Ankylosaurus
Ankylosaurus is a Thyreophora dionosaur genus that lived during the Cretaceous Period, approximately 66-68 million years ago and it was one of the last dinosaurs before the Cretaceous-Paleogene mass extinction. Ankylosaurus belongs to the suborder Ankylosauria, which is grouped together with Stegosauria within a clade called Thyreophora. We know its fossils only from the western part of North America. Ankylosaurus magniventris is the sole species within the genus. The name Ankylosaurus means 'fused lizard', and the species name, magniventris means great 'belly'. The fossils of the species are known from Hell Creek, Lance, Scollard, Frenchman, and Ferris formations - however, Ankylosaurus seems to be a relatively rare species compared to other dinosaurs of its age.
Ankylosaurus magniventris. By Nobu Tamura (http://spinops.blogspot.com) - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=19460220
Known occurrences of Ankylosaurus fossils.
The type specimen of Ankylosaurus magniventris was discovered by Barnum Brown, in 1906, during an expedition of the American Museum of Natural History, near Gilbert Creek, in the Hell Creek Formation, and it was described two years later, in 1908[1]. During the 20th century, a number of further Ankylosaurus specimen fossils were discovered in the above mentioned localities. The genus was redescribed by Kenneth Carpenter in 2004[2]. Due to newer fossils, Victoria M. Arbour and Jordan Mallon gave an updated, even more modern redescription for Ankylosaurus in 2017[3].
Description and anatomy
Ankylosaurus is considered to have been a usually slow animal which was capable of quick movements in case of danger. Ankylosaurus was 6-8 meters long, robust quadrupedal dinosaur, having longer hindlegs than its forelegs, and its weight was between 4.8 and 8 tons. It is the largest amongst the known ankylosaurids[3]. As of 2021 there are three known Ankylosaurus skulls. The skull of this species was wide and low, and beared two horns on the top-back of its head, which were pointing backward, and further two (jugal) horns below these, which were pointed backward-down. The nostrils of Ankylosaurus faced sideways. It had small, leaf-shaped teeth, and the anterior part of its jaws was covered by a beak. The skull and back of Ankylosaurus was covered by osteoderms (bony plates), it had half-ring shaped osteoderms covering the neck, and a huge bony club on the end of the tail. The snout region of the skull was quite special compared to its relatives due to a serious transformation it had undergone. The snout was spanned and truncated anteriorly, with downward and outward directed elliptical nostrils (in all the other known ankylosaurids these where directed forward or upward).
The name Ankylosaurus refers to the fused state of the bones of skull and some other body parts, which gave an extremely massive body structure to this dinosaur. Most old popular depictions show this dinosaur in a squatting posture, dragging its tail with the bony club on the ground. However, the modern reconstructions show that Ankylosaurus had its legs in a rather upright position, and held its tail off the ground. It is also interesting, that the large bony spines on the animal's sides which can be seen in most traditional depictions, are not based in true fossil Ankylosaurus remnants[4].
It is important to mention, that some structures (e.g. tail, feet, pelvis) of the Ankylosaurus skeleton are incompletely known[2], as there were no complete fossilized ankylosaur skeleton found. The armor of Ankylosaurus consisted of bony knobsand plates (called osteoderms or scutes), which were embedded in the skin. As none of these have been found in articulation to each other or to the skeleton, their exact configuration on the body is unknown. The proposed placement configurations we know today are mostly based on inferences based on related dinosaurs. These length of osteoderms ranged from 1 cm to 35.5 cm and were various in shape. However, they were usually relatively thin and their ventral side was hollowed. These were supplemented by cushions of collagen fibers, embedded directly into the osteoderms, providing a lightweight and at the same time strongly resistant armor covering. As their armor was heavily vascularized, it might have played an important role in theromregulation as well[5].
Ankylosaurus also had some half-ring shaped armor plates around the upper side of its neck. The tail club consisted of two huge osteoderms, having a row of smaller ones in the midline and two further small at the tip. The osteoderms of the club technically enclosed the last vertebra of the tail. There is only one fossilized Ankylosaurus tail club known, and that is 60cm long, 49cm wide and 19 cm high. The last seven vertebrae of the tail were immobile (sometimes even co-ossified), and formed a "handle" for the club. Probably it used the bony club on the end of its tail against predators, and maybe also against its conspecifics. Ankylosaurus was able to swipe with its tail club with such an impact that it was able to break the bones of an attacker, e.g. a huge theropod dinosaur[6].
Ankylosaurus magniventris skull. By Tim Evanson - https://www.flickr.com/photos/23165290@N00/9336678044/, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=27596767
Paleobiology
Ankylosaurus was a herbivorous dinosaur, and probably browsed the low-growing vegetation and dropped or low-growing fruits. Probably this dinosaur was a non-selective browser. If we presume that it was warm-blooded, Ankylosaurus needed approximately 60kg plant material per day to eat, which is similar to the amount that a today's elephant consumes. However, eating fruits could have been a more effective way of feeding in terms of endothermic metabolism, and the beak and teeth of this dinosaur seem to have been well adapted for this kind of nutrition. As in case of this dinosaur its centre of gravity was low, probably it was not able to knocking down trees in order to feed their foliage. This also means that Ankylosaurus was probably unable to form the landscape of its habitat like in todays elephants.
Investigations on the reconstructed inner ear of Ankylosaurus suggest that this dinosaur was capable of hearing even the low-toned resonant sounds at low frequencies. This dinosaur also had an enlarged olfactory region, which means that probably it had a well-developed sense of smell[7]. Due to the triangular shape of its skull, the eyes of Ankylosaurus faced not direcly sideways, therefore, this dinosaur might have had a stereoscopic vision[2]. Some research indicates that the forelimbs were quite strong, and Ankylosaurus may have been able to digging[8].
Ankylosaurus magniventris skeleton. By Gary Todd from Xinzheng, China - Ankylosaurus Skeleton, CC0, https://commons.wikimedia.org/w/index.php?curid=105166156
Scientific references
[1] 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.
[2] Carpenter, K. (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 (8): 961-86. doi:10.1139/e04-043.
[3] Arbour, V.M.; Mallon, J.C. (2017): Unusual cranial and postcranial anatomy in the archetypal ankylosaur Ankylosaurus magniventris. FACETS. 2 (2): 764-794. doi:10.1139/facets-2017-0063.
[4] Glut, D. F. (1997): Ankylosaurus. Dinosaurs, the encyclopedia. McFarland & Company, Inc. Publishers. pp. 141-143. ISBN 978-0-375-82419-7.
[5] 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. Bibcode:1982CaJES..19..689C. doi:10.1139/e82-058.
[6] Coombs, W. (1979): Osteology and myology of the hindlimb in the Ankylosauria (Reptillia, Ornithischia). Journal of Paleontology. 53 (3): 666-84.
[7] Miyashita, T.; Arbour V. M.; Witmer L. M.; Currie, P. J. (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.
[8] Coombs, W. (1978): Forelimb muscles of the Ankylosauria (Reptilia, Ornithischia). Journal of Paleontology. 52 (3): 642-57.