At the end of the early Triassic, new larger land animals appeared, evolving from Lystrosaur-vinatge ancestors.
The Kannemeyeriid community type was characterised by the ox-sized megaherbivore Kannemeyeria, and the squat, stock, wolf-sized carnivore Cynognathus. In addition to these two characteristic genera there were a number of other widespread small and large animals, including the including the diademodontid cynodonts, herbivorous descendents of small omnivorous cynodonts. This is the last Mesozoic tetrapod community type in which theropsids have the upper hand, and it will not be until the extinction of the dinosaurs some 170 million plus years later, that Cynognathus' descendents will remerge to inherit the world.
The Archosaurs meanwhile had evolved into the very large Erythrosuchids (3 to 5 meters), although these may not have competed ecologically with the Cynognathids. There was also a diverse range of small carnivores and insectivores, including new forms like the facultively bipedal Euparkeria
The Rhynchosaur-Traversodontid empire was a development of the Kannemeyeriid community type, in which archosaurs came to play an increasingly dominant role. It lasted for much of the middle Triassic. Here there is a definite increase in diversity. The herbivorous diademodontids disappear but are replaced by big Rhynchosaurid archosauromorphs and more specialised Traversodontids cynodonts. To the original Kannemeyeriines are added several more large dicynodont subfamilies, including the shansiodontines and stahleckeriines. But these large animals, which, with lengths of 3 meters and adult weights of a tonne, were diminished in numbers, being largely replaced by rhynchosaurs and traversodontids, whose dental specialization enabled them to handle the tougher, Dicrodium-dominated vegetation. Mike Benton refers to this as an example of differential survival, but the dicynodonts did not become extinct, and indeed were to cotinue right up until the end of the Carnian age. All the herbivores were squat quadrupeds that fed no more than a meter above ground.
The Middle Triassic Manda Formation in East Africa provides a sample of an early stage of the Rhynchosaur-Traversodontid community type, and the South American sequence fills in the later period. Therapsid herbivores and carnivores maintained their ecological dominance in lowland basins with the widespread Dicrodium flora. However, they had to contend with the increasingly sucessful archosaurs, whose reptilian metabolism gave them an edge over the paramammalian therapsids in the arid Triassic climate. In Laurasia Dicrodium is absent, and the tetrapod fauna is somewhat differnt, although still sharing many of the same elements as the Gondwana forms.
As early as the Anisian age (faunal stage 8 of Fig. 2) a standing diversity of four big herbivore families was reached, with biomass D between 2 and 4 in local basins; turnover continued in big herbivores in Ladinian and Carnian, but the number of big herbivore families remained at about 4.
Medium to large quadrupedal chiniquodotids replaced the Cynognathids as the primary therapsid carnivores. These well-adapted, dog-sized predators had to contend increasingly with several types of carnivorous archosaurs, which become ecologically important in this community type. These included the Prestosuchids and Rauisuchids, the largest terrestrial predators of the Triassic, with improved limb posture and hence increased locomotor capability. Alongside these were the small protodinosaurian Lagosuchids, early bipedal Sphenosuchid crocodylomorphs which looked (and probably acted) more like small dinosaurs and thecodonts then crocodiles, and a number of other types of uncertain relationships.
The archosaurian's morphological improvements in limb posture and locomotor capacity would certainly have benefited them in the open and dry savannah-like environments. It may be that the numerical rarity of some archosaurian taxa was the result of their only occasional emigration from other, perhaps more upland or drier, environments with poor preservation potential.
The third and final community type could perhaps be called the Aetosaur-Rhynchosaur-Traversodontid Community, because while almost all the previous elements are present, they are joined by the the large, armour-plated and spiny Aetosaurs (Stagonolepidae), the only thecodont family to evolve herbivory.
This period, which lasted throughout the extent of the Carnian age, saw a further diminishing of the therapsids, with only the Traversodontids continuing in large numbers. The mighty Kannemeyeriid dicynodonts that had remained the sole megaherbivore type throughout almost the entire Triassic were so diminished that in one local fauna (Chinle, late Carnian, stage 12 of Fig. 2) only one big herbivore genus is common, the placerine kannemeyerid Placerias. However, new herbivores included not only the large aetosaurs but at least two lineages of small bipedal dinosaur.
The archosaurian carnivores of the middle Triassic are joined by new large forms like the Postosuchids and Ornithosuchids, both of which were facultative bipeds (i.e. capable of both bipedal and quadrapedal walking and running), several further small gracile terrestrially adapted crocodylomorphs, as well as the very first dinosaurs, all bipedal and mostly small, and represented by the three main dinosaurian lineages of Theropoda, Sauropodomorpha, and Predentata/Ornithischia. Only the Theropods are carnivorou, but these are represented by several families (Staurikosauridae, Herrerosauridae, and possibly Coelophysidae). All the archosaurs - herbivores and carnivores - had an efficient, vertical limb posture superior to that of the contemporary therapsids.
Meanwhile, the rivers, lakes and swamps were populated by new types of amphibians (Metoposaurs) and the astonishingly crocodile-like phytosaurian thecodonts, which, unlike their terrestrial relatives, had a primitive, typically reptilain sprawling posture.
The Kannemeyeriid-Rhynchosaur-Traversodontid-Aetosaur megaherbivore assemblage died out suddenly at the end of the Carnian epoch, victims of the most serious of several mass-extinctions that rocked the Triassic world. Only the Aetosaurs continued, and they were so reduced in size as to no longer qualify for the title megaherbivore. In the place of these diverse forms appeared the prosauropod dinosaurs, at least four families of very similiar animals ranging in size from small to huge (upto 1500-2000 kg). The reign of the therapsids and thecodonts was over and teh dinosaurs had inherited the world.
The following map by Anderson and Cruikshank give the known distribution of The Kannemeyeriid-Rhynchosaur-Traversodontid 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
Robert T. Bakker, "The Need for Endothermic Archosaurs", in R.D.K.Thomas and E.C.Olson, eds, A Cold Look at the Warm Blooded Dinosaurs, AAAS Selected Symposium 28, p.366
M.J Benton, 1983, "Dinosaur success in the Triassic: a non-competitive ecological model" Quart. Rev. Biol. 58, 29-55
J.F. Bonaparte, 1982, "Faunal Replacement in the Triassic of South America", Journal of Vertebrate Paleontology 2 (3): 362-371, December 1982
Michael A. Cluver, 1978, Fossil Reptiles of the South African Karoo, South African Museum, Cape Town
Professor Paul Eric Olsen, The Triassic World
Professor Paul Eric Olsen, Great Triassic Assemblages Pt 1 - The Chinle and Newark
New Blood - about the very end of the dynasty, the animals on the way out and the others on the way in, from the superb BBC series.
J.M. Zawiskie, 1986 "Terrestrial vertebrate faunal succession during the Triassic", in Kevin Padia ed., The Beginning of the Age of Dinosaurs - faunal changes across the Triasic-Jurassic boundary, Cambridge University Press, Cambridge