propecia canadian
propecia canadian
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propecia canadian |
propecia canadian
In 1822 the Belgian geologist D'Omalius d'Halloy gave the name Terrain Cretace, for the chalk and rock outcrops of the Paris Basin, and for similar deposits in Belgium and Holland and traceable also from England eastward into Sweden and Poland.
This term "Cretaceous" "chalk-bearing" (from Creta, the Latin word for chalk) later came to be used. The famous White Cliffs of Dover, are typical of this rock formation. Extensive chalk deposits were laid down in Europe and parts of North America during this period. The chalk itself is actually formed from the shells of countless micro-organisms.
William Smith had previously mapped four strata between the lower clay (= early Tertiary) and the "Portland Stone" (= late Jurassic), namely White Chalk, Brown or Grey Chalk, Greensand, and Micaceous Clay or brick earth (later referred to as Blue Marl or Gault). In 1822 Conybeare and Phillips arranged these in two groups, the Chalk and the earlier strata, a division that has remained to the present day. In 1841 Leymerie introduced the term Neocomian for the lower division. Senonian was coined by d'Orbigny in 1842 for the later Cretaceous. The name "Gallic" has also been used for times that do not fall conveniently into either of the above two categories. The present scheme, however, recognizes only two epochs within the Cretaceous: and .
propecia canadian
Much of the land masses are covered by shallow continental oceans and inland seas. , , and are all a series of islands. Marked regional differences in floras and faunas between the northern and southern continents developed, and indeed there were even differences in flora and fauna from continent to continent.
The breakup of the world-continent , which began during the , continued apace.
The Cretaceous saw the lengthening and widening of Atlantic Ocean, which especially spread further south, while the Alps were formed in Europe. broke free of and became an island continent, much like is now. Africa and split apart, Africa moving north and closing the gap that was once the . The continents began to take on their modern forms.
propecia canadian
| Period | Epoch | Age | Base | Duration |
|---|---|---|---|---|
| Paleogene | Paleocene | Danian | 65.5 | 3.8 |
| Cretaceous | Late Cretaceous | Maastrichtian | 70.6 | 5.1 |
| Campanian | 83.5 | 12.9 | ||
| Santonian | 85.8 | 2.3 | ||
| Coniacian | 89.3 | 3.5 | ||
| Turonian | 93.5 | 4.2 | ||
| Cenomanian | 99.6 | 6.1 | ||
| Early Cretaceous | Albian | 112.0 | 12.4 | |
| Aptian | 125.0 | 13.0 | ||
| Barremian | 130.0 | 5.0 | ||
| Hauterivian | 136.4 | 6.4 | ||
| Valanginian | 140.2 | 3.8 | ||
| Berriasian | 145.5 | 5.3 | ||
| Jurassic | Late Jurassic | Tithonian | 150.8 | 5.3 |
propecia canadian
Plants
Although the Jurassic floras of ferns, cycads, and conifers continued, the most dramatic occurrence during the Cretaceous was the first appearance of the flowering plants, the . By the end of the Cretaceous, a number of modern plant forms had evolved.
Marine invertebrates
, , and other were abundant, as were sponges, and . Several new groups of echinoids appear during this time, and the families Echinocorythidae and Spatangidae become abundant. The meanwhile declined in diversity. Higher such as lobsters become common. were of essentially modern types. Modern groups (especially ) appear. These differ from all previous gastropods by being largely carnivorous, and, like the , are easily able to live in sandy environments.
The normally conservatively coiled ammonoids take on bizarre shapes, which was at one time attributed to a sort of racial senility. This idea is no longer considered valid. In addition to uncoiled ammonoids, and normal coiled forms, both with complex suture patterns, there are those types found in areas of North Africa, Syria, South America and southern USA in which the suture-lines had become simplified so that they closely resembled those of the . These are, naturally, called "pseudo-ceratites".
Cretaceous reefs are largely made up of rudists, huge bivalves with one cone-shaped valve and the other reduced to a small lid-like structure. A single animal could be a meter in height. Rudists grew rapidly, most certainly with the help of symbiotic algae. The early Cretaceous forms like Monopleura and Caprotina were replaced during the later Cretaceous by Hippurites, whose growth-habit paralleled that of Rugose corals. Especially in the Mediterranean region, these were an important part of the marine ecosystem.
Fish
Modern fishes become widespread around the middle and especially towards the end of the period, and also appear during this time.
Terrestrial invertebrates
The appearance of flowering plants greatly stimulated evolution, as the plants used insects for pollination. Many modern groups of insects appeared and began to diversify, including the oldest known ants and butterflies.
Amphibia
Apart from a single lineage of giant confined to the rift valleys of south-east , the amphibians are represented only by existing groups such as frogs and salamanders
Reptilia (non-archosauria)
The (turtles and tortoises) are common, and include representatives of both living and extinct groups. Some marine forms attained enormous size, such as the 3 meter Archelon.
The tuatara-like are much reduced, their ecological role being taken by the (lizards and snakes). One line of aquatic lizards evolve into gigantic marine forms, the predatory , with lengths of up to 10 meters and more.
The are greatly reduced, with only a single family straggling through to the early part of the late Cretaceous. It is likely that they were unable to adapt to the new fast-swimming teleost fish. They died out long before the Cretaceous ended.
are represented by a number of new forms, including the Polycotylidae, Cimoliasauridae, and huge long-necked elasmosaurids.
Crocodiles
Modern crocodilians () evolve for the first time, while a number of more primitive groups continue. The sea crocodiles die out early in the Cretaceous. The warm tropic conditions however are good news for the fresh-water crocs, some of which (e.g. Sarcosuchus, Deinosuchus) attain huge size. These gigantic crocodiles doubtless fed on unwary dinosaurs in the same way that large modern crocodiles will grab an ungulate mammal (such as a zebra or wildebeest) that approaches the water to drink.
Dinosaurs and birds
Like the , the Cretaceous Period is distinguished in the popular imagination by the presence of great dinosaurs, marine reptiles, and flying pterosaurs. Many new types of dinosaurs evolved at this time. Among the greatest of the giants, the brachiosaurs and diplodocids died out, while new sauropod dinosaurs, specifically the , continued to flourish in , but became rare in . The plated were replaced by the heavily armoured . The (horned dinosaurs) evolved and flourished in but (apart from one or two questionable forms) are unknown elsewhere. The seem to have been similarly limited in distribution. Curiously, the most abundant and diverse of the Cretaceous herbivores were the unarmoured ornithopod dinosaurs, specifically the hypsilophodontids and iguanodontian lines, all of which achieved cosmopolitan distribution.
Among the carnivores there was an astonishing radiation of large and small bird-like forms, ranging from the giant carcharodontosaurs, spinosaurs, and tyrannosaurs, through the medium-sized sickle-clawed deinonychosaurs to the bird-like ornithomimosaurs and the enigmatic segnosaurs. Alongside these medium to large forms were an extraordinary range of birds and dinosaur-like protobirds. Indeed, during the Cretaceous it would have been impossible to tell where dinosaurs ended and birds began, such is the continuum between them.
The birds seem to have pushed the (flying reptiles) progressively to extinction. As the Cretaceous progressed the pterosaurs declined steadily, until at the end of the period only one or two aberrant giant forms (Pteranodon, Quetzalcoatlus) remained.
Many new groups also appeared during this time, including the three living groups of mammals (, and ) which appear during the mid Cretaceous.
The Cretaceous-Tertiary Extinction
The Cretaceous and the entire Mesozoic was brought to an end by one of the greatest mass extinctions of all time, the K-T event or terminal Mesozoic extinction. About one half of all animal families died out, including dinosaurs, many marine reptiles, several lines of archaic birds, the ammonoid and most belemnite cephalopods, rudist clams, and many microorganisms.
propecia canadian
More Links: and have good, very accessible pages on the Cretaceous. has detailed stratigraphic information and some maps. The covers North American stratigraphy, at a less technical level. has very well-organized links and some connecting material. We find the tone of the site irritatingly slick -- but, then, we are very easily irritated. ATW060107
| propecia canadian 146-65.5 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 146-99.6 | 99.6-65.5 | ||||||||||
| 146-140 | 140-136 | 136-130 | 130-125 | 125-112 | 112-99.6 | 99.6-93.5 | 93.5-89.3 | 89.3-85.8 | 85.8-83.5 | 83.5-70.6 | 70.6-65.5 |
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page uploaded on Kheper Site on 28 May 1998, page uploaded on Palaeos Site 9 April 2002, wikified HAJ090124
last modified ATW060107
checked ATW060117
text content © M. Alan Kazlev 1998-2002, except as stated
this material may be freely used for non-commercial purposes