Re: [dinosaur] Diplodocus status

Surely it's bad practice to change the type species of a genus because the type species _might_ be non-diagnostic and there _could_ be taxonomic confusion in the future? If there's a credible case for the validity of the type species, and no evidence that the best-known referred species is not congeneric with the type species, what's the need for changing the type species right now? We could always try to change it later if the problem actually does arise. Nonetheless, the fact that considerable disagreement exists on whether designating D. carnegii the type species is a good idea is in itself an argument in favor of following the rules that exist to resolve these disagreements rather than requesting a special exemption to them.

Philosophy of nomenclature aside, what should happen if it is concluded that YPM 1920 (the holotype of D. longus) and CM 84 (the holotype of D. carnegii) are conspecific? That outcome seems more likely to me than the nearly identical D. longus and D. carnegii winding up in separate genera, and in this scenario designating D. carnegii the type species wouldn't help at all.

There are three possibilities here: we abandon D. longus in favor of D. carnegii, which would require an ICZN petition for reversal of precedence, we use D. longus for what has historically been called D. carnegii (and continue using D. hallorum for what has historically been called D. longus), or we use D. longus for what is currently called D. hallorum and keep D. carnegii, which would require an ICZN petition to designate a neotype for D. longus (specifically, AMNH 223, which has historically been treated as the effective type specimen of D. longus).

On Thu, Oct 15, 2020 at 9:24 PM Mickey Mortimer <mickey_mortimer111@msn.com> wrote:

The crux of our disagreement might be based on your answer to this.Â Take these two statements which you support-

"from a phylogenetic taxonomy standpoint, it's a really BAD idea for a taxonomically valid genus to have a nomen dubium as a type species."

"Because _D. longus_ is the type species of the genus Diplodocus, but is itself not diagnosable, a retention of _D. longus_ as type species would create insecurities and confusion concerning the use of _Diplodocus_ as a genus."

My question is- What bad outcome could occur because a species undiagnosable within its genus is the type species?

Mickey Mortimer

From: dinosaur-l-request@mymaillists.usc.edu <dinosaur-l-request@mymaillists.usc.edu> on behalf of Tim Williams <tijawi@gmail.com>
Sent: Thursday, October 15, 2020 7:06 PM
To: dinosaur-l@usc.edu <dinosaur-l@usc.edu>
Subject: Re: [dinosaur] Diplodocus status
Â

Mickey Mortimer <mickey_mortimer111@msn.com> wrote:

> There isn't even agreement that Diplodocus longus IS undiagnostic.

True, but beside the point.Â The assessment that a particular taxon is
a nomen dubium is always subjective.

> - Even IF Diplodocus longus was undiagnostic, undiagnostic species CAN be the type species of diagnostic genera.Â There's nothing in the ICZN arguing against that.

Just because it's not proscribed by the ICZN, doesn't necessarily mean
it's a good idea.Â In fact, from a phylogenetic taxonomy standpoint,
it's a really BAD idea for a taxonomically valid genus to have a nomen
dubium as a type species.Â A nomen dubium is a valid name
(nomenclature), but not a valid species (taxonomy).Â Scientifically,
there's no point pretending that a nomen dubium is a valid species
simply for the purpose of nomenclature.

> - The argument about future-proofing for nomenclatural stability has zero urgency because all parties involved agree Diplodocus longus is definitely the same genus as D. carnegii, even the authors who
> petitioned the ICZN in the first place.Â Tim et al. would be arguing for an official nomenclatural change based on a case that nobody thinks is true or even likely.

See response immediately above.

> - If we continue with this philosophy of designating the most complete species as the type species or the most complete specimen the neotype [*snip* Mickey's voluminous list of taxa] It's a bad
> philosophy.

I didn't say (or even imply) that we needed the most complete specimen
as the type species, or the most complete specimen as the neotype.
But when we have a scientifically well-known genus with a dodgy
holotype (as in the case of _Diplodocus longus_), it would be helpful
to have a species based on an unambiguously diagnostic specimen as the
new type species.

I thought Tschopp and Mateus (2016) presented a strong case in their
petition, especially this statement: "Because _D. longus_ is the type
species of the genus Diplodocus, but is itself not diagnosable, a
retention of _D. longus_ as type species would create insecurities and
confusion concerning the use of _Diplodocus_ as a genus."Â Thus, the
petition aimed to resolve the "insecurities and confusion" of
_Diplodocus_.ÂÂ Rebutting the petition by scouring the _D. longus_
holotype for perceived diagnostic characters does nothing to prevent
_Diplodocus_ from one day becoming a dubious genus.