Gravity wasn't so ad back in the day

You're assuming that these giant herbivores didn't live in semi-aquatic and aquatic conditions, which would dramatically reduce blood-pressure and muscle requirements for having such a long neck.

 

The 200 lb. bird still flew

 

Another huge assumption. How do they know that the bird would have weighed 200lbs. and how do they know the atmosphere wasn't more dense?

 

The bird had a 25 foot wingspan, 200 bs was an estimate but still a far cry from the 30 lbers we have nowadays. How dense would the atmosphere have to be to make up that difference?

 

Not much. The increase in atmospheric density would be multiplied by total lift generated by the wing.

 

Are you so sure about that, Diogenes? The bird still weighs 200 lbs, and our 30 lb birds already have trouble taking off because they're so damn fat and juicy. MmMmM...

 

Damn it smurf stole my identity!