Systematic palaeontology

Class Theropoda Marsh, 1881

Order Maniraptora Gauthier, 1986

Suborder Troodontidae Gilmore, 1924

Eosinopteryx brevipenna gen. et sp. nov.

Etymology

Eo, Greek for daybreak, dawn; sino, Latin for Chinese; pteryx, Greek for feather, wing; brevi, Latin for short; penna, Latin for feather.

Holotype

YFGP (Yizhou Fossil & Geology Park)-T5197, a complete articulated skeleton with associated integumentary structures (Fig. 1; see Methods for provenance and authenticity of the holotype specimen).

Figure 1: Eosipnopteryx brevipenna YFGP-T5197. (a) Photograph and (b) line drawing. cav, caudal vertebrae, cev, cervical vertebrae; dv, dorsal vertebrae; fu, furcula; ga, gastralia; lf, left femur; lh, left humerus; lil, left ilium; lis, left ischium; lm, left manus, lp, left pes; lr, left radius; ls, left scapula; lt, left tibia; lu, left ulna; ma, mandible; rcor, right coracoid; rf, right femur; rh, right humerus; ril, right ilium; ris, right ischium; rm, right manus; rp, right pes; rr, right radius, rs, right scapula; rt, right tibia; ru, right ulna; sk, skull. Full size image

Locality and horizon

Yaolugou, Jianchang, western Liaoning, China; Middle-Late Jurassic Tiaojishan Formation1,2,3.

Diagnosis

A small paravian maniraptoran theropod that possesses the following osteological autapomorphies: a short snout, about 82% the length of the orbit; a lacrimal with a long posterior process participating in about half the length of the dorsal margin of the orbit and a vestigial anterior process; a short tail, composed of 20 caudal vertebrae, 2.7 times the length of the femur; chevrons reduced to small rod-like elements below the proximal 8th or 9th caudal; an ilium with a proportionally long, low and distally tapering postacetabular process (ratio ‘length/height at mid-length’=5), and pedal unguals shorter than corresponding penultimate phalanges. The plumage of this taxon is characterized by the absence of rectrices (versus other paravians with preserved plumage) and feathers on metatarsus and pes (versus other troodontids with preserved plumage on the hindlimb).

Description

The holotype of E. brevipenna is small, just 30 cm long (Fig. 1). The skull has a subtriangular lateral profile produced by a shallow snout and expanded postorbital region (Fig. 2). Unlike Anchiornis2, the snout of Eosinopteryx is distinctly shorter (about 82% of orbital length) than its orbit. Both the rostral plate and the maxillary process of the premaxilla are particularly short. The posterior process of the lacrimal is elongated and straight, participating in about half the length of the dorsal margin of the orbit, whereas the anterior process is vestigial. The postorbital process of the jugal is broader than in Anchiornis2, whereas the triradiate postorbital is larger than in Archaeopteryx4 and forms a complete postorbital bar with the jugal. There is no trace of a mandibular fenestra. The dentary has a groove that widens posteriorly and contains a row of foramina as in Archaeopteryx, Anchiornis, Xiaotingia, troodontids and some basal dromaeosaurids2,3. The teeth of Eosinopteryx are unserrated and sharp in contrast to the bulbous teeth of Xiaotingia3. As characteristic of troodontids5, the premaxillary and the dentary teeth in the symphyseal region are more closely packed than are the teeth in the posterior part of the dentary.

Figure 2: Selected skeletal elements and associated feathers of Eosipnopteryx brevipenna YFGP-T5197. (a) Photograph of skull and mandible in right lateral view. (b) Line drawing of skull and mandible in right lateral view. (c) Photograph of pelvis in right lateral view. (d) Line drawing of right ilium in lateral view. (e) Line drawing of right ischium in lateral view. (f) Photograph of right metatarsus and pes. (g) Line drawing of right metatarsus and pes. (h) Detail of secondary remiges. (i) Detail of plumulaceous feathers under the middle part of the tail. ac, acetabulum; lpm, left premaxilla; mt, metatarsal; obt, obturator process; postpr, posterodistal process; prac, preacetabular process; ra, right angular; rdt, right dentary; rect, right ectopterygoid; rfr, right frontal; rj, right jugal; rl, right lacrimal; rmx, right maxilla; rna, right nasal; rpa, right parietal; rpal, right palatine; rpm, right premaxilla; rpo, right postorbital; rq, right quadrate; rsq, right squamosal; rsa, right surangular; scl, scleral plate. Full size image

There are seven postaxial cervical vertebrae in YFGP-T5197. The cervical ribs are as long as their corresponding centra, contrasting with the shorter cervical ribs seen in Troodon formosus5 and the much longer ones of Archaeopteryx6. The trunk is about 32% the length of the hindlimb, as in Anchiornis2 and Mei7. The middle and posterior dorsals are proportionally elongate as in Anchiornis, dromaeosaurids and Archaeopteryx8. As in the latter, Anchiornis and basal deinonychosaurs, the dorsal centra do not have distinct pneumatic foramina just shallow depressions on their lateral surfaces3. Numerous, very slender, abdominal ribs are preserved in YFGP-T5197, as in Archaeopteryx6, dromaeosaurids9, the basal pygostylian bird Confuciusornis10 and many enantiornithines (for example, Sinornis). The tail of Eosinopteryx is particularly short, 2.7 times the length of the femur (contrasting with 3.2 in Mei7 and 3.9 in the Eichstätt Archaeopteryx6), and only composed of 20 caudal vertebrae (contrasting with 21–23 in Archaeopteryx6, ca. 30 in Anchiornis1 and 24–26 in Microraptor11). The anterior caudals are proportionally short, their transverse processes longer than corresponding centra and particularly narrow. Neural spines are developed only on the anteriormost third or fourth caudals. As in Anchiornis and Archaeopteryx2,3, the middle and posterior caudal vertebrae each bear a distinct groove on the lateral surface near the junction of the centrum and neural arch. The chevrons are reduced to small rod-like elements and are only present below the proximal eighth or ninth caudal centra, contrasting with the larger hook-like proximal chevrons of Anchiornis1 and with the vertically oriented rectangular proximal chevrons of Archaeopteryx6.

The subrectangular coracoid of Eosinopteryx bears a distinct subglenoid fossa and has a wider proximal end than Xiaotingia3 (Supplementary Fig. S1). The scapula (scapula/femur=0.49) is proportionally much shorter than in Anchiornis (0.68). The forelimb of this small paravian (0.73 times hindlimb length) is slightly shorter than that of Anchiornis (0.8) (ref. 2) and Archaeopteryx (0.9–1.0) (ref. 6) but is much longer than Mei (0.5) (ref. 7). The humerus of Eosinopteryx is also shorter (0.8 times) than its femur; the humerus is slightly longer than the femur in Anchiornis, distinctly longer (1.1–1.2) (ref. 6) in Archaeopteryx, and is only half the femur length in Mei. As in Anchiornis, the radius and ulna of Eosinopteryx are straight with only a narrow gap between them. The manus is slightly longer than the femur (manus/femur=1.2), contrasting with the short hand of Mei (0.8) (ref. 7) and with the proportionally more elongate hands of Anchiornis (1.55) (ref. 2) and Archaeopteryx (1.4–1.6) (ref. 6). Metacarpal II is about one third the length of metacarpal III; in contrast to Xiaotingia3, metacarpal IV is shorter and is much more slender than metacarpal III (Supplementary Fig. S2). The phalangeal portion of the manus is also proportionally shorter, relative to the metacarpus, than in Xiaotingia3. As in Anchiornis2, the long manual phalanx II-1 of Eosinopteryx is as robust as the radius; this element is much more gracile in Sinornithoides12 and Archaeopteryx6. Contrasting with Xiaotingia3, phalanx III-2 is shorter than metacarpal III, whereas the manual unguals of Eosinopteryx, sheathed by long keratinous claws, are less curved than in Archaeopteryx6, Anchiornis1 and Xiaotingia3.

The ilium of Eosinopteryx is characterized by a long, low and distally tapering postacetabular process, with a ratio ‘length/height at mid-length’ of 5 (Fig. 2). The ischium is particularly short, about 28% the femur length. With its distally displaced obturator process and its long and pointed posterodistal process, this element closely resembles that of Anchiornis1,2 (Fig. 2). The tibiotarsus of Eosinopteryx (1.4 × femur length) is proportionally shorter than Anchiornis (1.60), whereas pes (metatarsus+phalanges) proportions (1.3 × femur length) are similar to those of Mei7 and shorter than Anchiornis (1.56) (ref. 2). The proximal end of metatarsal III is transversely compressed, suggesting a subarctometatarsalian condition. Pedal digit I lies on the medioplantar side of metatarsal II (Fig. 2), as in Archaeopteryx6,13,14, but contrasts with the position of pedal digit I in Anchiornis, which is medial to metatarsal II1. The phalanges of pedal toes II, III and IV decrease gradually in length proximodistally, as in Archaeopteryx and terrestrial cursorial birds15. The pedal unguals of Eosinopteryx are much shorter than those of Archaeopteryx6, Anchiornis1,2, troodontids5 and dromaeosaurids9, even shorter than the corresponding penultimate phalanges.

As in Anchiornis2, the longest remiges of Eosinopteryx are at least 150% the length of the humerus (Fig. 2) and are symmetrical on the forelimbs. Long, densely packed and symmetrical pennaceous feathers are present along the posterior part of the thigh and crus, consistent in distribution with the tetrapterygian condition of several other basal paravian taxa2,16. However, careful preparation shows that feathers were absent from the metatarsus and pes of YFGP-T5197. Feathers covering other regions of the body (head, neck, thorax, back and tail) are plumulaceous and ‘rachis-like’ structures described on the plumulaceous feathers at the base of the tail and on the neck in Mei7 are absent in Eosinopteryx. The feathers on the tail are short, <20 mm long; all are plumulaceous, composed of a bundle of filaments that are joined together proximally and remain nearly parallel as they pass distally (Fig. 2). Pennaceous rectrices are absent.