Nothing stay the same: not even 'living fossils' like crocodiles.
In a recent study published in Paleobiology, a pair of paleontologists at the Sorbonne University in Paris examined bone from a 237-million-year-old reptile and found signs that the ancestors of the crocodile family had remarkably active metabolisms — and that the creatures evolved them far earlier than expected.
Crocodiles are [...] members of the wider archosaur lineage, a group that includes endothermic pterosaurs, dinosaurs and birds. The clues to that relationship are written in their anatomy, said Roger Seymour, an evolutionary biologist at the University of Adelaide in Australia. Crocodilians can adopt a more straight-limbed posture than other reptiles, have gizzards, a four-chambered heart and birdlike respiratory systems.

Many researchers took these features to mean that the (thoroughly ectothermic) modern crocodiles are a relic of the transition from older coldblooded reptiles to endothermic dinosaurs and birds, a point of view that strengthened as paleontologists began accepting the idea of warmblooded dinosaurs.

But others argued that crocodiles might have been closer to endothermy than suspected. The crocodilian heart was a vital clue. Researchers like Christina Bennett-Stamper, a research microscopist at the federal Environmental Protection Agency, have described a process by which alligator embryos develop a unique opening just before hatching that allows blood to bypass the lung. The process turns “an endothermic heart into an ectothermic one,” Dr. Seymour said."
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The New York Times: Why Crocodiles Are Not Just Living Fossils (By ASHER ELBEIN)