Yi qi

Yi qi was a small scansoriopterygid theropod dinosaur from the Jurassic of China, known from a single fossil specimen recovered from the Tiaojishan Formation in Hebei Province. Formally described by Xu and colleagues in 2015, this taxon represents one of the strangest and most surprising forms among maniraptoran dinosaurs, not only because of its tiny size, but also due to the presence of a membranous wing system previously unknown among theropods. Its name comes from Mandarin Chinese and may be translated as “strange wing,” a direct reference to the extraordinary structure of its forelimbs.

Yi qi belongs to Scansoriopterygidae, a small group of paravian theropods characterized by reduced body size, short skulls, elongated forelimbs, and adaptations probably related to an arboreal or climbing lifestyle. The holotype specimen, catalogued as STM 31-2, preserves part of the skeleton associated with soft-tissue impressions, including simple feathers and remains of a wing membrane. This exceptional preservation made it possible to recognize that Yi qi did not develop a wing based solely on flight feathers, as in birds, but a far more unusual structure formed by a skin membrane supported by elongated fingers and by a rod-like bony or cartilaginous element projecting from the wrist.

From a morphometric perspective, Yi qi was a very small dinosaur, with an estimated body mass close to 380 grams and body dimensions comparable to those of a small bird or a small arboreal mammal. Its body was lightweight, with a short skull, blunt snout, relatively short neck, gracile hind limbs, and a short tail compared with other non-avian theropods. The forelimbs were proportionally long, with elongated fingers — especially the third finger — and were associated with a soft-tissue membrane extending between the hand region and the styliform element. This anatomical arrangement gave it a silhouette very different from that of other feathered dinosaurs from the Jurassic.

One of the most important features of Yi qi is the combination of feathers and wing membranes. The fossil preserves an abundant covering of simple feathers, filamentous or brush-like in appearance, distributed over much of the body, especially the head, neck, and trunk. However, the main aerodynamic surfaces appear to have included skin membranes, more comparable in functional concept to the wings of bats or certain gliding animals than to the wings of modern birds. This morphology demonstrates that the evolution of flight or gliding among paravian dinosaurs explored highly diverse anatomical solutions, some of which did not persist evolutionarily.

The locomotor capability of Yi qi remains debated. The presence of wing membranes suggests that it may have been able to glide or perform short aerial movements between trees, although later aerodynamic studies have proposed that its performance would have been limited and that it was probably not an active flyer comparable to birds. A cautious interpretation considers it a small arboreal or scansorial dinosaur, perhaps capable of controlled gliding or of using its forelimbs for assisted movement within a three-dimensional vegetated environment. This condition makes it a key example of evolutionary experimentation among dinosaurs close to the origin of birds.

Yi qi inhabited Jurassic ecosystems of northeastern China, in an environment associated with forests, lakes, and volcanic systems where numerous vertebrates, insects, and plants have also been preserved in exceptional detail. In this context, it probably occupied the niche of a small arboreal animal, feeding on insects, small invertebrates, or other small-sized resources. This replica represents Yi qi as a lightweight and extraordinarily specialized scansoriopterygid: a body covered with simple feathers, short skull, long forelimbs, developed fingers, and wing membranes supported by a unique styliform element. Its reconstruction integrates the available information on its osteology, soft tissues, and possible ecology, highlighting one of the most singular dinosaurs from the Jurassic of China.

Approximate measurements of Yi qi:

Scale 1:4 — Complete model
Length: 110 mm
Height: 59 mm
Width: 80 mm
Wingspan: 145 mm