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Re: Megalosaurus and Titanosaurus

In a message dated 11/20/02 10:41:39 AM Pacific Standard Time, 
TiJaWi@agron.iastate.edu writes:

<< Damn - another one bites the dust.  Though I had thought the type dentary
 for _Megalosaurus buklandii_ was diagnostic at the genus level.  Let's see
 what Rauhut has to say. >>

Not quite so fast.

In my opinion, a species may be declared a nomen dubium if and only if one 
can produce two other distinct species that cannot be distinguished from it. 
For example, Deinodon horridus is based on a type series of teeth that cannot 
(presently) be distinguished from the teeth of Albertosaurus libratus or 
Albertosaurus torosus. This is what makes it a nomen dubium. (It may become 
possible in the near future to tell whether Deinodon horridus teeth are more 
like those of A. torosus or A. libratus, and this would help remove the 
"dubium" from the "nomen" Deinodon horridus.) Trachodon mirabilis is a nomen 
dubium because the type tooth could have belonged to any of a number of 
different lambeosaurid genera.

With regard to Megalosaurus bucklandii, no one has yet produced two distinct 
theropod species from Middle Jurassic England that each have dentaries 
indistinguishable from the Megalosaurus bucklandii type dentary. So 
Megalosaurus bucklandii is >not< a nomen dubium, merely a species defined on 
scanty material. Just because someone publishes the notion that a species is 
a nomen dubium, perhaps because there is not enough material for a good 
diagnosis, doesn't make it a real nomen dubium. A species may be 
undiagnosable on known material without necessarily being a nomen dubium.

For another example, Majungasaurus crenatissimus was declared a nomen dubium 
"in favor of" Majungatholus atopus, but I have no doubt that the former is a 
senior subjective synonym of the latter. The recently discovered topotype 
good skull bridges the type material of the two species almost perfectly.

Once a species is defined, zoologists are (or should be) obliged to bend over 
backward to retain the species, not blow it off as a nomen dubium at the 
first opportunity. A species may be retained by, for example, discovery of 
better-preserved topotype specimens, or of more material from the same 
horizon/locality. This is how Troodon formosus was "saved" from the nomen 
dubium dustbin. I definitely find fault with the contempt some modern 
paleontologists evince toward the work of their predecessors, who for the 
most part did very well with the material available to them.