Hell Creek birds

NOTICE: The animals depicted here are known from very scarce material. These restorations are speculative, or even inaccurate, and should not be taken as reference. Critiques are always welcome.

The Hell Creek Formation is a very famous fossil site from North America, and the fossils discovered here show us how were Montana and the Dakotas at the end of the Age of Dinosaurs. Tyrannosaurus, Triceratops, Edmontosaurus or Ankylosaurus were the most famous inhabitants of this ecosystem, but scientists have also found remains of smaller dinosaurs that lived alongside the giant ones:birds. Because of their small size and fragile bones, bird bodies fossilize rarely, and we know these species only from scrappy material. Here are depicted some of Hell Creek Formation birds, which belong to the three main lineages of Cretaceous avialans.

On the left side of the picture is Avisaurus archibaldi, a giant member of the Enantiornithes group, distant relatives of modern birds. Avisaurus is known from some bones of the foot, the tarsometatarsus, which are some of the biggest in the enantiornithine group. Due to the size and robustness of these bones, this bird may have had a raptorial lifestyle, chasing small animals in the forests and swamps of Laramidia.

Bottom right is Potamornis skutchi. This bird, only known from scrappy remains, is probably a close relative of the giant flightless bird Hesperornis, although this one was much smaller (weighting around 1-2kg). It lived in both fresh water and the sea, as it's remains have also been found in coastal deposits of the Lance Formation. The picture shows a juvenile individual at the back and an adult with breeding plumage, similar to the one of grebes and loons. Hesperornithines were highly derived aquatic birds, and probably spent most of their lives in water.

At last, on top right there is a restoration of Cimolopteryx maxima. Known only from coracoid bones, this bird has been classified as an early charadriiforme, a diverse group of modern birds that include seagulls, terns and waders of all kind (plovers, jacanas, stilts, oystercatchers, sandpipers…). Cimolopteryx is depicted here as a small shorebird, similar to plovers. In the picture, an adult is followed by two chicks. Charadriiformes show great diversity in parental care behavior, so this adult could be the mother or the father of the chicks.