polacanthidae

Polacanthidae

   Polacanthidae, as noted before, has members with large spikes and spines, plus lateral protuberances that in ways resemble side-pointing Stegosaurus plates, plus a sacral shield over the hips, but no tail club. The few members known before 1994's publication of Mymoorapelta had been classified as nodosaurids, mostly because they were so poorly known up to that point (Polacanthus was the best known, with the back end well represented, but no skulls were known). Then, they typically were classified as a basal subfamily of ankylosaurids, but it seems just as likely that they form a third family. (Unfortunately, lately they often get broken up as various basal things) Of course, all of this family-subfamily talk is arbitrary, when you get right down to it, but it's still a popular topic.
    These are possibly the coolest-looking ankylosaurians, because of their bizarre armor. They really have to be seen to be believed. They're something like a nodosaurid with an ankylosaurid skull. In the mid 1990s, it was believed that they had small tail clubs, but this was later shown to be based on a misinterpretation of a section of caudals, ossified tendons, and armor. Polacanthids first appeared in the late Jurassic and apparently reached their zenith in the Barremian with strong representation in North America and Europe, then, in a disappointing move for fans of armored dinosaurs, essentially disappearing from the fossil record.
    An undescribed giant polacanthid has been mentioned in the press from Utah.

Polacanthidae:

Taxon or Taxa:		Time\Place:	Comments:
Gargoyleosaurus parkpinorum Carpenter, Miles, and Cloward, 1998		Kimmeridgian (LJ) of Wyoming	The second named ankylosaurian from the Morrison Formation (Mymoorapelta being the first), this animal's remains shows features of both ankylosaurids and nodosaurids. Its remains include a good skull and skeleton with much armor, that suggest it wasn't quite as spiky across the shoulders and back as later members of the group (personal observation).
?Hylaeosaurus armatus Mantell, 1833		late Valanginian (EK) of England, ?France, and ?Spain	With Iguanodon and Megalosaurus a founding member of Owen's Dinosauria, Hylaeosaurus has always been the obscure one. It has often been thought to be a senior synonym of the better-known Polacanthus, but better remains have supported separation. Usually classified as a nodosaurid, it was probably a polacanthid. It is mostly known from the holotype, a partial front end missing most of the head, but including much armor. Unlike Polacanthus, its scapula and coracoid were not fused, and there is no evidence yet for a sacral shield. Mainland European material referred to it probably belongs to Polacanthus instead.
Mymoorapelta maysi Kirkland and Carpenter, 1994		Kimmeridgian (LJ) of Colorado	Mymoorapelta is the first named ankylosaurian from the Morrison Formation. It is based on much of a postcranium from a small individual, who may have been a female; what may be an egg has been found with the skeleton. The discovery of this animal helped lead to the recognition of Polacanthidae, through its similarities to Polacanthus.
Gastonia burgei Kirkland, 1998		early Aptian (EK) of Utah	Gastonia may have been the most heavily-armored land vertebrate to have ever existed. Among its other protective features, it had large spikes along the sides of the body, and sharp plates sticking from its tail that were shaped in such a way they could cut objects placed between them if the dinosaur flexed its body. The skull was reinforced and angled in such a way to suggest individuals of this species engaged in head-pushing contests. The eyes faced somewhat forward, and there were ankylosaurid-type horns on the rear corners of the skull. It was a common animal of its time and place.
Hoplitosaurus marshi Lucas, 1902a (originally Stegosaurus marshi Lucas, 1901)		Barremian (EK) of South Dakota	Hoplitosaurus is a problematic taxon based on damaged remains, including a strange femur. It has been synonymized with Polacanthus in the past, but that appears to be incorrect. They probably looked rather similar, though. Hoplitosaurus had very prominent spines and spikes.
Polacanthus: Owen, 1865	P. foxii (type) Hulke, 1881	Barremian-early Aptian (EK) of England (possibly to as late as Cenomanian [LK])	Polacanthus, a common Wealden dinosaur, is traditionally depicted as having a lateral row of large spines from the neck up to the hip, a sacral shield composed of fused armor plates over the hips, and two rows of curved plates sticking up along the top of the tail. This is a decent rough picture of this taxon, although new finds have increased and complicated the armor arrangement. It was often incorrectly synonymized with Hylaeosaurus.
Polacanthus: Owen, 1865	P. rudgwickensis Blows, 1996	Barremian (EK) of England	P. rudgwickensis was a larger, more robust species than the type, but is not as well-known, based on some postcranial remains from the mainland of England.

Polacanthidae i.s.:

Taxon or Taxa:	Time\Place:	Comments:
?Acanthopholis horridus (N.D.) Huxley, 1867	late Albian-Cenomanian (EK-LK) of England	Acanthopholis is one of those dinosaurs you see illustrated in children's books that is a lot more poorly known than the pictures make it out to be; it seems to be have been most like the polacanthids. It appears to have possessed hefty spines. A number of dubious species have been referred here, often based on mixes of ankylosaurian and ornithopod material.

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