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Ankylosaurus

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Ankylosaurus
Temporal range: Late Cretaceous, 68–66 Ma
Brown skull cast on a pedestal
Cast of Ankylosaurus skull (AMNH 5214) in front view, Museum of the Rockies
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Suborder: Ankylosauria
Family: Ankylosauridae
Subfamily: Ankylosaurinae
Genus: Ankylosaurus
Brown, 1908
Type species
Ankylosaurus magniventris
Brown, 1908

Ankylosaurus (/ˌæŋkɨlɵˈsɔrəs/ ANG-ki-lo-SAWR-əs or /æŋˌklɵˈsɔrəs/ ang-KY-lo-SAWR-əs, meaning "fused lizard") is a genus of ankylosaurid dinosaur. Fossils of Ankylosaurus are found in geologic formations dating to the very end of the Cretaceous Period, between about 68–66 million years ago, in western North America. Ankylosaurus was named by Barnum Brown in 1908, and the only classified species in the genus is A. magniventris. Five specimens have been escavated to date, but a complete skeleton has not been discovered. Though other ankylosaurs are represented by more extensive fossil material, Ankylosaurus is often considered the archetypal member of the group.

Ankylosaurus is the largest known ankylosaurid, and measured up to 6.25 m (20.5 feet) in length. It had a long, low skull, with backwards pointed horns. The front part of the jaws were covered in a beak, with rows of small teeth further behind it. It was covered in armor plates, or osteoderms, with a bony half ring covering part of its neck and shoulders, and had a large club on the end of its tail. The tail club is thought to have been used in defense against predators or in intraspecific combat.

Ankylosaurus is a member of the subfamily Ankylosaurinae, and its closest relatives appear to be Euoplocephalus and Anodontosaurus. Ankylosaurus has been found in the Hellcreek, Lance, and Scollard formations, but appears to have been rare in its environment. Although it lived alongside another ankylosaur, Edmontonia, their ranges and ecological niches do not appear to have overlapped, and Ankylosaurus may have inhabited upland areas. Ankylosaurus lived alongside dinosaurs such as Tyrannosaurus, Triceratops, and Edmontosaurus.

Description[edit]

Outline of human superimposed on outline of Ankylosaurus
Size of the largest known skull (black) with estimated body size (green), compared to a human

Ankylosaurus is the largest known ankylosaurid dinosaur, estimated to have been up to 6.25 m (20.5 feet) long, 1.5 m (4.9 feet) wide, and 1.7 m (5.6 feet) tall at the hip. This length is based on the largest known skull (specimen NMC 8880), which is 64.5 cm (25.4 inches) long and 74.5 cm (29.3 inches) wide. The smallest known skull (specimen AMNH 5214) is 55.5 cm long and 64.5 cm wide.[1] The animal has also been estimated to be 9 m (30 ft) long,[2] and weighing up to 6 tonnes (13,000 lb).[3]

Image of restored specimen of a squat quadruped with knobby back
Life restoration

The body was low-slung and quite wide. It was quadrupedal, with the hind limbs longer than the forelimbs. Although the form of its feet is still unknown, comparisons with other ankylosaurs suggest Ankylosaurus probably had five toes on each foot. The skull was low and triangular in shape, wider than it was long. Like other ankylosaurs, Ankylosaurus had small, leaf-shaped teeth, which were the smallest relative to the body size among ankylosaurid species.[4] Ankylosaurus lacked the tooth batteries of the contemporaneous ceratopsid and hadrosaurid dinosaurs. Bones in the skull and other parts of the body were fused, increasing their strength.[1] [5]

Armor[edit]

Two fossilized knobs of bone, black with white streaks
Osteoderms of the holotype specimen (AMNH 5895)

A prominent feature of Ankylosaurus is its armor, consisting of massive knobs and plates of bone known as osteoderms or scutes embedded in the skin. Osteoderms are also found in the skin of crocodilians, armadillos and some lizards. The bone was probably covered by a tough, horny layer of keratin. The osteoderms varied greatly in shape, from wide, flat plates to small, round nodules. The plates were aligned in regular horizontal rows down the animal's neck, back, and hips, with the many smaller nodules protecting the areas between the large plates. Smaller plates may have been arranged on the limbs and tail. Compared to the slightly older ankylosaurid Euoplocephalus, the plates of Ankylosaurus were smoother in texture, and without the high keels found on the armor of the contemporaneous nodosaurid Edmontonia. A row of flat, triangular spikes may have protruded laterally along each side of the tail. Tough, rounded scales protected the top of the skull, while four large pyramidal horns projected outwards from its rear corners.[1]

Ankylosaurus tail club (AMNH 5214)

The famous tail club of Ankylosaurus was also composed of several large osteoderms, which were fused to the last few tail vertebrae. It was heavy and supported by the last seven tail vertebrae, which interlocked to form a stiff rod at the base of the club. Thick tendons which attached to these vertebrae have been preserved in the fossil record.[1]

History of discovery[edit]

Photograph of dorsal view of fossilized skull next to sketch of the same
Skull of holotype AMNH 5895 and reconstruction diagram

In 1906, an American Museum of Natural History expedition led by paleontologist Barnum Brown discovered the type specimen of Ankylosaurus magniventris (AMNH 5895) in the Hell Creek Formation, near Gilbert Creek, Montana. The specimen (found by collector Peter Kaisen) consisted of the upper part of a skull, two teeth, part of the shoulder girdle, cervical, dorsal, and caudal vertebrae, ribs, and more than thirty osteoderms.[6] Brown scientifically described the animal 1908; the genus name is derived from the Greek words 'αγκυλος/ankulos ('bent' or 'crooked'), referring to the medical term ankylosis, the stiffness produced by the fusion of bones in the skull and body, and σαυρος/sauros ('lizard'). The name can be translated as "fused lizard", "stiff lizard", or "curved lizard". The type species name magniventris is derived from the Latin magnus ('great') and venter ('belly'), referring to the great width of the animal's body.[7][8]

Images of skeleton; side view facing left, dorsal view, and dorsal view of dorsal plates
1908 skeletal reconstruction of AMNH 5895, with missing parts restored after Stegosaurus

The skeletal reconstruction accompanying the 1908 description restored the missing parts in a fashion similar to Stegosaurus, and Brown likened the result to the extinct armored mammal Glyptodon.[6] In a 1908 review of Brown's Ankylosaurus description, Samuel Wendell Williston criticised the skeletal reconstruction as being based on too scanty remains, and claimed that Ankylosaurus was merely a synonym of the genus Stegopelta, which Williston had named in 1905. Williston also stated that a skeletal reconstruction of the related Polacanthus by Franz Nopcsa was a better example of how ankylosaurs would have appeared in life.[9] The claim of synonymy was not accepted by other researchers, and the two genera are now considered distinct.[10]

Brown had collected seventy-seven osteoderms while excavating a Tyrannosaurus specimen in the Lance Formation of Wyoming in 1900. He mentioned these osteoderms (AMNH 5866) in his description of Ankylosaurus, but thought they belonged to the Tyrannosaurus instead. Henry Fairfield Osborn also expressed this view when he described the Tyrannosaurus specimen as the now invalid genus Dynamosaurus in 1905. Later examination has shown them to be similar to those of Ankylosaurus, and that Brown had compared them with some Euoplocephalus osteoderms, which had been erroneously catalogued as belonging to Ankylosaurus at the AMNH.[1][11]

Photograph in black and white of rugged, fissured cliff face
Excavation of AMNH 5214 (centre, above the pick), 1910

In 1910, another AMNH expedition led by Brown discovered an Ankylosaurus specimen (AMNH 5214) in the Scollard Formation by the Red Deer River in Alberta, Canada. This specimen included a complete skull, mandibles, and the first and only tail club known of this genus, as well as ribs, vertebrae, limb bones, and armor. In 1947, Charles M. Sternberg and T.P. Channey collected a skull and mandible (NMC 8880), a kilometre north of where the 1910 specimen was found. This is the largest known Ankylosaurus skull, but is badly preserved. A section of caudal vertebrae (CCM V03) was discovered in the 1960s, in the Powder River drainage, Montana, also part of the Hell Creek Formation. In addition, many other isolated osteoderms and teeth have been found.[1] In 1990, Walter P. Coombs pointed out that the teeth of two skulls referred to A. magniventris differed from those of the holotype specimen in some details, and though he expressed a "considerate temptation" to name a new species of Ankylosaurus for these, he refrained from doing so, as the range of variation in the species was not completely documented.[5]

Most of the five known incomplete Ankylosaurus specimens were not described at length, though several palaeontologists planned to do so, until American palaeontologist Kenneth Carpenter redescribed the genus in 2004. Carpenter noted that Ankylosaurus has become the archetypal ankylosaur, and the best known member of the group in popular culture, perhaps due to a life-sized reconstruction of the animal being featured at the 1964 World's Fair in New York City. The structure of much of the skeleton, including the pelvis and most of the tail and feet, is still unknown.[1]

Classification[edit]

Ribs and vertebra of AMNH 5895; broad bodies are typical of ankylosaurs

Brown considered Ankylosaurus so distinct that he made it the type genus of a new family, Ankylosauridae (members of which are called ankylosaurids), typified by massive, triangular skulls, stiff backs, broad bodies, and osteoderms. He also classified Palaeoscincus (only known from teeth), and Euoplocephalus (then only known from a partial skull and osteoderms) as part of the family. Due to the fragmentary remains, Brown was unable to fully distinguish between Euoplocephalus and Ankylosaurus. The genus Troodon was included as well, at the time only known from a single tooth, which Brown found to be similar to those of the ankylosaurs. Due to the few remains known of this family, he believed the group was part of the suborder Stegosauria.[6] In 1923, Osborn coined the name Ankylosauria (members of which are called ankylosaurs), thereby giving the ankylosaurids their own suborder.[12]

Two views of Ankylosaurus skull casts, from above and from the left
Skull (AMNH 5214) from the side and above, showing the triangular shape typical of the group

Ankylosauria and Stegosauria are now grouped together within the clade Thyreophora, consisting of armored dinosaurs. Ankylosauromorphs first appeared in the Sinemurian age, and survived for 135 million years, until disappearing in the Maastrichtian. They were widespread and inhabited a broad range of environments.[1][13] As more complete specimens and new taxa have been discovered, theories about ankylosaurian interrelatedness have become more complex, and hypotheses have often changed between studies. In addition to Ankylosauridae, Ankylosauria has been divided into the families Nodosauridae, and sometimes Polacanthidae (these families lacked tail clubs). Ankylosaurus is considered part of the subfamily Ankylosaurinae within Ankylosauridae.[14] Ankylosaurus and Euoplocephalus are sometimes thought to be sister taxa, but have also been found in different positions.[2][15] The following cladogram is based on a 2013 phylogenetic analysis of the Ankylosaurinae conducted by Victoria M. Arbour and Philip J. Currie:[16]

Ankylosaurinae

Scolosaurus





Ankylosaurus




Anodontosaurus



Euoplocephalus







Dyoplosaurus



Talarurus



Tsagantegia





Minotaurasaurus



Nodocephalosaurus



Tianzhenosaurus





Pinacosaurus





Tarchia



Saichania










Palaeobiology[edit]

Tooth of AMNH 5895 in inside and outside view

Like other ornithischians, Ankylosaurus was herbivorous. Its wide muzzle was adapted for non-selective low-browse cropping.[1] Paleontologist Georg Haas (1969) concluded that despite the large size of the skulls, the associated musculature was relatively weak. He also thought jaw movement was limited to up and down movements. Extrapolating from this, Haas suggested that ankylosaurids ate relatively soft non-abrasive vegetation.[17] However, later research on Euoplocephalus indicates that forward and sideways jaw movement was possible.[18] Ankylosaurus may have had a hindgut fermentation system like modern herbivorous lizards, based on the features of the ribcage.[1]

Diagram of internal chambers of a skull
Diagram showing nasal chambers inside the skull (AMNH 5895)

Nasal passages of Ankylosaurus indicate that airflow was unidirectional, (looping through the lungs during inhalation and exhalation), although it may also have been bidirectional in the posterior nasal chamber. The complex air passages may have acted as a chamber for vocal resonance but this hypothesis was rejected by Carpenter (2004).[1] A 2011 study of the air passages of the related genus Euoplocephalus suggested this function. In addition, the researchers suggested that ankylosaurids had well-developed olfactory systems and ears adapted to hearing at low frequencies, such as the low-tuned resonant sounds produced by the nasal passages.[19] The position of the orbits of Ankylosaurus suggest some stereoscopic vision.[1]

Reconstructions of the forelimb musculature made by Coombs in 1978 suggest that the forelimbs bore the majority of the animal's weight, and were adapted for high force delivery of the front feet, possibly for food gathering. In addition, Coombs suggested that ankylosaurids may have been capable diggers, though the hoof-like structure of the manus would have limited fossorial activity. Ankylosaurids were likely to have been slow-moving and sluggish animals.[20][21]

Defense[edit]

Depiction of Ankylosaurus displaying its tail club

The osteoderms of ankylosaurids were thin in comparison to those of other ankylosaurs, and appear to have been strengthened by randomly distributed cushions of collagen fibers. These were structurally similar to Sharpey's fibers, and were embedded directly into the bone tissue, a feature unique to ankylosaurids. This would have provided the ankylosaurids with an armor covering which was both lightweight and highly durable, being resistant to breakage and penetration by the teeth of predators.[22] In addition to protection, Carpenter (1982) suggests that the heavily-vascularized armor may have had a role in thermoregulation as in modern crocodilians.[23]

The tail club of Ankylosaurus seems to have been an active defensive weapon, capable of producing enough of an impact to break the bones of an assailant. The tendons of the tail were partially ossified (or bony) and were not very elastic, allowing great force to be transmitted to the club when it was swung.[1] Coombs suggested in 1979 that the large hindlimb muscles would have controlled the swinging of the tail, and that violent thrusts of the club would have been able to break the metatarsal bones of large theropods.[21] A 2009 study found that large ankylosaurid tail clubs were capable of breaking bones, but medium and small clubs were not. Despite the feasibility of tail swinging, the researchers could not determine whether ankylosaurids used their clubs for defense against potential predators, in intraspecific combat or both.[24] Thulborn (1993) has proposed that the tail club acted as a decoy for the head,[25] although this idea is disputed.[1]

Palaeoecology[edit]

Ankylosaurus and other animals of the Hell Creek Formation

Ankylosaurus existed between 68 to 66 million years ago, in the final, or Maastrichtian, stage of the Late Cretaceous Period. It was among the last dinosaur species that appeared before the Cretaceous–Paleogene extinction event. The type specimen is from the Hell Creek Formation of Montana, while other specimens have been found in the Lance Formation of Wyoming and the Scollard Formation in Alberta, Canada, all of which date to the end of the Cretaceous.[2][26] Fossils of Ankylosaurus are rare in these sediments, and the distribution of its remains suggest that it was restricted to the uplands of the formations, rather than the coastal lowlands. Another ankylosaur, the nodosaurid Edmontonia, is also found in the same formations, but the range of the two genera does not seem to have overlapped. Their remains have so far not been found in the same localities, and Edmontonia appears to have inhabited the lowlands. The narrow muzzle of Edmontonia suggests it had a more selective diet than of Ankylosaurus, further indicating ecological separation.[1]

The Hell Creek, Lance and Scollard Formations represent different sections of the western shore of the shallow sea that divided western and eastern North America during the Cretaceous. They represent a broad coastal plain, extending westward from the seaway to the newly formed Rocky Mountains. These formations are composed largely of sandstone and mudstone, which have been attributed to floodplain environments.[27][28][29] The Hell Creek is the best studied of these ancient environments. At the time, this region was subtropical, with a warm and humid climate. Many plant species were supported, primarily angiosperms, with less common conifers, ferns and cycads. An abundance of fossil leaves found at dozens of different sites indicates that the area was largely forested by small trees.[30] Ankylosaurus shared its environment with dinosaurs including the ceratopsids Triceratops and Torosaurus, the hypsilophodont Thescelosaurus, the hadrosaurid Edmontosaurus, Edmontonia, the pachycephalosaurian Pachycephalosaurus, and the theropods Struthiomimus, Ornithomimus, Troodon, and Tyrannosaurus.[26][31]

See also[edit]

References[edit]

  1. ^ a b c d e f g h i j k l m n o Carpenter, Kenneth (2004). "Redescription of Ankylosaurus magniventris Brown 1908 (Ankylosauridae) from the Upper Cretaceous of the Western Interior of North America". Canadian Journal of Earth Sciences 41: 961–986. doi:10.1139/e04-043. 
  2. ^ a b c Vickaryous, M.K., Maryanska, T., & Weishampel, D.B. (2004). "Ankylosauria". In: Weishampel, D. B.; Dodson, P.; Osmólska, H., eds. (2004). The Dinosauria (2nd ed.). University of California Press. pp. 363–392. ISBN 0-520-24209-2. 
  3. ^ Coombs, Walter P. (December 1978). "Theoretical Aspects of Cursorial Adaptations in Dinosaurs". The Quarterly Review of Biology 53 (4): 393–418. doi:10.1086/410790. 
  4. ^ Carpenter, Kenneth (2001). The Armored Dinosaurs. Indiana University Press. p. 255. ISBN 0-253-33964-2. 
  5. ^ a b Coombs, W.P., Jr. (1990). Teeth and taxonomy in ankylosaurs. In K. Carpenter & P.J. Currie (Eds.), Dinosaur systematics: Approaches and perspectives (pp. 269-279). Cambridge; New York: Cambridge University Press.
  6. ^ a b c Brown, B. (1908). "The Ankylosauridae, a new family of armored dinosaurs from the Upper Cretaceous". Bulletin of the American Museum of Natural History. 24: 187–201. 
  7. ^ Creisler, Ben (July 7, 2003). "Dinosauria Translation and Pronunciation Guide A". Archived from the original on August 18, 2010. Retrieved September 3, 2010. 
  8. ^ Liddell, Henry George; Scott, Robert (1980) [1871]. A Greek-English Lexicon (abridged ed.). Oxford, United Kingdom: Oxford University Press. p. 5. ISBN 0-19-910207-4. 
  9. ^ Williston, S. W. (1908). "Review: The Ankylosauridae". The American Naturalist 42 (501): 629–630. doi:10.2307/2455817. JSTOR 2455817.  edit
  10. ^ Carpenter, Kenneth (2001). "Chapter 21: Phylogenetic analysis of the Ankylosauria". In Carpenter, Kenneth. The Armored Dinosaurs. Indiana University Press. pp. 454–483. 
  11. ^ Osborn, H. F. (1905). "Tyrannosaurus and other Cretaceous carnivorous dinosaurs". Bulletin of the AMNH (New York City: American Museum of Natural History) 21 (14): 259–265. hdl:2246/1464.  Retrieved October 6, 2008.
  12. ^ Osborn, H. F. (1923). "Two Lower Cretaceous dinosaurs of Mongolia." American Museum Novitates, 95: 1–10.[1]
  13. ^ Coombs, W. 1978. The families of the ornithischian dinosaur order Ankylosauria. Journal of Paleontology, 21:143-170.
  14. ^ Thompson, R. S.; Parish, J. C.; Maidment, S. C. R.; Barrett, P. M. (2012). "Phylogeny of the ankylosaurian dinosaurs (Ornithischia: Thyreophora)". Journal of Systematic Palaeontology 10 (2): 301. doi:10.1080/14772019.2011.569091.  edit
  15. ^ Hill, R. V.; Witmer, L. M.; Norell, M. A. (2003). "A new specimen of Pinacosaurus grangeri (Dinosauria: Ornithischia) from the Late Cretaceous of Mongolia: ontogeny and phylogeny of ankylosaurs". American Museum Novitates. 3395: 1–29. 
  16. ^ Arbour V.M. and Currie P.J., 2013, "Euoplocephalus tutus and the Diversity of Ankylosaurid Dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA", PLoS ONE 8(5): e62421. doi:10.1371/journal.pone.0062421
  17. ^ Haas G. (1969). "On the jaw musculature of Ankylosaurus". American Museum Novitates 2399: 1–11. 
  18. ^ Rybczynski, N. and M. K. Vickaryous (2001). "Evidence of Complex Jaw Movement in the Late Cretaceous Ankylosaurid, Euoplocephalus tutus (Dinosauria: Thyreophora)". In K. Carpenter. The Armored Dinosaurs. Indiana University Press. pp. 299–317. ISBN 978-0253339645. 
  19. ^ Miyashita T, Arbour VM, Witmer LM, Currie PJ, (2011). "The internal cranial morphology of an armoured dinosaur Euoplocephalus corroborated by X-ray computed tomographic reconstruction". Journal of Anatomy 219 (6): 661–75. doi:10.1111/j.1469-7580.2011.01427.x. 
  20. ^ Coombs, W. (1978). "Forelimb muscles of the Ankylosauria (Reptilia, Ornithischia)". Journal of Paleontology 52 (3): 642–57. JSTOR 1303969. 
  21. ^ a b Coombs, W. (1979). "Osteology and myology of the hindlimb in the Ankylosauria (Reptillia, Ornithischia)". Journal of Paleontology 53 (3): 666–84. JSTOR 1304004. 
  22. ^ Scheyer, T. M.; Sander, P. M. (2004). "Histology of ankylosaur osteoderms: implications for systematics and function". Journal of Vertebrate Paleontology 24 (4): 874–93. JSTOR 4524782. 
  23. ^ Carpenter, K. (1982). "Skeletal and dermal armor reconstruction of Euoplocephalus tutus (Ornithischia: Ankylosauridae) from the Late Cretaceous Oldman Formation of Alberta". Canadian Journal of Earth Sciences 19 (4): 689–97. doi:10.1139/e82-058. 
  24. ^ Arbour, V. M. (2009). Farke, Andrew Allen, ed. "Estimating Impact Forces of Tail Club Strikes by Ankylosaurid Dinosaurs". PLoS ONE 4 (8): e6738. doi:10.1371/journal.pone.0006738. PMC 2726940. PMID 19707581.  edit
  25. ^ Thulborn, T. (1993). "Mimicry in ankylosaurid dinosaurs". Record of the South Australian Museum. 27: 151–158. 
  26. ^ a b Weishampel, D.B.; Barrett, P.M.; Coria, R.A.; Le Loeuff, J.; Xu X.; Zhao X.; Sahni, A.; Gomani, E.M.P.; Noto, C.R. "Dinosaur Distribution"". In Weishampel, D.B.; Dodson, P. & Osmolska, H.. The Dinosauria (2nd). University of California Press. pp. 517–606. ISBN 0-520-24209-2. 
  27. ^ Lofgren, D.F. (1997). "Hell Creek Formation". In: Currie, P.J. & Padian, K. The Encyclopedia of Dinosaurs. Academic Press. pp. 302–303. ISBN 978-0-122-26810-6. 
  28. ^ Breithaupt, B.H. (1997). "Lance Formation". In: Currie, P.J. & Padian, K. The Encyclopedia of Dinosaurs. Academic Press. pp. 394–395. ISBN 978-0-122-26810-6. 
  29. ^ Eberth, D.A. (1997). "Edmonton Group". In: Currie, P.J. & Padian, K. The Encyclopedia of Dinosaurs. Academic Press. pp. 199–204. ISBN 978-0-122-26810-6. 
  30. ^ Johnson, K.R. (1997). "Hell Creek Flora". In: Currie, P.J. & Padian, K. The Encyclopedia of Dinosaurs. Academic Press. pp. 300–302. ISBN 978-0-122-26810-6. 
  31. ^ Bigelow, Phillip. "Cretaceous 'Hell Creek Faunal Facies'; Late Maastrichtian". Archived from the original on 24 January 2007. Retrieved 2014-03-24.