Protocycloceras whitfieldi (left) and P. lamarcki (right); figures from Furnish & Glenister (1964).

Belongs within: Cephalopoda.

Protocycloceras is a genus of early cephalopods from the Lower Ordovician. Like many early cephalopods, Protocycloceras had a relatively small shell (perhaps about 5-10 cm long, guesstimating from the figures in Furnish & Glenister 1964) with numerous closely-spaced septa. Whether Protocycloceras was nektic like later cephalopods or still benthic is unknown, but the latter seems likely.

Characteristics (from Furnish & Glenister 1964): Strongly annulate orthocones with circular cross-section; annulations and sutures transverse and essentially straight; siphuncle approximately 0.3× conch diameter, ventral but not marginal; septal necks probably orthochoanitic, siphuncular segments tubular.

<==Protocycloceras Hyatt in Zittel 1900 [=Orotocycloceras (l. c.); incl. Dresseroceras Ulrich, Foerste et al. 1944] FG64
    |--*P. lamarcki (Billings 1859) [=Orthoceras lamarcki] FG64
    |--P. abnorme Ulrich, Foerste et al. 1944 P68
    |--P. affine Ulrich, Foerste et al. 1944 P68
    |--P. arkansasense Ulrich, Foerste et al. 1944 P68
    |--P. doniphonense Ulrich, Foerste et al. 1944 P68
    |--P. foerstei Butts 1926 P68
    |--P. geronticum Ulrich, Foerste et al. 1944 P68
    |--P. manitouense Ulrich, Foerste et al. 1944 P68
    |--P. odenvillense Ulrich, Foerste et al. 1944 P68
    |--P. philipsburgense Ulrich, Foerste et al. 1944 P68
    |--P. rushense P68
    |    |--P. r. rushense P68
    |    `--P. r. crebricinctum Girty 1911 P68
    |--P. smithvillense Ulrich, Foerste et al. 1944 P68
    `--P. whitfieldi FG64

*Type species of generic name indicated


[FG64] Furnish, W. M., & B. F. Glenister. 1964. Nautiloidea - Ellesmerocerida. In Treatise on Invertebrate Paleontology pt. K. Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea (R. C. Moore, ed.) pp. K129-K159. The Geological Society of America and the 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.

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