What is a dinosaur? It is the common name given to a group of reptiles, often very large, that first appeared nearly 250 million years ago i.e., the beginning of the Middle Triassic Epoch and flourished across the world for nearly 180 million years. Unfortunately, majority of them died by the end of the Cretaceous Period, about 66 million years ago, but researches on the topic revel that one lineage evolved into birds about 155 million years ago. The Triassic is a geologic period which began 50.6 million years ago, while the Cretaceous Period began 145.0 million years ago and ended 66 million years ago.
The name dinosaur was derived from the Greek words deinos which meant “terrible” or “fearfully great” and sauros which meant reptile” or “lizard”. The great English scientist Richard Owen used the formal term ‘Dinosauria’ in 1842 to include three giant extinct animals i.e., Megalosaurus, Iguanodon, and Hylaeosaurus and were represented by the large fossilized bones that had been unearthed at several locations in southern England during the early part of the 19th century. Owen was of the belief that these reptiles were far different from other known reptiles found in the present and the past as they were large but terrestrial, unlike the aquatic ichthyosaurs and plesiosaurs that were already known. They had five vertebrae in their hips, whereas most known reptiles had only two; and, rather than holding their limbs sprawled out to the side similar to lizards, dinosaurs held their limbs under the body in columnar fashion, like other large mammals. The question is where they reptiles or mammals?
They were originally used for just a handful of incomplete specimens, the clade Dinosauria now includes more than 800 generic names and at least 1,000 species, with new names being added to the roster every year as the result of scientific discoveries around the world. However, not all of these names are valid. A great many of them have been based on fragmentary or incomplete material that may actually have come from two or more different dinosaurs. In addition, their bones have sometimes been mistaken as dinosaurs when they are not.
There is a misunderstanding commonly described in popular books and media that all the dinosaurs became extinct at the same time and apparently quite suddenly at the end of the Cretaceous Period. This is not true, because birds are a living branch of dinosaurian lineage. The best records, which are almost exclusively from North America, show that dinosaurs were already in decline during the latest portion of the Cretaceous. The causes of this decline are complex and difficult to attribute to a single source. In order to understand extinction, it is necessary to understand the basic fossil record of dinosaurs.
Around 160 million years or so of the Mesozoic Era i,e., 252.2 million to 66 million years ago, from which dinosaurs are known, there were constant changes in dinosaur communities. Different species evolved quickly and were quickly replaced by others throughout the Mesozoic age. It is not possible that any particular type of dinosaur survived from one geologic formation into the next.
It is important to note that extinction is a normal, universal occurrence. Mass extinctions often come to mind when the term extinction is mentioned, but the normal background extinctions that occur throughout geologic time probably account for most losses of biodiversity. Just as new species constantly split from existing ones, existing species are constantly becoming extinct. The speciation rate of a group must, on balance, exceed the extinction rate in the long run, or that group will become extinct. The history of animal and plant life is full with successions as early forms were replaced by the new and often more advanced forms. In most instances the layered nature of the fossil record gives too little information to show whether the old forms were actually displaced by the new successors from the effects of competition, predation, or other ecological processes or if the new kinds simply expanded into the declining population’s ecological positions.
However, because of the knowledge of the various dinosaur groups is somewhat incomplete, the duration of any particular dinosaur can be gauged only approximately by stratigraphic boundaries and presumed “first” and “last” occurrences. The “moments” of apparently high extinction levels among dinosaurs occurred at two points in the Triassic i.e., about 221 million and 210 million years ago, or perhaps at the end of the Jurassic around 145 million years ago, and, of course, at the end of the Cretaceous period.
The next question is whether the extinctions were simultaneous and instantaneous or whether they were non-synchronous and spread over a long time. The precision with which geologic time can be measured leaves much to be desired no matter what means are used. Only rarely does an “instantaneous” event leave a worldwide, or even regional signature in the geologic record in the way that a volcanic eruption does locally.
It is believed that dinosaurs left no descendants. However, it is not true as Archaeopteryx, discovered in 1861, and Xiaotingia, which was discovered in 2011, give compelling evidence that birds of class Aves evolved from the small theropod dinosaurs. Following the principles of genealogy that are applied to humans as much as to other organisms, and they are classified at a higher level within the groups from which they evolved. Archaeopteryx and Xiaotingia which are the oldest birds known are therefore classified as both dinosaurs and birds, just as humans are both primates and mammals.
It is still unclear how dinosaurs were extinct? Hope we will solve this mystery soon!