Cruralispennia

Skull and vertebrae

Although most of the skull is poorly preserved, several elements are identifiable. The premaxilla is short and tall, while the rear part of the frontal is domed. The left dentary preserves fourteen teeth, a characteristically high number compared to other enantiornitheans. Although none of the teeth preserve complete crowns, there is a visible trend of the teeth becoming more buccolingually (side-to-side) flattened towards the back of the jaw.^[1]

The spine is incomplete, but several articulated thoracic vertebrae preserve a lateral groove, characteristic of enantiornitheans. The fused pygostyle of IVPP 21711 is characteristically short and broad, tapering distally into an upturned tip. This is entirely unlike the long rod- or spear-like pygostyles known in other enantiornitheans, as well as confuciusornithids and sapeornithids. Instead, its pygostyle is most similar to the plow-shaped pygostyles of ornithuromorphs. In modern birds, this form of pygostyle attaches to tail muscles which control the advanced pennaceous rectrices of their fan-shaped tails. However, its presence in IVPP 21711, which has simple, hair-like tail feathers, throws doubt on the hypothesis that a plow-shaped pygostyle always correlates to an advanced, fan-shaped tail.^[1]

Forelimbs

Each coracoid of IVPP 21711 is slender, similar to those of advanced enantiornitheans. The thinnest part of the coracoid is the third which is furthest away from the sternum, while the widest is the half nearest the sternum. However, the edges of the coracoid are almost parallel near the sternum, unlike the coracoids of other enantiornitheans, which drastically splay outwards near the sternum, forming concave outer margins.^[1]

The front of sternum has a pointed rostral spine, which is known in a few other enantiornitheans. The rear edge of the sternum has two pairs of thin rod-like structures. Although these structures are known in all but the most basal enantiornitheans, only IVPP 21711 has the inner pair extend the same distance as the outer pair. On the other hand, the V-shaped xiphoid region is similar to basal enantiornitheans such as Protopteryx and pengornithids. The forelimb is rather short, with a robust humerus possessing enantiornithean features. The hand is shorter than the humerus and the alular digit is reduced, both features characteristic of advanced enantiornitheans.^[1]

Hind limbs

The postacetabular process of the ilium is characteristically short and tapering, and extends further downward than the ischiadic peduncle. The ischium is shorter than the pubis and has an upward pointing process midway down the shaft, similar to that of basal ornithuromorphs. There is no pubic foot, as in enantiornitheans. The tibiotarsus and tarsometatarsus are long and slender, but the fibula is short and tapering (as in advanced enantiornitheans and ornithuromorphs). The toe claws are sharp and curved.^[1]

Feathers

Well-preserved feathers surround every part of the skeleton except the snout and feet. The body and tail feathers are short, hair-like, and do not have rachises. The feathers on the upper side of the neck are longer than those on the lower side. Asymmetrical pennaceous feathers are preserved attached to the wings, although they are shorter than in other enantiornitheans, being only twice the length of the hand.^[1]

The most characteristic features of IVPP 21711, which the genus Cruralispennia was named after, are its crural (leg) feathers. These feathers are present on the tibiotarsus and possibly the femur, although overlap from body feathers makes the femur's integument uncertain. They are long and wire-like, consisting of a rachis-like structure seemingly formed from the fusion of a bundle of parallel barbs. This rachis-like structure forms 90% of the length of the feather. In the distal 10% of the feather's length, the individual parallel barbs become distinguishable, forming a brush-like tip. This form of feather morphology, dubbed "proximally wire-like part with a short filamentous distal tip (PWFDT)" , is also present in feathers projecting along the front edge of the left wing. PWFDT feathers are currently only known in Cruralispennia, although they are similar to the ribbon-like tail feathers of the oviraptorosaur Similicaudipteryx. It is probable that PWFDT feathers are a result of a genetic overexpression of the BMP4 gene (which promotes barb fusion and rachis formation) or a suppression of the Sonic hedgehog gene (which promotes the formation of individual barbs).^[3]

Another possibility is that the PWFDT feathers are not a novel type of feather unique to Cruralispennia, but instead are immature pennaceous feathers formed after a recent molt. There are several lines of reasoning supporting this hypothesis. With a long, thick rachis and minimal barbs at the tip, the PWFDT feathers resemble moderately developed pin feathers, which are known from modern birds as well as newly described juvenile enantiornitheans. In addition, they are interspersed with mature feathers, while truly unique feather types are expected to be separated from typical feathers. Many of the specimen's feathers are loosely attached or separated from the rest of the specimen, adding further evidence to the hypothesis that it was molting when it died.^[2]

Color

Structures believed to be fossilized melanosomes were found in five feather samples from the specimen using scanning electron microscopy.^[1] Due to their rod-like shape, they were identified as eumelanosomes, which correspond to dark shades. Although specific colors were not stated in the analysis, other studies have shown that coloration in extant birds correlates to the length and aspect ratio (length to width ratio) of their eumelanosomes.^[4] A sample taken from the crural feathers had eumelanosomes with the shortest aspect ratio, which may have corresponded to dark brown coloration. The highest aspect ratio eumelanosomes were found in a sample from the head feathers. High aspect ratios have been known to correlate with glossy or iridescent colors, although without knowing the structure of a feather's keratin layer (which does not fossilize well), no hue can be assigned for certain.^[4] The wing and tail samples also had high aspect ratios, while the tail's eumelanosomes were the largest sampled.^[1]