Final answer:
There may be a size limit to warm-blooded animals with endoskeletons due to the metabolic and temperature regulation demands that come with endothermy. Endoskeletons provide support and protection, but at diminutive sizes, they might be inefficient compared to other support systems. This could explain why the smallest vertebrates are not less than 0.01 grams in weight.
Step-by-step explanation:
The question refers to why endoskeletons have not evolved in very small warm-blooded animals and if there are physical reasons for this. Amphibians, which include frogs, are the first true tetrapods and the earliest vertebrates to live on land. They exhibit a range of sizes from as small as less than half an inch to as large as 1.5 meters, but even the smallest known vertebrates, some frogs, do not weigh less than 0.01 grams. Endoskeletons provide structural support and protection, but they also add weight and require energy and resources to grow and maintain. Therefore, it may be more energetically efficient for smaller organisms to have simpler, less structured support systems like exoskeletons or hydrostatic skeletons.
The evolution of vertebrates started with fish, which later gave rise to amphibians on land. These amphibians had to adapt to gravitational forces and needed supportive skeletal structures. However, a lower limit in size becomes apparent when considering the metabolic demands and temperature regulation requirements of endothermic, or warm-blooded, animals. Larger bodies can retain heat more efficiently, which may partly explain why we observe a lower size limit in warm-blooded vertebrates compared to cold-blooded ones.
The presence of a bony endoskeleton can be traced back to the early evolution of chordates, of which echinoderms, though invertebrates, have internal ossicles as a type of endoskeleton. As vertebrates evolved, structures like jaws and vertebral columns developed, culminating in amphibians, such as frogs, which then gave rise to amniotic vertebrates like reptiles.