In one timeline crown birds didn’t make it past the KT event, but ichthyornithids did. Like our birds they quickly radiated into a massive array of species; most of the initial niches were raptorial and insectivorous, but gradually aquatic plant eaters and arboreal frugivores kickstarted an herbivorous revolution, and these would eventually pale with the arrival of blunt-beaked herbivores, the toothed parrots.
The ancestor of the toothed parrots was a semi-aquatic, swamphen like ichthyornithid. It waded in the warm swamps of the Eocene, shifting from a duck or crane-like diet of soft aquatic plants into hardier reeds. To these ends it developed two key adaptations:
- The upper jaw completely lost its teeth, becoming a powerful, deep beak. By contrast, the lower jaw essentially lost its rhamphoteca, and became lined with blunt teeth, similar to those of long gone sauropod dinosaurs.
- The feet lost their webbing, and became prehensile, allowing the birds to manipulate food items.
The swamphen like forms quickly expanded into larger land herbivores similar to our gastornithids and dromornithids, and their reign wasn’t short either, having remained large browsers until their demise in the Pliocene when grass dominance was too much.
But their bigger success story was when these birds took to the trees, and quickly exploded into a variety of seed eaters and tough vegetation processors, dominating the volant herbivorous flying guilds in the arboreal realm.
Most remained anisodactyl, but some developed zygodactylous and some even heterodactylous feet multiple times. Their teeth remained peg-like in more folivorous and frugivorous niches, but for the more derived granivorous taxa they became increasingly bulkier and encased in bone much like those of the long gone dsungaripterids, making conventional reptilian tooth replacement impossible. In some taxa, some of the teeth moved sideways and lost their role in food processing, instead becoming powerful tusks.
One example of a derived toothed parrot are the elephant-parrots (genus Loxodontornis), birds endemic to African rainforests and dry forests. These gray birds have a hooked upper beak and large lower jaw tusks jutting to the sides, making them look like elephants from the front, helped further by their broad wings. More typical (for the group anyways) dsungaripteroid-like teeth process hard seeds and fruit as well as the occasional bone, while their heterodactylous feet both help manipulate food items as well as provide a stronger grip while climbing. Like all ichthyornithids they lack derived brain anatomy seen in our birds, so they are somewhat less agile flyers and in this particular species rather reluctant at flying.
A clade, the “p-parakeets”, have seen increased miniaturization, specializing on parakeet and finch-like niches. These birds are zygodactylous and have lost the lower jaw teeth, replacing them with a proper hard rhamphotecae; further, they have developed more complex birds, allowing for both increased aerial agility as well as higher intelligence. For now they are small, but these splitting images of our true parrots may yet rule the world.