The available data are important for understanding the evolution of the avian beak

Limusaurus is the only known reptile to lose its teeth and form a beak after birth

Ontogenetic variation is documented within many dinosaur species, but extreme ontogenetic changes are rare among dinosaurs, particularly among theropods. Here, we analyze 19 specimens of the Jurassic ceratosaurian theropod Limusaurus inextricabilis, representing six ontogenetic stages based on body size and histological data. Among 78 ontogenetic changes we identify 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. Jaw morphological data, including those derived from Mi-CT and SR-μCT scanning of Limusaurus specimens, reveal dental alveolar vestiges and indicate that ontogenetic tooth loss in Limusaurus is a gradual, complex process. Our discovery has significant implications for understanding the evolution of the beak, an important feeding structure present in several tetrapod clades, including modern birds. This radical morphological change suggests a dietary shift, probably from omnivory for juvenile Limusaurus to herbivory for adult Limusaurus, which is also supported by additional evidence from gastroliths and stable isotopes. Incorporating new ontogenetic information from Limusaurus into phylogenetic analyses demonstrates surprisingly little effect on its placement when data from different stages are used exclusively, in contrast to previous analyses of tyrannosaurids, but produces subtle differences extending beyond the placement of Limusaurus.

Results

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et al. A Jurassic ceratosaur from China helps clarify avian digital homologies. Figure 1 Six Ontogenetic Stages and Major Ontogenetic Variations of Limusaurus inextricabilis Show full caption (A–F) Diagrams of six specimens of L. inextricabilis, as preserved, and their bone histology for skeletochronology, representing six ontogenetic stages (A, IVPP V20100; B, IVPP V15301; C, IVPP V20098; D, IVPP V15923; E, IVPP V15297; F, IVPP V20099; IVPP refers to the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences). White arrows mark the LAGs (lines of arrested growth); black arrow marks the EFS (external fundamental system). (G) Juvenile (upper) and subadult (lower) L. inextricabilis skeletons highlighting some ontogenetically variable features: (1) straight (juvenile) or ventrally deflected (subadult) anterior end of the dentary; (2) relatively deep (juvenile) or elongate (subadult) skull; (3) gastroliths absent (juvenile) or present (subadult); (4) short (juvenile) or elongate (subadult) posterior process of the pubic boot. See also Figure S1 and Tables S1 S2 , and S3 Cranial ontogenetic changes are known in many dinosaur clades and are sometimes extreme in Ornithischia [], but non-avian theropod skulls generally exhibit minor or moderate ontogenetic changes related to negative allometry of the orbit [] and/or increased robusticity through ontogeny []. Limusaurus inextricabilis is a ceratosaurian theropod from the Upper Jurassic (Oxfordian) Shishugou Formation of northwestern China, which represents the only known Jurassic theropod with the combination of a fully developed rhamphotheca and gastric mill []. To date, 19 skeletons with various body sizes have been recovered from three separate miring aggregations, two at the same level and one 6.5 m stratigraphically higher ( Figure S1 A). All newly referred skeletons are identified as L. inextricabilis based on a suite of features unique to this taxon [], and this taxonomic identification is further supported by our phylogenetic analysis ( Figures S2 and S3 ). Skeletochronologic analyses of 13 of these specimens indicate an age range from less than 1 year to nearly 10 years, and combined with body-size measurements ( Table S1 ), these data suggest that these 19 individuals represent six different ontogenetic stages ( Figure 1 and Table S2 ) ranging from hatchling through adult. Therefore, these specimens comprise a growth series of L. inextricabilis and represent one of the best-documented growth series for an extinct theropod species, which provides a rare opportunity to investigate theropod ontogeny and its effect on phylogenetic analyses.

A total of 78 ontogenetically variable features have been identified in these specimens ( Table S3 ; see also Supplemental Information ). Compared to the juveniles, the subadults and adults have a proportionally shallower head, a proportionally longer metacarpal II, and an elongate posterior process of the pubic boot ( Figure 1 ; see also Supplemental Information ). Furthermore, the adult differs from the hatchling in having a straight quadrate shaft (curved in juveniles), a dentary with a downturned anterior end (straight in juveniles), and presence of gastroliths (absent in juveniles).

Figure 2 Jaw Bones of L. inextricabilis Illustrated by CT Data, and Diagrammed Tooth-Loss Pattern in L. inextricabilis Show full caption (A–E) Jaw bones of L. inextricabilis: premaxilla (right), maxilla (middle), and dentary (left) of stage I Limusaurus (A, IVPP V20100), stage II Limusaurus (B, IVPP V15301), and stage IV Limusaurus (C, IVPP V15923) in right lateral and ventral views; transverse view of middle portion of dentary of stage II Limusaurus (D) and stage IV Limusaurus (E). av, alveolar vestiges; nc, neurovascular canal; t, tooth. (F–H) Diagrammed tooth-loss pattern in L. inextricabilis. Scale bars in (A), (B), and (C) represent 1 cm. Bars in (D)–(H) are not to scale. See also Table S2 The most striking change is from fully toothed jaws in juvenile L. inextricabilis to completely toothless jaws in more mature individuals ( Figure 2 ). Stage I Limusaurus has one premaxillary, eight maxillary, and at least 12 dentary teeth ( Figures 2 A and 2F). Stage II has one premaxillary tooth and only five maxillary and 11 dentary teeth ( Figures 2 B and 2G). No erupted teeth preserve wear facets or resorption pits, suggesting that dental function may have been reduced and that normal tooth replacement was inactive. CT data show that the lost maxillary teeth in stage II are the first, sixth, and eighth and that the lost dentary tooth is a middle one (the sixth in the right dentary and the seventh in the left dentary). The corresponding alveoli remain hollow but enclosed on the occlusal margins (though a shallow fossa is present; Figure 2 B). Interestingly, a small replacement tooth is present within a fully enclosed dentary alveolus ( Figure 2 D). It should be noted that an extra dentary tooth may have been lost even earlier, as a shallow fossa is present on the occlusal margin of the dentary immediately anterior to the first dentary tooth in both stage I and stage II Limusaurus ( Figures 2 A and 2B). Specimens of stage IV and more mature individuals have completely toothless jaws. CT data of stages IV and V specimens indicate that the alveolus for the lost premaxillary tooth has been completely filled and that the alveoli for the lost maxillary teeth are ventrally closed hollows ( Figures 2 C and 2H). Individual dentary alveolar vestiges, however, are absent and modified to a canal that runs dorsal to the neurovascular canal inside the dentary ( Figures 2 E and 2H). Consequently, at least three distinct stages of ontogenetic dentition change are documented in known Limusaurus specimens: stage I Limusaurus has at least 42 teeth, stage II has 34 teeth, and stages IV and V are toothless.

Figure 3 Carbon Isotope Compositions of L. inextricabilis, Sauropod, Ornithischian, and Theropod Apatites Plotted against Their Corresponding Oxygen Isotope Compositions of Apatite Carbonate Show full caption Convex polygons show ranges for carnivorous (red) and herbivorous (blue) dinosaurs from the upper Shishugou Formation. Numerals inside Limusaurus data points indicate ontogenetic stages. Dashed line bounds subadult Limusaurus ontogenetic stages with edentulous skulls and gastric mills. See also Figure S2 and Table S4 Gastroliths are absent in stages I and II Limusaurus but are present in individuals of more advanced ontogenetic stages. Furthermore, the gastroliths increase in size and quantity in more mature individuals (see Supplemental Information for further description). We further investigate the diet of Limusaurus by analyzing stable carbon and oxygen isotope compositions of apatite carbonate from a sample set consisting of 28 specimens, including 13 individuals of L. inextricabilis ( Figures 3 and S4 ). The stable isotope signature of stage V Limusaurus is most similar to that of presumed dinosaur herbivores, but less mature individuals (stages II to IV) display a wide range of isotope values ( Figure 3 and Table S4 ).