I wrote a post to the Dinosaur Mailing List I think went detailed enough it could serve to be here.
Understand that the provenances for proposed neotypes should, if at all possible, derive from the same formation as the holotype. This occurs with regard to Carrano et al. 2009’s propoal for a neotype of Majungasaurus crenatissimus, using Lavocat’s material over Depéret ’s holotype (presumably to avoid using Sues & Galton ’s Majungatholus atopus). I mention this (with citations) here. Both taxa occur in the Maeverano Formation, and may have been collected close to one another in similar levels of the formation.
The situation is similar to Titanosaurus indicus (Lydekker, 1877) where the holotype of Titanosaurus colberti (Jain & Bandyopadhyay, 1998, granted Isisaurus by Wilson & Upchurch, 2003) comes from the same hill and formation as the type species/specimen, but eas excluded merely for purposes of implied poor stability on the part of Titanosaurus indicus and slight variation in the preserved caudals (ignoring variation in the holotype and paratype caudals of the rejected taxon). We could reasonably state that Isisaurus colberti is a junior synonym of Titanosaurus indicus by designating the holotype of the former as the neotype of the latter. This situation would be similar to what was done for Coelophysis bauri, despite being rather different. In that case (ICZN, 1996, proposed by Colbert et al., 1992), specimens from a different formation entirely, of a different stratigraphic position (only slightly different), of a different locality, made the basis of new taxonomy (Rioarribasaurus colberti Hunt & Lucas, 1991), in favor of nomenclature that was older, the basis of further employed taxonomy (Coelophysidae, Coleophysoidea, etc.), and more recognizable.
Greater issues are present when it comes to Troodon formosus of the Judith River Formation of Montana (leidy, 1856), from which body fossils of troodontids are unknown but which exist in laterally continguous Two Medicine Formation beds and in less laterally contiguous but of likely same-age Dinosaur Park Formation. Here, a person can make a geographic argument that body fossils and the variation that exists among dentition leads to doubt about taxonomic overlap, especially as there are very few teeth that are identical in morphology and size to the holotype (this is a “taste” issue, though — I am overly picky on this point, but I have a particular reason). The best options (in my opinion) for a neotype would be material from the Two Medicine, which have failed to be published on for nearly two decades, but I would think a more or less complete cranial sample would be best. Otherwise, cranial material referred to Stenonychosaurus inequalis (Sternberg, 1932) from the RTMP of Dinosaur Park Formation (Currie, 1987) is the best non-dental-bearing cranial material found so far, and would also be useful. I do not think any of this material should bear the name, though, and we should simply stop using the name for the material from Alberta, Montana, Wyoming, and Alaska formations, aside from the type tooth.
Note that some taxa for which neotypes seem preferable are problematic: Spinosaurus aegyptiacus (von Stromer, 1915) is known (apparently) from material from Algeria, Tunisia, and Morocco, but good cranial or spinal material does not come from Egypt, and the Kem Kem and other formations are not in the same stratigraphic level as the Baharija. The same is true of Carcharodontosaurus saharicus (Depéret, 1856), which while known for the skull from Morocco (and partial skeleton from Egypt), is based on teeth from Algeria (?Albian) — the possible neotype material comes from younger sediments (Cenomanian). Such temporal distance can be used as a proxy for taxonomic distance, at least in well-sampled and broad ranging sediments such as Dinosaur Park Formation and Hell Creek Formation, so why then should this not be true elsewhere? It is, in fact, the reason I don’t think the neotype of Coelophysis bauri was properly selected: Padian’s 1986 specimen should have been better, being a whole specimen and from the type locality and formation. The same should then be true of other taxa. That we do not have good material from type formations is irrelevant, and the nomenclature may simply be set aside until then.
So, two errors: First, I misplaced the locality of Isisaurus colberti and inferred it to the type hill from whence Titanosaurus indicus derives; while both are from the Lameta Formation, neither of them come from the same region of India. However confusing, Antarctosaurus septentrionalis does come from the same hill, and does have similar caudals to Titanosaurus indicus, but Isisaurus colberti does not, so I wonder if I simply confused them.
Second, Padian’s Coelophysis comes from the Petrified Forest Formation (or Petrified Forest Member of the Chinle Formation) of Arizona, while the Coelophysis type localities are in the Petrified Forest (Member/Formation) of New Mexico. I was in error in noting they were from the same locality.
Carrano, M. T., Krause, D. W., O’Connor, P. M. & Sampson, S. D. 2009. Case 3487. Megalosaurus crenatissimus Depéret, 1896 (currently Majungasaurus crenatissimus; Dinosauria, Theropoda): proposed replacement of the holotype by a neotype. Bulletin of Zoological Nomenclature 66(3):261-264.
Colbert, E. H., Charig, A. J., Dodson, P., Gillette, D. D., Ostrom, J.H. & Weishampel, D.B. 1992. Case 2840. Coelurus bauri Cope, 1887 (currently Coelophysis bauri; Reptilia, Saurischia): Proposed replacement of the lectotype by a neotype. Bulletin of Zoological Nomenclature 49(4):276-279.
Currie, P. J. 1987. Bird-like characteristics of the jaws and teeth of troodontid theropods (Dinosauria, Saurischia). Journal of Vertebrate Paleontology 7:72–81.
de Lapparent, A. F. & Lavocat, R. 1955. Dinosauriens [Dinosaurs]. pg.785-962 in Piveteau (ed.) Traité de Paléontologie. Tome V. La Sortie des Eaux. Naissance de la Tétrapodie. L’Exubérance de la Vie Végétative. La Conquête de l’Air Amphibiens. Reptiles. Oiseaux. (Masson et Cie, Paris).
Depéret, C. 1896. Note sur les Dinosauriens Sauropodes et Théropodes du Crétacé supérieur de Madagascar [Note on sauropod and theropod dinosaurs from the Upper Cretaceous of Madagascar]. Bulletin de la Société Géologique de France 21:176–194.
Hunt, A. P. & Lucas, S. G. 1991. Rioarribasaurus, a new name for a Late Triassic dinosaur from New Mexico (USA). Paläontologische Zeitschrift 65(1/2):191-198.
International Commission on Zoological Nomenclature, 1996. Opinion 1842: Coelurus bauri Cope, 1887 (currently Coelophysis bauri; Reptilia, Saurischia): lectotype replaced by a neotype. Bulletin of Zoological Nomenclature 53(2):142-144.
Jain, S. L. & Bandyopadhyay, S. 1997. New titanosaurid (Dinosauria: Sauropoda) from the Late Cretaceous of central India. Journal of Vertebrate Paleontology 17(1):114-136.
Lavocat, R. 1955. Sur une portion de mandibule de Théropode provenant du Crétacé supérieur de Madagascar [On a partial theropod mandible coming from the Upper Cretaceous of Madagascar]. Bulletin du Muséum National d’Histoire Naturelle 27:256–259.
Leidy, J. 1856. Notices of remains of extinct reptiles and fishes, discovered by Dr. F. V. Hayden in the bad lands of the Judith River, Nebraska Territory. Proceedings of the Academy of Natural Sciences of Philadelphia 8:72-73.
Lydekker, R. 1877. Notices of new and other Vertebrata from Indian Tertiary and Secondary rocks. Records of the Geological Survey of India 10(1):30-43.
Padian, K. 1986. On the type material of Coelophysis Cope (Saurischia: Theropoda), and a new specimen from the Petrified Forest of Arizona (Late Triassic: Chinle Formation). p.45-60 in Padian (ed.) The Beginning of the Age of Dinosaurs: Faunal Change Across the Triassic-Jurassic Boundary. Cambridge University Press (Cambridge).
von Stromer, E. 1915. Wirbeltier-Reste der baharije-Stufe (unterstes Cenoman). 3. Das Original des Theropoden Spinosaurus aegyptiacus nov. gen. et nov. spec. [Vertebrate remains of the Baharije Formation (lowest Cenomanian). 3. The original of the theropod Spinosaurus aegyptiacus nov. gen. et nov. spec.] Abhandlungen der Königlichen Bayerischen Akademie der Wissenschaften Mathematisch-physikalische Klasse Abhandlung 28(4):1–32.
Sternberg, C. M. 1932. Two new theropod dinosaurs from the Belly River Formation of Alberta. Canadian Field-Naturalist 46(5):99-105.
Sues, H.-D. & Taquet, P. 1979. A pachycephalosaurid dinosaur from Madagascar and a Laurasia-Gondwanaland connection in the Cretaceous. Nature 279:633–635.