Limusaurus is a genus of theropod dinosaur that lived in what is now China during the Late Jurassic, around 161 to 157 million years ago. The type and only species Limusaurus inextricabilis was described in 2009 from specimens found in the Upper Shishugou Formation in the Junggar Basin of China. The genus name consists of the Latin words for "mud" and "lizard", and the species name means "impossible to extricate", both referring to these specimens possibly dying after being mired. Limusaurus was a small, slender animal, about in length and in weight, which had a long neck and legs but very small forelimbs (with reduced first and fourth fingers). It underwent a drastic morphological transformation as it aged: while juveniles were toothed, these teeth were completely lost and replaced by a beak with age. Several of these features were convergently similar to the later ornithomimid theropods as well as the earlier non-dinosaurian shuvosaurids.
Limusaurus was the first known member of the group Ceratosauria from Asia. It belonged to the Noasauridae, a family of small and lightly built ceratosaurs, along with its closest relative Elaphrosaurus. The pattern of digit reduction in Limusaurus has been used to support the hypothesis that the three-fingered hand of tetanuran theropods is the result of the loss of the first and fifth digits from the ancestral five-fingered theropod hand, a contested hypothesis which is relevant to the evolution of birds. The change to toothlessness in adults probably corresponded to a dietary shift from omnivory to herbivory, which is confirmed by the presence of gastroliths (stomach stones) in adults. Since many specimens were found together, it is possible Limusaurus lived in groups. Its fossils were discovered in rocks dated to the Oxfordian age. Specimens of Limusaurus (along with other small animals) appear to have been mired in mud pits created by the footprints of giant sauropod dinosaurs.
Discovery and naming
Between 2001 and 2006, a Chinese-American team of paleontologists examining the Wucaiwan locality in the Shishugou Formation, in the northeastern Junggar Basin of Xinjiang, China, discovered three bone beds (numbered TBB 2001, TBB 2002, and TBB 2005, found by T. Yu and J. Mo). The bone beds were dominated by the remains of small theropod dinosaurs, representing at least three genera, with most belonging to a small ceratosaur, the first member of this group found in Asia. Stacked skeletons from these bone beds were removed from the field in blocks, jacketed by plaster, and encased in crates. A resin cast of block TBB 2001 was made, making it available for study after the specimens had been extracted from the original matrix. Except one, all specimens from this block are mounted in a cast of the block in its semi-prepared state.
In 2009, the small ceratosaur was described by paleontologist Xu Xing and colleagues, who named it Limusaurus inextricabilis. The genus name consists of the words limus, Latin for "mud" or "mire", and saurus, Greek for "lizard", and the species name means "impossible to extricate"; both names refer to how these specimens appear to have died after being mired. The name has also been translated as "mire lizard who could not escape". The description incorporated data from two specimens both stored at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing: the holotype (a subadult cataloged under the specimen number IVPP V 15923) is an almost complete and (still connected) skeleton, missing only the hindmost tail vertebrae, and is preserved next to another specimen (a large juvenile, IVPP V 20098) which is missing the front part of the skeleton. The other (an adult, IVPP V 15304, inaccurately referred to by the number IVPP V 16134 in the original description) is a likewise almost complete and articulated specimen that is missing only the skull, and is larger than the holotype.
Seventeen additional Limusaurus specimens were described by paleontologist Shuo Wang and colleagues in 2017, excavated from the same blocks as those described in 2009. These specimens include six juveniles (one year in age or less), ten subadults (two to six years in age), and one adult (more than six years in age). These specimens are also stored at the IVPP. The toothless adults and toothed juveniles were initially thought to be different kinds of dinosaurs, and were studied separately, until it was realized they represent the same species.
Description
Limusaurus was a small and slender animal. The holotype (which was originally considered an adult based on the level of fusion of its bones, but later as a subadult when analyzed along with other specimens) is estimated to have been about in length and the weight of the animal has been estimated at . One adult specimen is estimated to have been 15% larger than the holotype. Several features of the animal, such as the small head with large orbits (eye openings), toothless jaws, and the long neck and legs, were very similar to the ornithomimid theropods of the Cretaceous, as well as the Triassic non-dinosaurian shuvosaurids, representing a significant case of convergent evolution among these three distinct groups of archosaurs. While Limusaurus has sometimes been depicted with feathers and may have had them, there is no direct evidence of such structures.
Skull
The skull of Limusaurus was relatively tall and short, roughly half the length of the femur (upper thigh bone). The tip of its jaws was covered by a beak, a feature that was previously unknown in non-coelurosaurian theropods like Limusaurus (the coelurosaurs include the most bird-like dinosaurs). As in most dinosaurs, the skull featured five principal fenestrae (openings): the (bony nostril), orbit, antorbital fenestra (between the nostril and eye), as well as the and (on the top and on the side of the skull's rear, respectively). As in other ceratosaurians, parts of the bony nostril were formed by the maxilla (upper jaw bone); also, the antorbital fenestra was proportionally small, and the rear part of the nasal bone formed parts of the cavity which contained this opening. The external naris was large and located in a hindwards position, similar to tetanuran theropods. The orbit was large, while the lateral temporal fenestra was not as large as would be expected from more derived (or "advanced") members of the Ceratosauria. Uniquely to Limusaurus, the inner bottom edge of the premaxilla, the frontmost bone of the upper jaw, was convex. The nasal bone was distinct in having a "shelf" on its side, was short, wide, less than one-third of the length of the , and twice as long as it was wide. The lower part of the lacrimal, the bone that formed the front margin of the eye opening, was unique in being strongly inclined forwards. The jugal bone, which formed the floor of the eye opening, was slender, and its rami (or branches) were rod-like, which is also unique to this genus.
The lower jaw of ceratosaurians was pierced by a generally large . In Limusaurus, it was especially large, accounting for 40% of the length of the entire lower jaw, a distinguishing feature of the genus. The (tooth-bearing bone at the front of the lower jaw) was short compared to the rest of the lower jaw, as in other ceratosaurians. The front end of the dentary was down-turned and had a convex inner margin, similar to the related Masiakasaurus. The angular bone of the lower jaw was positioned significantly forwards in relation to the hind end of the mandible, similar to other ceratosaurians. Juveniles had nine teeth in each side of the upper jaw and twelve in each side of the lower; they were gradually lost as they grew, disappearing by adulthood.
Postcranial skeleton
The (neck vertebrae) of Limusaurus were elongated as in the closely related Elaphrosaurus, and the neck itself was long. The axis (second neck vertebra) lacked the (excavation) on its front end and the foramina (openings) in its neural arch that are seen in derived ceratosaurians. As in other noasaurids, the neural spines of the cervicals were positioned more towards the front end of their vertebrae than is the case in other theropods.
Distinctively, the scapula (shoulder blade) bore a prominent ridge at its front edge. It also had a comparatively high . The sternum was fused into a single, large, continuous plate, another feature that evolved independently in coelurosaurs (convergent evolution). Limusaurus also had a furcula, or wishbone, which previously was unknown among ceratosaurians. The head of the humerus (upper arm bone) was bulging, and the , a forward-directed bony flange of the humerus that served for muscle attachment, was long and angled; these features were typical for ceratosaurians. In the forearm, the radius was longer than the ulna, and the , a bony extension on the upper end of the ulna that served for muscle attachment, was absent in Limusaurus. Both features are considered distinctive features of the genus. As in other ceratosaurians, ossified wrist bones were absent.
As is typical for ceratosaurs, the arms and hands of Limusaurus were considerably reduced, even more so than in Ceratosaurus. Limusaurus had three fingers (the middle three), as compared to the five fingers of more basal relatives; it was unique in that the first finger was missing entirely, and the first metacarpal was shorter than the other metacarpals. The second metacarpal was more robust than the other metacarpals, which is another distinctive feature of the genus. The second finger had three phalanges (finger bones). The third finger also only had three phalanges, as opposed to four in other early theropods. Although the fourth finger is not preserved, the tip of the fourth metacarpal indicates the presence of a joint and therefore the presence of a phalanx; it is likely that this was the only phalanx of the fourth finger. The (claw bones) of the fingers were short, stout, and expanded at their base. They had two grooves on their sides, a feature also found in Masiakasaurus.
Among the pelvic bones, the ilium was small and tilted towards the midline of the body, as was the case in Elaphrosaurus. As in other ceratosaurians, the lower end ("boot") of the pubis was large and expanded. Unique to the genus, it pointed backwards in a hook-like shape and had a ridge on each side. The elongated legs of Limusaurus had proportions that were well-adapted to running, with their lower segments much longer than the femur: the tibiotarsus, the fusion of the tibia (shin bone) and tarsal bones, was 1.2 times the length of the femur, and the foot was 1.3 times the length of the femur. The legs were 1.8 times the length of the torso. The upper half of the femur was triangular in cross section, a feature shared with Masiakasaurus. The metatarsals of the three weight-bearing toes were arranged in an arc, with the fourth metatarsal straight and adhering tightly to the third for its entire length; these features are unique to Limusaurus. The hallux (the first toe or dewclaw) was reduced, being only 17% the length of the third metatarsal, another unique feature. As in other ceratosaurians, the unguals of the foot had two grooves on their sides.
Classification and evolution Limusaurus was classified as a basal member of Ceratosauria by Xu and colleagues in 2009 (who also considered the closely related Elaphrosaurus as such). It had several skull features in common with basal theropods such as other ceratosaurs and coelophysoids, but it also shared a number of traits, including the beak and the fused sternum, convergently with the later coelurosaurs. A 2012 study by paleontologists Diego Pol and Oliver Rauhut also found Limusaurus and Elaphrosaurus to be basal ceratosaurians in their phylogenetic analysis, while a 2010 study by paleontologist Martin Ezcurra and colleagues placed them in the more derived group Abelisauroidea within Ceratosauria.