Potamornis

Potamornis is a prehistoric bird genus that dated back to the late Maastrichtian. Its scrappy remains were found in the Lance Formation at Buck Creek, USA, and a single species has been described: Potamornis skutchi.

This was almost certainly a member of the Hesperornithes, the hefty and toothed flightless diving birds of the Mesozoic seas. Its precise relationships are not all to clear; the quadrate bone is quite unique in some respects but apparently shares more apomorphies with the family Hesperornithidae - the "typical" Hesperornithes - in cladistic analysis. Consequently, it might be considered a fossil hesperornithid with a different feeding specialization. Though it was heavily built like many (flying and flightless) diving birds, it weighed perhaps 1.5 or 2 kg. This raises the possibility that the Hesperornithes not only included flying members (see also Enaliornis), but that their families might have evolved flightlessness independently.