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 1.7 m (5 ft 7 in) in length and 15 kg (33 lb) 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 articulated (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 1.7 m (5 ft 7 in) in length and the weight of the animal has been estimated at 15 kg (33 lb). 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
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