"Most modern birds (Ornithurae) of Hell Creek occupy a watermarginal habitat, much like modern ‘wader’ type birds. Wide, thin toes allow them to walk over soft mud, while a narrow bill is used to probe for seeds and small invertebrates. One of the more widespread groups of modern birds near the end Cretaceous was the Cimolopterygidae (Cretaceous Wings). This family includes animals such as Cimolopteryx and Lamarqueavis. Like the unnamed ornithurine, this group includes animals that greatly resemble modern shorebirds. These animals were present in Hell Creek, but more common farther south in the Lance Formation."
―Saurian: A Field Guide to Hell Creek
Cimolopteryx is a Cretaceous ornithuran found in the Lance Formation. The type species is C.rara but there is another, dubious, described species tentatively named "Cimolopteryx" maxima, which is known from the Lance Formation and the Hell Creek Formation.
Cimolopteryx was believed to have been a member of the bird order Charadriiformes, in the extinct family Cimolopterygidae, however cimolopterygid relationships with Charadriiformes are dubious and inconclusive at best, so they might not be related to any defined modern bird group. It was around the size of a small gull. Although cimolopterygids are known from the Hell Creek Formation, these were tentatively placed in the genus Cimolopteryx, under the dubious species "Cimolopteryx" maxima. C.maxima was around twice the size of C.rara, but given its fragmentary nature, there is not enough evidence to name it as a Cimolopteryx or as a different genus.
In Saurian[]
Concept art Cimolopteryx feeding on a breached Mosasaurus carcass
Cimolopteryx flying
Its only possible future existence in Saurian is justified in concept art by RJ Palmer of a flock feeding on a beached Mosasaurus carcass.[1] It is possible that Cimolopteryx is or has been replaced by the currently undescribed ornithurine found in Hell Creek. Cimolopterygid remains are known from the Hell Creek Formation, but these are currently found in Montana, so the inclusion of the unnamed ornithurine, found in South Dakota, where the game is set, makes perfect sense.