The Earth may well have had far more CO2 in the past. I've read that it has a carbon inventory capable of producing 70 atmosphere's of pressure which compares well with Venus although I don't understand why Earth forming further out should have less unless it was another fortuitous result of the Moon forming impact. However, the dinosaurs were barely more than a blink of an eye in the past compared to Earth's 4.5 billion year history. A simple linear reduction from 70 to 1 would indicate an atmosphere about 3 times as heavy as today's. I would bet that the process is highly nonlinear as most chemical reactions are. How much denser are you talking about?

Regards,
Mark