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RE: Question about Ankylosauria, or at least Polacanthidae

Jamie Stearns wrote:

Generally, groups of dinosaurs tend to be based on which members of the group were named first; this is, I take it, why Saurornithoididae was renamed to Troodontidae anfter Stenonychosaurus and Troodon were found to be the same thing.

The situation is a little more complicated. With the Troodontidae-Saurornithoididae thing, the synonymy came about when it was realized that _Troodon_, _Stenonychosaurus_ and _Saurornithoides_ all belonged in the same family, so the name Troodontidae had priority (by 50 years). But if Troodontidae had never been named, then Saurornithoididae would be the valid name, even though _Troodon_ was named long before _Saurornithoides_. It's the oldest available *family* name that's important in this context, not the oldest named genus that belongs in this family.

Even here there's wiggle-room, because if _Ornithodesmus_ belongs in the family Troodontidae, then the older-named Ornithodesmidae might have prioirity (Hooley 1913 vs Gilmore 1924). For various reasons, this hasn't happened (current usage; doubts about _O_'s validity and referral to Troodontidae; Ornithodesmidae was originally a pterosaur family; etc).

This was confirmed when the thought of naming a clade "Deinonychosauria" for
Troodontidae+Dromaeosauridae came up and it was remarked that it would more correctly
be "Dromaeosauria" as Dromaeosaurus was named before Deinonychus (though it should actually
be "Troodontia" as Troodon was named before either).

I was not aware that this was ever remarked upon. I had thought that the name Deinonychosauria was intended to be descriptive, given that it references the "terrible claw" shared by both dromaeosaurids and troodontids - as well as incorporating the genus name _Deinonychus_.

With that in mind, it should stand to reason that Polacanthidae should have been called
Hylaeosauridae, and Ankylosauria possibly renamed Hylaeosauria, as Hylaeosaurus was named >before either Polacanthus or Ankylosaurus was.

There is no rule or convention to this effect. Priority depends upon when the *family* was named, not the genus it's named after.

Nevertheless, if _Hylaeosaurus_ and _Polacanthus_ belong to a family to the exclusion of _Nodosaurus_, then the name Hylaeosauridae (Nopcsa 1902) should have priority over Polacanthidae (Wieland 1911). However, Polacanthidae has appeared more often in the recent literature than Hylaeosauridae, so for the sake of nomenclaural stability, Polacanthidae would probably win out anyway.

I don't think Hylaeosaurus being relatively incomplete would have anything to do with it, as Ceratops isn't all that complete either and still has Ceratopsidae and Ceratopsia named after it...

Personally, I'm not so sure it's a good idea to have a family-level clade named after _Ceratops_, given that it's (a) probably a nomen dubium, (b) excluded from all phylogenetic definitions of Ceratopsidae; and (c) not demonstrably a member of the family Ceratopsidae. But most people disagree and are OK with Ceratopsidae. :-)



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