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Polacanthidae
Polacanthidae is a grouping of ankylosaurs, possibly primitive nodosaurids. Polacanthids are Late Jurassic to Late Cretaceous in age. From North America, polacanthids are observed to be 155 to 126 million years old, from Europe they are 140 to 126 mya, and, until recently, no polacanthids have been known from any other continents. Before polacanthids from other continents were known, Kirkland observed they appeared to become extinct about the same time a land bridge opened between Asia and North America.〔 Now though, ''Taohelong'', ''Dongyangopelta'' and ''Zhejiangosaurus'' are known from Asia and are dated at 113 to 93.9 mya.
Polacanthids were somewhat more lightly armoured than more advanced ankylosaurids and nodosaurids. Their spikes were made up of thin, compact bone with less reinforcing collagen than in the heavily armoured nodosaurids. The relative fragility of polacanthine armour suggests that it may have been as much for display as defense.〔
==Classification==
The family Polacanthidae was named by Wieland in 1911 to refer to a group of ankylosaurs which seemed to him intermediate between the ankylosaurids and nodosaurids. This grouping was ignored by most researchers until the late 1990s, when it was used as a subfamily (Polacanthinae) by Kirkland for a natural group recovered by his 1998 analysis suggesting that ''Polacanthus'', ''Gastonia'', and ''Mymoorapelta'' were closely related within the family Ankylosauridae. Kenneth Carpenter resurrected the name Polacanthidae for a similar group which he also found to be closer to ankylosaurids than to nodosaurids. Carpenter became the first to define Polacanthidae as all dinosaurs closer to ''Gastonia'' than to either ''Edmontonia'' or ''Euoplocephalus''.〔 Most subsequent researchers placed polacanthines as primitive ankylosaurids, though mostly without any rigorous study to demonstrate this idea. The first comprehensive study of 'polacanthid' relationships, published in 2012, found that they are ither an unnatural grouping of primitive nodosaurids, or a valid subfamily at the base of Nodosauridae.〔Thompson, R.S., Parish, J.C., Maidment, S.C.R. and Barrett, P.M. (2012). "Phylogeny of the ankylosaurian dinosaurs (Ornithischia: Thyreophora)." ''Journal of Systematic Palaeontology'', 10(2): 301-312. 〕 In 2013, Polacanthinae was deemed the improper rank for the group by Kirkland ''et al.''.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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