Belongs within: Ammonoidea.
The Marathonitidae are a group of more or less globular ammonoids known from the Upper Carboniferous to the Upper Permian (Miller et al. 1957).
Characters (from Miller et al. 1957, as Marathonitinae): Conch subglobular to globular. Lateral lobes of sutures tending to be trifid.
<==Marathonitidae RR79
|–Marathonites smithi P68
|–Kargalites RR79
|–Tabantalites RR79
|–Demarezites RR79
|–Hyattoceras subgeinitzi RR79
|–Pseudovidrioceras RR79
`–Peritrochia RR79
|–P. erebus Girty 1908 P68
|–P. novomexicana Miller & Parizek 1948 P68
`–P. sellardsi P68
*Type species of generic name indicated
REFERENCES
Miller, A. K., W. M. Furnish & O. H. Schindewolf. 1957. Paleozoic Ammonoidea. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt L. Mollusca 4. Cephalopoda: Ammonoidea pp. L11–L79. Geological Society of America, and University of Kansas Press.
[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.
[RR79] Ross, C. A., & J. R. P. Ross. 1979. Permian. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A291–A350. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).