Report
Extreme Ontogenetic Changes in a Ceratosaurian Theropod Highlights d
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Wang et al. report 78 ontogenetically variable features of the theropod Limusaurus Limusaurus is the only known reptile to lose its teeth and form a beak after birth
Authors Shuo Wang, Josef Stiegler, Romain Amiot, Xu Wang, Guo-hao Du, James M. Clark, Xing Xu
Correspondence
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The available data are important for understanding the evolution of the avian beak
[email protected] (S.W.),
[email protected] (X.X.)
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The ontogenetically variable features have little effect on its phylogenetic position
In Brief
Wang et al., 2017, Current Biology 27, 144–148 January 9, 2017 ª 2017 Elsevier Ltd. http://dx.doi.org/10.1016/j.cub.2016.10.043
Wang et al. report 78 ontogenetically variable features of the theropod dinosaur Limusaurus, among which the loss of teeth and formation of a beak was previously unknown in any reptile. The extreme ontogenetic changes have surprisingly little effect on phylogenetic analyses and are important in understanding the evolution of the avian beak.
Current Biology
Report Extreme Ontogenetic Changes in a Ceratosaurian Theropod Shuo Wang,1,2,7,* Josef Stiegler,3 Romain Amiot,4 Xu Wang,5 Guo-hao Du,6 James M. Clark,3 and Xing Xu2,* 1Laboratory of Vertebrate Evolution, College of Life Science, Capital Normal University, 105 West 3rd Ring Road North, Beijing 100048, China 2Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, 142 Xi-Zhi-Men-Wai Street, Beijing 100044, China 3Department of Biological Sciences, The George Washington University, Washington, DC 20052, USA 4CNRS UMR 5276, Universite Claude Bernard Lyon 1 and Ecole Normale Supe rieure de Lyon, Villeurbanne Cedex 69622, France 5Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Bei-Tu-Cheng-Xi Road, Beijing 100029, China 6Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 239 Zhangheng Road, Shanghai 201204, China 7Lead Contact *Correspondence:
[email protected] (S.W.),
[email protected] (X.X.) http://dx.doi.org/10.1016/j.cub.2016.10.043
SUMMARY
RESULTS
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-mCT 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.
Cranial ontogenetic changes are known in many dinosaur clades and are sometimes extreme in Ornithischia [1, 2], but non-avian theropod skulls generally exhibit minor or moderate ontogenetic changes related to negative allometry of the orbit [3] and/or increased robusticity through ontogeny [4]. 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 [5, 6]. 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 S1A). All newly referred skeletons are identified as L. inextricabilis based on a suite of features unique to this taxon [5], 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).
144 Current Biology 27, 144–148, January 9, 2017 ª 2017 Elsevier Ltd.
Figure 1. Six Ontogenetic Stages and Major Ontogenetic Variations of Limusaurus inextricabilis (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.
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 2A and 2F). Stage II has one premaxillary tooth and only five maxillary and 11 dentary teeth (Figures 2B 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 2B). Interestingly, a small replacement tooth is present within a fully enclosed dentary alveolus (Figure 2D). 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 2A 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 2C 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 2E 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. 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). We conducted several phylogenetic analyses using a new theropod dataset (Figure 4; Figures S2 and S3). An analysis based on scoring only subadult and adult specimens of Limusaurus recovered this genus as the sister taxon of Elaphrosaurus, both being members of the Noasauridae (Figure 4A). Including data from only juveniles yielded a similar hypothesis but produced character conflict resulting in reduced resolution within Ceratosauria and clades not recovered in the standard analysis (Figure 4B). Limusaurus was never recovered in an unexpectedly Current Biology 27, 144–148, January 9, 2017 145
Figure 2. Jaw Bones of L. inextricabilis Illustrated by CT Data, and Diagrammed Tooth-Loss Pattern in L. inextricabilis (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.
early diverging position as has likely occurred in analyses of juvenile tyrannosaurids [7, 8]. DISCUSSION Moderate ontogenetic dentition change has been noticed in several theropod clades. For example, it has been suggested that the tooth count increases during ontogeny in Allosaurus [9] and in many coelurosaurian taxa [10, 11], while tooth count is known to increase or decrease in different tyrannosaurid theropods [4]. With a fully developed dental system in hatchling and juvenile individuals, but completely toothless jaws in more mature individuals, Limusaurus exhibits the most radical ontogenetic change of dental system among dinosaurs. In fact, Limusaurus represents the first known reptile with ontogenetic edentulism and is the only known example of ontogenetic edentulism in the fossil record of jawed vertebrates. Otherwise, ontogenetic edentulism is known only in a few extant species of teleost fish [12, 13] and the platypus [14]. Comparatively, onto146 Current Biology 27, 144–148, January 9, 2017
genetic edentulism of Limusaurus is more radical than in some other known examples given that juvenile Limusaurus has a fully developed dentition, in contrast to the partially developed dentitions of juvenile fish [12, 13] and mammals [14] with postnatal ontogenetic edentulism. Ontogenetic edentulism in Limusaurus is a gradual, complex process. When compared with the dentition of their closest relatives [15], the jaw morphology of Limusaurus specimens of different ontogenetic stages indicates that Limusaurus loses teeth from both the anterior and posterior ends of the jaws, combined with tooth loss caused by lack of replacement in alternate teeth along a replacement wave [16]. This pattern differs from the anterior reduction pattern seen in some dinosaurs [17, 18] and Cretaceous ornithurine birds [19, 20] and from the posterior reduction pattern seen in some theropod clades [20, 21]. It is most similar to that seen in jeholornithid birds [20, 22], which also display a combination of anterior and posterior tooth reduction. Considering that reptilian replacement teeth arise from the successional lamina, we hypothesize that the early cessation of tooth
Figure 3. Carbon Isotope Compositions of L. inextricabilis, Sauropod, Ornithischian, and Theropod Apatites Plotted against Their Corresponding Oxygen Isotope Compositions of Apatite Carbonate 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.
replacement in Limusaurus may have resulted from the regression of successional lamina during the first year after hatching, similar to that present in a monophyodont lizard [23]. However, the detailed mechanism behind the alveolar remodeling needs further investigation. The discovery of ontogenetic edentulism in the theropod Limusaurus is potentially significant in understanding the development and evolution of the beak, an important feeding structure present in several tetrapod clades, including modern birds [20, 24]. Early signaling pathways involved in odontogenesis remain inducible in modern birds [25]; the fully developed dentition present in early postnatal ontogenetic stages of Limusaurus indicates that the complete signaling pathways responsible for odontogensis are present and viable in at least one fully beaked theropod. This suggests that dental developmental regressions might have occurred heterochronically in the evolution of beaked-animal lineages, including beaked theropod taxa; future investigations of ontogenetic changes of jaw morphology, including dental morphology and genetic development of beaks, are needed to test this hypothesis. The radical change in the dental system suggests a dietary shift during Limusaurus ontogeny, as in some vertebrates [12–14, 26]. For example, the ontogenetic loss of premaxillary teeth in some extant armored catfish and mullets has been suggested to be related to a change from predatory feeding in juveniles to benthic feeding in adults [12, 13], representing a dietary shift corresponding to the ontogenetic edentulism. This conjecture is also supported by additional evidence from gastroliths and stable isotopes. In living birds, gastric mills are known in herbivorous, insectivorous, and omnivorous taxa, but not carnivores
Figure 4. Phylogenetic Hypotheses for Ceratosauria, and the Effects of Juvenile Morphology on Phylogenetic Analysis (A) Strict consensus relationships based on subadult and adult scorings for Limusaurus. (B) Reduced strict consensus based on morphology from juvenile Limusaurus. The sister clade to Ceratosauridae was collapsed for clarity. Clades not recovered in the standard analysis are marked with asterisks. Dotted gray lines show possible positions for Laevisuchus. Abelisauridae, orange; Abelisauroidea, red; Ceratosauridae, blue; Elaphrosaurinae, purple; Noasauridae, yellow; Noasaurinae, green. See also Figures S3 and S4.
[27], and the large body size of adult Limusaurus suggests that insectivory is unlikely. The fact that small individuals (stages I–III) display a wider range of isotope values than large individuals (stages IV and V) suggests that juvenile Limusaurus occupied a more varied dietary niche than mature individuals and were probably omnivores. On theoretical grounds, omnivory ensures a greater survival rate of immature individuals, and an ontogenetic dietary niche shift can reduce the competition for food between juvenile and adult individuals within a population. Theropod species are often established based only on juvenile specimens, but taxonomic evaluation of such specimens can be problematic [28, 29]. The results of the present phylogenetic analyses suggest that a cladistic approach for taxonomic assignment of juvenile specimens is warranted, though local Current Biology 27, 144–148, January 9, 2017 147
topological accuracy and character optimization may be compromised. The recovery of species represented by juvenile remains in early-diverging positions in tyrannosaurids may be attributable to a protracted ontogeny and corresponding morphological disparity. Morphological changes due to ontogenetic size disparity may be predictable, but dramatically variable ontogenies such as in Limusaurus are unpredictable when only juveniles are known and may mislead phylogenetic analyses in a more cryptic fashion. SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, four figures, and four tables and can be found with this article online at http://dx.doi.org/10.1016/j.cub.2016.10.043. AUTHOR CONTRIBUTIONS X.X., J.M.C., S.W., and J.S. designed the research; S.W., J.S., R.A., X.W., and G.D. performed the research; S.W., J.S., R.A., and X.W. analyzed the data; S.W., J.S., R.A., X.X., and J.M.C. wrote the paper. ACKNOWLEDGMENTS We thank L. Xiang, T. Yu, and X. Ding for preparing the specimens, R. Li and H. Zang for illustrating and photographing, T. Xiao, Y. Fu, Y. He, G. Peng, Y. Wang, G. Zhou, Y. Ren, B. Deng, H. Tan, and X. Yu for SR-mCT imaging, Y. Feng and Y. Hou for Mi-CT imaging and 3D reconstruction, and L. Cui for geochemical analyzing. This study was supported by the National Natural Science Foundation of China (41602013 and 41120124002), the U.S. National Science Foundation (NSF EAR 0310217 and 0228559), and the Special Funds for Major States Basic Research Projects of China (2012CB821903); J.S was also supported by NSF 1311000, R.A and X.W by the National Basic Research Program of China (2012CB821900), and X.W by the ‘‘Strategic Priority Research Program’’ of the Chinese Academy of Sciences (XDB03020500). Received: August 22, 2016 Revised: October 3, 2016 Accepted: October 20, 2016 Published: December 22, 2016 REFERENCES 1. Horner, J.R., and Goodwin, M.B. (2009). Extreme cranial ontogeny in the upper cretaceous dinosaur pachycephalosaurus. PLoS ONE 4, e7626. 2. Fiorillo, A.R., and Tykoski, R.S. (2013). An immature Pachyrhinosaurus perotorum (Dinosauria: Ceratopsidae) nasal reveals unexpected complexity of craniofacial ontogeny and integument in Pachyrhinosaurus. PLoS ONE 8, e65802. 3. Bhullar, B.-A.S., Maruga´n-Lobo´n, J., Racimo, F., Bever, G.S., Rowe, T.B., Norell, M.A., and Abzhanov, A. (2012). Birds have paedomorphic dinosaur skulls. Nature 487, 223–226. 4. Carr, T.D. (1999). Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). J. Vertebr. Paleontol. 19, 497–520. 5. Xu, X., Clark, J.M., Mo, J., Choiniere, J., Forster, C.A., Erickson, G.M., Hone, D.W.E., Sullivan, C., Eberth, D.A., Nesbitt, S., et al. (2009). A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature 459, 940–944. 6. Eberth, D.A., Xing, X., and Clark, J.M. (2010). Dinosaur death pits from the Jurassic of China. Palaios 25, 112–125. 7. Fowler, D.W., Woodward, H.N., Freedman, E.A., Larson, P.L., and Horner, J.R. (2011). Reanalysis of ‘‘Raptorex kriegsteini’’: a juvenile tyrannosaurid dinosaur from Mongolia. PLoS ONE 6, e21376. 8. Tsuihiji, T., Watabe, M., Tsogtbaatar, K., Tsubamoto, T., Barsbold, R., Suzuki, S., Lee, A.H., Ridgely, R.C., Kawahara, Y., and Witmer, L.M.
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