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Thescelosaurus

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Thescelosaurus
Temporal range: Late Cretaceous
Thescelosaurus neglectus
Fossil
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Sauropsida
Superorder:
Dinosauria
Order:
Ornithischia
Suborder:
Ornithopoda
Family:
Hypsilophodontidae
Subfamily:
Thescelosaurinae
Genus:
Thescelosaurus
Binomial name
Thescelosaurus neglectus
Gilmore, 1913

Thescelosaurus (from the Greek meaning "wonderful", "marvelous", or "surprising" "lizard") was a genus of small ornithopod dinosaur that appeared at the very end of the Late Cretaceous period in North America, and was a member of the last dinosaurian fauna before the extinction at the Cretaceous-Tertiary boundary at around 65.5 million years ago. It is known from several partial skeletons and skulls that show it to have been around 2.4-3.7 meters (8-12 feet) long on average, with sturdy hind limbs, small wide hands, a head with an elongate pointed snout, and possibly small armor scutes along the midline of the back. It is regarded as a specialized hypsilophodont and a herbivore. The genus also attracted media attention in 2000, when a specimen was interpreted as including a fossilized heart.

Discovery and species

The type specimen of Thescelosaurus (USNM 7757) was first discovered in 1891 by paleontologists John Bell Hatcher and William H. Utterback, from beds of the late Maastrichtian-age Upper Cretaceous Lance Formation of Niobrara County, Wyoming, USA. The skeleton, however, remained in its shipping crates, undescribed, until Charles Gilmore of the National Museum of Natural History (Smithsonian Institution) had it prepared and described it, first naming it as T. neglectus ("Neglected wonderful, surprising, or marvelous lizard") in a short paper in 1913.[1] He provided a detailed monograph in 1915, describing its well-preserved skeleton. The type specimen was found largely in natural articulation and was missing only the head and neck (lost due to erosion).[2] The name comes from the surprise Gilmore felt at finding such a good specimen that had been unattended to for so long. He considered it to be a light, agile creature, and assigned it to the Hypsilophodontidae, a family of small bipedal dinosaurs.[2]

Charles Gilmore's 1915 reconstruction of Thescelosaurus.

Other remains of similar animals were found throughout the late 1800s and 1900s, although they didn't receive much attention. Another very good skeleton from slightly older beds in Alberta, Canada, was named T. warreni by William Parks in 1926,[3] but this animal had notable differences from T. neglectus, and so Charles M. Sternberg renamed it Parksosaurus in 1937.[4] Sternberg also named an additional species, T. edmontonensis, based on another articulated skeleton, this time including a partial skull (NMC 8537), and drew attention to the genus' heavy build and thick bones; due to these differences from the regular light hypsilophodont build, he suggested that the genus warranted its own subfamily, Thescelosaurinae.[5] T. edmontonensis has generally been considered a more robust individual (possibly the opposite sex of the type individual) of T. neglectus since Peter Galton's 1974 review,[6][7][8] with the notable exception of William Morris (1976), who considered the ankles of the two species to be significantly different.[9]

In his paper, Morris described a partial skull with heavy ridges on the lower jaw and cheek (SDSM 7210) as T. sp, from the late Maastrichtian-age Hell Creek Formation of Harding County, South Dakota, USA.[9] This skull was recognized as an unnamed hypsilophodont for many years,[7] until Galton renamed it Bugenasaura infernalis.[10] Morris also named a new species, on the basis of vertebrae and limb remains (LACM 33542): T. garbanii, from the Hell Creek Formation of Garfield County, Montana, USA. Because he believed that its ankles compared favorably to those of T. edmontonensis, he assigned it to Thescelosaurus. T. garbanii would have been about 4.5 m (15 feet) long, much larger than average T. neglectus.[9] However, the literature favored Galton's view that T. edmontonensis was not different from T. neglectus (see above), and that the ankle was actually damaged.[10] To better accommodate this species, Galton in 1995 suggested that it belonged to his new genus Bugenasaura as B. garbanii (although noting that it could also be the leg of similarly sized pachycephalosaurid Stygimoloch).[10] Only one valid species of Thescelosaurus is currently recognized: T. neglectus.[10][11]

Classification

Thescelosaurus has almost invariably been allied to Hypsilophodon and other small ornithopods as a hypsilophodontid,[2][9] although usually recognized as being distinct among them for its robust build, unusual hindlimbs (the femur is longer than the tibia, the opposite of Hypsilophodon and running animals in general),[2] and (more recently) its unusual skull.[5][6][7][8] Galton (1974) presented one twist to the classic arrangment, suggesting that because of its hindlimb structure and heavy build (not cursorial by his definition), it should be included in Iguanodontidae. This has not been followed, Morris arguing strongly against Galton's classification scheme;[9] at any rate, Galton's Iguanodontidae was polyphyletic, and so would not be recognized under modern cladistic usage.

Today, it is generally thought, although little tested by cladistics, that Bugenasaura and Thescelosaurus are closely related,[11] and/or that Thescelosaurus belongs in its own family or subfamily, Thescelosauridae or Thescelosaurinae (Norman et al [2004] found this result in their study, although they didn't name the clade).[8][12][13][14] Two recent studies have found it to be a close relative of Parksosaurus.[15][8] This area of the dinosaur family tree is complicated by a lack of research, with some papers finding Hypsilophodontidae to be a natural group,[7][16] and others finding it to be a paraphyletic assemblage leading into Iguanodontia.[17][18][19][15][8] Oddly, Thescelosaurus has been regarded as both very basal[16] and very derived.[8]

Paleobiology

Head and arms of Thescelosaurus.

Thescelosaurus was a heavily-built bipedal animal, probably mostly herbivorous,[8] but possibly omnivorous.[20] It would have browsed in the first meter or so from the ground,[8] with food held in the mouth by cheeks while chewing.[6] Aside from the long narrow beak, the skull also had premaxillary teeth (a primitive trait among ornithopods) and long palpebrals over the eyes, giving it heavy bony eyebrows.[21][8] Its teeth were of two types: small pointed premaxillary teeth, and leaf-shaped cheek teeth.[5] The exact number of teeth is unknown, as complete jaws have not been described. It had short, broad, five-fingered hands, four-toed feet with hoof-like unguals, and a long tail surrounded by ossified tendons, making it very stiff.[2] The rib cage was broad, giving it a wide back, and the limbs were robust.[5] This animal may have been able to move on all fours,[6] given fairly long arms and wide hands, but this idea has not been followed up. It was probably slower than other hypsilophodonts, due to its heavier build and leg structure.[5] Large thin flat bony plates have been found next to the sides of the ribs,[22] similar to those in Talenkauen,[23] with an unknown function. Gilmore described patches of carbonized material near the shoulders as possible epidermis, with a "punctured" texture but no regular pattern,[2] but no further reports of scalation or skin has been published. Morris suggested that armor was present, in the form of small scutes present at least along the midline of the neck,[9] but no further reports have surfaced. The size has been estimated as in the 2.5-4 m range for length (8.2-13.1 ft)[6][24] for various specimens, and a mass of 200-300 kilograms (450-660 pounds).[24] As mentioned in "Discovery and species", it may have been sexually dimorphic, with one sex larger than the other.[6] Juvenile remains are known from several locations, mostly based on teeth.[25][14]

Conflicting reports have been made as to its preferred habitat; one paper suggests it preferred channels to floodplains,[26] but another suggests it preferred the opposite.[13]

The skeletal anatomy of this genus is well-documented (except for the head), and restorations have been published in several papers, including skeletal restorations,[2][6][27][28] and models.[2][5] One paper, by Alfred Sherwood Romer, offers a detailed reconstruction of the pelvic and hindlimb muscles.[29]

"Heart of stone"

In 2000, a skeleton of this genus informally known as "Willo", now on display at the North Carolina Museum of Natural Sciences, was described as including the remnants of a four-chambered heart, and an aorta. The authors had found this internal detail through CT imagery. They suggested that the heart had been saponified (turned to soap) under anaerobic burial conditions, and then permineralized by goethite, an iron mineral. The authors suggested that the structure of the heart indicated an elevated metabolic rate for Thescelosaurus (i.e. greater than reptilian ectothermy).[22]

The possible heart of "Willo" (center).

Their conclusions have been disputed; another team of researchers published a paper wherein they assert that the heart is really a concretion; as they note, the anatomy given for the object is incorrect (for example, the "aorta" narrows coming into the "heart" and lacks arteries coming from it), it partially engulfs one of the ribs, it has an internal structure of concentric layers in some places, and another concrection is preserved behind the right leg.[30] The original authors defended their position; they agreed that it was a type of concretion, but one that had formed around and partially preserved the more muscular portions of the heart and aorta.[31] The question of how this find reflects on metabolic rate and dinosaur internal anatomy is moot, though; Regardless of the object's identity, dinosaurs probably had four-chambered hearts because both crocodilians and birds, the closest living relatives of dinosaurs, have them, so the structure is not tied to metabolic rate.[32]

Temporal and geographic range

True Thescelosaurus remains are known definitely only from late Maastrichtian-age rocks, from Alberta (Scollard Formation) and Saskatchewan (Frenchman Formation) (Canada), and Wyoming (Lance Formation), South Dakota (Hell Creek Formation), Montana (Hell Creek), and Colorado (Laramie Formation), USA.[6][8] It was one of the last genera of nonavian dinosaurs, its remains being found as close as 3 meters to the boundary clay containing the iridium layer that closes the Cretaceous.[33] Records of teeth from older, Judithian-age rocks, particularly the Dinosaur Park Formation of Alberta, Canada,[34] are not from Thescelosaurus and are much more like those of Orodromeus.[10] More specimens are known than have been officially described for this genus (for example "Willo", with its complete skull, and the Triebold specimen,[20] which has been the source of several skeletal casts for museums). Other dinosaurs that shared its time and age include Bugenasaura, the ceratopsids Triceratops and Torosaurus, hadrosaurids Edmontosaurus and Anatotitan, ankylosaurid Ankylosaurus, pachycephalosaurians Pachycephalosaurus and Stygimoloch, and the theropods Ornithomimus, Troodon, and Tyrannosaurus.[35][36]

Miscellaneous

Despite its reputation for slowness,[5][6] Thescelosaurus has been a featured animal in a math problem given to undergraduate students, based around the question of if its agility is enough to outmatch either one or two Velociraptor (for the curious, here is one detailed solution[37]).[38]

References

  1. ^ Gilmore, C.W. (1913). A new dinosaur from the Lance Formation of Wyoming. Smithsonian Miscellaneous Collections 61(5):1-5.
  2. ^ a b c d e f g h Gilmore, C.W. (1915). Osteology of Thescelosaurus, an orthopodus dinosaur from the Lance Formation of Wyoming. Proceedings of the U.S. National Museum 49(2127):591-616.
  3. ^ Parks, W.A. Thescelosaurus warreni, a new species of orthopodous dinosaur from the Edmonton Formation of Alberta. University of Toronto Studies (Geological Series) 21:1-42.
  4. ^ Sternberg, C.M. (1937). Classification of Thescelosaurus, with a description of a new species. Geological Society of America Proceedings for 1936:365.
  5. ^ a b c d e f g Sternberg, C.M. (1940). Thescelosaurus edmontonensis, n. sp., and classification of the Hypsilophodontidae. Journal of Paleontology 14(5):481-494.
  6. ^ a b c d e f g h i Galton, P.M. (1974). Notes on Thescelosaurus, a conservative ornithopod dinosaur from the Upper Cretaceous of North America, with comments on ornithopod classification. Journal of Paleontology 48(5):1048-1067.
  7. ^ a b c d Sues, H.-D., and Norman, D.B. (1990). Hypsilophodontidae, Tenontosaurus, Dryosauridae. In: Weishampel, D.B., Osmólska, H., and Dodson, P. (eds.). The Dinosauria. University of California Press:Berkeley, 498-509. ISBN 0-520-06727-4
  8. ^ a b c d e f g h i j Norman, D.B., Sues, H.D., Witmer, L.M., and Coria, R.A. (2004). Basal Ornithopoda. In: Weishampel, D.B., Osmólska, H., and Dodson, P. (eds.). The Dinosauria (second edition). University of California Press:Berkeley, 393-412. ISBN 0-520-24209-2
  9. ^ a b c d e f Morris, W.J. (1976). Hypsilophodont dinosaurs: a new species and comments on their systematics. In: Churcher, C.S. (ed.). Athlon. Royal Ontario Museum:Toronto, Canada, 93-113. ISBN 0888541570
  10. ^ a b c d e Galton, P.M. (1995). The species of the basal hypsilophodontid dinosaur Thescelosaurus Gilmore (Ornithischia: Ornithopoda) from the Late Cretaceous of North America. Neues Jahrbuch fèur Geologie und Palèaontologie Abhandlungen 198(3): 297-311.
  11. ^ a b Galton, P.M. (1999). Cranial anatomy of the hypsilophodont dinosaur Bugenasaura infernalis (Ornithischia: Ornithopoda) from the Upper Cretaceous of North America. Revue Paléobiologie, Genève 18(2):517-534.
  12. ^ Holtz, Jr., T.R. (2000). Classification and Evolution of the Dinosaur Groups. In: Paul, G.S. (ed.). The Scientific American Book of Dinosaurs. St. Martin's Press:New York, 140-168. ISBN 0-312-26226-4.
  13. ^ a b Carpenter, K., and Young, B.D. (2002). Late Cretaceous dinosaurs from the Denver Basin, Colorado. Rocky Mountain Geology 37(2):237-254.
  14. ^ a b Russell, D.A., and Manabe, M. (2002). Synopsis of the Hell Creek (uppermost Cretaceous) dinosaur assemblage. In: Hartman, J.H., Johnson, K.R., and Nichols, D.J. (eds.). The Hell Creek Formation and the Cretaceous-Tertiary Boundary in the Northern Great Plains: An Integrated Continental Record of the End of the Cretaceous. Geological Society of America Special Paper 361. Geological Society of America:Boulder, Colorado, 169-176.
  15. ^ a b Weishampel, D.B., Jianu, C.-M., Csiki, Z., and Norman, D.B. (2003). Osteology and phylogeny of Zalmoxes (n.g.), an unusual euornithopod dinosaur from the latest Cretaceous of Romania. Journal of Systematic Palaeontology 1(2):1-56.
  16. ^ a b Weishampel, D.B., and Heinrich, R.E. (1992). Systematics of Hypsilophodontidae and Basal Iguanodontia (Dinosauria: Ornithopoda). Historical Biology 6:159-184.
  17. ^ Scheetz, R.D. (1998). Phylogeny of basal ornithopod dinosaurs and the dissolution of the Hypsilophodontidae. Journal of Vertebrate Paleontology 18(3, Suppl.):75A.
  18. ^ Winkler, D.A., Murry, P.A., and Jacobs, L.L. (1998). The new ornithopod dinosaur from Proctor Lake, Texas, and the deconstruction of the family Hypsilophodontidae. Journal of Vertebrate Paleontology 18(3, Suppl.):87A.
  19. ^ Buchholz, P.W. (2002). Phylogeny and biogeography of basal Ornithischia. In: The Mesozoic in Wyoming, Tate 2002. The Geological Museum, Casper College:Casper, Wyoming, 18-34.
  20. ^ a b Triebold Paleontology, Inc. "Thescelosaurus". Retrieved 2007-01-26.
  21. ^ Galton, P.M. (1997). Cranial anatomy of the basal hypsilophodontid dinosaur Thescelosaurus neglectus Gilmore (Ornithischia; Ornithopoda) from the Upper Cretaceous of North America. Revue Paléobiologie, Genève 16(1):231-258.
  22. ^ a b Fisher, P.E., Russell, D.A., Stoskopf, M.K., Barrick, R.E., Hammer, M., and Kuzmitz, A.A. (2000). Cardiovascular evidence for an intermediate or higher metabolic rate in an ornithischian dinosaur. Science 288(April):503-505.
  23. ^ Novas, F.E., Cambiaso, A.V., and Ambrosio, A., (2004). A new basal iguanodontian (Dinosauria, Ornithischia) from the Upper Cretaceous of Patagonia. Ameghiniana 41(1):75-82.
  24. ^ a b Erickson, B.R. (2003). Dinosaurs of the Science Museum of Minnesota. The Science Museum of Minnesota:St. Paul, Minnesota, x-86. ISBN 911338-54-3
  25. ^ Carpenter, K. (1982). Baby dinosaurs from the Late Cretaceous Lance and Hell Creek formations and a description of a new species of theropod. Contributions to Geology 20(2):123-134.
  26. ^ Pearson, D.A., Schaefer, T., Johnson, K.R., Nichols, D.J., and Hunter, J.P. (2002). Vertebrate biostratigraphy of the Hell Creek Formation in southwestern North Dakota and northwestern South Dakota. In: Hartman, J.H., Johnson, K.R., and Nichols, D.J. (eds.). The Hell Creek Formation and the Cretaceous-Tertiary Boundary in the Northern Great Plains: An Integrated Continental Record of the End of the Cretaceous. Geological Society of America Special Paper 361. Geological Society of America:Boulder, Colorado, 145-167.
  27. ^ Brett-Surman, M.K. (1997). Ornithopods. In: Farlow, J.O., and Brett-Surman, M.K. (eds.). The Complete Dinosaur. Indiana University Press:Bloomington and Indianapolis, 330-346. ISBN 0-253-33349-0
  28. ^ Paul, G.S. (2000). The Scientific American Book of Dinosaurs. St. Martin's Press:New York, 401. ISBN 0-312-26226-4.
  29. ^ Romer, A.S. (1927). The pelvic musculature of ornithischian dinosaurs. Acta Zoologica 8:225-275.
  30. ^ Rowe, T., McBride, E.F., and Sereno, P.C. (2001). Technical comment: dinosaur with a heart of stone. Science 291(February):783a.
  31. ^ Russell, D.A., Fisher, P.E., Barrick, R.E., and Stoskopf, M.K. (2001). Reply: dinosaur with a heart of stone. Science 291(February):783a.
  32. ^ Chinsamy, A., and Hillenius, W.J. (2004). Physiology of nonavian dinosaurs. In: Weishampel, D.B., Osmólska, H., and Dodson, P. (eds.). The Dinosauria (second edition). University of California Press:Berkeley, 643-659. ISBN 0-520-24209-2
  33. ^ Carpenter, K., and Breithaupt, B.H. (1986). Latest Cretaceous occurrences of nodosaurid ankylosaurs (Dinosauria, Ornithischia) in Western North America and the gradual extinction of the dinosaurs. Journal of Vertebrate Paleontology 6(3):251-257.
  34. ^ Sahni, A. (1972). The vertebrate fauna of the Judith River Formation, Montana. Bulletin of the American Museum of Natural History 147:321-412.
  35. ^ Weishampel, D.B., Barrett, P.M., Coria, R.A., Le Loeuff, J., Xu Xing, Zhao Xijin, Sahni, A., Gomani, E.M.P., and Noto, C.R. (2004). Dinosaur Distribution: in Weishampel, D.B., Dodson, P., and Osmólska, H., (eds.). The Dinosauria (2nd edition). University of California Press:Berkeley, 517-606. ISBN 0-520-24209-2
  36. ^ Phillip Bigelow. "Cretaceous "Hell Creek Faunal Facies; Late Maastrichtian". Retrieved 2007-01-26.
  37. ^ Amrita Das. "The Hunting Strategy of the Velociraptor — Amrita Das". Retrieved 2007-01-26.
  38. ^ John David Stone. "Mathematical Contest in Modeling, 1997". Retrieved 2007-01-29.
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