Limusaurus

Limusaurus is an herbivorous toothless abelisauroid dinosaur from the Jurassic Upper Shishugou Formation in the Junggar Basin of western China. Limusaurus is also the first definitely known ceratosaur from Eastern Asia, including China.^[1]^[2]^[3]^[4]^[5]

A 2016 study determined that the young of this dinosaur had teeth that they lost as they grew. This could further eloborate that how certain species of dinosaur lost their teeth when they evolved into birds.

Description[]

Limusaurus inextricabilis (‘mire lizard who could not escape’) is a noasaurid theropod dinosaur that lived during the Upper Jurassic (Oxfordian) in what is now northwestern China. It was found in 159 million-year-old deposits located in the Junggar Basin of Xinjiang and earned its name from the way its skeletons were preserved, stacked on top of each other in fossilized mire pits.

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 those of the Cretaceous ornithomimid theropods, 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.

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.

The research team, led by Dr. Xing Xu from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and Dr. Shuo Wang of Capital Normal University in Beijing, studied 19 skeletons of Limusaurus inextricabilis. The individuals ranged in age from baby to adult, showing the pattern of tooth loss over time. The baby skeleton had small, sharp teeth, and the adult skeletons were consistently toothless. “We analyzed 19 specimens of Limusaurus inextricabilis, representing six ontogenetic stages based on body size and histological data,” the authors explained. “Among 78 ontogenetic changes we identified in these specimens, the most unexpected one is the change from fully toothed jaws in the hatchling and juvenile individuals to a completely toothless beaked jaw in the more mature individuals, representing the first fossil record of ontogenetic edentulism among the jawed vertebrates.”

Significance[]

“This discovery is important for two reasons,” said co-author Prof. James Clark, from the George Washington University’s Columbian College of Arts and Sciences. “First, it’s very rare to find a growth series from baby to adult dinosaurs. Second, this unusually dramatic change in anatomy suggests there was a big shift in Limusaurus’ diet from adolescence to adulthood.” “For most dinosaur species we have few specimens and a very incomplete understanding of their developmental biology,” said co-author Josef Stiegler, a graduate student at George Washington University.

“The large sample size of Limusaurus inextricabilis allowed us to use several lines of evidence including the morphology, microstructure and stable isotopic composition of the fossil bones to understand developmental and dietary changes in this animal.” Limusaurus inextricabilis is part of the theropod group of dinosaurs, the evolutionary ancestors of birds. The team’s earlier research of Limusaurus inextricabilis described the species’ hand development and notes that the dinosaur’s reduced first finger may have been transitional and that later theropods lost the first and fifth fingers. Similarly, bird hands consist of the equivalent of a human’s second, third and fourth fingers.

The fossils indicate that baby Limusaurus inextricabilis could have been carnivores or omnivores while the adults were herbivores, as they would have needed teeth to chew meat but not plants. The fossils also could help to show how theropods such as birds lost their teeth, initially through changes during their development from babies to adults. “Tooth loss isn’t so unusual in animals alive today. There are fish and an amphibian that lose teeth as they grow. Platypuses lose their teeth too,” Dr. Wang said. “But the discovery is still a first for the fossil record and for reptiles.”

History[]

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.

Museum exhibit of two skeletons in the original position in which they were found Holotype and assigned specimen (upper right) exhibited in Tokyo 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".

Other Wikis[]

https://prehistoric-wiki.fandom.com/wiki/Limusaurus

References[]

Gallery[]

[1] ttps://blog.everythingdinosaur.co.uk/blog/_archives/2016/12/24/limusaurus-dinosaur-species-lost-teeth-as-it-grew-up.html

[2] ttps://www.eurekalert.org/news-releases/677524

[3] ttps://indianexpress.com/article/technology/science/newly-discovered-dinoasaur-species-explains-why-birds-are-toothless-4442871/

[4] ttps://www.livescience.com/57300-dinosaur-lost-teeth-as-it-aged.html

[5] ttps://www.cbc.ca/news/science/toothless-dinosaur-birds-1.3914277

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