The Pelagornis was a very large sea bird that took the ecological niche of the Pteradactyl. It had an enormous wing span of up to 5-6 meters (15-20 feet). Their prey was slipperey, hence the bony-teeth on its imposing bill. The bony-toothed birds died out, yet today, there are still plenty of sea birds who dine on slippery squid and fish. So the question remains why they had teeth at all.
The Pelagornis has mostly been depicted looking more like a pelican, but there is some scientific thought that they may be more like the albatross, hence my illustration.