Why aren't reptiles as big as dinosaurs?

This sizeable question came from Jake on twitter. Jason Head from Cambride Univeristy joined Chris Smith in the studio to rattle on about reptiles...

Jason - The reason that we don't have terrestrial reptiles the size of Mesozoic dinosaurs today, is that that particular history stopped in a series of extinction events throughout the Mesozoic. Living dinosaurs of course are birds, and there have been some really big birds throughout their history but birds are specialised for being comparatively lightweight. Their bird bodies are adapted for flying for the most part. Other groups of large reptiles, things like snakes, crocodiles, turtles, lizards all have these different histories of large body size evolution through time and actually one of my graduate students is working on this very question right now about whether they're big trends. But really what's happened in the Cenozoic, since since the end of the Cretaceous, the last 66 million years is that the large body size new is pretty much occupied by mammals right now. So we don't really see opportunities for any of the living reptile groups to adapt to any of those environments. And it really has to do with eating plants and how mammals, modern mammals, are specialised to eat plants in a way that living reptiles aren't.