Ankylosaurus magniventris

Ankylosaurus magniventris was the largest known ankylosaurid and a rare inhabitant of latest-Cretaceous western North America. It carried tile-like bones over the skull, half-rings around the neck and osteoderms across the body. Referred fossils reveal a stiffened tail ending in a bony club, but the holotype itself has no club. Its exceptionally broad skull, sideways-facing nostrils and proportionally tiny teeth distinguish it from earlier ankylosaurids such as Euoplocephalus.

Its fossils occur between approximately 68 and 66 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The body was low, wide and supported by four robust limbs. Fused skull bones and polygonal caputegulae formed a heavily reinforced head, while bony cervical half-rings guarded the neck. Numerous osteoderms were embedded in the skin, but no specimen preserves their entire arrangement in life. In the tail, interlocking vertebrae and elongated tendons created a stiff handle for a terminal knob of enlarged osteoderms. The only well-preserved club belongs to referred specimen AMNH 5214, not the holotype AMNH 5895.

• Exceptionally broad, heavily fused skull
• Polygonal caputegulae and horn-like projections over the skull
• Sideways- and downward-facing nostrils with complex internal passages
• Bony cervical half-rings protecting the neck
• Rows of keeled and flattened osteoderms embedded in the skin
• Stiff distal tail forming a handle for a bony club knob
• Small leaf-shaped teeth relative to the enormous skull

Peter Kaisen found the holotype AMNH 5895 during Barnum Brown's 1906 American Museum expedition near Gilbert Creek, Montana, in the Hell Creek Formation. Brown named Ankylosaurus magniventris in 1908. Ankylosaurus refers to the extensive stiffening and fusion in the skeleton; magniventris means 'great belly' and reflects the animal's remarkable body width. The partial holotype includes a skull roof, teeth, vertebrae, a shoulder element, ribs and armour, but no limbs or tail club. Later specimens from Alberta, Montana and Wyoming supplied the better skull and club familiar from modern reconstructions.

Discovery credit: Peter Kaisen, Barnum Brown.

Naming authors: Barnum Brown.

A. magniventris inhabited river floodplains, forested lowlands and coastal-plain environments represented by the Hell Creek, Lance, Scollard and Frenchman formations. Flowering plants, conifers, ferns and shrubs formed the vegetation. Triceratops, Edmontosaurus, Thescelosaurus and Tyrannosaurus shared portions of this latest-Maastrichtian landscape. Ankylosaurus fossils are extremely rare within these otherwise well-sampled assemblages.

Its broad muzzle, very small teeth and massive gut region indicate low browsing and fermentation of large amounts of vegetation, although the preferred plants are unknown. Biomechanical work shows that large ankylosaurid clubs could deliver forceful lateral strikes. Defence against predators is plausible, but combat between ankylosaurids is also possible; injuries in the related Zuul strengthen that broader hypothesis without directly documenting it in A. magniventris. The exact arrangement of body armour and any social behaviour remain unknown.

• The iconic tail club is absent from the holotype; it is known from the smaller referred specimen AMNH 5214.
• The largest known skull, CMN 8880, is about 67 centimetres long and unusually broad, but no body bones were found with it.
• A 2017 study found previously unrecognised pieces of skull, pelvis and neck armour stored with fragments of the 1906 holotype.
• The largest individual may have approached ten metres and eight tonnes, but that estimate scales an entire body from an isolated skull.
• Ankylosaurus lived close to the end-Cretaceous extinction and shared its world with Tyrannosaurus and Triceratops.

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2. 02
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   Barnum Brown · Bulletin of the American Museum of Natural History · 1908

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3. 03
   Unusual cranial and postcranial anatomy in the archetypal ankylosaur Ankylosaurus magniventris

   Victoria M. Arbour, Jordan C. Mallon · FACETS · 2017

   Open source
4. 04
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   Victoria M. Arbour · PLOS ONE · 2009

   Open source
5. 05
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   Victoria M. Arbour, Philip J. Currie · Journal of Anatomy · 2015

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   Open source
7. 07
   Paleontology Collections

   American Museum of Natural History

   Open source
8. 08
   Palaeobiology Collections

   Canadian Museum of Nature

   Open source