Belongs within: Orthoceratia.
The Lower Ordovician Bassleroceratidae were the earliest cephalopods to evolve an exogastrically curved shell. They may have been the ancestors of the later Tarphycerida and Oncocerida, and through the latter of the modern Nautilus.
Characteristics (from Furnish & Glenister 1964): Gradually expanded, smooth cyrtocones with slight to moderate exogastric curvature; conch strongly compressed to subcircular in section; siphuncle characteristically marginal and moderate in size, orthochoanitic.
Bassleroceratidae [Basslerocerida]
|--Dwightoceras Ulrich & Foerste 1936FG64
| `--*D. dactyloides (Dwight 1884) [=Cyrtoceras dactyloides]FG64
|--Dyscritoceras Ulrich & Foerste 1936FG64
| `--*D. metellus (Billings 1865)FG64 [=Cyrtoceras metellusFG64, Maelonoceras metellusH84]
|--Anguloceras Unklesbay & Young 1956FG64
| |--*A. ovatum Unklesbay & Young 1956FG64
| |--A. depressum Unklesbay & Young 1956K93
| `--A. rotundum Unklesbay & Young 1956K93
|--Monogonoceras Ulrich, Foerste et al. 1944FG64
| |--*M. alabamense Ulrich, Foerste et al. 1944FG64
| |--M. magnisiphonatum Ulrich, Foerste et al. 1944P68
| `--M. subrectum Ulrich, Foerste et al. 1944P68
|--Diaphoroceras Ulrich, Foerste et al. 1944FG64
| |--*D. avaense Ulrich, Foerste et al. 1944FG64
| |--D. belliFG64
| `--D. buffaloense Ulrich, Foerste et al. 1944P68
|--Lawrenceoceras Ulrich, Foerste et al. 1944 (see below for synonymy)FG64
| |--*L. collinsi Ulrich, Foerste et al. 1944FG64
| |--‘Onychoceras’ australe Ulrich, Foerste et al. 1944P68
| |--‘Onychoceras’ breve Ulrich, Foerste et al. 1944P68
| |--L. confertissimumFG64
| `--‘Onychoceras’ subrotundum Ulrich, Foerste et al. 1944P68
|--Bassleroceras Ulrich & Foerste 1936FG64
| |--*B. perseus (Billings 1865) [=Orthoceras perseus]FG64
| |--B. bridgei Ulrich, Foerste et al. 1944P68
| |--B. dalevillense Ulrich, Foerste et al. 1944P68
| |--B. lasallense Ulrich, Foerste et al. 1944P68
| |--B. tenuiculum Ulrich, Foerste et al. 1944P68
| `--B. xintaiense Chen 1976K93
`--Avaoceras Ulrich, Foerste et al. 1944 (see below for synonymy)FG64
|--*A. elongatum Ulrich, Foerste et al. 1944FG64
|--‘Stylocyrtoceras’ avaense Ulrich, Foerste et al. 1944P68
|--‘Leptocyrtoceras’ curvatum Ulrich, Foerste et al. 1944P68
|--A. longidomum Ulrich, Foerste et al. 1944P68
|--A. magnisiphonatum Ulrich, Foerste et al. 1944P68
|--‘Leptocyrtoceras’ powellense Ulrich, Foerste et al. 1944P68
|--‘Leptocyrtoceras’ virginianum Ulrich, Foerste et al. 1944P68
`--A. wellsense Ulrich, Foerste et al. 1944P68
Avaoceras Ulrich, Foerste et al. 1944 [incl. Leptocyrtoceras Ulrich, Foerste et al. 1944, Stylocyrtoceras Ulrich, Foerste et al. 1944]FG64
Lawrenceoceras Ulrich, Foerste et al. 1944 [=Lawrensoceras (l. c.); incl. Onychoceras Ulrich & Foerste 1936 non Wunstorf 1905]FG64
*Type species of generic name indicated
References
[FG64] Furnish, W. M., & B. F. Glenister. 1964. Nautiloidea—Ellesmerocerida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K129–K159. The Geological Society of America and the University of Kansas Press.
[H84] Hyatt, A. 1883–1884. Genera of fossil cephalopods. Boston Soc. Nat. History, Proc. 22: 253–338.
[K93] King, A. H. 1993. Mollusca: Cephalopoda (Nautiloidea). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 169–188. Chapman & Hall: London.
[P68] Purnell, L. R. 1968. Catalog of the type specimens of invertebrate fossils. Part I: Paleozoic Cephalopoda. United States National Museum Bulletin 262: 1–198.