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Re: Megalosaurus and Titanosaurus
In a message dated 11/20/02 10:41:39 AM Pacific Standard Time,
<< 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.