Dinosaur Systematics Approaches and Perspectives
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Dinosaur Systematics Approaches and Perspectives Edited by Kenneth Carpenter Denver Museum of Natural History and Philip J. Currie r.„«..u Tynell Museum of Palaeontology SEP 20 1991 The right of the University of Cambridge to print and sell alt manner of hooks was granted by Henry VIU in 1534. The University has printed and published continuously since 1584. Cambridge University Press Cambridge New York Port Chester Melbourne Sydney Morphometric observations on hadrosaurid ornithopods RALPH E. CHAPMAN AND MICHAEL K. BRETT-SURMAN Abstract because they are represented by large numbers of well- Results are presented of preliminary morphometric documented specimens with both cranial and postcranial analyses on hadrosaurs using the landmark shape analysis material. Contrast this with the pachycephalosaurs, for method Resistant-Fit Theta-Rho-Analysis (RFTRA). The analy- example, for which little postcranial material is avail- ses were performed on both cranial and postcranial material. able, and most taxa and specimens are represented by They show this approach to be useful for the analysis of hadro- only incomplete crania (Maryariska and Osmolska 1974; saur morphology and provide insight into how this morphology varies within the context of the phylogenetic structure of the Sues and Galton 1987; Goodwin this volume). In fact, family. Further, the patterns are related to two other groups of hadrosaur material can be so abundant that it is often Euornithopods, the iguanodontids and camptosaurids. The left uncollected when resources restrict the number of results highlight the distinct morphology of the lambeosaurine specimens that can be removed during a field season hadrosaurs, confirm that most of the significant morphological (P. Currie pers. comm. 1986). variation in hadrosaur crania is concentrated in the muzzle and In addition to this abundance, hadrosaurs exhibit narial regions, and indicate that pelvic element shape should be a high degree of morphological variability, especially in useful for taxonomic identification and discrimination. In gen- cranial structures. These are thought by some to play a eral cranial shape, the lambeosaurines are shown to be most role in social behavior (Dodson 1975; Hopson 1975; closely related to the hadrosaurines, supporting a monophyletic Molnar 1977). Dodson (1975), for example, analyzed Hadrosauridae. morphometric data for the crania of 36 specimens of Introduction lambeosaurine hadrosaurs referable to three genera {Corythosaurus, Lambeosaurus, and "Procheneosaurus") Hadrosaurs have one of the most complex taxo- and 12 species. The approaches used included standard nomic histories of all the dinosaurs; over 100 species bivariate allometric analyses and principal coordinates representing 44 genera have been named. This unusu- analysis. Dodson concluded that the taxon "Procheneo- ally high taxonomic diversity is a consequence of the saurus" includes only juvenile forms of other taxa, and interplay between the taxonomic philosophies of the that only one species of Corythosaurus and two of Lam- many researchers studying hadrosaurs, the high level of beosaurus were represented, as well as both sexes. real taxonomic diversity, the unusually abundant mate- Clearly, the wide range of body size, the great diversity rial available, and the high degree of morphological of display structures, and sexual dimorphism combined variability within populations and between age groups. to produce a great deal of morphological variation among The latter is the result of allometric and ontogenetic individuals of a single species. effects over a wide range of sizes (see Dodson 1975; The taxonomy of hadrosaurids has been the sub- Hopson 1975; Molnar 1977). Herein, we will present the ject of discussion recently. Sereno (1986), in his cladis- results of a series of preliminary shape analyses of hadro- tic analysis of the ornithischians, retained the hadrosaurs saur crania and pelves, and discuss these in the context of as a monophyletic taxon, whereas Homer (this volume) hadrosaur taxonomy, phytogeny, and identification. has suggested that the group is diphyletic. Hadrosaurs are unusual among the dinosaurs Dinosaur morphometric analyses have been spo- In Dinosaur Systematics: Perspectives and Approaches, Kenneth radic and limited by the small numbers of specimens Carpenter and Philip J. Currie, eds. Copyright © Cambridge University available for most groups (Chapman this volume). The Press, 1990. most comprehensive studies to date are Dodson's (1975) Ralph E. Chapman and Michael K. Brett-Surman 164 work on lambeosaurine hadrosaurs, Dodson's (1976) coefficients (the estimate of the closeness of fit between allometric analysis of growth and sexual dimorphism in the two specimens based on the superimposition). Protoceratops, the study of Chapman et al. (1981) on The morphometric approaches used here were cranial allometry and sexual dimorphism in the pachy- applied to provide insight into the morphology of the cephalosaurian Stegoceras, the study of variation in hadrosaurs and how the different hadrosaur morpholo- Plateosaurus femora by Weishampel and Chapman (this gies interrelate. Iguanodontids were used as the primary volume), and the review by Chapman (this volume) outgroup to provide examples of the most closely related demonstrating the application of shape analysis meth- group to the hadrosaurs, and two specimens of camp- ods, specifically Resistant-Fit Theta-Rho-Analysis, to tosaurids also were used as the second outgroup to give general problems of dinosaur paleobiology. an indication of the morphological trajectory of the Generally, morphometric methods can provide euornithopods (sensu Sereno 1986). A specimen of important insights into the morphology of dinosaurs, "Procheneosaurus," considered by us to be a juvenile and the implication of this morphology for interpreting lambeosaurine (fide Dodson 1975), was included to phylogeny, ontogeny, paleoecology, and taphonomy. show how an immature form would compare with the Morphometric methods already have provided impor- adults, and whether it would cluster with the lambeo- tant information on lambeosaurine variability (Dodson saurines. 1975). Here, we apply it more generally to hadrosaur One characteristic that morphometric analyses morphology and to the interpretation of hadrosaur taxo- share with modern phylogenetic analyses is that they are nomic structure. both dialetic in nature: the results of an analysis are not considered to be the final answer but, instead, indicate Materials and methods the direction that further analyses should take. Because Resistant-Fit Theta-Rho-Analysis (RFTRA) is a of this, we recognize these results as a first approach in form of landmark shape analysis that provides superim- the ongoing analysis of both cranial and postcranial ele- posed figures representing the fit of one specimen onto ments of hadrosaurs and other euornithopods. The num- another after size and positional differences are removed, ber of complete and articulated specimens available is and gives distance values representing an estimate of the exceedingly small. However, the results do indicate "goodness of fit." The fit is made using the relative where expanded studies should concentrate. positions of groups of landmarks (homologous or geo- The results of RFTRA within this context do pro- metrically equivalent points) and the original geometry vide information relevant to the interpretation of taxo- of the landmarks is maintained without the specimens nomic and phylogenetic structure. If the results of a being distorted during the analysis (Benson et al. 1982). morphometric analysis do not agree with conventional Landmark shape analysis methods are derived phylogenetic reconstructions, then they raise questions from the transformation grids developed by D'Arcy that must be addressed before those phylogenies can be Thompson (1942), and include tensor methods (see accepted. Often the differences can be recognized as Bookstein et al. 1985, and references therein) and vector convergence, providing information that may be rele- methods (Sneath 1967). RFTRA was developed by vant to the functional morphology or ecology of the Siegel and Benson (1982) and Benson et al. (1982), who taxon. Where convergence is not apparent, however, the modified Sneath's least-squares approach by applying characters used in the phylogenetic analysis should be more robust statistical methods (Siegel and Benson reconsidered and additional morphometric analyses 1982). The RFTRA algorithm is a major improvement developed to try to reconcile the differences. Agreement on the least-squares method (referred herein as LSTRA) between the two provides support in much the same because it allows localized change to be identified. way that the addition of new characters strengthens a Chapman (this volume) presents a detailed dis- phylogenetic analysis. It is more parsimonious to accept cussion on the philosophy and mechanics of the method. morphological similarity between the members of two In this study, a series of photographs/illustrations repre- groups as the result of recent ancestry rather than just a senting the same view was obtained for the specimens, chance convergence, if other factors independently sug- and the questions to be addressed by this analysis were gest the connection. In this way, morphometric analyses developed. Next, a series of homologous landmarks or can help in the choice between two phylogenies devel- equivalent points were located on each illustration. The oped