The fossilized remains of dinosaurs (with the notable exception of their lineal descendants the birds - see Chapter 6) have been found in rocks identified as belonging to the Mesozoic Era. Mesozoic rocks range in age from 245 to 65 million years ago (abbreviated to Ma from now on). In order to put the time during which dinosaurs lived into context, since such numbers are so large as to be quite literally unimaginable, it is simpler to refer the reader to the geological timescale.

During the 19th and a considerable part of the 20th centuries, the age of the Earth, and the relative ages of the different rocks of which it is composed, had been the subject of intense scrutiny. During the early part of the 19th century it was becoming recognized (though not without dispute) that the rocks of the Earth, and the fossils that they contained, could be divided into qualitatively different types. There were rocks that appeared to contain no fossils (often referred to as igneous, or 'basement'). Positioned above these apparently lifeless basement rocks was a sequence of four types of rocks that signified four ages of the Earth. During much of the 19th century these were named Primary, Secondary, Tertiary, and Quaternary - quite literally the first, second, third, and fourth ages. The ones that contained traces of ancient shelled and simple fish-like creatures were 'Primary' (now more commonly called Palaeozoic, literally indicative of 'ancient life'). Above the palaeozoics was a sequence of rocks that contained a combination of shells, fish, and land-living saurians (or 'crawlers', which today would include amphibians and reptiles); these rocks were designated broadly as 'Secondary' (nowadays Mesozoic, 'middle life'). Above the mesozoics were found rocks that contain creatures more similar to those living today, notably because they include mammals and birds; these were named 'Tertiary' (now also called Cenozoic, 'recent life'). And finally, there was the 'Quaternary' (or Recent) that charted the appearance of recognizably modern plants and animals and the influence of the great ice ages.

This general pattern has stood the test of time remarkably well. All modern geological timescales continue to recognize these relatively crude, but fundamental, subdivisions: Paleozoic, Mesozoic, Cenozoic, Recent. However, refinements in the way the fossil record can be examined for example, through the use of high-resolution microscopy, the identification of chemical signatures associated with life, and the more accurate dating of rocks enabled by radioactive isotope techniques have led to a more precise timescale of Earth history.

The part of the timescale that we are most concerned with in this book is the Mesozoic Era, comprising three geological periods: the Triassic (245-200 Ma), the Jurassic (200-144 Ma), and the Cretaceous (144-65 Ma). Note that these periods of time are not by any means equal in duration. Geologists were not able to identify a metronome-like tick of the clock measuring the passing of Earth time. The boundaries between the periods were mapped out in the last two centuries by geologists who were able to define particular rock types and, very often, their constituent fossils, and this is usually reflected in the names chosen for the periods. The term 'Triassic' originates from a triplet of distinctive rock types (known as the Lias, Malm, and Dogger); the 'Jurassic' hails from a sequence of rocks identified in the Jura Mountains of France; while the name 'Cretaceous' was chosen to reflect the great thickness of chalk (known as Kreta in Greek) such as that which forms the White Cliffs of Dover and is found widely across Eurasia and North America.

The earliest dinosaurs known have been identified in rocks dated at 225 Ma, from the close of the Triassic (a period known as the Carnian), in Argentina and Madagascar. Rather disconcertingly, these earliest remains are not rare, solitary examples of one type of creature: the common ancestor of all later dinosaurs. To date at least four, possibly five, different creatures have been identified: three meat-eaters (Eoraptor, Herrerasaurus, and Staurikosaurus), a tantalizingly incomplete plant-eater named Pisanosaurus, and an as-yet-unnamed omnivore. One conclusion is obvious: these are not the earliest dinosaurs. In the Carnian there was clearly a diversity of early dinosaurs. This indicates that there must have been dinosaurs living in the Middle Triassic (Ladinian-Anisian) that had 'fathered' the Carnian diversity. So we know for a fact that the story of dinosaur origins, both the time and the place, is incomplete.

SOURCE: http://about-dinasaurs.blogspot.com/2007/06/dinosaurs-in-perspective.html

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