kranz syndrome seems to have occurred only two or three times early in grass evolution. This assumption, which is fundamental in this discussion, is based on the uniformity of tribes and subfamilies as Kranz or non-Kranz, and the proven predictability of the syndrome from single aspects of it (in this work from the 13C/12C ratio), from various other characters, or from classification3. The only exception reported, of Kranz and non-Kranz species within one grass genus, is within Panicum4 where the subgenus Dichanthelium is non-Kranz whereas the more typical species of the subgenus Eupanicum are characterized by the Kranz syndrome (Table 1). Recently we found, by determination of 13C/12C ratios of over 180 species, in 110 genera, of forty tribes of grasses, evidence of both types also among species of Eupanicum as well as among genera of the tribe Paniceae and among tribes of the subfamily Panicoideae (Table 1). It is this diversity of panicoid tribes (many of which are small and endemic), of the tribe Paniceae, and the genus Panicum that provides clues to the likely evolution of the grass family, superimposed, of course, on a great deal of other morphological, anatomical, cytological, and physiological evidence3,5. Furthermore, now that geology has accepted Wegenerian drifting continents and the break-up of an ancient single world-continent (Pangaea) by rifting under the concept of plate tectonics, evolution of the Gramineae can be discussed in terms of that concept; that is, Upper Jurassic–Middle Cretaceous separation of East Gondwana and the Middle to Late Cretaceous separation of South America from Africa, with the tropical belt approximately in its present relationship to the continents since Early Jurassic time6.

The interpretation of past movements of land areas and their degree of emergence involves a number of suppositions and uncertainties which are briefly discussed. Evidence seems to indicate that from at least the Mesozoic to the present, water barriers have been present and have served as an isolating mechanism for terrestrial organisms. Further data and interpretation are needed before much of our speculation on the past movements of organisms can be said to have a firm factual basis. WITH THE CURRENT PLETHORA of geological papers discussing continental drift it is not surprising that the evidence and theories relating to drift have influenced thinking in other disciplines. For many years some biogeographers have felt the necessity of proposing either continental drift or land bridges to explain certain types of distribution patterns, particularly those of terrestrial organisms. Now with the current geophysical evidence supporting continental drift, many biologists seem to feel that continental movements can be used to explain most disjunct patterns found in the older groups of terrestrial organisms. While continental movements may help to explain some disjunct distributions, a number of factors that could influence both old and present distribution patterns frequently seem to be overlooked. The purpose of this paper is to summarize some of these factors and consider some of the inherent problems and uncertainties, particularly in relation to continental drift and the occurrence of epicontinental seas. For most biologists the interpretation of geological evidence as an aid to understanding biological patterns and diversity presents many problems. Not the least of these is the voluminous literature concerning continental drift and related subjects. Because of this situation, biologists have tended to utilize review or summary papers such as the series from the Scientific American entitled "Continents Adrift" with an introduction by J. Tuzo Wilson (1972). Unfortunately many of the papers are oversimplifications and tend to present the positive evidence for plate tectonics and drift while neglecting the contrary evidence. A second aspect is that the geological evidence has usually not been presented in a format of optimal usefulness for biologists. The vast majority of geologists now accept the broad picture of continental plate movements. However, there are objections that need to be answered, and biologists should be aware of these. Meyerhoff and Meyerhoff (1972) and Meyerhoff, Meyerhoff, and Briggs (1972) have thoughtfully discussed a number of inconsistencies that need answering such as the position of coal and evaporite beds which can be related to present continental positions and climatic belts but which form an unexplainable mosaic when plotted on drift maps of Pangaea. Ilich (1972) has briefly summarized major problem areas, and Wesson (1972), in a summary manner, has listed six major groupings of problems and 74 objections. Both Meyerhoff and Meyerhoff (1972) and Keith (1972) have proposed alternate interpretations of present evidence, which involve cooling and contraction movements of the earth's crust (e.g., a sea floor narrowing). Of the various objections to present drift theories, Meyerhoff's numerous papers (see bibliography in Meyerhoff et al. 1972) are the best documented. If one assumes that most of the objections can be answered, a consideration of prodrift evidence still poses problems for both geologists and biologists. The Caribbean area presents numerous interpretive problems (Khudoley and Meyerhoff 1971, list 46 major unsolved geological problems) as does the Beringean area (Churkin 1972). One specific example of a problem area involves the debate about the placement of Madagascar (the Malagasy Republic), its direction of movement and time of separation. At least three different positions have been proposed (Heirtzler and Burroughs 1971): first, that it has remained in its present position from Mesozoic time; second, that it has moved eastward since the Cretaceous period from a position adjacent to Mozambique; and third, that it has been moving BIOTROPICA 6(1): 1-6 1974 1 This content downloaded from 157.55.39.243 on Wed, 05 Oct 2016 04:36:49 UTC All use subject to http://about.jstor.org/terms MAP 1. Approximate extent of exposed land (black) and epicontinental seas (white) about 150 million years B.P. (Jurassic) plotted on a continental drift configuration from Wilson (in "Continents Adrift," 1972). The "?" indicates a possibly seaway. south and slightly east from a position against Somalia since the early Cenozoic. In addition, northward movements from a more southerly position have also been proposed. At present the type and direction of movements are unresolved as is the time of separation. Hammond (1973) has stated that recent evidence has shown that Madagascar has been "an independent minicontinent for at least the last 100 million years." Epicontinental sea sediments surrounding the north-south mountain spine of Madagascar seem also to indicate that there has been no dry-land connection to Madagascar from other continental plates for at least the same period of time or longer. Some biologists seem to question this assumption, since according to Fooden (1972) the mammalian relicts indicate that the separation of Madagascar may date from the Paleocene-Eocene. Differences in the time sequence for continental separations are numerous, both among geologists and biologists, with biologists often tending to utilize the dates proposed by geologists that seem most favorable to explain a particular pattern. Recently Phillips and Forsyth (1972) have dated the opening of the North Atlantic at 150 million years B.P. and the opening of the South Atlantic at 130 million years B.P., while some earlier authors have suggested a much later opening. Current geological literature seems generally in accord with the Phillips and Forsyth dates. (Maps 1 and 2 herein show a later separation.) Timing of the Mesozoic (and earlier) continental movements will undoubtedly be debated for some time along with their effect on terrestrial dispersal patterns. However, if the overall major pattern of continental movements is accepted, other geological events must still be considered in assessing the distributional patterns of terrestrial organisms. Present theories of plate movements and continental fits are based, in part, on the shapes of the continental margins. These margins often are now submerged

The thick section (greater than 5 km) of flat-lying Cretaceous and younger carbonates capping the Bahama platform implies an unusual geotectonic history, characterized by great subsidence. We suggest that this platform is underlain neither by sial nor by a volcanic foundation creating a “mega-atoll.” Instead we propose a basement of oceanic crust about 11 km down which has undergone slow subsidence. Triassic rifting in the Atlantic probably initially created a closed small ocean basin, or mediterranean, in the Bahama region. This was accomplished by a rotational movement of the North American plate away from North Africa accompanied by shearing across the top of the South American plate which remained stationary and attached to Africa. A wedge-shaped sphenochasm resulted which became a sediment trap within Pangaea and quickly filled to sea level with turbidites. With renewed continental drift, the Bahama platform became a subsiding marginal plateau attached to the North American craton upon which algal-coral growth explosively flourished under holo-oceanic conditions, providing sufficient upbuilding to offset subsidence and maintain a sea level freeboard. This interpretation obviates the overlap of the Bahama salient onto the African craton when the Atlantic Ocean is closed under continental drift reconstruction. It accounts for the long history of subsidence without calling on “oceanization.” It also explains the inference, based on some Cuban-Soviet geophysical evidence, that there is a thick sedimentary section beneath the Bahamian carbonates. An underlying clastic sequence could also include the source of the probable salt domes at the bottom of Exuma Sound.