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THYREPHORA: ANKYLOSAURS

     The Ankylosauria are known from all continents except South America and are divided into two groups: the ankylosaurs and the nodosaurs.  They were low browsers, not able to eat anything more than 1 meter from ground.  The large hyoid bone indicates that they had large tongues.    The nodosaurs had a narrower beak and might have been more selective in their diet.  They had a very deep rib cage, perhaps for separate gut compartments involved in the fermentation of plant material.   Their limb size/body weight ratios give estimates that they could walk 3 km/hour and run 10 km/hour.  A trackway in Canada gives an estimate of 3 km/hr.  They also some of the smallest brains relative to body size known in dinosaurs (Lambert, 1990, Fastovsky, 1996).

     For their defense, they had heavy body armor and (in ankylosaurs), a tail club.  If attacked, they could squat down and, at 3500 kg, they would be very difficult for a predator to flip them over.

nodosaur
The family Nodosauridae, or “node lizards”, were more primitive than the family Ankylosauridae and lack a tail club.  They possessed bands of bony plates, spines, and nodules along their backs. The earliest known nodosaur, Sarcolestes, is dated from the Mid-Jurassic (Czerkas, 1990).   Hylaeosaurus (perhaps the same genus as Polacanthus) had curved spines on neck and shoulders.  Their eyes were located more posteriorly; this is an adaptation for spotting predators often seen in animals which browse low to the ground.  Higher nodosaurs evolved in the Early Cretaceous. Acanthopholis was 3-4 meters long and Struthiosaurus was 2-3 meters long.  Edmontonia (see image above) had spikes on shoulders which might have been used for lunging at attackers.  Edmontonia also had bigger shoulder plates than other nodosaurs; the bone of these armor plates was probably covered by horn in life.  Nodosaurus is actually poorly known and might be an ankylosaur instead of a nodosaur.  Of the 2 great groups of nodosaurs, one (Panoplosaurinae) had squat snouts and lumpy armor while the other (Edmontoninae) had flat armor and unswollen snouts (Bakker, 1988)
sauropelta pinacosaurus
Silvisaurus
     The family Ankylosauridae, or “fused lizards”, possessed fused back vertebrae.  Most of the derived features of this group involve their armor. Although the elements of the armor are present in fossils, often the exact arrangement of spikes and spines on living organisms is unknown.  At least some had bands of hollow armor plates across back and tail.  The plates in skull were firmly attached to skull bones and the various fenestrae in the skull were closed.  Lateral spikes projected from the back of skull, known as squamosal horns.  Some had horn-covered bony plates projecting from back and shoulders (Lambert, 1990, Fastovsky, 1996). 
ankylosaurus
tail club
The last tail vertebrae were fused in a terminal tail club with 4 additional bony plates, 2 large and 2 small.
nasal passages
Ankylosaurs possessed a unique folding of their nasal passages, unlike nodosaurs.

The number of metarsals and phalanges in the feet varied among ankylosaurs.

 

Metatarsals

Phalanges

Euoplocephalus

3

0-3-4-5-0

Pinacosaurus

4

2-3-4-4-0

Talarurus

4

2-3-4-4-0

Sauropelta

5

2-3-4-4/5-0

Nodosaurus

4 or 5

2-3-4-5-4 (?)

(Weishampel, p. 473)

     Ankylosaurs may have evolved in the Upper Jurassic in Europe (one possible specimen is known from Portugal).  Shamosaurus from the Early Cretaceous of Mongolia is the earliest ankylosaur and has small squamosal horns.  In the Mid-Late Cretaceous, ankylosaurs are known from Mongolia and North America.  Pinacosaurus, Saichania (which also possessed armor on its belly) and Tarchia (which possessed a much larger tail club) are known from Mongolia while Ankylosaurus (up to 9 m), Euoplocephales, and Talarurus are known from North America.

tarchia shamosaurus
ankylosaurus
    Are the ankylosaurs and nodosaurs the only 2 groups of Ankylosauria?  One animal named Minmi may be an ankylosaur, a nodosaur, or perhaps a member of a third group of armored dinosaurs.  It had small armor bones on its underside and each vertebra had a plate called a paravertebrae and bony rods for reinforcement.  Gastonia is another species that appears to belong to a third group of Ankylosauria (Lambert, 1990, Fastovsky, 1996; Lucas, 2004).