Systematic palaeontology

Dinosauria Owen 1842. Saurischia Seeley 1887. Theropoda Marsh 1881. Coelurosauria Huene 1914. Maniraptora Gauthier 1986. Dromaeosauridae Matthew and Brown 1922. Zhenyuanlong suni gen. et sp. nov.

Etymology

“Long”, from the Chinese Pinyin, means dragon. The generic and specific names are in honor of Mr. Zhenyuan Sun, who secured the specimen for study.

Holotype

A nearly complete skeleton with skull and lower jaws preserved (JPM-0008), curated at the Jinzhou Paleontlogical museum. It is likely a sub-adult, as neural arches and centra are not fused in some anterior dorsal vertebrae and the sacral vertebrae and the anterior sacral vertebrae are not completely fused to each other. The individual is fairly mature, however, as the more posterior sacrals are fused to each other and the neural arches and centra of the cervical vertebra, caudal vertebrae and some dorsal vertebrae are fused.

Type Locality and Horizon

Sihedang of Jianchang County, Liaoning Province; Yixian Formation16.

Diagnosis

Dromaeosaurid theropod with the following unique combination of characters, with autapomorphies among dromaeosaurids indicated with an asterisk and autapomorphies among Liaoning dromaeosaurids indicated with a double asterisk: extremely slender radius whose shaft is thinner than the shaft of manual phalanax I-1*; a shortened metacarpal II, which is shorter than the combined lengths of metacarpal I and manual phalanax I-1** (differing from Changyuraptor, Graciliraptor, Microraptor, Sinornithosaurus and Tianyuraptor, in which metacarpal II is longer than metacarpal I + phalanx I-1); six sacral vertebrae** (Microraptor, Sinornithosaurus and Tianyuraptor have five or fewer, although additional unfused sacrals may be present and unpreserved in the latter); a shortened forelimb that is approximately half the length of the hindlimb, with a humerus:femur ratio less of than 0.65, an ulna:femur ratio of less than 0.55 and a manus:femur ratio of less than 0.90 (these features are shared with only Tianyuraptor among Liaoning dromaeosaurids, although this may be due to allometry given their shared larger size than other Liaoning dromaeosaurids17 and additionally Mahakala among all dromaeosaurids); absence of a prominent lateral tubercle at the midpoint of the lateral surface of the pubic shaft (also absent in Tianyuraptor and non-Liaoning dromaeosaurids, but present in Changyuraptor, Microraptor and Sinornithosaurus9,10,18); absence of a distally placed dorsal process along the posterior edge of the ischial shaft (also absent in Tianyuraptor and most non-Liaoning dromaeosaurids, but present in Microraptor and Sinornithosaurus).

Zhenyuanlong differs further from Tianyuraptor in possessing an antorbital fossa bordered ventrally by a sharp rim (as in Sinornithosaurus); a pointed posterior end of the iliac postacetabular process (as in Sinornithosaurus, but lacking the lobate brevis shelf that projects beyond the end of the postacetabular lamina in Microraptor and Tianyuraptor); and a pubis with an anteriorly convex shaft (as in Changyuraptor, Microraptor and Sinornithosaurus, but not straight as in Tianyuraptor). Zhenyuanlong also lacks ossified uncinated processes, which are present in Tianyuraptor and Microraptor, But this is not a definitive diagnostic difference because some Liaoning theropods are known from multiple specimens, some of which preserve uncinate processes and others of which do not.

Limb ratios are also slightly different in Zhenyuanlong and Tianyuraptor: the forelimb is shorter relative to the hindlimb (ratio of 0.48 vs. 0.53) and the manus is shorter relative to the femur (ratio of 0.76 vs. 0.86) in Zhenyuanlong. However, given that only two specimens are being compared, these differences are of questionable diagnostic value, without more information on intraspecific variation and possibly variation caused by taphonomic distortion. We note that the difference forelimb:hindlimb ratio is within the range of intraspecific variation in two paravian taxa known from multiple specimens, Microraptor17 and Archaeopteryx19,20 and is therefore not likely to be a clearly distinguishing feature between Tianyuraptor and Zhanyuanlong.

Zhenyuanlong differs further from Microraptor in possessing a manual phalanx III-1 that is less than twice the length of III-2 (the Microraptor condition is also present in Sinornithosaurus) and an iliac preacetabular process with a pointed anterior end (as in Tianyuraptor, but unlike the strongly convex and lobate morphology of Microraptor). Furthermore, Zhenyuanlong lacks two autapomorphies of Microraptor: distal maxillary teeth with a pronounced constriction and very strongly recurved and slender pedal unguals with prominent flexor tubercles9,12.

Zhenyuanlong differs further from Sinornithosaurus in possessing a first premaxillary tooth considerably smaller than the second and third (not approximately equal in size as in Sinornithosaurus and Changyuraptor), more elongate middle-posterior caudal vertebral centra, which are three times the length or greater than the dorsal vertebrae (as is also the case in Microraptor and Tianyuraptor) and an iliac postacetabular process that is acuminate (as in Microraptor and Tianyuraptor), not squared off. Zhenyuanlong also lacks the heavily pitted antorbital fossa anterolateral to the antorbital fenestra that is autapomorphic for Sinornithosaurus21.

Zhenyuanlong differs further from Changyuraptor in possessing a second premaxillary tooth that is considerably larger than the third and fourth teeth (as in Sinornithosaurus); a transition point between the 7th and 10th caudal vertebrae (as in Microraptor and Tianyuraptor, not proximal to the 7th caudal vertebra as in Changyuraptor); a small semilunate carpal that covers approximately half of the bases of metacarpals I and II (as in Microraptor and Tianyuraptor, not the larger carpal that covers the entire proximal ends of metacarpals I and II in Changyuraptor); a straight metacarpal III (as in Graciliraptor and Sinornithosaurus, not the bowed morphology of Changyuraptor); a metatarsal IV that is approximately the same width as metatarsals II and III (as in Microraptor and Sinornithosaurus, unlike the proportionally broader metatarsal IV of Changyuraptor); and a metatarsal II that extends distally to the same level as metatarsal IV (as in Sinornithosaurus, but unlike the further distally extending metatarsal IV of Microraptor and Changyuraptor).

Zhenyuanlong differs further from Graciliraptor in that it lacks the extremely elongate and slender tibiotarsus that is autapomorphic of the latter taxon9,11.

Description

The type specimen of Zhenyuanlong is a large animal for a Liaoning dromaeosaurid, measuring 126.6 cm in total body length as preserved (Fig. 1). However, based on comparison with Tianyuraptor15, approximately half of the tail is considered missing, meaning that the individual would have been closer to 165 cm in total body length in life. Femur length is a convenient proxy for comparing the size of Liaoning dromaeosaurids, as all Liaoning taxa are known from at least one femur and femur length is a strong correlate with body mass22. The femur of Zhenyuanlong is 193.4 mm long, compared with 212.5 mm in Tianyulong15, 153 mm in Changyuraptor10, 109.8–148 mm in Sinornithosaurus13,23, 107.7 mm in Graciliraptor11 and 72–97 mm in Microraptor5,24. Therefore, Zhenyuanlong is larger than all Liaoning dromaeosaurids except for Tianyuraptor and is considerably larger than most Liaoning dromaeosaurid specimens.

The skull of Zhenyuanlong is generally well preserved, although the bones of the back portion of the cranium surrounding the lateral temporal fenestra and comprising the braincase are heavily crushed (Fig. 2). On the maxilla, the antorbital fossa is bordered ventrally by a sharp rim, as in Sinornithosaurus21 but differing from Tianyuraptor and most other dromaeosaurids. The lateral surface of the fossa underneath the antorbital fenestra is marked by a series of pits, as is characteristic for Liaoning dromaeosaurids25. However, the degree of sculpturing is weak, similar to Tianyuraptor and distinct from Sinornithosaurus, in which the pits are deeper, more numerous and extend across almost the entire maxillary antorbital fossa21. The promaxillary fenestra is huge, almost equal in size to the maxillary fenestra and located dorsally at approximately the same level as the maxillary fenestra (Fig. 2). A large promaxillary fenestra is also seen in other Liaoning dromaeosaurids such as Sinornithosaurus21 and Tianyuraptor15, but it is much smaller in most other dromaeosaurids, such as Velociraptor26 and Tsaagan27. Like Sinornithosaurus, but unusual for dromaeosaurids, the maxillary fenestra is not recessed into its own fossa9. The lateral lamina of the ascending ramus of the maxilla is extremely reduced to a small triangular exposure in lateral view, as in Sinornithosaurus and Microraptor, but unlike the much broader lamina in most other dromaeosaurids9.

Figure 2 The skull of the large-bodied, short-armed Liaoning dromaeosaurid Zhenyuanlong suni gen et. sp. nov. (JPM-0008). (A) photograph and (B) line drawing. Abbreviations: ang, angular; den, dentary; emf, external mandibular fenestra; en, external naris; fr, frontal; hy, hyoid; jug, jugal; lac, lacrimal; max, maxilla; mf, maxillary fenestra; nas, nasal; pa, parietal; pmf, promaxillary fenestra; pmx, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; sa, surangular; sp, splenial; tex, pitted texture on lateral surface of antorbital fossa. Full size image

The T-shaped lacrimal appears to have a pneumatic excavation on its lateral surface, where the anterior ramus and ventral ramus merge. The quadratojugal has a pronounced posterior process and a thin dorsal ramus. A pronounced lateral crest on the nasals and frontals forms the lateral edge of the skull roof. The frontals are elongate and the supratemporal fossa extends far anterior onto the dorsal surface of the bone. The parietals are broken but the preserved portions indicate that the left and right elements were fused in life, an unusual feature for dromaeosaurids that is also seen in Tianyuraptor and Sinornithosaurus9. There is a series of large foramina extending across the middle of the lateral surface of the dentary, but these are not set into a distinct groove. The external mandibular fenestra is large and is bordered by the dentary, angular and surangular. There is a small posterior surangular foramen immediately anteroventral to the glenoid. The splenial is widely exposed in lateral view and ventral to the posterior end of the lower jaw a long, thin hyoid is preserved.

The neck is complete and cervical neural arches are X-shaped in dorsal view, with a small centrally placed sheet-like neural spine and widely divergent zygapophyses. The dorsal vertebrae have anteroposteriorly elongate rectangular neural spines, which lack a swelling (‘spine table’) on their dorsal margins. This is also the case in Tianyuraptor and Microraptor, whereas many other dromaeosaurids have a swelling dorsally9. There are six vertebrae in the sacrum, four of which are clearly fused. A complex network of interwoven bony rods reinforces the tail, as is characteristic of dromaeosaurids8 and extends proximally nearly to the base of the tail. The middle caudal vertebrae are approximately three times as long as the dorsal vertebrae, similar to most other Liaoning dromaeosaurids but proportionally longer than in most other dromaeosaurids, in which these caudals are roughly twice as long as the dorsals9.

Both pectoral girdles and forelimbs are preserved in articulation (Fig. 3A,E). Ossified sternal plates are present, but the left and right elements are not fused to each other. The scapular blade is long and strap-like, does not expand distally and is straight as in Sinornithosaurus, not dorsoventrally curved as in most other dromaeosaurids9. Little can be said about the coracoid because of poor preservation. The most notable feature of the Zhenyuanlong forearm is that it is extremely short. The ratio of the entire forelimb (humerus, ulna, digit II) to the hindlimb (femur, tibia, digit III) is 0.48, which is the shortest of any Liaoning dromaeosaurid. In comparison, this ratio measures 0.53 in the short-armed Tianyuraptor and more than 0.80 in every other Liaoning dromaeosaurid specimen10,15. Similarly, the humerus of Zhenyuanlong is the shortest relative to the femur of not only any Liaoning dromaeosaurid, but any dromaeosaurid in general except for the very basal taxon Mahakala28 and the aberrant unenlagiine Austroraptor29. The humerus-to-femur ratio of Zhenyuanlong is 0.626, compared to 0.65 in Tianyuraptor and a ratio greater than 0.75 in all other Liaoning dromaeosaurids10,24.

Figure 3 The postcranium of the large-bodied, short-armed Liaoning dromaeosaurid Zhenyuanlong suni gen et. sp. nov. (JPM-0008). (A) right forearm; (B) distal hindlimbs; (C) thorax; (D) pelvis; (E) pectoral girdles. Full size image

The humerus bears a short deltopectoral crest, but other anatomical details are obscured by crushing. The ulna is robust and curved, with a small but marked olecranon process. One of the most distinctive features of Zhenyuanlong is its extremely slender radius, which is much thinner than the ulna and also thinner at mid-shaft than the shaft of manual phalanx I-1. The latter is a highly unusual feature among theropods, as such a thin radius relative to the first digit is not seen in any other dromaeosaurids and is only present in alvarezsauroids and a handful of basal coelurosaurs9. Zhenyuanlong is also unique among Liaoning dromaeosaurids in that the second metacarpal (55 mm) is shorter than the combined lengths of metacarpal I plus the first phalanx of digit I (60 mm).

The pelvic girdles and hindlimbs are essentially complete (Fig. 3B–D). The ilium is anteroposteriorly long and dorsoventrally shallow, with a preacetabular process that is slightly longer than the postacetabular process. The anterior end of the preacetabular process and the posterior edge of the postacetabular process are both pointed and there is no lobate brevis shelf that projects posteriorly beyond the lateral lamina of the postacetabular process as a discrete flange. The acetabulum is small but widely open medially and it is bordered dorsally by a prominent supracetabular crest and posteriorly by a large antitrochanter on the ischial peduncle of the ilium, both of which would have prevented the femur from splaying far laterally30.

The pubic shaft is anteriorly convex as in Changyuraptor, Microraptor and Sinornithosaurus, not straight as in Tianyuraptor9. There is no prominent lateral tubercle on the anterior margin of the pubic shaft. This tubercle, which causes the pubis to have a ‘kinked’ appearance, is present in most Liaoning dromaeosaurids and is often held to be a synapomorphy of a Liaoning dromaeosaurid clade9,18. It is also absent, however, in Tianyuraptor15. The ischium is much shorter than the pubis and lacks a distally placed process along the posterior edge of the shaft, which is also absent in Tianyuraptor but present in Microraptor and Sinornithosaurus9.

The femur (193.4 mm) is shorter than the tibiotarsus (260.3 mm). A similar ratio of tibiotarsus-to-femur length of approximately 1.30 is seen in most other Liaoning dromaeosaurids, except for the large four-winged Changyuraptor (1.10), but differs from the proportions of most other dromaeosaurids in which the tibiotarsus is relatively shorter compared to the femur24. The metatarsus is elongate, with the three central metatarsals bunched closely together. Large and curved claws cap the three central digits and digit I is present but reduced in size, attaching to metatarsal II far distally.

Feathers are present and well preserved on several portions of the body, particularly the arms and tail (Fig. 4). Large pennaceous feathers with a central rachis and barbs form broad wings on the forearms. The shape of the wings is unclear due to taphonomic distortion, but they were evidently quite large, as the preserved portion of the right wing is approximately 800 cm2 in area and the left wing is 1120 cm2. Fine details of the feathers are better seen on the right wing, where coverts, primaries and secondaries are visible (Fig. 4C–E). The size, presumed shape and feather architecture of the wings is generally similar to that of Microraptor5,6, Changyuraptor10, Anchiornis31, Eosinopteryx32 and basal avialans like Archaeopteryx20,33,34. The feathered Liaoning dromaeosaurid Sinornithosaurus, on the other hand, is known from a specimen that apparently had smaller wings comprised of more simple pennaceous feathers and not a more orderly sequence of coverts, primaries and secondaries23. However, the integument of this specimen is not well preserved and probably does not accurately reflect the wing shape of this species.

Figure 4 The integument of the large-bodied, short-armed Liaoning dromaeosaurid Zhenyuanlong suni gen et. sp. nov. (JPM-0008). (A) overview of the skeleton with regions of integument indicated with grey highlight; (B) proximal tail; (C) left forearm; (D) right forearm; (E) closeup of coverts on right forearm. Full size image

On the right forearm wing of Zhenyuanlong, a series of approximately 30 small feathers (approximately 25–35 mm long) are preserved attaching to the ulna and metacarpal III. These are the major coverts, the outer layer of feathers that cover the primaries and secondaries dorsally. Most are oriented approximately perpendicular to the ulna, but the most distal ones that connect to metacarpal III are oriented obliquely, such that they approximately parallel the long axis of the hand. It is not clear whether these coverts are complete or broken at their distal ends. However, the fact that nearly all of them are the same length and consistently preserved as broad carbonized impressions, unlike the more posterior wing feathers that are represented almost entirely by faint impressions of the rhachides, suggests that their size and shape are genuine. If so, these coverts are short as in modern birds, not elongate and covering much of the wing as has been hypothesized for the basal avialan Archaeopteryx and the close avialan outgroup Anchiornis30 (but see Foth et al.20 for an alternative view).

Pennaceous primary and secondary remiges are also identifiable, but are not as well preserved as the coverts. These feathers evidently comprised most of the surface area of the wing and extend far posterior to the coverts. Counting these feathers is difficult, but there appears to be approximately 10 primaries and 20 secondaries. It is also difficult to measure the lengths of these feathers, both because of poor preservation and because both wings are broken posteriorly. It is clear, however, that both the primaries and secondaries are more than twice the length of the humerus, as in Microraptor5,6 and modern birds, but unlike the shorter primaries and secondaries of the close avialan outgroups Anchiornis and Eosinopteryx, which are approximately 1.5 times the length of the humerus31,32. On the better preserved right wing of Zhenyuanlong the primaries are longer than the secondaries. The secondaries are oriented approximately perpendicular to the ulna whereas the primaries are positioned at an acute angle to the manus, which becomes less acute and approaches 90 degrees as the feathers progress proximally up the arm. A very similar arrangement is also seen in Microraptor5. Some primaries and secondaries appear to have an asymmetric shape, but preservation makes this difficult to assess for most of the feathers. There is no sign of any flight feathers attaching to manual digit I to form the alula of modern birds, many Cretaceous birds and perhaps Microraptor5.

Large pennaceous feathers are also present on the tail of Zhenyuanlong (Fig. 4B). These are preserved only dorsal to the tail. The rhachides are preserved as a series of thin (ca. 1 mm thick), long (up to 8 mm), straight lineations that project approximately 45 degrees posterodorsally from the long axis of the tail. The preservation of the remainder of the feathers is too poor to determine whether they were symmetrical or asymmetrical. The size and orientation of these feathers is similar to the condition in many coelurosaurs bracketing the dinosaur-bird transition, including Sinornithosaurus23, Jinfengopteryx35, Anchiornis31, Eosinopteryx32 and likely Archaeopteryx20. However, the rhachides in the proximal portion of the tail of Zhenyuanlong appear to be longer than those of Microraptor5,6 and Changyuraptor10, which had relatively small and simple feathers proximally on the tail but much larger and more complex feathers distally, which expanded outwards into a large fan. Because the distal tail is not preserved in Zhenyuanlong, the presence or absence of this fan cannot be assessed.

Small regions of feathery integument are also visible above the back and in front of the neck (Fig. 4C). These regions are very poorly preserved but do contain small, simple filaments that are less than one millimeter thick and at most 30 millimeters long. It is uncertain whether these are simple non-shafted feathers or if they had a more complex series of filaments branching laterally from a small central rachis, as has been observed in the body (non-wing, leg, or tail) feathers of Sinornithosaurus21 and Changyuraptor10, among other non-avialan dinosaurs36.

There is no sign of any feathers on the legs of Zhenyuanlong. Given that feathers are preserved across wide portions of the remainder of the specimen, this may indicate a genuine lack of hindlimb feathers in this new taxon. If so, this would immediately distinguish Zhenyuanlong from a growing collection of early avialans and close dinosaurian relatives that had extensive, pennaceous and often asymmetrical feathers on the hindlimbs, including Microraptor5, Changyuraptor10, Pedopenna37, Anchiornis31, Eosinopteryx32 and Xiaotingia38 among non-avialan dinosaurs and Archaeopteryx20,34 and Sapeornis39 among early avialans. However, it must be noted that the hindlimbs are part of the second major piece of the slab and no feathers are visible on this piece, whereas numerous feathers are seen on pieces one and three. The lack of feathers on the second piece could be an artifact of taphonomy or have been erased during preparation. Therefore, it cannot be totally ruled out that hindlimb feathers were present in life.

Phylogenetic Analysis

A phylogenetic analysis recovers the six Liaoning dromaeosaurids (Changyuraptor, Graciliraptor, Microraptor, Sinornithosaurus, Tianyuraptor, Zhenyuanlong) in a basal polytomy with the clade of dromaeosaurines and velociraptorines (Fig. 5). This demonstrates that Zhenyuanlong is a dromaeosaurid and is more closely related to other Liaoning dromaeosaurids and the Laurasian dromaeosaurines and velociraptorines than it is to Mahakala and the unenlagiines. However, this topology is considerably less resolved than recovered by earlier versions of this analysis that did not include Zhenyuanlong, in which the Liaoning dromaeosaurids are placed in a clade (Microraptorinae) that is sister taxon to Dromaeosaurinae + Velociraptorinae9. Enforcing the six Liaoning dromaeosaurids to form a clade results in MPTs one step longer than the shortest trees in the unconstrained analysis. Furthermore, it is noteworthy that Zhenyuanlong and Tianyuraptor do not form a clade, despite their shared possession of a unique short-armed bauplan.