page_title: Limusaurus
section_title: Growth
hierarchical_section_title: Limusaurus / Paleobiology / Growth
caption_attribution_description: Left lateral side of the skull of Limusaurus (IVPP 20093 V; Institute of Vertebrate Paleontology and Paleoanthropology, Beijin, China) (scales bar: 5 cm).
context_page_description: 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 genus and species Limusaurus inextricabilis was described in 2009 from specimens found in the Upper Shishugou Formation in the Junggar Basin. 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 in length and 15 kg in weight, which had a long neck and legs but very small forelimbs. 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. Along with its closest relative, Elaphrosaurus, it was a member of the family Noasauridae, a group of small and lightly built abelisaurs.
context_section_description: Wang and colleagues analyzed growth rings (visible in bone cross-sections and analogous to the growth rings of trees) of the tibiae from the various developmental stages of Limusaurus in 2017, and found that the animal was skeletally mature at six years of age. In older specimens, the outermost growth rings are very close together (forming what is known as the external fundamental system), indicating that rapid growth had ceased in these individuals. Specimens of Limusaurus show 78 different anatomical changes that occurred as the animals grew. In particular, their heads became proportionally shallower, their middle hand bones lengthened, and the "hook" of their pubis grew longer. The shaft of the quadrate bone in the skull also straightened in adults, and the tips of their lower jaws became more downturned.
  The most obvious change that happened during the growth of Limusaurus was the complete loss of teeth from juveniles to adults. Juveniles began with one tooth in the premaxilla, eight in the maxilla, and eleven in the lower jaw (42 teeth in total). At the next stage, the first, sixth, and eighth teeth in the maxilla, as well as the sixth in the lower jaw had all been lost (34 teeth in total), although the sockets were still present, and there was a small replacement tooth in the socket of the sixth lower tooth. During this stage, use of teeth and normal tooth replacement likely ceased or became reduced, since none of the still-erupted teeth bear any wear marks or resorption. As the specimens grew, the transformation became more radical. In subadult and adult specimens older than one year, all the teeth were missing. CT scanning shows that only five empty tooth sockets remained in the adult maxilla; all the sockets in the lower jaw were fused into a single, hollow canal, and the rest of the tooth sockets were obliterated.
  The loss of teeth with age in Limusaurus is the most extreme case of tooth morphology changing with age recorded among dinosaurs. Limusaurus is one of the only known jawed vertebrates where teeth are completely lost during growth. The other known examples are the red mullet and striped red mullet, several armored catfish, and the platypus. Its complicated pattern of losing teeth from both the front and the back is most similar to that of the avialan Jeholornis. The early halt in tooth replacement possibly resulted from the regression of the replacement tooth buds during the first year, as in the veiled chameleon. The replacement of teeth by a beak during growth in Limusaurus suggests that beaks in other lineages of theropods, and indeed beaked animals in general, may have evolved heterochronically, i.e. that beaks first occurred in adults and then gradually extended to juveniles. This is in accordance with genetic signal pathways (molecular processes that control the formation of teeth) still being present in birds.
caption: Skull of juvenile specimen IVPP 20093 V; the specimen has teeth, which were lost in adults
exif: {"Image Orientation": "Horizontal (normal)", "Image XResolution": "1080", "Image YResolution": "1080", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS3 Windows", "Image DateTime": "2020:03:03 19:33:13", "Image ExifOffset": "164", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "302", "Thumbnail JPEGInterchangeFormatLength": "5390", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "374", "EXIF ExifImageLength": "222"}