Jean-Claude Rage
Laboratoire de paléontologie des vertébrés (URA 1761 du CNRS), case 106, Université Paris 6, 4 place Jussieu, 75252 Paris cedex 05, France
Prior to its final opening, the Tethys appeared as a gulf which separated the eastern parts of Laurasia and Gondwana. At that time, dispersals of land vertebrates perhaps occurred across the Tethyan gulf, but data are very poor and this part of the history is not dealt with here. The western advance of the Tethys led to the completion of its opening during the late Jurassic, apparently during the Kimmeridgian ( Cecca et al.,1993). However, before this completion, epicontinental seas linked the Pacific Ocean and the Tethys on the site of Central America; they permitted exchanges of marine faunas between these two realms. In other words, as far as terrestrial faunas are concerned, the Tethys barrier was in place before the Kimmeridgian, at least as early as the Callovian (Enay et al., 1993).
Although fully opened, during the latest Jurassic the Tethys was still very narrow in its western part. After the opening, crossings occurred several times on various places. Four stages may be distinguished in the history of these crossings: (1) after the final opening, up to the Santonian, the Tethys acted as an efficient barrier, (2) from the Campanian to the Palaeocene, trans-Tethyan crossings occurred on three sites, (3) no crossing occurred during the Oligocene, and (4) from the early Miocene onward, present trans-Tethyan connections have arisen. It should be noted that from seemingly the latest Cretaceous, the Tethys has progressively broken up and it has been no longer recognizable as such. However, marine areas have still separated Laurasian from Gondwanan continents.
Trans-Tethyan dispersals by land vertebrates: the main events
The Tethys barrier (Kimmeridgian-Santonian)During this period, rare faunas are reported to be common to Laurasian and Gondwanan areas. The record of terrestrial vertebrates from the late Jurassic is poor. Nevertheless, four dinosaur genera are shared by latest Jurassic faunas from both Gondwana and Laurasia (Russell, 1993). In North America, they come from the Kimmeridgian or Tithonian of the Morrison Formation (U.S.A.) whereas in Africa they are known in the Kimmeridgian of Tendaguru, Tanzania. A mammal family, the Peramuridae is also known on both sides of the Tethys during the Jurassic: at Tendaguru and in the Kimmeridgian-Tithonian of Europe. However, because of the age of these fossils, which is very close to that of the final opening, these distributions may result from vicariance, more especially as Peramuridae were possibly present in the middle Jurassic of Europe. But, if dispersal events were implied, various land routes appear to have been possible in the western Tethyan area: Africa-South America-North America, Africa-Europe-North America, or even a direct route Africa-North America. Anyhow, the direction of the dispersals, if any, remains unknown. A similar problem, either vicariance or dispersal, muddles the early Cretaceous history (Le Loeuff, manuscript). During the Berriasian and Valanginian, only mammals, the Spalacotheriidae give evidence of possible trans-Tethyan dispersals (Berriasian of Morocco and Berriasian or Valanginian of England). But, Spalacotheriidae are also known from the late Jurassic of Laurasia; therefore, vicariance cannot be ruled out. From the Hauterivian to the Albian, a few possible dispersals are documented by rare dinosaurs. A few dinosaurs from the Hauterivian-Barremian interval could argue for dispersals. They are a titanosaurid from England (the family is of Gondwanan origin), iguanodontids known in North America and Europe at that time and which also occur in the Aptian of Africa, spinosaurids present in the Barremian of Europe and later in Africa (Cenomanian). However, Titanosauridae are known as early as the late Jurassic, that is as far as they are concerned vicariance cannot be excluded. As for Iguanodontidae and Spinosauridae, dispersals appear more probable (but they are not ascertained), apparently from Europe to Africa owing to the Mediterranean seuil (a seuil is a narrow oceanic zone partly occluded by small parts of continents, platforms…; Vrielynck et al., 1994). As mentioned above, the presence of Spinosauridae in the Cenomanian of Africa could result from a dispersal of northern origin which took place during the early Cretaceous. Caudate amphibians from the Cenomanian of Africa apparently represent a Gondwanan subgroup of Sirenidae (Evans et al., 1996) which are primarily Laurasian (North American). Their presence in Africa is seemingly the result of a dispersal which could have taken place during either the Cenomanian or the early Cretaceous. |
Campanian-Palaeocene: trans-Tethyan land routes
From the Campanian to the Palaeocene, the Tethys was crossed by terrestrial forms on three sites: between South and North America, between Eurasia and Africa, and between India and Eurasia.
(1) During the Campanian, an important period of interchanges began between South and North America; it ended during the Palaeocene. Rather numerous terrestrial taxa were involved (Gayet et al., 1992). Dispersals from the south markedly outnumbered dispersals of northern origin. These interchanges did not take place on the Panamanian bridge which was not in existence at that time. The terrestrial connection probably comprised the Greater Antilles, the Bahamas platform, the Aves Ridge which is submerged today, and perhaps the site of the present Lesser Antilles. Apparently, this route was not absolutely continuous.
(2) At the same period, various land vertebrates of Gondwanan origin reached southern Europe (Buffetaut, 1989; Le Loeuff, 1991). During the Campanian and Maastrichtian, these immigrants from the south were reptiles. Some mammals also dispersed between Europe and Africa during the latest Maastrichtian and the Palaeocene (Gheerbrant, 1990). As for mammals only, some interchanges extended in the Eocene. Whereas dispersals of reptiles were only of southern origin, mammals dispersed in both directions. These interchanges took place on the Mediterranean seuil which afforded a probably discontinuous route.
(3) By the late Maastrichtian, a terrestrial connection linked India to Asia. This is evidenced by the presence in the late Maastrichtian of India of taxa of Laurasian origin (Rage and Jaeger, 1995). Among them, Amphibia (Pelobatidae and Discoglossidae) appear to be highly significant because salt water forms a full barrier to their dispersals. Palaeomagnetic data corroborate that India was already connected to Asia by Cretaceous/Tertiary times (Klootwijk et al., 1992). This has been a final connection, although geographically different from the present one. The main part of the connection was seemingly made by the present Southeastern Asia which occupied a more western position (Rage, 1988) and the structure of which was different from the present one (Jaeger et al., 1989).
It may be wondered whether the Asia-India route reached Madagascar at that time. This hypothesis rests only on the distribution of a few living reptiles (Iguanidae and Boinae); it remains flimsy. A pachycepalosaurid dinosaur is the only fossil evidence of a Cretaceous dispersal of Laurasian origin to Madagascar (Sues and Taquet, 1979); if this fossil actually comes from the late Cretaceous of Madagascar, neither the route nor the age of the dispersal can be established.
Oligocene: Absence of interchanges
After the last interchanges of the Eocene, no crossings of the Tethys can be evidenced during Oligocene times.
The modern connections
(1) The final Eurasia-Africa connection: The collision between Eurasia and the Arabo-African plate was initiated by the Oligocene, but interchanges of terrestrial faunas between these two continents have begun only during the early Miocene, on the site of the present Middle East (Bernor et al., 1987). (Another miocene connection, of very short duration, linked Europe and Africa on the site of Gibraltar, during the Messinian. The interchanges were of limited geographic range, the dispersants remained in restricted areas on both sides).
(2) The collision between Autralia and Southeastern Asia has apparently begun during the middle Miocene. At the same time, the Indonesian-Australian seuil appeared which provided a possible route between Australia and Asia. Prior to the collision, continental fragments were scattered between Asia and Australia, they apparently permitted the first exchanges (from Asia to Australia only). The first Asian immigrant into Australia was a bat from the early Eocene of Australia (Hand et al., 1994); obviously this is not significant. The first true terrestrial immigrants in Australia appear to have been pythons (boid snakes), known in the late Oligocene or early Miocene of Australia. Exchanges between Asia and Australia have never been important; only dispersals from Asia to Australia are recorded. The Asia-Australia connection is still in its initial stages.
(3) Finally, the last trans-Tethyan route, the Panamanian land bridge, appeared during the Pliocene. South America and North America were separated since the Palaeocene. Here, the first terrestrial interchanges have begun during the middle/late Pliocene. They were heralded by a few immigrants (in both directions) during the middle/late Miocene which probably crossed through water gaps.
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