Reconstruction of Hexameroceras hertzeri, from a larger drawing by Avancna (whom I believe is Stanton Fink).

Belongs within: Oncoceratidae.

The Middle to Upper Silurian (possibly to Middle Devonian) Hemiphragmoceratidae were a very distinctive family of oncocerid cephalopods. At maturity, large lappets developed at the aperture of the body cavity, restricting the shell's opening. Exactly what sort of animal inhabited such a shell and how it lived are currently unknown.

Characters (from Sweet 1964): Compressed endogastric brevicones, curved adapically, straight adorally, with ventral, nummuloidal, actinosiphonate siphuncle and elaborately visored aperture. Actinosiphonate deposits continuous between segments. Mature peristome with hyponomic sinus on spoutlike process, paired lateral sinuses and mid-dorsal salient.

    |--Hemiphragmoceras Hyatt in Zittel 1900 [=Hemiphraygmoceras (l. c.)] S64
    |    `--*H. pusillum (Barrande 1865) [=Phragmoceras pusillum] S64
    |--Conradoceras Foerste 1926 S64
    |    `--*C. pseudoconradi Foerste 1926 S64
    |--Octamerella Teichert & Sweet 1962 S64
    |    `--*O. callistomoides (Foerste 1926) [=Octameroceras callistomoides] S64
    |--Tetrameroceras Hyatt 1884 S64
    |    `--*T. bicinctum (Barrande 1865) [=Phragmoceras bicinctum] S64
    `--Hexameroceras Hyatt 1884 (see below for synonymy) S64
         |--*H. panderi (Barrande 1865) [=Phragmoceras panderi] S64
         |--H. erectum Foerste 1936 P68
         `--H. hertzeri P68

Hexameroceras Hyatt 1884 [=Hexamoceras (l. c.); incl. Octameroceras Hyatt in Zittel 1900, Septameroceras Hyatt 1884, Heptameroceras (l. c.)] S64

*Type species of generic name indicated


[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.

[S64] Sweet, W. C. 1964. Nautiloidea – Oncocerida. In Treatise on Invertebrate Paleontology pt. K. Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea (R. C. Moore, ed.) pp. K277-K319. The Geological Society of America and the University of Kansas Press.


  1. Bizarre. this looks a bit like the queen alien in Starship Troopers.

  2. Unfortunately, the soft parts of the animal are speculative, but I think Stanton's done a reasonably good job here of considering what a more shell-confined nautiloid would have looked like. Personally, I've been wondering if there's any way of demonstrating whether or not this arrangement would work better with fewer, larger tentacles like a squid or octopus. We don't currently know whether the coleoid or nautilid arrangement of tentacles more resembles the primitive arrangement for cephalopods; if we could somehow demonstrate the presence of the former in a stem-nautilid, it would clarify things considerably.


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