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The marsupial and placental mammals originated at a time when the pattern …

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- Plate Tectonics, Seaways and Climate in the Historical Biogeography of Mammals

The implications of the word "mammal" _ Mammals first evolved in the Late Triassic, about 210 mya, and about a dozen different groups of Mesozoic mammal are known to have existed over the following 130 my Their osteological characters are similar to those that we find in the ancestors of the later marsupial and placental mammals, so it is likely that these Mesozoic mammals, too, were hairy and warm-blooded, like their living descendants the monotremes of Australia. They were, almost certainly, also like the latter in being egg-laying, rather than either marsupial or placental in their manner of reproduction. Most were about the size of a mouse, and were insectivorous (i.e. eating invertebrates), though one group became omnivorous and as large as a wood-chuck. The Mesozoic mammals are, however, worth noting because they provided the first opportunity for parasites to become adapted to the mammalian type of physiology.

Mesozoic marsupials and placentals can be distinguished from one another, and from the other Mesozoic mammals, primarily by characteristics of their dentition. Fossil evidence suggests that the two modern groups diverged from one another in the Early Cretaceous, 110-120 mya, but the earliest forms that can be positively identified as belonging to one group or the other are from the Late Cretaceous (for geological time-scale, see Fig. 1) . The placental method of reproduction probably evolved from the marsupial method, rather than directly from the egg-laying method. This implies that the earliest forms that are identified as placental (on the basis of their tooth structure) were probably still marsupial in their method of reproduction.

Times of first appearance of groups - In trying to show the pattern of appearance of the different mammalian orders and families, one can only give the geological level, and corresponding date, of the earliest known specimen; almost certainly, the taxon will in reality have evolved at an earlier date. It is common to find, in non-palaeontological literature, much earlier dates of appearance hypothesized where these better suit the needs of the theory being propounded. But one needs to be prudent in this. In particular, few different types of mammal are known before the end of the Cretaceous, and it is possible that some others may have evolved before that date. But the pattern of first-appearances (see Table, derived from data in Benton 1993) does suggest that the radiation of the mammals took place very rapidly after the end of the Cretaceous. This was the time at which the sudden extinction of the dinosaurs removed the restriction on mammalian ecological diversity that predation by the dinosaurs had previously imposed. Until then, the mammals had been limited to being nocturnal, small, insectivorous or frugivorous, probably mainly arboreal, animals whose way of life made them unattractive, inaccessible or invisible as a food-source for dinosaurs, who were much larger and diurnal. A few Cretaceous placentals have been placed in Cenozoic groups that are otherwise composed of large herbivores or carnivores. This is not because the Cretaceous forms were necessarily identical with them in size or diet, but only because they show characteristics which suggest that they may have been ancestral to those Cenozoic groups.

Times of continental fragmentation or collision - The start of the break-up of a continental mass is announced by the appearance of volcanic activity along the line of its future separation into two daughter continents. New volcanic rock is continually produced along this line, so that the two new continents are continually and gradually moved apart. The presence of radio-active minerals in these rocks makes it easy to date them. The band of rocks closest to the line of volcanic activity, which have just appeared from the depths of the earth, contains the youngest rocks, while the band that lies closest to the edge of the continent is the oldest. The age of these oldest rocks therefore indicates the age of separation of the two continents.

The gap between the two separating continents becomes filled by ocean waters, and the rocks become covered by ocean sediments that contain the skeletons of minute marine planktonic organisms. Sometimes the line that separates the two continents is irregular, so that in some areas the two continents are sliding past one another, instead of simply moving apart _ for example, the northern edge of South America moved westwards along the southern edge of West Africa for some time after its eastern edge had separated from Africa. The date of final ending of any land connection between the two continents is then given by the date at which the two marine faunas become united, as shown by the fossil marine planktonic skeletons in the sea sediments.

The date of coalescence of two continents is more difficult to establish, but is indicated by the nature of the rocks laid down between them. As the two continents approach one another, the ocean between them first becomes a shallower sea, and then dries up altogether, and these changes can be detected from the nature of the fossil marine organisms that are laid down in the rocks. But biological contacts between the two areas may begin earlier than the time of union of the continents, because islands may form in the intervening seaway and provide a pathway for dispersal.

Types of faunal link - Biogeographers distinguish three different types of link between faunas or floras. The first is a corridor, which contains a wide variety of habitats, so that most types of organism are able to pass through it. The wide expanse of central Asia is a good example of this, forming a corridor linking the faunas of western Europe with those of China. The second is a filter route, which contains only a limited variety of habitats, so that only those organisms that can exist in those habitats will be able to disperse through it. The exclusively tropical lowlands of the Panama Isthmus provide a good example of such a link. Finally, a sweepstakes route separates areas that are so difficult of access that very few organisms ever succeed in reaching them. The ocean barriers that surround the scattered islands of the Pacific are good examples of this type of link.

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