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Chapter 1. The North Atlantic connection

Chapter 1. The North Atlantic connection

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consistent with Kurten’s conclusions and appears to indicate the presence of a

migration corridor at the beginning of the Eocene which was probably interrupted

by a water gap sometime during the early Eocene. Thiede (1980) concluded that the

presence of late Paleocene to early Eocene marine fossils on Svalbard (Spitsbergen)

indicated the first opening of a shallow-water seaway between the Arctic Ocean and

the North Atlantic.

Most recently, Savage and Russell (1983) observed that the early Eocene in

Europe witnessed a mass immigration of land mammals from North America. As

a result, about 50-60% of the genera of mammals from the Sparnacian (Lower

Eocene) Stage of the Paris Basin are the same as genera that have been found in

the Wasatch strata of Wyoming. At about the time the immigrants arrived, some

80% of the genera known from the late Paleocene of Europe became extinct.

Representatives of several lizard families (helodermatids, varanids, gekkonids, and

agamids) apparently also took the same route at about the same time (Estes, 1983).

The salamanders (urodele amphibians) demonstrate some interesting transatlantic

relationships. The North American genera of the family Salamandridae are considered to be a derived subgroup of a predominantly Eurasian family. It has been

suggested that they originated in Europe and dispersed to North America in the early Cenozoic (Milner, 1983). The family Proteidae has living genera in southeast

Europe and eastern North America. Fossil proteids are known from the Upper

Paleocene of North America and from the Miocene of southwest Russia and Germany. An extinct related family, the Batrachosauroididae, is known from the

Cretaceous to the Miocene of North America and the Paleocene and Eocene of

Europe. Both families, therefore, inhabited Euramerica prior to the early Eocene

division of that continent. Unlike the salamandrids, they may have arisen in North

America.

The distribution patterns of some of the freshwater fishes are useful. Within the

oculutus species group of fossil and recent gars (Lepisosteidae), there is a

European - North American relationship that Wiley (1976) attributed to an early

Eocene continuity. Two species of bowfins (Amiidae) were common to North

America and Europe in the early Tertiary (Boreske, 1974) and Eocene remains of

gars and bowfins have recently been found on Ellesmere Island, northwest of

Greenland (Patterson, 1981a). A study of the systematics of the family Percidae by

Collette and Banarescu (1977) indicated that the family probably originated in

Europe and then dispersed over a North Atlantic land route sometime between the

end of the Cretaceous and the beginning of the Eocene. An Eocene interruption of

contact between the two continents then allowed their percid faunas to develop independently. In North America, the percid tribe Ethiostomatini had undergone a

remarkable evolutionary radiation resulting in three genera and about 150 species.

The North Atlantic connection must have been important for much of the

plantlife of the Mesozoic and early Tertiary. A number of conifers and early

angiosperms, that now exist only as relicts, once had broad holarctic distributions.

Examples are such genera as Ginko, Sequoia, Liriodendron, and Ceridiphyllum

(Axelrod, 1983). Fossils of these genera have been found on Greenland as well as

either Spitsbergen or Iceland so the evidence for their once continuous distribution

across the North Atlantic seems very good. Another example is the section Aigeros



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of the genus Populus. The species of this section d o not occur in Asia but do have

a disjunct distribution across the Atlantic.

In regard to the marine fauna of the North Atlantic, Fallow (1979) published a

paper showing a strong, positive correlation between the width of the ocean basin

and the degree of similarity of the invertebrate animals that inhabit the continental

shelf on each side. His data, on the width of the ocean basin from the early Jurassic

to Neogene times, were taken from the plate tectonic work of Sclater et al. (1977).

In this case, the earth sciences data has produced a broad outline of the evolution

of the North Atlantic Basin that is consistent with the information from marine

biology. However, it is evident that general studies of plate movements have not

been able to focus with sufficient accuracy on the details of the relationship among

the terrestrial areas of Europe, Greenland and North America. Thus, the studies of

Sclater and Tapscott (1979) and Barron et al. (1981) showed these areas to be

separated by sea passages from the mid-Jurassic to the present while the biological

data, based on the distributions of terrestrial mammals, amphibians, reptiles,

shallow marine fossils, and freshwater fishes, indicate that land connections must

have persisted from the Mesozoic into the Cenozoic as late as the early Eocene.

Additional information that has a bearing on this problem became available in

1983 with the publication of the proceedings of a NATO Advanced Research Institute held in Italy in 1981. The meeting was devoted to the structure and development of the Greenland-Scotland Ridge. There were two papers dealing with paleontology (Hoch, 1983; McKenna, 1983a). Both recognized the presence of two early

Tertiary land bridges. One, called the Thulian route, extended from Labrador and

Baffin Island through Greenland, The Faeroe Islands and Scotland. The other, the

DeGeer route, connected Ellesmere Island, Greenland, Spitsbergen, and Scandanavia. A geophysical account (Nunns, 1983) appeared to show that the separation

of the Greenland area from Europe began in the early Paleocene.



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Chapter 2

THE NORTH PACIFIC CONNECTION

Widely ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely extended homes will have the best chance of seizing on new places,

when they spread into new countries.

Charles Darwin, The Origin of Species, 1859



As the tectonic plates on each side of the North Atlantic began to separate, those

of the North Pacific area began to draw closer to one another. By the late

Cretaceous (80 Ma), the maps of Smith and Briden (1977) still show a large gap between Asia and North America but those of Barron et al. (1981) indicate that the

two continents have made contact. Also, Fujita (1978) described a series of

Cretaceous collisions among several small plates between Siberia and North

America.

Lillegraven et al. (1979) found fossil evidence of land vertebrate exchanges in the

region of Beringia which were determined to have followed a continental collision

that took place in the late Cretaceous. The Rocky Mountain, Bug Creek Fauna of

that time has revealed new mammalian genera (multituberculate and placental)

which have been recognized as probable immigrants, presumably from Asia (Webb,

1985a). This Bering land connection, once established, evidently endured for a long

time instead of being interrupted in the early Cenozoic as indicated by Barron et al.

(1981). Durham and McNeil (1967) were unable to find any evidence for the migration of marine invertebrates between the North Pacific and the Arctic - North

Atlantic in the early Tertiary. Also, during this time, the marine mammal faunas

on each side of the land bridge were strikingly different; the desmostylians, sea

lions, and ancestral walruses were confined to the North Pacific, while the true seals

of the family Phocidae were found in the Arctic - Atlantic (Hopkins, 1967). Early

Tertiary marine fossils from northern Alaska have been interpreted to indicate an

isolation or near isolation of the Arctic Ocean that lasted from the end of the

Cretaceous until sometime in the Eocene (Marincovich et al., 1985).



ANIMAL MIGRATIONS



The broad expanse of Beringia that emerged in the late Cretaceous provided an

almost continuous highway for the dispersal of terrestrial and freshwater fauna that

lasted through almost all the Tertiary and was again available during the glacial

stages of the Pleistocene. Relatively brief inundations apparently took place in the

late Pliocene, and during the interglacial stages (Hopkins, 1967; Herman and

Hopkins, 1980). The biogeographic importance of the Beringia connection has been

noted in works dealing with the distribution of mammals, reptiles, various insect

groups, and freshwater fishes.

The extensive review work on the mammalian paleofaunas of the world by Savage



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and Russell (1983) and a chapter on mammalian diversification in North America

by Webb (1985a) are most useful in determining the role of Beringia in the history

of the Tertiary mammalian fauna of the northern hemisphere. In early Paleocene

time there was a sudden (explosive) origin of orders, families, and genera. The order

Condylarthra has been traced from one genus in the late Cretaceous Bug Creek

fauna to 20 genera in the early Paleocene (Van Valen, 1978). The explosive early

Paleocene interval may have generated the following five orders: Condylarthra,

Cimolesta, Insectivora, Dermoptera, and Carnivora, in addition to the Arctocyonia, Primates, Leptictida, and Taeniodonta that had already appeared in the

latest Cretaceous (Webb, 1985a). By the mid-Paleocene, three genera of

mesonychids had appeared that represent the order Acreodi, which had been known

from earlier deposits in Asia. As many as three genera of large herbivores,

representing the order Pantodonta, also evidently immigrated into North America

from an earlier origin in Asia (Simons, 1960). An important event of the late

Paleocene was the appearance of three families in North America that showed relationships to both South America and Asia. All three groups, the edentates, notoungulates, and xenungulates, were best developed in South America. It has been

suggested (Gingerich, 1985) that they originated in South America then migrated,

in the late Paleocene, to North America and then to Asia via Beringia.

In the early Eocene, several genera, apparently immigrants from Asia, appeared

in North America for the first time (Webb, 1985a). These include genera belonging

to the orders Tillodontia, Rodentia, and Pantodonta. A little later in the early

Eocene, the first perissodactyls, artiodactyls, adapid and omomyid primates,

hyaenodontids, and tapiroids, all possible immigrants from Asia, appeared. In the

mid-Eocene, the North American mammalian fauna became considerably more

endemic. But, in the late Eocene, the Asiatic influence was renewed. The new immigrants appear to be adapted to woodland savanna and scrubby habitats (Webb,

1985a) and included several groups of lophodont rodents, pig-like entelodonts,

several families of artiodactyls including the Camelidae, and several groups of

perissodactyls including tapiroids, rhinocerotids, and chalicotheres.

With the onset of the Oligocene, came the most impressive faunal turnover in the

whole age of mammals (Webb, 1985a). In the larger mammal groups, there was the

first American appearance of the Canidae, Felidae, Mustelidae, Tapiridae, Castoridae, Rhinocerotidae, Anthracotheriidae, and Tayassuidae. Many of the new groups

arrived by way of the Bering Strait. According to Webb, the groups most traceable

to Asiatic stock are the Castoridae, Anthracotheriidae, and Tapiridae. A great wave

of immigration from Asia came in the early Miocene when some 16 genera established themselves in North America. Among the more conspicuous forms were species

of cats, bears, pronghorn antelopes, beavers, and flying squirrels. By the midMiocene, the proboscidian genera Miomastodon and Gomphotherium arrived along

with crecitid rodents but, in general, this was a time when the Asian influence had

become slowed. Finally, toward the end of the Miocene, there was a renewed surge

of immigrants including several large carnivores, large ungulates, and small herbivores.

In the Pliocene, there occurred another burst of intercontinental migration

resulting in an extensive faunal turnover at the generic level. Some 72 new genera



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