Some perceive dinosaurs as entertainment for children. Dinosaurs are actually subject to scientific investigation. As they are described today they make sense to me. They make as much sense as platypuses, kangaroos and bats. The platypus has the reputation of being really weird. However, its closest living relatives are comparable in weirdness. Kangaroos look so special the word is the world’s most loaned. Bats are mammals which front legs have evolved into wings. All these can arise through gradual modification of a vaguely rat-like animal. I think the latter constitute the mammals’ basic shape. Still today there are many mammals vaguely resembling rats. This despite not being murids or even rodents.
The world were the dinosaurs arose came out of the Permian-Triassic mass extinction. There were no large land predators with higher metabolism. Neither were there any flying animals larger than today’s insects. In the early Triassic reptiles evolved a range of crocodile-like forms. In at least two cases they began to evolve higher metabolism. One gave rise to the rauisuchids (Rauisuchidae) which first seem to have taken over as large predators. Another gave rise to the dinosaurs.
Gizzards already existed in the common ancestor of dinosaurs and crocodiles. Now hind legs evolved which were held strait beneath the trunk. The front legs were bowed backwards instead of to the sides. Breathing with air sacs made the lungs much more efficient. This had the advantage that the skeleton could become partially hollow too. Also the kidneys become much more efficient. To keep warm they evolved hollow bristles. These gave rise to the pterosaurs’ fur and later the bird’s feathers.
In the middle of the Triassic pterosaurs had taken over as larger flying animals. Their wings were stretched out from a single finger. These animals were also in to control of how taut their wing membranes were. The rest of the toes of their front legs were made for walking on. Same applies to their hind legs. There was only two small species having griping ability at all. These are thought to have been adapted to climbing trees. All could throw themselves into the air with both their front and hind legs. The same method is today used by certain species of bats. By mostly using their front legs even the largest ones could fly.
Most dinosaurs descended from small, bipedal animals. As a consequence large dinosaurs also had smaller front legs than hind legs. Already in the late Triassic they had divided into three main groups. One consisted of carnivorous dinosaurs ranging from the size of a human and up. Another was bipedal herbivores which could weight several tons and gave rise to the long-necks. A third one I have chosen to call billed dinosaurs. These gave rise to a wide variety of dinosaurs as time went on.
At the transition from the Triassic to the Jurassic a smaller mass extinction happened. It was the violent birth of the
Largest were of cause the long-necks which legs worked like those of an elephant. Their tails were used as a whip or for touching each other. The neck primarily filled the function of letting them reach more without moving around. A lot of energy was still needed to move such large animals. The heaviest we have extant evidence of is the South American Argentinosaurus. (Fossils of even larger dinosaurs have disappeared or been destroyed. So their size can’t be verified.) I think it weighted about 75 tonnes (74 long tons or 83 shots tons). With air cooling though the previously mentioned air sacs such an animal is entirely possible.
The size of other dinosaurs should probably not be overestimated. The heaviest carnivorous dinosaur was likely Giganotosaurus. I don’t think it weighted more than at most 14 tonnes (14 long tons or 15 short tons). The heaviest billed dinosaur is thought to be Shantungosaurus. 13 tonnes (13 long tons or 14 short tons) seem to be the weight most agree on. The sizes of both then become comparable to the heaviest elephants seen by humans. So nothing extraordinary is needed to explain their size.
Just because the dinosaurs were so big their eggs were not gigantic. The largest dinosaur egg found was just 61 centimetres (2 feet). Moreover, it was very elongated. I think this was necessary for the egg to function. Most eggs laid by dinosaurs were considerably smaller. This means newly hatched young oftentimes were very small compared to the adults. Juveniles then filled an entirely different niche than the adult animals did. (The number of species and genera may have been overestimated due to this.) For this reason there were few dinosaurs of what can be called medium size. The juveniles of larger dinosaurs usually outcompeted them.
This competition made certain small carnivorous dinosaurs to begin living in the trees. They already had feathers filling the same function as fur. Individual with longer feathers on their arms and/or legs could jump longer between trees. This led to many of them ending up evolving gliding. One group evolved powered flight in the late Jurassic. In this way the birds arose. Many of the early birds still had teeth. However, such birds went extinct at the same time as the dinosaurs.
Please note that I purely systematically consider dinosaurs to be birds. Then I mean the animals more closely related to today’s birds than today’s crocodilians. Present-day birds are sufficiently distinct to warrant their own class. But on a geological time scale the distinction does not become obvious. Over hundreds of millions of years step-by-step changes accumulate. To me the most important step is starting to evolve warm-bloodiness. Consequentially I count proto-mammals as mammals. If we should now continue to use classical systematics. Something I promote for purely practical reasons.
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