Variation in the Late Triassic Canjilon quarry (Upper Chinle Group, New Mexico) phytosaur skulls: a case for sexual dimorphism

  • Kate E. Zeigler
  • Spencer G. Lucas
  • Andrew B. Heckert
Article

Abstract

The Canjilon quarry, located in north-central New Mexico near Ghost Ranch, contains a death assemblage of phytosaurs located stratigraphically high within the Petrified Forest Formation of the Chinle Group (Revueltian = early-mid Norian). The site has yielded numerous fossils ofPseudopalatus-grade phytosaurs, including at least 10 skulls collected from the locality byCharles Camp in 1928 and 1933 and another collected more recently byAlex Downs, curator of paleontology at Ghost Ranch. A re-examination of these skulls reveals two morphotypes that differ only in the relative lengths and relative robustness of their premaxillae. In these two morphotypes, the premaxillae define the shape and length of the rostral crest, the dimensions of which are independent of skull size. In one morphotype, the premaxillae are long, thin bones that lead to an abrupt, volcano-like narial crest. In the second morphotype, the premaxillae are of approximately the same length, but expand dorsoventrally halfway along their lengths, creating a longer and more robust crest. The most probable explanation of these two variants in rostral crest morphology in a Single, catastrophic death assemblage is thatPseudopalatus-grade phytosaurs are sexually dimorphic. Thus, the larger, more robust crest of the first morphotype is probably a display feature, most likely of the male animal. The more gracile snout änderest characterize the female morph. In the sample of phytosaurs examined, there are three individuals of the more robust (male) morphotype, five individuals of the gracile (female) morphotype, and three individuals (2 adult, 1 juvenile) that cannot be assigned to either morphotype because the skulls are too damaged to make an aecurate assessment. This is the first clear evidence of sexual dimorphism in phytosaurs, and has important implications for phytosaur species-level taxonomy, as well as for understanding aspects of their paleobiology (e.g., population dynamics).

Keywords

Phytosaurs sexual dimorphism Norian Chinle Group Petrified Forest Formation New Mexico 

Kurzfassung

Die Canjilon-Fundstelle, die im nördlichen New Mexico in der Nähe von Ghost Ranch liegt, hat eine Taphozönose von Phytosauriern geliefert, die sich stratigraphisch hoch in der Petrified Forest-Formation befindet (Revueltium = frühes bis mittleres Nor). Es handelt sich um zahlreiche Funde von Phytosauriern auf der Evolutionshöhe von Pseudopalatus, darunter 11 Schädel. Von diesen wurden 10 von Charles Camp in den Jahren 1928 and 1933 geborgen und ein weiterer in jüngerer Zeit von Alex Downs vom paläontologischen Museum Ghost Ranch. Eine Neubearbeitung dieser Schädel zeigt, dass zwei Morphotypen vorhanden sind, die sich nur in der relativen Länge und relativen Robustheit des Prämaxillare unterscheiden. In beiden Morphotypen bestimmen die Prämaxillaria die Form und Länge des Rostralkammes, dessen Proportionen unabhängig von der Schädelgröße sind. Bei dem einen Morphotypus sind die Prämaxillaria lange, dünne Knochen, die in einem vulkanartigen Nasenkamm führen. Bei dem zweiten Morphotyp haben die Prämaxillaria zwar die gleiche Länge, aber nehmen auf der halben Länge in dorsoventraler Richtung an Größe zu, so dass ein längerer und robusterer Kamm entsteht. Die wahrscheinlichste Erklärung für diese beiden Varianten in einer durch eine Katastrophe verursachten Taphozönose ist, dass diese Phytosaurier sexuell dimorph waren. So ist der größere und robustere Kamm wohl eine Display struktur, vermutlich des männlichen Tieres. Die grazilere Schnauze und Kamm kennzeichnen das Weibchen. Im Material befinden sich 3 Individuen des robusteren Morphotyps (Männchen), 5 Individuen des grazileren Typs (Weibchen) und 3 Individuen (2 Adulttiere, 1 Juveniles), die zu keinem der Morphotypen gestellt werden können, weil die Schädel für eine sichere Ansprache zu stark beschädigt sind. Der Befund ist der erste eindeutige Beleg für einen Sexualdimorphismus bei Phytosauriern. Er hat bedeutende Implikationen für die Taxonomie der Phytosaurier auf dem Artniveau sowie für das Verständnis ihrer Paläobiologie (z.B. Populationsdynamik).

Schlüsselwörter

Phytosauria Sexualdimorphismus Nor Chinle-Gruppe Petrified Forest-Formation Südwesten der USA 

Literature

  1. Andersson, M. 1994. Sexual selection. — 599 p., Princeton (Princeton University Press).Google Scholar
  2. Ballew, K.L. 1985. A phylogenetic analysis of Phytosauria (Reptilia: Archosauria) from the Late Triassic of the western United States. — 75 p., Berkeley (Masters thesis, University of California).Google Scholar
  3. Ballew, K.L. 1989. A phylogenetic analysis of the Phytosauria from the Late Triassic of the western United States. — In:Lucas, S.G. &Hunt, A.P., eds., Dawn of the age of the dinosaurs in the American Southwest: 309–339, Albuquerque (New Mexico Museum of Natural History and Science).Google Scholar
  4. Behrensmeyer, A.K. 1978. Taphonomic and ecologic information from bone weathering. — Paleobiology4 (2): 150–162.Google Scholar
  5. Behrensmeyer, A.K. 1982. Time resolution in fluvial vertebrate assemblages. — Paleobiology8 (3): 211–227.Google Scholar
  6. Benton, M.J. &Clark, J.M. 1988. Archosaur phylogeny and the relationships of the Crocodylia. — In:Benton, M.J., ed., The phylogeny and Classification of the tetrapods: 295–338, Oxford (Clarendon Press).Google Scholar
  7. Camp, C.L. 1930. A study of the phytosaurs with description of new material from western North America. — 174 p., Berkeley (University of California Press).Google Scholar
  8. Chapman, R.E.;Galton, P.;Sepkoski, J.J. &Wall, W.P. 1981. A morphometric study of the cranium of the pachycephalosaurid dinosaurStegoceras. — Journal of Paleontology55: 608–618.Google Scholar
  9. Chapman, R.E.;Weishampel, D.B.;Hunt, G. &Rasskin-Gutman, D. 1997. Sexual dimorphism in dinosaurs. — In:Wolberg, D.L., ed., Dinofest: Proceedings of a Symposium Held at the Academy of Natural Sciences, Philadelphia: 83–93, Philadelphia (Academy of Natural Sciences).Google Scholar
  10. Colbert, E.H. 1989. The Triassic dinosaurCoelophysis. — Museum of Northern Arizona Bulletin57: 1–160.Google Scholar
  11. Colbert, E.H. 1990. Variation inCoelophysis. — In:Carpenter, K. &Currie, P.J., eds., Dinosaur systematics: approaches and perspectives: 80–90, Cambridge (Cambridge University Press).Google Scholar
  12. Cope, E.D. 1881.Belodon in New Mexico. — American Naturalist15: 922–923.Google Scholar
  13. Davitaschvili, L.S. 1961. The theory of sexual selection. — 538 p., Moscow (Academy of Sciences).Google Scholar
  14. Dodson, P. 1975. Taxonomic implications of relative growth in lambeosaurine hadrosaurs. — Systematic Zoology24: 37–54.CrossRefGoogle Scholar
  15. Dodson, P. 1976. Quantitative aspects of relative growth and sexual dimorphism inProtoceratops. — Journal of Paleontology50 (5): 929–940.Google Scholar
  16. Farlow, J.O. &Dodson, P. 1975. The behavioral significance of frill and hörn morphology in ceratopsian dinosaurs. — Evolution29: 353–361.CrossRefGoogle Scholar
  17. Forster, CA. 1996. Species resolution inTriceratops: cladistic and morphometric approaches. — Journal of Vertebrate Paleontology16: 259–270.Google Scholar
  18. Galton, P.M. 1997. Comments on sexual dimorphism in the prosauropod dinosaurPlateosaurus engelhardti (Upper Triassic, Trossingen). — Neues Jahrbuch für Geologie und Paläontologie, Monatshefte1997 (11): 674–682.Google Scholar
  19. Galton, P.M. 1999. Sex, sacra andSellosaurus gracilis (Saurischia, Sauropodomorpha, Upper Triassic, Germany) — or why the character “two sacral vertebrae” is plesiomorphic for Dinosauria. — Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen213 (1): 19–55.Google Scholar
  20. Galton, P.M. 2001. Prosauropod dinosaurSellosaurus gracilis (Upper Triassic, German): Third sacral vertebra as either a dorsosacral or a caudosacral. — Neues Jahrbuch für Geologie und Paläontologie, Monatshefte2001 (11): 688–704.Google Scholar
  21. Geist, V. 1966. The evolution of horn-like organs. — Behaviour27: 175–214.Google Scholar
  22. Gregory, J.T. 1962a. The genera of phytosaurs. — American Journal of Science260: 652–690.Google Scholar
  23. Gregory, J.T. 1962b. The relationships of the American phytosaurRutiodon. — American Museum Novitates2095: 1–22.Google Scholar
  24. Hungerbühler, A. 1998. Cranial anatomy and diversity of the Norian phytosaurs of Southwestern Germany. — 453 p., Bristol (Ph. D. thesis, University of Bristol).Google Scholar
  25. Hungerbühler, A. 2002. The Late Triassic phytosaurMystriosuchus westphali, with a revision of the genus. — Palaeontology45 (1): 377–418.CrossRefGoogle Scholar
  26. Hunt, A.P. 1989. Cranial morphology and ecology among phytosaurs. — In:Lucas, S.G. &Hunt, A.P., eds., Dawn of the age of the dinosaurs in the American Southwest: 349–354, Albuquerque (New Mexico Museum of Natural History and Science).Google Scholar
  27. Hunt, A.P. 1994. Vertebrate paleontology and biostratigraphy of the Bull Canyon Formation (Chinle Group, Upper Triassic), east-central New Mexico with revisions of the families Metoposauridae (Amphibia: Temnospondyli) and Parasuchidae (Reptilia: Archosauria). — 404 p., Albuquerque (Ph. D. thesis, University of New Mexico).Google Scholar
  28. Hunt, A.P. &Downs, A. 2002. Taphonomy of the Late Triassic Canjilon quarry (Petrified Forest Formation: Chinle Group), north-central New Mexico: Data from new excavations. — New Mexico Museum of Natural History, Bulletin21: 291–295.Google Scholar
  29. Hunt, A.P. &Lucas, S.G. 1989. Late Triassic vertebrate localities in New Mexico. — In:Lucas, S.G. &Hunt, A.P., eds., Dawn of the age of the dinosaurs in the American Southwest: 72–101, Albuquerque (New Mexico Museum of Natural History and Science).Google Scholar
  30. Hunt, A.P. &Lucas, S.G. 1993. Triassic vertebrate paleontology and biochronology of New Mexico. — New Mexico Museum of Natural History, Bulletin2: 49–60.Google Scholar
  31. Jungers, W.L., ed., 1985. Size and scaling in primate biology. — 491 p., New York (Plenum Press).Google Scholar
  32. Kurzanov, S.M. 1972. Sexual dimorphism in protoceratopsians. —Paleontological Journal1972 (1): 91–97.Google Scholar
  33. Lanoston, W. &Gasparini, Z. 1997. Crocodilians,Gryposuchus, and the South American gavials. — In:Kay, R.F.;Madden, R.H.;Ciffelli, R.L. &Flynn, J.J., eds., Vertebrate paleontology in the Neotropics: the Miocene fauna of La Venta, Colombia: 113–154, Washington, D.C. (Smithsonian Institution).Google Scholar
  34. Lawler, D.A. 1974. Osteological Variation in the phytosaurRutiodon tenuis from Ghost Ranch, New Mexico. — 137 p., Berkeley (Masters thesis, University of California).Google Scholar
  35. Lehman, T.M. 1990. The ceratopsian subfamily Chasmosaurinae: sexual dimorphism and systematics. — In:Carpenter, K. &Currie, P.J., eds., Dinosaur systematics: approaches and perspectives: 211–229, Cambridge (Cambridge University Press).Google Scholar
  36. Long, R.A. &Murry, P.A. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. — 254 p., Albuquerque (New Mexico Museum of Natural History and Science).Google Scholar
  37. Long, R.A.;Lucas, S.G.;Hunt, A.P. &McCrea, R.T. 1989. Charles Camp: collecting Late Triassic vertebrates in the American Southwest during the 1920s and 1930s. — In:Lucas, S.G. &Hunt, A.P., eds., Dawn of the age of the dinosaurs in the American Southwest: 65–71, Albuquerque (New Mexico Museum of Natural History and Science).Google Scholar
  38. Lucas, S.G. 1998. Global Triassic tetrapod biostratigraphy and biochronology. — Palaeogeography, Palaeoclimatology, Palaeoecology143: 347–384.CrossRefGoogle Scholar
  39. Lucas, S.G. &Hunt, A.P. 1992. Triassic stratigraphy and paleontology, Chama basin and adjacent areas, north-central New Mexico. — In:Lucas, S.G.;Kues, B.S. &Williamson, T.E., eds., New Mexico Geological Society 43rd Field Conference Guidebook: 151–172, Socorro (New Mexico Bureau of Geology and Mineral Resources).Google Scholar
  40. Lucas, S.G. &Hunt, A.P. 1993. Tetrapod biochronology of the Chinle Group (Upper Triassic), western United States. — In:Lucas, S.G. &Murales, M., eds., The nonmarine Triassic: 327–329, Albuquerque (New Mexico Museum of Natural History and Science).Google Scholar
  41. Lucas, S.G.;Heckert, A.B.;Zeigler, K.E. &Hunt, A.P. 2002. The type locality ofBelodon buceros Cope, 1881, a phytosaur (Archosauria: Parasuchidae) from the Upper Triassic of north-central New Mexico. — New Mexico Museum of Natural History, Bulletin21: 189–192.Google Scholar
  42. Martin, R.E. 1999. Taphonomy: a process approach. — 508 p. Cambridge (Cambridge University Press).Google Scholar
  43. McGregor, J.H. 1906. The Phytosauria with especial reference toMystriosuchus andRutiodon. — Memoirs of the American Museum of Natural History9: 29–101.Google Scholar
  44. Neill, W.T. 1971. The last of the ruling reptiles: alligators, crocodiles, and their kin. — 486 p, New York (Columbia University Press).Google Scholar
  45. Nopsca, F.B. 1929. Sexual differences in ornithopodous dinosaurs. — Palaeobiologica2: 187–201.Google Scholar
  46. Olson, E.C. 1968. Sexual dimorphism in extinct amphibians and reptiles. — In:Westermann, G.E.G., ed., Sexual dimorphism in fossil Metazoa and taxonomic implications: 223–225 (Series A, no. 1), Ottawa (International Union of Geological Sciences).Google Scholar
  47. Raath, M. 1977. The anatomy of the Triassic theropodSyntarsus rhodesiensis (Saurischia: Podokesauridae) and a consideration of its biology. — 353 p., London (Ph. D. thesis, Rhodes University).Google Scholar
  48. Raath, M.A. 1990. Morphological Variation in small theropods and its meaning in systematics. — In:Carpenter, K. &Currie, P.J., eds., Dinosaur systematics: approaches and perspectives: 91–106, Cambridge (Cambridge University Press).Google Scholar
  49. Rinehart, L.F.;Lucas, S.G. & Heckert, A.B. 2001. Preliminary Statistical analysis defining the juvenile, robust and gracile forms of the Triassic dinosaurCoelophysis. — Journal of Vertebrate Paleontology,21 (Supp. to 3): 93A.Google Scholar
  50. Sampson, S.D. 1999. Sex and destiny: the role of mating Signals in speciation and macroevolution. — Historical Biology13: 173–197.CrossRefGoogle Scholar
  51. Sander, P.M. 1989. The Pachypleurosaurids (Reptilia: Nothosauria) from the Middle Triassic of Monte San Giorgio (Switzerland) with the description of a new species. — Philosophical Transactions of the Royal Society of London (B)325: 561–666.CrossRefGoogle Scholar
  52. Tereshchenko, V.S. 2001. Sexual dimorphism in the postcranial skeleton of protoceratopsids (Neoceratopsia, Protoceratopsidae) from Mongolia. — Paleontological Journal35 (4): 415–425.Google Scholar
  53. Webb, G.J.W. &Messel, H. 1978. Morphometric analysis ofCrocodylus porosus from the north coast of Arnhem Land, northern Australia. — Australian Journal of Zoology26: 1–27.CrossRefGoogle Scholar
  54. Westphal, F. 1976. Phytosauria. — Handbuch der Paläoherpetologie13: 99–120.Google Scholar

Copyright information

© E. Schweizerbart’sche Verlagsbuchhandlung 2003

Authors and Affiliations

  • Kate E. Zeigler
    • 1
  • Spencer G. Lucas
    • 2
  • Andrew B. Heckert
    • 2
  1. 1.Department of Earth and Planetary SciencesUniversity of New MexicoAlbuquerqueU.S.A.
  2. 2.New Mexico Museum of Natural HistoryAlbuquerqueU.S.A.

Personalised recommendations