What is Ankylosauria?

The most inclusive clade containing Ankylosaurus magniventris but not Stegosaurus stenops. Ankylosauria is a group of Middle Jurassic to Late Cretaceous-aged thyreophoran ornithischian dinosaurs, whose members (ankylosaurs) are affectionately known as tank dinosaurs because of their wide, low slung appearance and oval to oblong armour plates arranged in side to side bands across their shoulders, back and tail. Smaller armour nodules protect their legs and underside and fill the gaps between larger plates, and their heavily-ornamented skulls were fortified with yet more plates that were fused to the underlying bone. Two out of the five pairs of windows (fenestrae) which are present in other dinosaur skulls for load-lightening have been fused shut, and in extreme cases even their cheeks and/or eyelids were armoured. To carry this extra weight ankylosaurs had short but powerful limbs and particularly strong shoulder and pelvic girdles, though not all were created equal; Ankylosauria houses several armoured battalions which sport unique features of their own.

Ankylosauridae: The heavy-duty ankylosaurs with heavy, as wide or wider than long, almost triangular heads when viewed from above, large armour plates, and a clubbed tail for self-defence. Short wide snouts suggest they were unfussy, low-foliage grazers.

Nodosauridae: Nodosaurids don't have a clubbed tail, and have simpler armour 'nodes' but this is counter-balanced by a series of nasty-looking spikes on their shoulders and neck. Their skulls are longer than they are wide, and somewhat pear-shaped when viewed from above. Slender snouts suggest they were picky, low foliage browsers.

Polacanthidae: This group was coined by Wieland way back in 1911 during a rant at the unnecessary raising of new family names. But he didn't provide a definition because he thought it was synonymous with Nodosauridae, making the name itself unnecessary, and that is where its members were assigned by most palaeontologists until it was resurrected by Kenneth Carpenter in 2001. Described as having "the skull of an ankylosaurid on the skeleton of a nodosaurid" which really doesn't do them justice, polacanthids are the most lightly armoured ankylosaurs and lack a clubbed tail but sport triangular plates along their neck, back and tail. Some experts suspect that Polacanthidae is a sub-group of primitive critters belonging to Nodosauridae, in which case it would adopt the sub-family suffix and become Polacanthinae.

It's an indication of ankylosaur "intelligence" that the much larger Tyrannosaurus rex was a genius by comparison but had a brain the size of a banana! Plus, they were probably stinking wind-bags: a rather unfortunate side effect of belly-fulls of fermenting foliage, but they were none the worse for it.
The first ankylosaurs rose in the early Jurassic of China then diversified and farted their way across every continent including Antarctica right up to the end of the age of dinosaurs when it took a six-mile-wide asteroid on a collision course with Earth to stop them dead in their tracks. But the fossil record suggests their numbers were being thinned out a couple of million years before that.

Click here to view Dinochecker's full A-Z list of all Ankylosaurs. Ankylosauria is derived from the Greek "ankylos" (fused), "sauros" (lizard), and "-ia" (neuter plural). They're a family anchored by Ankylosaurus -- the "fused lizard".