|The holotype of Nothosaurops/Nothosaurus occiduus, from Plate 15 in Leidy (1873), with a couple of other fossils erased.|
In the Nothosaurops corner we have Hay (1902), Williston (1903, 1908), and Spamer et al. (1995). In the Nothosaurus corner we have Cope (1875), Erickson (1972), and Gao and Fox (1998). To cut to the chase, if it's Nothosaurus occiduus, it's a lot easier to ignore than if it's Nothosaurops occiduus. In the first case, we've already got the genus Nothosaurus, which is a Triassic aquatic reptile much more closely related to plesiosaurs, so we can nudge "N. occiduus" into the pile of marginal nineteenth-century species. In the second case, we've got a unique genus which needs to be dealt with. Williston (1908) flat-out stated that Nothosaurops should be given precedence, which obviously has not been done. Erickson (1972) and Gao and Fox (1998) used Leidy (1873) as the publication of record. They considered Nothosaurops a typographical error and dismissed "N. occiduus" as either a nomen vanum (an "empty name", a term no longer used much because nomen dubium has taken over its territory; Erickson 1972) or a nomen oblitum (a name which can be considered lapsed because of disuse; Gao and Fox 1998). Although I believe that Champsosaurus would practically be a lock for conservation in the face of Leidy's nearly forgotten saurian if the case were put before the International Commission on Zoological Nomenclature, I disagree with Nothosaurops being a typo. Based on Leidy's 1873 description, I suspect that Leidy originally intended a separate genus (1870), but after a few years passed he changed his mind and designated a species within Nothosaurus (1873), and appearances of Nothosaurops are cases where he forgot to revise the captions. (This is not difficult to do, especially in the days before computerized word processors with replace functions.) Furthermore, I don't think that the 1870 publication can be dismissed, especially when most of the text is reused for the 1873 description. Edward Drinker Cope, who named Champsosaurus, had no particular concern about Leidy's saurian; he classified the specimen as Plesiosaurus occiduus (Cope 1875). This wasn't a case of conflict of interest, either, given he wouldn't name Champsosaurus until 1876.
(Curiously, the Paleobiology Database has separate entries for Nothosaurus occiduus and Nothosaurops occiduus. Nothosaurus occiduus is classified as a champsosaurid using Erickson (1972), and Nothosaurops occiduus is classified as a plesiosaur using Williston (1903), apparently having missed his 1908 statement.)
This is not where the fun ends. In a twist that should surprise no one who is familiar with Edward Drinker Cope's practice of nomenclature and taxonomy, Champsosaurus itself hides a bundle of very marginal species. As noted above, Cope named Champsosaurus in 1876. As was his custom, he gave the world not only one species but several, in this case four species based on a grab-bag of Judith River vertebrae differentiated by proportions. For the record, they are C. annectens (the type species), C. brevicollis, C. profundus, and C. vaccinsulensis. The first three are probably all variants on the same thing (Erickson 1972; Gao and Fox 1998), and C. vaccinsulensis is not a champsosaur (Brown 1905; Erickson 1972). For good measure, he later pulled the same trick with New Mexican fossils, coining C. australis (Cope 1881), C. puercoensis, and C. saponensis (Cope 1882). Also in the 1881 article, he accidentally created the completely undescribed and undefined name "C. laticollis" when he noted that the dimensions of the C. australis cervical vertebrae "are about those of the C. laticollis". The usefulness of this comparison is somewhat lessened by his complete omission of the dimensions of cervical vertebrae belonging to "C. laticollis". The one other hint we have to the identity of "C. laticollis" is buried in an anonymous author's "Scientific Intelligence" note in the November 1881 issue of the American Journal of Science. This short piece summarizing Cope (1881) reported that "C. laticollis" is from the Laramie beds (uppermost Cretaceous), but then also states that Cope based the genus on it, so maybe it's just all wet.
Out of Cope's work, we are left with six formally described species still considered to be Champsosaurus, one species outside of Champsosaurus, and one "species" that could be anything from a faulty memory to a typo for C. brevicollis to a species he wanted to name but never got around to. The six actual Champsosaurus species are all based on various vertebrae, all useful in illustrating the presence of Champsosaurus, but not so helpful for doing taxonomic comparisons. More recent authorities such as Erickson (1972) and Gao and Fox (1998) have made the case that these early species can be regarded as belonging to the genus Champsosaurus, but none of them are diagnostic within Champsosaurus. This includes C. annectens, the type species. To get to a species that can be diagnosed, you have to wait until Brown (1905), with C. ambulator and C. laramiensis.
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