[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Ceratops (was RE: Pterosaur diversity (was: Re: Waimanu))

Mickey Mortimer wrote:

Well, not so fast. Horn core size isn't necessarily the only character which can be evaluated for Ceratops.

_Ceratops montanus_ is known from a pair of horn cores and an occipital condyle. That's not much to go by. If anyone can glean some more characters other than "big horn cores" from the type material then I haven't heard.

Perhaps other skull roof or occipital characters can distinguish ceratopsids from more basal taxa, or centrosaurines from chasmosaurines.

I think you may have misinterpreted what I meant. What I was trying to say is that large brow horns are not exclusive to chasmosaurines, so using this character *alone* to refer a specimen to this group is a bad idea.

In fact, Ryan's thesis finds that his unpublished taxon is extremely similar with Ceratops and from equivalent beds. I wouldn't be surprised if they were synonymous, though Ceratops' holotype is not as obviously diagnostic.

Is this the new ceratopsian from Milk River?

If this new taxon is not a chasmosaurine, but is potentially the same as _Ceratops_, then we cannot assume that _Ceratops_ is a chasmosaurine. This undermines Sereno's argument that Chasmosaurinae should be renamed Ceratopsinae.

True, but again we have other characters to consider (e.g. neural arch placement). I don't think anyone doubts Titanosaurus is a titanosaurid (in the sense of being more closely related to Saltasaurus than Andesaurus; not a phylogenetic definition, but you get the clade I mean). I'd agree with Salgado. There's nothing in the ICZN saying family names shouldn't be based on nomina dubia.

Yeah, _Titanosaurus_ is probably a titanosaurid under those terms. However, there is no guarantee that this will always remain so. For example, new sauropod material may convince us that procoelous mid-caudals and forward-sitting neural arches evolved more than once, and so have a wider distribution than we previously thought.

Using just one or two characters to refer a taxon to a group (e.g., _Titanosaurus_ to Titanosauridae, _Ceratops_ to Ceratopsidae) reminds me of the old "key characters" rationale for classification. This is the same rationale that phylogenetic taxonomy tries to avoid, by using as many characters as possible to ascertain relationships.

Salgado subdivides Titanosauridae into two subfamilies: Epachthosaurinae and Saltasaurinae. So, he doesn't even use Titanosaurinae. That shows you what little faith he has in the genus _Titanosaurus_. By contrast, the advantage of using a certain genus as BOTH the name-giver and the internal specifier for a clade is that the taxon's inclusion in the clade is guaranteed. This is the advantage that Saltasauridae has over Titanosauridae.

To play devil's advocate, both Ceratops and Titanosaurus were very distinctive when they were first found. It's only with hindsight that we know strongly procoelous caudals and large brow horns have wide distributions.

I agree. It is not the fault of either the _Ceratops_ or _Titanosaurus_ type material that their significance was eclipsed by later discoveries. But if we want stability in phylogenetic usage, then we should be anchoring our clades in the genera that we name them after. (This only applies to family-level clades.) And if a certain taxon is too poorly-known to be used as a specifier, then we should name the clade after a taxon that can be used as a specifier.