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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"
Troodontidae+Dromaeosauridae came up and it was remarked that it would more
be "Dromaeosauria" as Dromaeosaurus was named before Deinonychus (though it
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
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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