Until recently, the success of the dinosaurs over the rhynchosaurs, dicynodonts and cynodonts was explained by a competitive model. It was assumed that the erect gait of the dinosaurs, and other supposed advantages, allowed them to vanquish other Triassic animals and drive them to extinction.
There was a major crisis about 225 million years ago, some five million years after the origin of the first small dinosaurs. Numerous groups of animals died out in the sea and on land, as a result of a great climatic change or some other catastrophe. There is evidence that plants underwent major evolutionary upheavals about this time, and the rhynchosaurs and dicynodonts may have died out when they lost their essential plant foods. Whatever the cause, there was a mass extinction 225 million years ago. A mass extinction is the disappearance of a broad cross-section of plant and animal groups in a relatively short time. A dozen or more reptile groups died out then, including several significant ones such as the rhynchosaurs, dicynodonts, aetosaurs, and various carnivorous cynodont and ‘thecodontian’ groups. This left a large number of gaps in the ecology and possible lifestyles of terrestrial plants and animals, giving great opportunities for the surviving groups to take over and fill the gaps. The rare early dinosaurs, never more than one or two percent of their communities before the mass extinction, blossomed to represent 50 percent or more within a few million years.
This model for the origin of the dinosaurs – their opportunistic radiation into ‘empty ecospace’ is very different from the old competitive model. There is no long-term battle, in which whole groups are pitted against each other globally. The dinosaurs were lucky to be around at the right time, and they seized the opportunity. Competitive advantage no doubt played a part, however. The small Lagosuchus-like dinosaurs had an effective erect gait, with all of its advantages, and they were agile carnivores able to hunt a variety of prey. Just as the mammals replaced the dinosaurs opportunistically after the latter’s extinction, some 160 million years later, so the dinosaurs probably owed 95 percent of their success to being in the right place at the right time, and five percent to their natural competitive attributes.
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