Hesperornithiforms, the sea parrots and seaguins, most likely evolved during the Cretaceous and are the only group of toothed birds which still have survived to the present day.  Throughout the intervening time between the clades inception and the present day, the hesperornithiforms have proven quite conservative, and their basic body type has changed little.

    A hesperornithiforme resembles nothing so much as a loon, which has been stretched longer and then flattened.  These creatures lack the wings of their cousins, the modern birds, and indeed, their foremost appendages have been reduced to jointless paddles. Seaguins' propulsive force is applied by the two hind legs, which are stoutly built, and end in enormous, lobed, feet. 

    Hesperornithiforms once enjoyed a planet-wide distribution, but by the end of the Mesozoic, they were already withdrawing from the South.  Now, due to competition from mosasaurs and (in the Atlantic) suchotheriids, these creatures are entirely relegated to the Northern Hemisphere, and only really successful around the Arctic.

    Sea parrots and seaguins rarely venture onto land, but are forced to make the excursion, periodically, to give birth.  These birds are ovoviviparous, but their young are born still encased in the sack-like allentois, and would suffocate in an underwater birth.  Thus, seaguin females must climb up onto the land, propelling themselves with sweeps of the feet and a snake-like wriggling of the body, lay their "egg" and then pull the chick free of the confining birth membrane.  Further elaboration of this process varies from species to species.

(Text by Daniel Bensen)
    Unlike the seaquins (see below), sea parrots have retained the teeth in their upper jaw. Based on this character, sea parrots are believed to have diverged from the seaguins in the Eocene or early Miocene.
(Text by Matti Aumala)
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