Ankylosaurus is a genus of ankylosaurid dinosaur, containing one species, A. magniventris. Fossils
of Ankylosaurus are found in geologic formations dating to the very end of the Cretaceous Period
in western North America. Although a complete skeleton has not been
discovered and several other dinosaurs are represented by more extensive fossil material,
Ankylosaurus is often considered the archetypal armored dinosaur. Other ankylosaurids shared
its well-known features—the heavily armored body and massive bony tail club—but Ankylosaurus
was the largest known member of the family.
Description
Ankylosaurus was a large animal by modern standards, estimated at lengths of 8 to 9 m
and weighing up to 6 tonnes. However, following a significant redescription of the fossils
in 2004, a much smaller size has been suggested, based on the largest known skull, at 64.5 cm
long and 74.5 cm wide, the whole animal was estimated at 6.25 m long, 1.5 m wide and
1.7 m tall at the hip. The body shape was low-slung and quite wide. It was quadrupedal,
with the hind limbs longer than the forelimbs. Although its feet are 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
was herbivorous, with small, leaf-shaped teeth suitable for cropping vegetation. These teeth
were smaller, relative to the body size, than in any other ankylosaurid species. Ankylosaurus
did not share the grinding tooth batteries of the contemporaneous ceratopsid and hadrosaurid
dinosaurs, indicating that very little chewing occurred. Bones in the skull and other parts
of the body were fused, increasing their strength. Armor
The most obvious 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 crocodiles, armadillos and some lizards. The bone was probably overlain
by a tough, horny layer of keratin. These osteoderms ranged greatly in size, 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 more ancient ankylosaurid Euoplocephalus, the plates of Ankylosaurus
were smooth in texture, 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.
Tail club
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 have been preserved, which attached to these vertebrae. These tendons were partially
ossified and were not very elastic, allowing great force to be transmitted to the end of
the tail when it was swung. It seems to have been an active defensive weapon, capable of
producing enough of a devastating impact to break the bones of an assailant. A 2009 study
showed that "large tail knobs could generate sufficient force to break bone during impacts,
but average and small knobs could not", and that "tail swinging behavior is feasible in
ankylosaurids, but it remains unknown whether the tail was used for interspecific defense,
intraspecific combat, or both". It has also been proposed that the tail club acted as
a decoy for the head, although this idea is now largely discredited.
Classification
Ankylosaurus was named as the type genus of the family Ankylosauridae. Ankylosaurids are
members of the larger taxon Ankylosauria, which also contains the nodosaurids. Ankylosaur
phylogeny is a contentious topic, with several mutually exclusive analyses presented in recent
years, so the exact position of Ankylosaurus within Ankylosauridae is unknown. Ankylosaurus
and Euoplocephalus are often thought to be sister taxa. However, other analyses have
found these genera in different positions. Further discoveries or research may clarify
the situation. The following cladogram is based on the phylogenetic
analysis of the Ankylosaurinae conducted by Arbour and Currie:
The cladogram below follows the most resolved topology from a 2011 analysis by paleontologists
Richard S. Thompson, Jolyon C. Parish, Susannah C. R. Maidment and Paul M. Barrett.
History of discovery
Ankylosaurus was named by American paleontologist Barnum Brown, in 1908. The generic name is
derived from the Greek words αγκυλος/ankulos and σαυρος/sauros. Brown intended this
name in the same sense as the medical term ankylosis, to refer to the stiffness produced
by the fusion of many bones in the skull and body, so the name is often translated as 'stiffened
lizard.' The type species is A. magniventris, from the Latin magnus and venter, referring
to the great width of the animal's body.
A team led by Brown discovered the type specimen of A. magniventris in the Hell Creek Formation
of Montana, in 1906. This consisted of the top of the skull, as well as vertebrae, ribs,
part of the shoulder girdle and armor. Six years earlier, Brown found the skeleton of
a large theropod dinosaur in the Lance Formation of Wyoming. This specimen was named Dynamosaurus
imperiosus in 1905 but is now thought to belong to Tyrannosaurus rex. Associated with AMNH
5866 were more than 75 osteoderms of various sizes, which were also attributed to Dynamosaurus.
However, these osteoderms are nearly identical in form to those of A. magniventris and most
probably belong to this species. In 1910, while on an expedition to Alberta, Barnum
Brown recovered his third specimen of A. magniventris, from the Scollard Formation. AMNH 5214 includes
a complete skull and the first known tail club, as well as ribs, limb bones and armor.
All three of the above specimens are now housed at the American Museum of Natural History
in New York City. The largest known skull of this animal was collected in Alberta by
Charles M. Sternberg, in 1947, and is now housed at the Canadian Museum of Nature. Many
other isolated bones, armor plates and teeth have been found over the years.
Palaeoecology
Ankylosaurus existed between 66.5 to 66 million years ago, in the final Maastrichtian stage
of the Late Cretaceous Period, and 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.
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. 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. Ankylosaurus shared its environment
with dinosaurs including the ceratopsids Triceratops and Torosaurus, hypsilophodont Thescelosaurus,
hadrosaurid Edmontosaurus, nodosaurid Edmontonia, pachycephalosaurian Pachycephalosaurus, and
the theropods Ornithomimus, Troodon, and Tyrannosaurus. Fossils of Ankylosaurus are considerably rare
in these sediments, compared to Edmontosaurus and the super-abundant Triceratops, which
make up most of the large herbivore fauna. Another ankylosaur, Edmontonia, is also found
in the same formations. However, Ankylosaurus and Edmontonia seem to have been separated
both geographically and ecologically. Ankylosaurus had a wide muzzle, perhaps used for non-selective
grazing and thus may have been limited to the upland regions, away from the coast, while
Edmontonia had a narrower muzzle, indicating a more selective diet, and seems to have lived
at lower elevations, closer to the coast. In popular culture
Since its description in 1908, Ankylosaurus has been publicized as the archetypal armored
dinosaur, and due to its easily recognizable appearance and the intense public interest
in dinosaurs, it has been a feature of worldwide popular culture for many years. A life-sized
reconstruction of Ankylosaurus featured at the 1964 World's Fair in New York City greatly
contributed to its popularity. Ankylosaurus is named in the 1955 Japanese kaiju film Godzilla
Raids Again as the ancestor of the mutated monster Anguirus. Anguirus appeared in seven
Godzilla films between 1955 and 2004. Anguirus has also made numerous appearances in books,
on television shows, and in video games. References
External links
Ankylosaurus- The Bullet-Proof Dinosaur'