Understanding the evolution of endothermy requires the resolution of a paradox: Increasing the resting metabolic rate can provide heat to raise the body temperature, but without insulation (hair or feathers) that heat is rapidly lost to the

Question

environment. But a layer of insulation does not benefit a vertebrate unless the animal has a high metabolic rate. Discuss the evidence suggesting that Mesozoic synapsids and sauropsids had developed both high metabolic rates and insulation, and identify the puzzling gaps in the information currently available.

Answer 1

Synapsids: The presence of turbinate bones in the respiratory passages of nonmammalian therapsids and early mammals indicates that these animals had high rates of respiration and metabolism. We have no fossils that show the imprint of hair, however, so there is no direct evidence of insulation in these animals.
Sauropsids: Impressions of feathers show that feathers were well developed in many small carnivorous dinosaurs, indicating that they had a layer of insulation, but the absence of turbinal bones in the respiratory passages is inconsistent with the hypothesis that these animals had the high respiratory rates that would be expected for endotherms.