| Liaoningvenator | |
|---|---|
 | |
| Life reconstruction with hand for scale | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Troodontidae |
| Genus: | †Liaoningvenator Shenet al., 2017 |
| Type species | |
| †Liaoningvenator curriei Shenet al., 2017 | |
Liaoningvenator (meaning "Liaoning hunter") is agenus oftroodontidtheropoddinosaur from theEarly Cretaceous ofChina. It contains a single species,L. curriei, named afterpaleontologistPhillip J. Currie in 2017 by Shen Cai-Zhi and colleagues from an articulated, nearly complete skeleton, one of the most complete troodontid specimens known. Shen and colleagues found indicative traits that placedLiaoningvenator within the Troodontidae. These traits included its numerous, small, and closely packed teeth, as well as the vertebrae towards the end of its tail having shallow grooves in place ofneural spines on their top surfaces.
Within the Troodontidae, the closest relative ofLiaoningvenator wasEosinopteryx, and it was also closely related toAnchiornis andXiaotingia; while these have traditionally been placed outside the Troodontidae, thephylogenetic analysis conducted by Shen and colleagues offered evidence supporting the alternative identification of theseparavians as troodontids. Compared to its close relatives, however,Liaoningvenator had relatively long legs, in particular thefemora. As the fifth troodontid known from geographically and temporally comparablestrata,Liaoningvenator increases the diversity of Chinese troodontids.
With a total body length (estimated lengths of the skull, neck, trunk, and tail combined) of approximately 69 cm (27 in),Liaoningvenator is a smalltroodontid.[1]
Liaoningvenator possesses a long, narrow, and triangular skull that measured 9.8 cm (3.9 in) long. At the front of the snout, likeSinovenator andXixiasaurus, thepremaxilla excludes themaxilla from the rim of the nostril. There are three openings on the surface of the maxilla, the premaxillary, maxillary, andantorbital fenestrae. Below, the maxilla forms thesecondary palate as inByronosaurus,Gobivenator, andXixiasaurus. Uniquely among troodontids, thepostorbital bone is slender and radiates into three processes. LikeZanabazar, there is apneumatic diverticulum in thejugal bone where an air sac was present within the bone; there is also a pneumatic opening on the rear side of thequadrate bone, as in other troodontids.[2] UnlikeSauronithoides,Zanabazar, andStenonychosaurus, the crest separating theparietal bones does not participate in the border of the supratemporal (upper) temporal fenestra at the back of the skull.[1]
Characteristic of troodontids,[2]Liaoningvenator has a pitted groove on the outer edge of its shallow and triangular lower jaw. The bottom margin of the jaw is slightly convex; inSinornithoides, it is straight. Thedentary andangular bones may have formed a flexible joint within the jaw - that is, an intramandibular joint.[3] UnlikeXiaotingia, the dentary and maxilla terminate at the same position in the jaw. Also like other troodontids (withSinusonasus being an exception),Liaoningvenator has a number of small, closely spaced teeth, with at least 15 in the upper jaw and 23 in the lower jaw. The teeth towards the rear of the lower jaw are serrated, unlike a number ofbasal troodontids. The tooth row of the maxilla terminates below the front margin of the antorbital fenestra, whereas it terminates further forward - below the rear of the maxillary fenestra - inJinfengopteryx.[1]
There are tencervical vertebrae, twelvedorsal (trunk) vertebrae, and at least sixteen caudal (tail) vertebrae inLiaoningvenator. Out of the cervicals, the third to eighth are elongated, with the fifth being the longest; among thearticular processes known as prezygapophyses, the fifth cervical also has the longest. In the third and fourth cervicals, the latter like otherderived troodontids, the prezygapophyses are longer than another set of processes known as the postzygapophyses. In the dorsals, the pneumatic pitting is simplified relative toAnchiornis. In the tail, the transition point - the point where the sides of the caudals become more compressed such that they are sub-triangular instead of rectangular - occurs at the seventh caudal, further forward thanSinornithoides andMei (where it occurs at the ninth).[4][5] The longest of the caudals is the fourteenth, which is almost twice the length of the sixth. On the underside of the caudals, thechevrons are slightly curved and directed backwards, as is seen inDeinonychus.[1]
Theacromion process on thescapula ofLiaoningvenator is poorly developed, as in basal troodontids. Unlike basal troodontids, however, theglenoid - the arm socket - is directed vertically downwards instead of to the side. On thehumerus, the deltopectoral crest extends for 40% of the bone's length, and terminates halfway down the bone. The humerus is shorter, relative to thefemur, inLiaoningvenator (59% length) than inEosinopteryx (80% length).[6] Further below, the three-digited hand is unique in that the firstphalanx of the first digit is longer than the secondmetacarpal, at 1.49 times the length of the latter.[1]
As inMei, the top of theilium has a curved sinusoidal shape inLiaoningvenator. There is no crest over theacetabulum (hip socket) ofLiaoningvenator, unlikeAnchiornis. The rear portion of the ilium (the postacetabular process) also has a shorter bottom edge than bothAnchiornis andEosinopteryx. Uniquely, there is no process on the top end at the tip of theischium, and the bone also has a slenderobturator process. Thepubis points forwards inLiaoningvenator, but backwards inMei.[5] Additionally, the hindlimb ofLiaoningvenator is twice the length of the torso, while inMei it is 2.8 times the length of the torso. UnlikeSinovenator,Liaoningvenator has afourth trochanter on its femur. Thetibia is slender and 1.4 times the length of the femur, likeSinornithoides. The four-digited foot is highly compacted, with atarsus that narrows toward the bottom. The thirdmetatarsal is offset from the second and fourth, forming a trough between the latter two that is deeper than in other troodontids. Proportionally, the first phalanx of the second digit is shorter relative to the second phalanx inLiaoningvenator (135% length) than inSinovenator (150% length).[1][7]
Liaoningvenator is known from a single specimen, a nearly complete and well-preserved skeleton with most bones preserved in their original articulated positions. It was found in the Lujiatun Beds of theYixian Formation in Shangyuan,Beipiao,Liaoning,China; currently, it is stored at the Dalian Natural History Museum (DNHM) inDalian, Liaoning under the accession number DNHM D3012. Some of the specimen's snout bones are incomplete and some toe phalanges have been added by illegal fossil traders; asides from this, it is one of the most complete troodontid fossils ever found. Its head is curved forward, and its limbs are tucked in; this differs from both the classicdeath pose (where the head is flexed backwards), as well as the sleeping posture ofMei andSinornithoides.[1]
In 2017, DNHM D3012 was named as thetype specimen of the new genus and speciesLiaoningvenator curriei by Shen Caizhi, Zhao Bo, Gao Chunling,Lü Junchang, and Martin Kundrát. The genus nameLiaoningvenator combines Liaoning with the suffix-venator, meaning "hunter" inLatin; the specific namecurriei honors the contributions ofCanadian paleontologistPhilip John Currie to the research of small theropods.[1]
Shen and colleagues identifiedLiaoningvenator as a member of the Troodontidae based on its numerous, closely spaced teeth that are constricted below thecrown; the pneumatic opening on the rear of its quadrate; the oval shape of itsforamen magnum; the replacement ofneural spines by shallow midline grooves in the vertebrae towards the end of its tail; the tall ascending process on itsastragalus; and its asymmetrical andsubarctometatarsal (i.e. where the third metatarsal is somewhat pinched by the neighboring metatarsals) foot.[2][8] They further placed it in the "higher troodontid clade" based on the lack of a bulbous capsule-like structure on theparasphenoid of itspalate, and the presence of the promaxillary fenestra on its skull.[1]
Based on aphylogenetic analysis modified from a prior analysis by Takanobu Tsuihiji and colleagues in 2016, which was in turn modified from the modification by Gao and colleagues in 2012 of an analysis byXu Xing and colleagues in 2012,[9] Shen and colleagues found thatLiaoningvenator formed a unified group, orclade, withEosinopteryx,Anchiornis, andXiaotingia, thus offering contrary evidence to the traditional placement of thesetaxa as non-troodontid members of theParaves. They are united by the teeth being flattened and recurved, with the crowns in the middle of the tooth row having heights smaller than twice their widths; the front edge of the acromion being outturned; the presence of a pronounced notch between the acromion and thecoracoid; the presence of a flange on the first phalanx of the second digit on the finger; and the backward-projecting pubis.[1]
Within this clade, which Shen and colleagues did not name,Liaoningvenator formed a group withEosinopteryx whileAnchiornis formed a group withXiaotingia. The former two are united by the lack of serrated teeth at the front of the jaw; the skull being more than 90% of the length of the femur; the cervical ribs having slender shafts and being longer than their corresponding vertebrae; and the front end of the ilium being "gently straight". Meanwhile, the latter two share the tips of the neural spines on the dorsals being fan-shaped; the coracoid being sub-triangular; the claw on the first digit of the hand being strongly arched, being higher than the top of the articulating surface; the presence of a "lip" at the top end of the claws on the second and third digits; the front edge of the pubic shaft being convex; and the claws on the third and fourth digits of the foot being strongly curved. The results of the phylogenetic analysis are reproduced in the belowphylogenetic tree.[1]
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Thin sections from thetibia of theholotype specimen ofLiaoningvenator indicate that thecortical bone is 1.5 mm (0.059 in) thick. The cortex is divided into four zones bylines of arrested growth (LAGs), which indicates that the animal was at least four years old when it died (further LAGs may have been obliterated by the expansion of themedullary cavity). Shen and colleagues suggested, based on the thinness of the innermost zone 1 compared to zone 2, that it had partially eroded away. However, the complete zone 3 is even thinner, being only a quarter of the width of zone 2, and zone 4 is even thinner (albeit incomplete). This indicates that growth had slowed substantially by the end of the third year.[1]
Each LAG is surrounded by two bands of dense avascular (i.e. lacking openings for blood vessels) bone, which Shen and colleagues termed the "pre-annular" and "post-annular" bands. The first LAG differs from the others in that it consists of two LAGs, one weaker than the other, indicating that growth mildly slowed before the resumption of bone growth. There is no external fundamental system (EFS) on the outer rim of the bone, indicating that the holotype was still growing at the time of death. However, the decreasing thinness of zones, the presence of avascular bone in the outer layers, and evidence ofbone remodeling collectively suggest that it was close to skeletal maturity.[1]
According to Shen and colleagues,Liaoningvenator is one of eleven troodontids known from China, and the fifth Early Cretaceous Chinese troodontid afterSinovenator,Sinusonasus,Mei, andJinfengopteryx.[1] In a separate 2017 publication for which Shen was also the lead author, an additional troodontid was described,Daliansaurus, which forms theSinovenatorinae with the former three.[10] With the exception ofJinfengopteryx, all of these troodontids lived in the Lujiatun Beds. While Shen and colleagues assigned the Lujiatun Beds to theHauterivian epoch, newer date estimates published by Chang Su-chin and colleagues suggested a younger age of ~126Ma for the Lujiatun Beds, which dates to theBarremian epoch.[11]
Contemporaneous dinosaurs included themicroraptorinedromaeosauridGraciliraptor; theoviraptorosaurIncisivosaurus; theornithomimosaursShenzhousaurus[12] andHexing;[13] theproceratosauridtyrannosauroidDilong;[12] thetitanosauriformsauropodEuhelopus;[14] theornithopodJeholosaurus; andceratopsians such as the ubiquitousPsittacosaurus[15] as well asLiaoceratops.[12] Mammals present includedAcristatherium,[16]Gobiconodon,Juchilestes,Maotherium,Meemannodon, andRepenomamus.[17][18][19] Othertetrapods included the frogsLiaobatrachus[20] andMesophryne;[21] and the lizardDalinghosaurus.[22] The Lujiatun Beds consist offluvial andvolcaniclastic deposits, indicating a landscape of rivers bearing volcanoes,[1] which may have killed the preserved animals bylahar.[19] Mean annual air temperatures in the region reached a minimum of 10 °C (50 °F).[23]
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