During the early Norian age there is a sudden turn-over of large terrestrial herbivores. All or nearly all of the Rhynchosaur-Traversodontid big herbivores - the giant (upto one tonne) Kannemeyeriid dicynodonts, the stocky medium-sized Rhynchosaurs and Traversodontids, and the large armoured aetosaur thecodonts - become extinct within a very short period.
The Dinosaur-dominated Anchisaur-Plateosaur empire emerged as a result. The dinosaurs did not drive the earlier creatures to extinction. Early dinosaurs had co-existed with them, but had been unable to attain dominance until their overlords had vanished. It is the same story with the therapsids and the thecodonts, and the dinosaurs and the mammals. A dynasty or empire of creatures will rule until being exterminated by climatic factors, or perhahs extraterrestrial impact, and a new lot will emerge and grow large in their place.
These newcomers - anchisaurs, plateosaurs, and melanorosaurs - were all of a type of early plant eating dinosaur called a prosauropods. They were the ancestors and the grand-uncles of the gigantic sauropods like Brachiosaurus and Apatosaurus, and indeed they resembled much smaller and lightly built versions of those better known animals.
The pattern of emergence of the anchisaur-plateosaur empire follows very closely that of the earlier lystrosaurid-kannemeyerid empire. As Dr Bob Bakker explains, In the faunal zone following the Carnian extinctions (13 in Fig. 2) prosauropod dinosaurs of several closely related families (T and U in Fig. 2) make up nearly all big herbivore specimens.
Yet, as with the dicynodonts of the early Triassic, there is very little diversity. This is bvecause the newcomers have not yet had the opportunity to evolve into different forms. As Dr Bakker continues, "In any one local basin, the diversity appears to be very low, reminiscent of that of the Lystrosaurus Zone; usually one genus of big prosauropod dominates the collections, although various growth stages sometimes have been recognized as distinct genera (Rozdestvenski). Surprisingly, the top predators of these early prosauropod zones are holdovers from the mid-Trassic - ornithosuchid and rausiuchid thecodonts (H and L in Fig. 2). Advanced theropod dinosaurs take over this trophic role at the Triassic-Jurassic boundary"
These Prosauropods remained the dominant herbivores for millions of years, from the latest Triassic to end of the Early Jurassic. Apart from their rarer Sauropod cousin, the only other large herbivores were the heavily armoured aetosaurs, and even they are mostly reduced in size from their Carnian predecessors, and did not survive the end of the Triassic.
Thus, during the later Triassic and onwards, the terrestrial ecosystem is dominated by sauropodomorphs of all types and sizes, from slightly-built thecodontosaurs to giant (length to 9 meters, weight to 2 tonnes) plateosaurids, melanorosaurids (see above diagram) and proto-sauropods.
Preying on these herds of dinosaurian herbivores were several lineages of thecodontian and theropod carnivores. The thecodonts include the Ornithosuchids, Postosuchids, Rauisuchids, and Prestosuchids. These were the dominant animals of their day, often reaching 5 or 6 meters in length. Most of these could run on either two or four legs, as they chose (they were faculative but not obligatory bipeds). Among the theropods were a few surviving Herrerasaurs and the first of the big dinosaurian carnivores, the Megalosauridae and Dilophosaurinae. These were large obligatory bipedal preditors with lengths of upto 6 or 7 metres, but more lightly built than the thecodonts. Interestingly, the thecodonts die out during the terminal Rhaetic (end Triassic), but the theropods survive and flourish into the Jurassic.
In addition, there were many types of small reptilian, cynodontian, and mammalian animals in the undergrowth, including herbivores, insectivores, and small carnivores.
During the latest Triassic and early Jurassic then, evolution seems to have polarised: on the one hand there were the ruling land animals, the great dinosaurs, which filled the ecological roles now taken up by medium-sized and large mammals; on the other hand the first mammals had appeared, and together with the Tritylodont Therapsids they filled the small rodent and insectivore niche. The mammals were to remain small and individually insignificant - comparable to shrews, mice and rats of today - although doubtless very significant ecologically, for the 135 million years of the dinosaurs reign.
The following map by Anderson and Cruikshank give the known distribution of Anchisaur-Plateosaur-Melanorosaur faunas. This refers only to the location of fossil remains. The actual distribution would naturally have been much wider.
|some Links and References|
J. M. Anderson & A. R. I. Cruikshank, "The Biostratigraphy of the Permian and Triassic, Part 5, a review of the classification and distribution of Permo-Triassic Tetrapods," in Paleontologica Africana, 21, 15-44 (1978)
R.T. Bakker, 1977 "Tetrapod Mass Extinctions - A model of the regulation of speciation rates and immigration by cycles of topographic diversity" in A. Hallam, ed. Patterns of Evolution as illustrated by the Fossil Record, Elsevier Scientific Publishing Company, Amsterdam, Oxford, New York, pp.439-68
J.F. Bonaparte, 1982, "Faunal Replacement in the Triassic of South America", Journal of Vertebrate Paleontology 2 (3): 362-371, December 1982
Professor Paul Eric Olsen, Great Triassic Assemblages Pt 2 - The Keuper and Fleming Fjord Formation
Rozdestvenski, A.K., 1965, "Growth changes in Asian dinosaurs and some problems of their taxonomy", Paleont. Zh., 1965: 95-109 (in Russian)