American Scientist had a fine article on the evolution of filter feeding in whales. The whales first lost their teeth and became suction feeders. Then they developed baleen to become more efficient suction feeders.
It turns out that some modern marine mammals suction-feed without
teeth or baleen, swallowing prey without chewing or filtering.
Narwhals, for example, technically have teeth—their tusks—but they
don’t use them to chew. The Risso’s dolphin, sperm whales, and some
beaked whales suction-feed without using teeth or baleen. “
Could there be a large suction-feeding turtle, the transitional form to a full on filter feeder?
Not only is the skull from what must have been an enormous reptile,
but the shape of the lone fossil is unlike any other turtle. Wide at
the back, the 27 and a half inch long skull narrows in front of the
eyes into a flattened tube. Ocepechelon didn’t have the short-faced
look at modern sea turtles, but an unusual snout that recalls a
toothless, beaked crocodile... Among turtles, Ocepechelon is bizarre.
But the ancient swimmer’s skull shape isn’t totally unique among
vertebrates. The extinct turtle’s upper jaws, Bardet and colleagues
suggest, bear some resemblance to the mouths of small pipefish and the
larger, mammalian beaked whales. Since these animals feed by suction –
creating a vacuum to draw in small prey – Bardet and collaborators
suggest that Ocepechelon may have done the same.
If the turtle had a distensible throat (as I proposed might be the case for such turtles here, a nozzle like mouth would increase the velocity of incoming water and more effectively suck up prey that might be trying to escape.
The last step, to filter feeding. Flamingos are full on filter feeders that use a modified beak, as a turtle must. They accomplish this via a specialized and versatile tongue that keeps the food in the middle and expels the water to the sides, and lamella along the beak.
Filter feeding in flamingos
flamingoes . . . dragged their mandibles through the mud and water,
rocking their heads from side to side. . . pumping movements of the
gular region, and the twin jets of muddy water that were shot outward
from the comer of the mouth [were ob- served] . . . Certainly a
suction was being set up in the mouth opening . . . evidently a
combination of a pumping and a sucking through the opening of the
gullet. Then, as the upper mandible clamped shut, streams of mud and
water were found forced outward, certain portions being retained
within . . . processes along the upper edgeof the tongue . . . with an
additional straining effect from the lamellae on the mandibles.
So your turtle - a large suction filter feeder which gradually improves food extraction efficiency by evolving a specialized tongue and baleen-like ramifications on the beak.