Ectotherms Research Paper

It is well-known that modern birds, aside from being the living descendants of dinosaurs, are endotherms -meaning they have use metabolic processes to maintain an optimal internal body temperature. Unfortunately, there is an apparent controversy about the origin of endothermy in the evolutionary lineage leading to birds, given that dinosaurs were presumed to be ectothermic or ectotherm-like. In discussions about the physiology of extinct vertebrate species like dinosaurs, one factor that has always been used to distinguish ectotherms from endotherms is bone histology. Ectotherms are known to show signs of annual growth arrest cycles, coinciding with decreases in metabolic rate during less favorable dry/cold seasons, while endotherms were
just assumed to continuously grow into maturity due to a consistently high metabolic rate and body temperature. This means that while endotherms are expected to show rapid tissue growth, ectotherms –whose metabolic processes are dependent on external sources of heat-, would have zones of alternating bone growth and lines of arrested growth (LAGs). With this in mind, it’s no wonder that the appearance of zonal bone leads to conclusion of ectothermy or ectotherm-like regulation in fossil findings. This study, however, found the assumption of constant, regular growth pattern in endotherms to be false, as they observed clear cycles of seasonal arrested growth while studying extant ruminants across a wide geological range.