Ankylosaurus magniventris – Not Your Archetypal Ankylosaur

A newly-published study has provided fresh insights on Ankylosaurus.  This Ornithischian dinosaur, a contemporary of Tyrannosaurus rex in the Late Cretaceous of North America, is perhaps, one of the best-known of all the armoured dinosaurs in the minds of the public, however, this dinosaur star of stage and screen with such a high profile in the popular media, has actually a very fragmentary fossil record, when compared to its close relatives.  Palaeontologist and Ankylosauridae expert Victoria Arbour (Royal Ontario Museum), in collaboration with Jordan Mallon (Canadian Museum of Nature), writing in the Canadian open access science journal “Facets”, suggest that the dinosaur that gave its name to the family Ankylosauridae, is a very atypical member of this armoured dinosaur family.

A Model of an Ankylosaurus

An Ankylosaurus model.

 The armoured dinosaur – Ankylosaurus magniventris – not your typical armoured dinosaur.

Picture Credit: Everything Dinosaur

Examining Previously Unidentified Elements of the Holotype

Ankylosaurus (A. magniventris), is known from only a handful of fossil specimens excavated from Upper Cretaceous deposits in Montana, Wyoming, Saskatchewan and Alberta.  The researchers examined previously unidentified and not described fossil fragments associated with the holotype fossil AMNH 5895 (from Montana).  In addition, they revisited earlier research (Carpenter 2004), making further observations as to body mass, arrangement of the body armour, size of the tail club and the anatomy of Ankylosaurus.

Tail Club Comparison Anodontosaurus Compared to Ankylosaurus

Ankylosaur tail club comparisons.

Ankylosaur tail club comparisons (Anodontosaurus versus Ankylosaurus).

Picture Credit: “Facets Journal”

The picture above shows a comparison between the tail club of Anodontosaurus lambei, and the closely related Ankylosaurus magniventris (dorsal view).  The tail bones and club of Ankylosaurus are poorly known, although the specimen number AMNH 5214 includes a portion of the tail club and a well-preserved, bony knob.  The vertebrae that make up the handle are twice as wide as those corresponding vertebrae making up the handle on the Anodontosaurus club, but they are not longer.  The researchers suggest that the tail of Ankylosaurus may have been proportionately shorter than the tail of Anodontosaurus, or the tail may have had similar overall proportions but the Ankylosaurus tail club was smaller.  The handle vertebrae of Ankylosaurus are unique among ankylosaurids, with U-shaped neural spines in dorsal view compared with the V-shaped neural spines in Anodontosaurus, Euoplocephalus, Pinacosaurus, Talarurus, and most other ankylosaurids.  There may be an upward size limit for ankylosaurid clubs, the shape of the bony knobs (labelled “maj” and “min” for major and minor respectively in the diagram), are different between these two closely related genera, Ankylosaurus magniventris may have had an atypical tail club, one that was not representative of the Ankylosauridae.

One Large Skull Helping to Shape Our Understanding

The largest skull associated with Ankylosaurus is specimen CMN 8880.  It is huge and it was briefly described in 2004 (Carpenter), who regarded the dorsal surface as poorly preserved.  However, the skull was stored on its dorsal surface and it was not turned to permit a proper examination of what would have been the top of the dinosaur’s head.  In Arbour and Mallon’s new paper, they have had the chance to examine the dorsal surface of CMN 8880, which they found to be remarkably well-preserved.  As a result, the scientists have been able to compare and contrast the bony cranial morphology of the top of the skull and confirm that the arrangement of scales and scutes on the top of the skull was very different when compared to other North American ankylosaurids.

The Largest Skull of Ankylosaurus (Specimen Number CMN 8880)

Ankylosaurus cranial material.

Views of the largest Ankylosaurus skull found to date (CMN 8880).

Picture Credit: “Facets Journal”

The picture above shows the skull of CMN 8880, Ankylosaurus magniventris, in (A) dorsal, (B) ventral, (C) left lateral, and (D) right lateral views, note the scale bar equals ten centimetres.  The skull is well preserved on the dorsal and left lateral surfaces. The right lateral surface has caved inwards slightly, the researchers have measured the basal skull length as 671 millimetres, based on these measurements and other material reported in this scientific paper, the researchers were able to confirm that A. magniventris was much larger than other Late Cretaceous armoured dinosaurs.  The scientists reaffirmed the length of this dinosaur at around ten metres.

This review underscores the fact that although Ankylosaurus gave rise to the family name the Ankylosauridae, A. magniventris is far from typical of this family.  The teeth, the nares, the tail club and body size of Ankylosaurus tend to make it stand out from the other Laramidian Ankylosaurines.

Changing Views of Ankylosaurus magniventris

Changing views of Ankylosaurus (dorsal view).

Changing views of Ankylosaurus magniventris over the years.

Picture Credit: “Facets Journal”

In Competition with Edmontonia – Perhaps Not

It is thanks to this new study, that we have a better understanding of Ankylosaurus, it is not your typical Ankylosaur.  Intriguingly, the researchers postulate on the role of Ankylosaurus in the palaeoenvironment of Laramidia during the Late Cretaceous.  Fossils of this armoured dinosaur are very infrequently found and therefore it might have been ecologically rare, or just a very infrequent visitor to the coastal plain where fossilisation of corpses was much more likely than if these creatures habitually lived further inland away from rivers and large bodies of water.  The nodosaurid Edmontonia was contemporaneous with Ankylosaurus and the researchers comment on previous studies that have alluded to the fact that Edmontonia may have been ecologically separated from Ankylosaurus on the basis that Edmontonia seems to have been more abundant in coastal, lowland habitats.  It is likely that these animals did not compete directly with each other (different beak and tooth shapes – indicating niche partitioning).

Ecosystem Engineer Like a Modern Elephant – Unlikely

Ankylosaurus probably fed on low-growing vegetation, ferns, flowers and shrubs, with an estimated consumption of about 60 kilogrammes of vegetable matter per day, about the same as an elephant. It did not chew its food, food processing taking place in the enormous gut.

Modern elephants with their ability to knock down trees and strip bark, are regarded as ecosystem engineers, helping to shape the environment.  It is suggested that Ankylosaurus did not carry out this role, tree felling, bark stripping and environmental engineering was more likely to have been undertaken by the equally massive and much more ubiquitous hadrosaurids.

Although Ankylosaurines are typically categorised as herbivores, the unusual narial anatomy of Ankylosaurus could reflect a change in diet or feeding strategy relative to other Ankylosaurs and the researchers suggest this warrants further investigation.  The smaller, posteriorly set, and dorsally roofed external nares in Ankylosaurus could have evolved as these animals grubbed in the soil for nutritious grubs, earthworms, insects or plant tubers.  The broad muzzle and powerful front limbs would have made Ankylosaurus an accomplished digger.  So perhaps Ankylosaurus had a different lifestyle compared to other members of the Ankylosauridae, it may have foraged through leaf litter or turned over the soil, like a giant hog.

The scientific paper: “Unusual cranial and Postcranial Anatomy in the Archetypal Ankylosaur Ankylosaurus magniventris” by Victoria M. Arbour and Jordan C. Mallon, published in the Canadian open access journal “Facets”.

Link to the paper: Ankylosaurus Paper

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