Mammals appeared much later than the first dinosaurs

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A popular misconception is that mammals appeared much later than the first dinosaurs. Actually, protomammals appear the same time as protodinosaurs, at about 235 million years ago in the mid-Triassic, and the two groups shared the rest of the Mesozoic Era. Early dinosaurs were small-down to the size of weasels-which means they were potential competitors for the last therapsids as they rapidly diminished into tiny nocturnal protomammals. Animatronic dinosaur

 

Here arose a great dichotomy that would separate dinosaurs and mammals for the rest of the Mesozorc. Many, but not all, dinosaurs rapidly increased in size-only a few million years into their start one paleodinosaur had reached rhino bulk-and they were daylight-oriented. The mammals stayed small and nocturnal. There was strikingly little overlap in weight between the two groups, for throughout the Mesozoic few dinosaurs and few mammals were in the range between I to 5 kg. Again, we ask why. Both probably had high metabolic rates. In fact, the very first dinosaurs, the fully erect lagosuchians, may have already been at the avian level, which is as high as one can get. Some contend that dinosaurs were low-metabolic-rate animals that needed big size to maintain constant temperatures, big bodies being slower to cool and heat than smaller ones. This explanation fails for the obvious reason that many dinosaurs were smaller than the ton or so of body mass required for such temperature constanry, and as will be explained in Chapter 7, big size is advantageous for tropical warm-bloods too. walking dinosaur costume