Why did Spinosaurus evolve to be so big?

Spinosaurus was a semi-aquatic animal, like a crocodile today. However most Spinosaurus species were fish hunting specialists (or rather they hunted aquatic animals rather than ambushing land animals like many crocodiles do today). read more

Archeopteryx did not evolve into any other species, either. Its lineage also went extinct. Sometimes people say “every individual is a transitional”. That simply is not true. Spinosaurus was not transitional to anything. It was not evolving towards birds — it was way to big. Instead, it represents a dead-end, like so many lineages. read more

But it's unclear just how big Spinosaurus was, due to incomplete fossils. In a 2005 study in the Journal of Vertebrate Paleontology, researchers estimated Spinosaurus was 52 to 59 feet (16 to 18 m) long and weighed 7.7 to 9.9 tons (7 to 9 metric tons), based on extrapolations from skull measurements. read more