Above: Illustration of Manospondylus gigas by Matt Martyniuk, copyright.
In a previous post, I brought up the taboo subject of Manospondylus gigas, the large Lancian theropod named by E.D. Cope in 1886 and a potential senior synonym of Tyrannosaurus rex. Google "manospondylus" and the first hit is an old Q&A post by Mike Taylor called "So why hasn't Tyrannosaurus been renamed Manospondylus?"
Aside from cultural inertia, the answer Taylor gave is this: "As of 1st January 2000, a new ICZN ruling has come into effect, saying that a name that's been considered valid for fifty years can't now be replaced by one that's been considered invalid during that time." This is also the answer Mickey Mortimer gave in some (but not all) relevant entries on the current Theropod Database. Brian Switek, in an old Laelaps post, gave the same answer: Manospondylus hasn't been considered valid in 50 years, so it's a nomen oblitum ("forgotten name"). He also suggested that this means T. rex has "protected status" which would have to be overturned by the ICZN in order for M. gigas to become valid.
The problem is that none of that has any bearing on what "nomen oblitum" actually means according to the current ICZN code. The ICZN states that in order for a name to be declared a nomen oblitum, all of the following things need to be true:
1. the senior synonym or homonym has not been used as a valid name after 1899
2. the junior synonym or homonym has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years
3. a paper must be published citing evidence for #2, and citing both names together, declare that the junior synonym or homonym is being made a nomen protectum ("protected name") in accordance with ICZN article 23.9.
The confusion seems to come from #2. It's not a matter of the senior name not having been used in the past 50 years, but the junior name must have been used frequently enough in recent history in order to be eligible for conservation. Clearly, T. rex meets the second criteria. But what about the first and third?
For number one, the answer is I'm not sure, but I think so. It depends on what your definition if "is" is. Has Manospondylus been used since 1899? Definitely, yes, and in several papers, as Mickey Mortimer pointed out in the comments on my previous post on this topic. But was it used as valid? Most authors, even in the early 20th century, recognized that two vertebrae were pretty poor material to be basing a species on. Many of them considered it to be what we'd now call a nomen dubium ("dubious name"). For example, here's what Matthew & Brown (1922) had to say on the subject: "Osborn has already (1917) called attention to another fragmentary type, Manospondylus gigas, as possibly identical with Tyrannosaurus but based upon an inadequate type." They don't explicitly say it's valid or invalid (I'd like to know what Osborn 1917 actually aid on the matter but don't have that paper. Anyone?).
As for number three, it appears that the situation has never been adequately addressed in the literature, so the criterion is not satisfied. The issue came up in 2000, when Peter Larson claimed to have rediscovered the original Manospondylus locality, and more of the type specimen, confirming it is the same as T. rex. As far as I know, this has never seen print beyond an AP article (Anonymous, 2000. "Discovery could Endanger T.Rex Name." The Associated Press.)
In short, could M. gigas be a nomen oblitum? Maybe, depending on the meaning of "valid" and if somebody gets up the nerve to actually publish the case. Is Manospondylus a nomen oblitum as of right now? Definitely not, and it remains the valid senior synonym of Tyrannosaurus until and unless someone acts as revisor to the contrary.
Addendum: Just for fun, I looked up "valid" in the ICZN's glossary. Who better to determine what the ICZN means by valid than the ICZN? Here's the entry:
valid, a. (validity, n.) Of an available name or a nomenclatural act: one that is acceptable under the provisions of the Code and, in the case of a name, which is the correct name of a taxon in an author's taxonomic judgment.
Since the Code does not formally recognize nomina dubia as invalid names (or at all, really), this seems to indicate that M. gigas was considered valid by all those authors in the 20th century, at least under the ICZN's definition. Matthew and Brown, for example, did not consider M. gigas an invalid name (a junior homonym, improperly coined, etc.), just non-diagnostic. Which as far as the rules are concerned, is A-OK. Sorry kids, barring an act of ICZN, M. gigas looks like it's on solid footing as a currently valid name.