EOSINOPTERYX

The Jehol Biota—the entire Early Cretaceous ecosystem of northeastern China from 130 to 120 mya—has remarkable fossil preserving properties, with its Lagerstätten ("storage places") yielding such well-preserved small theropod dinosaurs that their feathers, even different types of feathers, are clearly visible. Unfortunately, Eosinopteryx is from the somewhat older, Late Jurassic Tiaojishan Formation, which is not an ideal environment for preservation due to its composition of different aged rocks all mixed together by folding, and erosion and volcanic activity. But even a blind dog can find a bone once in a while, and Eosinopteryx is quite well-preserved itself.

Eosinopteryx belongs to Troodontidae—a family of small carnivorous dinosaurs within Deinonychosauria—and is most closely related to Anchiornis who inhabited the same area. However, the two differ significantly in the form of their feathers and feet. Unlike Anchiornis and other feathered deinonychosaurs, Eosinopteryx lacked a curved "killing claw" on the second toe of each foot, and the rest of its claws were short and straight and attached to slender toes. Although similarly sized overall, the legs and tail of Eosinopteryx were less feathery than those of Anchiornis, and its feet lacked remiges (flight feathers) altogether. It seems likely that the pair occupied two different niches of the same ecosystem to minimize pressure on food, with the former's lack of stride-hindering plumage allowing it to run down its prey.