Let's summarize ..
The dicynodont herbivores radiate in the Permian and sink in diversity in the Middle Triassic. Simultaneously, the Late Paleozoic succulent plants (largely pteridophytes) decline in diversity. There is a brief hiccup in the mid Triassic of rhyncosaurs which are herbivorous dinosaur relatives. They are also low feeding and they seem to fill in as the dicynodont diversity drops down. They fill in briefly, then the dinosaurs start to radiate in the late Triassic. We are seeing the demise of one flora and with it the demise of a group of herbivores that evolved with it and the rise of another flora and of another group of herbivores. There is a major transition and it is driven by the floral change.
The new dinosaurian herbivores that come into this story are large, such as Plateosaurus. Again, remember my point about eating plants what you need to do is be fairly large in your stomach in order to hold a sufficient quantity of low-quality food. Yes, some like the Lesothosaurus are sort of German shepherd size herbivores, but they are the exception. The Plateosaurusis the rule - at ten tons. The bottom line point is we are making a very interesting transition. This slide shows the numbers of non-flying mammal species and non-flying dinosaur species in terms of their sizes (61). The count on the vertical axis is the number of species and the horizontal axis is the log of weight. Living mammals average around 10 gram to 100 grams. By contrast, if we plot of non-avian dinosaurs (remember dinosaurs are still with us, we see them on our bird feeders) the average size is one ton. The terrestrial herbivores of the Late Paleozoic are much more of the scale of the present day ecosystem dominated by mammals. The dinosaur ecosystem of the Mesozoic stands out as unlike either what preceded or followed. This is an enormous change. Why? I think in part because dinosaurs are being driven by the quality of the food.
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