In the introduction to his paper on trans-Pacific relationships, McCarthy (2003) agrees with an extreme view of the expanding earth theory that calls for a small, pre-Jurassic globe that was completely terrestrial. His reasoning is straight forward: the floor of all the world's oceans was formed less than 200 Ma, therefore oceans did not exist prior to that time. Of course, this is a reasonable thesis only if one is prepared to ignore a number of well-established facts: (1) the Precambrian to Paleozoic fossil record of marine life providing evidence of extensive oceans; (2) the failure of the expanding earth theory to pass a rigorous paleomagnetic test (McElhinny et al., 1978); (3) the absence of cracks across the planet caused by expansion (Hallam, 1994); (4) The absence of a drastic fall in sea level since the Triassic that would have been caused by expansion; (5) the abundant evidence of large-scale subduction that absorbed the older sea floor; (6) the lack of evidence for the generation of the internal energy necessary for expansion (Bursa & Hovorkova, 1994); and (7) no evidence of the rapid reduction in the earth's rotation that would be caused by expansion (Bursa, 1993).
A less extreme view of earth expansion was published by Owen (1976) and this was followed-up by his Atlas of Continental Displacement (Owen, 1983). Owen felt that he had geological evidence that the continents could fit together to form Pangaea only if the earth's diameter was 80% of its modern mean value. Otherwise, a reassembly of the continents from their present positions back to their pangaean beginning would leave a series of large V-shaped gaps. Owen's maps, illustrating a 20% expansion over the past 200 Ma, were of interest to a number of biogeographers. However, Weijermars (1986) showed that if a three-dimension globe is used, instead of map projections, the gaps will disappear. The concise review published by Cox (1990) should have laid the expanding earth theory to rest, but alas it has arisen once more.
The expansionist paper by McCarthy (2003) maintains that the Pacific Ocean was formed by the separation of the west coast of the New World from the east coast of Asia and that this event took place in less than 200 Ma. In support of this idea, he utilizes two kinds of evidence, geological and biological. The former consists of arbitrarily shifting continental blocks and oceanic trenches about so that, in the Triassic, the shorelines of the two sides would fit together. It may suffice to say that geophysical evidence of such an arrangement is lacking. The major part of his argument consists of extensive literature references to trans-Pacific animals and plants that are supposedly unable to achieve long-distance dispersal. The authors of the referenced articles are, in large part, people who share the expansionist philosophy. Even so, a general rebuttal to such information may be of interest to biogeographers.
The great bulk of the biological evidence (about five pages) is devoted to relationships across the Southern Ocean and, in particular, emphasizes the supposed juxtaposition of New Zealand and the southern part of Chile. Plate tectonic reconstructions (Lawver et al., 1992) for the late Cretaceous show that the tip of the Antarctic Peninsula was located close to Tierra del Fuego and that Tasmania, and the South Tasman Ridge, formed a connecting link between Australia and Antarctica. These positions remained relatively static until the late Eocene, about 40 Ma, after which Australia moved rapidly northward. The drop in sea-level at the end of the Cretaceous (Hallam, 1994) allowed considerable exposure of the continental shelves that lessened the distance between shore lines, and provided increased opportunity for migration of the terrestrial biota.
The window of migratory opportunity, that was available from about 60–40 Ma, permitted the formation of what is often called an ‘amphinotic track’. The strongest ties are between South America and Australia followed by New Zealand. More distant relations are shown by southern Africa, Madagascar, and New Caledonia (Briggs, 1995). Australian–South American relationships are demonstrated by a large fraction of the flora and fauna. The list includes vertebrates such as the marsupial mammals, xiphodont crocodiles, the frog family Leptodactylidae, the turtle family Chelidae, ratite birds, and mound birds. The invertebrates include freshwater mussels, crayfish, and aquatic insects such as the mayflies, stoneflies, caddiceflies, and others. Also included are a host of terrestrial invertebrates such as land snails, oligochaete worms, and most major groups of insects and spiders.
Like Australia, New Zealand probably received its earliest vertebrates (dinosaurs, a leiopelmatid frog and its tautara species) in the Triassic or Jurassic. But, during the early Tertiary, it too received an infusion of biota from South America via Antarctica and Australia. Unlike Australia, New Zealand received no vertebrate animals at that time except birds and possibly some lizards. The ratite birds, moas and kiwis, probably arrived in the early Tertiary. Their migration was facilitated because their ancestors were probably flying birds related to the South and Central American tinamous (Briggs, 2003). A review of the origins of the New Zealand flora (Pole, 1994) indicates a close relationship to Australia, and that the present vegetation of the former is entirely, or almost entirely, the result of long distance dispersal during the Tertiary. It should be noted that not all of the amphinotic migratory traffic moved from west to east. From southeast Asia to the general Australian–New Zealand region, there is a remarkable concentration of primitive conifer and angiosperm families. A few of them are found in South America and it is likely that they arrived from the east.
In general, the Southern Ocean botanical and marine animal references cited by McCarthy (2003) call attention to widely separated sister species. Disjunct living sister species or, in most cases, even disjunct related genera cannot be used as evidence for the alignment of Triassic continents or islands. Such species and genera are simply too young. The average species duration for terrestrial vertebrates is about 2 Ma (Avise et al., 1998), that of marine animals is about 4 Ma (Sepkoski, 1999), and that of plants probably not much different. The majority of living genera and many families do not extend back beyond the Tertiary. It is the distribution of the oldest families and orders that may be helpful in determining pre-drift continental position.
Considerable space is devoted to the freshwater fish family Galaxiidae that has a broad distribution in the cold-temperate waters of the southern hemisphere. Before the turn of the century, there was considerable speculation that the galaxiids had been distributed by continental dispersal in the Mesozoic. However, recent research has shown that many of the species are diadromous, moving back and forth between fresh and salt water at various stages in their life history. It seems that their evolution and distribution has occurred fairly recently, within the past 30 Ma, although there is a possibility that there may have been a trans-Antarctic dispersal in the earlier Tertiary (Waters et al., 2000). The general consensus is that the family has been distributed by migrating through the sea (McDowall, 2002).
In order to support his theory of the opening of the Pacific Ocean in the Jurassic, Shields (1998) cited a large number of terrestrial, Upper Triassic fossil records from each side of the ocean. Such records are difficult to evaluate, for the organisms in question might have been distributed across the width of Pangaea but appeared to be disjunct because of the lack of fossils or fossil bearing strata in the intervening areas. The Upper Triassic, prior to the break-up of Gondwana, is noted for the very broad distribution patterns of its fauna and flora. At that time, a Peri-tethyan Province occupied the major part of low and mid-latitude Pangaea (Rage, 1988).
Under the heading of ‘terrestrial tropical distributions’, a large amount of data are presented that deal primarily with trans-Pacific distributions at the generic level. Again, the question is, are such genera really old enough to bear on the problem of geographic relationships prior to 200 Ma? The cited references deal with a variety of groups such as composite plants, dipterid genera, iguanid lizards, and various birds including the ratites, galliformes, gruiformes, and several more advanced groups that were supposed to have had trans-Antarctic distribution patterns. In regard to the birds, Cracraft (2001) was probably correct in suggesting the Antarctic route but the time for the migrations was almost certainly the early Tertiary, not the pre-drift Mesozoic.
In the ‘marine tropical category’ the finding of trans-Pacific relationships among extant species of polychaete worms, squids, and certain fishes should not be at all surprising. After all, there are many living species of trans-Pacific fishes, molluscs, echinoderms, and crustaceans and their relationships indicate a frequent crossing of the East Pacific Barrier. In fact, a few of them even have circumtropical ranges. The late Cretaceous fossils described by Skelton (1988) probably do represent crossings of the Pacific from the Caribbean to the western side, rather than their being members of the same fauna. Skelton suggested that shallow-water plateaus and seamounts formed by mid-plate volcanism served as staging posts for their distribution. Since some of the taxa occur in slightly older strata in the Caribbean, that direction of dispersal seems reasonable (Hallam, 1994).
This expanding earth hypothesis says the Pacific Ocean began its initial formation in the Triassic and thereafter gradually increased in size. But a significant work on trans-Pacific faunal similarities among Mesozoic and Cenozoic invertebrates indicated a direct relationship to the recognized plate tectonic process (Fallow, 1983). That is, the faunal similarity between the east and west sides of the ocean has gradually increased over post-Triassic time indicating a decrease instead of an increase in the width of the ocean basin. The increase in resemblance is a reflection of the narrowing of the Pacific due to the expansion of the Atlantic and the westward movement of the American continents.
The final category of evidence is ‘northern trans-Pacific disjunctions’. This section begins with several paragraphs of references devoted to the distribution of Triassic to Lower Jurassic tetrapods. The main point is to emphasize the similarity of the East Asian and North American faunas, and to show that there must have been a connection between the two regions. But during this time, plate tectonic maps (Scotese, 2001) show a continuous, terrestrial habitat across the northern part of the globe from western North America to eastern Siberia, and almost all the tetrapod families were noted for their broad distributions. The tetrapod references are followed by references to insects, freshwater fishes, and many references on floral distribution. More recent than the cited botanical references, is the comprehensive volume Jurassic and Cretaceous floras and climates of the earth (Vakhrameev, 1991). This work delineated a ‘moderate-warm’ Siberian–Canadian Region that extended all the way across the combined continents. These works leave little excuse for advocating a connection across the North Pacific.
The next section discusses East Asian–North American relationships in the Cretaceous, especially the late Cretaceous. In the late Cretaceous, the closing of the Bering Strait provided a migration pathway for early mammals; necrosaur, varanid, and teiid lizards; tyrannosaurid dinosaurs; cryptobranchid and plethlodontid salamanders; asticid and cambarid crayfish; and polyodontid, lycopterid-hiodontid, and esocid fishes (Briggs, 1995). This was the beginning of a migratory flood that continued sporadically through most of the Cenozoic. Its origin was due to a new tectonic connection, not to the remnants of an historic amalgamation.
The final part of the evidence jumps backward in time to include an extended presentation on displaced terranes bearing Permian to Jurassic marine fossils. There is a voluminous literature on the terranes that were accreted to the Asian and American continents, mainly during the Cretaceous. At one time, due to the tropical nature of their fossils, the terranes were thought to have been of tethyan (Western Pacific) origin and had been transported northward for great distances. Now the accepted scenario is that, as the North Atlantic opened in the late Mesozoic, North America was carried westward and collided with an archipelago of East Pacific islands and submarine plateaus (Hallam, 1994). A series of strike-slip faults permitted a progressive northward displacement. Japan represents a collage of terranes that originated in the East Asian area. Therefore, it is not possible to relate the movement of displaced terranes to a theoretical opening of the Pacific Ocean.
The final part of the paper is devoted to a discussion that contains some naive observations: (1) if everyone agrees that the Atlantic and Indian Oceans were closed in pre-Jurassic times, why could not the Pacific Ocean also have been closed?; (2) chance trans-oceanic dispersal is an unfalsifiable hypothesis; (3) chance dispersal is antithetical to biogeographical patterns; (4) repeated postulating of different dispersal methods conflicts with the foundational premises of biogeography; and (5) objections to a seeming pattern of a conspiracy of trans-oceanic jaunts by a wide variety of poor-dispersing species. One is compelled to observe that these concluding remarks are simply complaints rather than useful biogeographic conclusions.