Large dinos
may not have needed (detectable) adaptations to stay
submerged for significant periods of time. Never seen
this mentioned in discussions of sauropod
lifestyles...
Large dinos needed large bodies of water to stay submerged for any periods
of time... the Morrison Fm environment had enough water to submerge 4 m long
lungfish, but apparently not enough to submerge 40 m long sauropods...