Why are there no aquatic dinosaurs?

Why are there no aquatic dinosaurs? Dinosaurs dominated Earth for around 165 million years. They ruled forests, deserts, floodplains, swamps, coastlines, and eventually even the air through their surviving descendants, birds. But for all that time, one environment remained almost completely out of reach. The sea. People often call mosasaurs, plesiosaurs, and ichthyosaurs “sea dinosaurs,” but that is not actually correct. These animals lived alongside dinosaurs, but they were not dinosaurs. Mosasaurs were giant marine lizards. Plesiosaurs were separate marine reptiles. Ichthyosaurs were dolphin-shaped ocean reptiles. They ruled the prehistoric oceans while true dinosaurs stayed mostly tied to land. So why were there no full-time aquatic dinosaurs? The answer comes down to anatomy, eggs, competition, and evolution. Dinosaurs were built for land. Their upright legs, rigid hips, and specialised skeletons made them incredibly efficient walkers and runners, but those same features made it difficult to become fully aquatic. A dinosaur body was not shaped like a dolphin, crocodile, ichthyosaur, or whale. It was a land machine. Then there was the bigger problem: eggs. Dinosaur eggs needed air. Their shells were porous, allowing oxygen in and carbon dioxide out. Put those eggs underwater and the embryo could not survive. That meant every dinosaur, no matter how aquatic it became, still had to return to solid land to reproduce. Spinosaurus came closer than any other known dinosaur. It had dense bones, a crocodile-like snout, conical teeth for catching fish, high nostrils, and a paddle-shaped tail. It likely spent huge amounts of time in rivers and lagoons, hunting fish in freshwater environments. But even Spinosaurus never became a true ocean animal. The prehistoric seas were already ruled by animals perfectly adapted for life in water: ichthyosaurs, plesiosaurs, mosasaurs, and later marine mammals. Dinosaurs were late to a world that was already occupied. But there