Limusaurus

Chapter • Meet the dinosaur

Overview

Limusaurus inextricabilis was a small, lightly built theropod from the end of the Jurassic in north-western China. It measured less than two metres from snout to tail tip and weighed roughly as much as a medium-sized dog. Its body was dominated by a long neck, extremely long and slender hind legs and a stiff, outstretched tail that balanced the animal in a fully horizontal posture. The head was small and shallow, and in adults the jaws were completely toothless, covered instead by a horny, beak-like sheath. The arms were strongly reduced and the innermost finger was almost lost — a feature that made Limusaurus a key piece of evidence in the debate over how the three-fingered bird hand evolved. As a ceratosaur, Limusaurus was not closely related to birds, but feather-like body coverings are known across several theropod groups, so a simple downy or bristle-like covering cannot be ruled out. No skin or feather impressions have actually been preserved with Limusaurus. The most remarkable thing about Limusaurus is that it swapped teeth for a beak during its own lifetime. A study of 19 individuals of different ages showed that the smallest hatchlings had small, pointed teeth in both jaws, that the teeth were progressively shed during growth, and that animals older than about one year were entirely toothless. No other known reptile shows such complete ontogenetic tooth loss. The researchers interpret this as a dietary shift with age: juveniles probably ate insects and other small prey, or a mixed omnivorous diet, while adults turned to plants. Support for adult herbivory comes from gastroliths — clusters of small, polished stomach stones found in the abdominal region of several specimens. Such stones help grind tough plant material inside a muscular gizzard, exactly as in many living birds. The long legs suggest that Limusaurus was a fast, agile runner that browsed on low vegetation along rivers and lakes and relied on speed to escape larger predators. The species was described in 2009 by Xu Xing and colleagues, based on fossils from the Shishugou Formation of the Junggar Basin in Xinjiang, China. The name means roughly "mire lizard that could not extricate itself": the individuals were found stacked on top of one another in funnel-shaped mud pits, probably collapsed mudholes or structures created by the footfalls of large sauropods, in which the animals became trapped and died. These extraordinary finds yielded an unusually complete growth series, which in 2017 allowed Shuo Wang and co-authors to document the tooth loss in the journal Current Biology.

Chapter • Where it lived

Habitat

Limusaurus lived about 161–158 million years ago, during the Oxfordian stage of the Late Jurassic, in what is now the Junggar Basin of Xinjiang, north-western China. All specimens come from the upper part of the Shishugou Formation, a landscape of floodplains, meandering rivers, lakes and wetlands vegetated with conifers, ginkgophytes, cycads and ferns. Volcanic ash and seasonally waterlogged ground were common, and it is precisely in such soft, funnel-shaped mud traps that the Limusaurus individuals were found piled on top of each other. The same fossil site has produced several other dinosaurs, including the early tyrannosauroid Guanlong and long-necked mamenchisaurid relatives.

Chapter • Wild behavior

Behavior & Ecology

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Chapter • Scientific place

Taxonomy