Ctenobactrites costatus, from Erben (1964).

Belongs within: Cephalopoda.
Contains: Parabactritidae, Coleoidea, Bactrites, Agoniatitina.

The Ammonoidea sensu stricto are a highly diverse group of shelled cephalopods known from the Devonian to the Cretaceous. Most forms had coiled shells though some had straight or irregular conches. The siphuncle usually has a ventral position. Early forms had simple sutures, but sutures in later forms became more complex, bent into a series of lobes and saddles.

Many authors derive the ammonoids from the Bactritida, more or less straight-shelled (sometimes curved) cephalopods known from the Ordovician to the Permian. As a result, many authors expand the Ammonoidea to include the bactritids. Bactritids had a globular protoconch separated by a distinct constriction from the younger shell. Members of the family Bactritidae have longiconic shells with a small apical angle (less than ten degrees) and orthochoanitic septal necks. The Lower Ordovician Eobactrites sandbergeri is distinguished a circular cross section, a relatively narrow and deep ventral lobe, and rectiradiate growth lines lacking a dorsal saddle (Erben 1964).

Relationship between the bactritids and early ammonoids is supported by the presence of a ventral siphuncle and of a ventral lobe in the suture (Erben 1964). The bactritids may also include the ancestors of the Coleoidea, as is suggested by similarities between the shell of the Parabactritidae with phragmocones of the Belemnoidea (Erben 1964). If so, then the ammonoid lineage may be regarded as surviving to the present day.

The soft anatomy of ammonoids remains largely unknown. Many basal forms are known to have had the body chamber closed by an anaptychus, a calcified hemispherical structure homologous to the lower jaw. The presence of this structure implies that the tentacle array was somehow reduced or modified compared to living cephalopods, in which the tentacles surround the jaws and would have prevented the anaptychus from closing (Engeser & Keupp 2002).

The horns of Ammon
Published 23 April 2012
Goniatite of the genus Girtyoceras, showing the relatively simple zig-zag sutures of this group, from here.

Ammonites are one of the few groups of fossil invertebrates that are known to the general public as animals with coiled shells, some of them reaching significant sizes. The name Ammonites means ‘image of Ammon’: Ammon was an Egyptian god whose sacred animal was the ram, the curled horns of which ammonites were supposed to resemble. Ammonites were Mesozoic representatives of a larger group of cephalopods, the Ammonoidea, which also included a number of Palaeozoic lineages.

Specimen of Ceratites, a Triassic ammonoid with a greater number of suture lobes than Girtyoceras, but with the lobes still relatively simple (if you look very closely, you may be able to see small crenulations in the lobes). From Drow male.

Among extant cephalopods, only extant members of the Nautilidae, the chambered nautiluses, have permanent external shells. Nautilus shells bear a general resemblance to those of ammonoids, and as a result ammonoids have often been assumed to have resembled nautiluses in life. However, there are numerous reasons to think that this may not have been the case. Ammonoids are more closely related to the other living cephalopods, the shell-less coleoids (octopods and squid). Study of the fossil record indicates that the coiled shells of ammonoids and nautiluses are due to convergence: both groups derived separately from straight-shelled ancestors. Soft-body remains of Michelinoceras, a straight-shelled cephalopod that was related to the ammonoid + coleoid clade, suggest that ammonoids probably possessed ten relatively large tentacles like modern squid, rather than the very numerous small tentacles of a nautilus (Jacobs & Landman 1993). Jacobs & Landman (1993) also argued that ammonoids are likely to have had an expansive mantle like that of coleoids, and could probably extend the body partially out of the shell. Many ammonoids had lateral extensions of the shell at the aperture that would have required some forward extension of the mantle to grow, and some even show evidence of external shell deposition. Palaeozoic ammonoids often have a sinus on the lower edge of the aperture like that of a nautilus: in the nautilus, this marks the position of the siphon used to propel the animal. Mesozoic ammonites, however, lack such a sinus, and may have had a more dorsally placed siphon, closer to the shell’s centre of buoyancy. This would have allowed more direct, steady propulsion than that of a nautilus, but would have restricted the nautilus’ ability to bend the siphon and use it to propel itself forwards as well as backwards.

The ammonite Phylloceras (Goretophylloceras) subalpinum, with greatly subdivided lobes, from here.

As generally presented, the story of ammonoid evolution is the story of sutures. The septa dividing the chambers within the shells of ammonoids had a tendency to become increasingly complex over time, and the form of the sutures between septa and shell are one of the main characteristics used in distinguishing ammonoids. In many species of goniatites, one of the more basal Palaeozoic ammonoid groups, the sutures had only a small number of simple lobes. In other ammonoids, the number of lobes increased, and the individual lobes tended to develop their own complications. By the appearance of the ammonites, the sutures had become massively complicated, with almost fractal-appearing folds and folds within folds. The reasons for this complexity are uncertain: one possibility is that, if the ammonoids were more mobile than the modern nautilus, the crenulated sutures may have helped the animal in withstanding the hydrodynamic pressures involved with faster movement, by breaking up the flow of water within the body chamber (Hewitt & Westermann 2003).

A mystery ammonoid
Published 28 June 2017

Looks like I drew another dud. For today’s semi-random post, I ended up tasking myself to write something about the Devonian ammonoid genus Heminautilinus. But as it turns out, there simply isn’t that much to say about this genus, and what there is isn’t really worth saying.

Münster’s (1834) figure of Goniatites hybridus.

Heminautilinus was established as a genus by A. Hyatt in 1884. He diagnosed it as including “species with whorls similar to those of Anarcestes, but with angular lateral lobes in the adults“, and designated George de Münster’s (1834) Goniatites hybridus as type species on the basis of that author’s original figure. The problem is that Münster’s figure is apparently not very reliable; the original specimen was only fragmentary and Münster himself expressed uncertainty as to just what section of the ammonoid conch he had on hand. So Hyatt’s assumption that Münster’s species retained some juvenile features to maturity should not be considered reliable.

As a result, Hyatt’s genus seems to have been pretty roundly ignored. Those authors who have made some speculation as to its identity have suggested that it is probably synonymous with some better known genus such as Cheiloceras or Imitoceras. This might present something of an issue because either one of these genera was published more recently than 1884, meaning that Heminautilinus should be considered the senior name. Because there would be little to be gained from replacing a familiar name with one that is all but forgotten, it seems most likely that, even if Heminautilinus‘ identity could be reliably established, it would be somehow suppressed. As such, Heminautilinus seems doomed to remain in obscurity.

Systematics of Ammonoidea
<==Ammonoidea (see below for synonymy)HK93
|--Eobactrites Schindewolf 1932E64
| `--*E. sandbergeri (Barrande 1867) [=Bactrites sandbergeri]E64
`--+--Sicilioceras Shimanskiy 1954E64
| `--*S. paternoi (Gemmellaro 1887) [=Orthoceras paternoi]E64
| |--Boggyoceras Mutvei 2017 [Boggyoceratidae]M17
| | `--*B. centrale Mutvei 2017M17
| |--Zhuravlevia insperata Doguzhaeva 1994M17
| `--Ctenobactrites Shimanskiy 1951E64
| | i. s.: C. lesliensis Mapes 1979 [=Ctenoceras lesliense]M17
| |--*C. (Ctenobactrites) costatus Shimanskiy 1951E64
| `--C. (Mirites Shimanskiy 1962)E64
| `--C. (*M.) mirus Shimanskiy 1954E64
| `--+--DonovanocoridaKVF11
| `--ColeoideaE64
| i. s.: Dillerites shastensis Gordon 1966HK93
| Devonobactrites Shimanskiy 1962H81, E64
| `--*D. obliquiseptatus (Sandberger & Sandberger 1852) [=Orthoceratites obliquiseptatum]E64
|--Pseudobactrites Ferronnière 1921H81, E64 [incl. Bojobactrites Horný 1957E64; Bojobactritidae]
| |--*P. bicarinatus Ferronnière 1921 [incl. Bojobactrites ammonitans Horný 1957]E64
| `--P. peneauiE64
`--+--Lobobactrites Schindewolf 1932E64
| |--*L. ellipticus (Frech 1897) (see below for synonymy)E64
| `--L. timanicusE64
`--Cyrtobactrites Erben 1960E64
|--*C. sinuatus Erben 1960E64
`--C. asinuatusE64

Ammonoidea incertae sedis:
Mithraxites Becker & House 1994KK02
|--A. knoxvillensis Stanton 1895TMNZ64
`--A. solidum Girty 1915P68
Bransonoceras bakeri Miller & Parizek 1948P68
|--E. discoidalis Gordon 1964P68
`--E. mooreiP68
Epicanites loeblichiP68
|--G. filifer Gordon 1964P68
`--G. matheri Gordon 1964P68
|--P. australis Delepine 1941F71
`--P. lyoniP68
|--P. pygmaeumP68
`--P. solidum Gordon 1964P68
Schuchertites grahami Smith 1903P68
|--S. cuyleriP68
`--S. simondsi Smith 1903P68
Stenopronorites arkansiensisP68
Mkuzeiella andersoni Klinger & Kennedy 2008KK08
Metalogoceras jacksoni (Etheridge 1907) [=Gastrioceras jacksoni]F71
Histrichoceras antipodeus Etheridge 1902F71
Tragophylloceras ibex (Quenstedt 1843) [=Ammonites ibex]CDH04
Haplophylloceras strigileH79
Nevadisculites tayloriFF13
Rherisites tubaKK12
Fidelites clariondiKK12
Pernoceras dorsatumKK12
Paragattendorfia patensKK12
Cochleiferoceras Shimanskiy 1962E64
`--*C. cochleiferum (Sandberger & Sandberger 1852) [=Orthoceras cochleiferum]E64
Cyclobactrites Shimanskiy 1955E64
`--*C. erbeni Shimanskiy 1955E64
|--Sinuobactrites morrowanensis Mapes 1979HK93
`--Dilatobactrites missouriensis Mapes 1979HK93
Heminautilinus Hyatt 1884 [Nautilinidae]H84
`--*H. hybridus [=Goniatites hybridus]H84
Subperrinites bakeriSG89
Caemisites turneriB81
Verneuilites pygmaeusR81
Hudsonoceras mooreiR81

Ammonoidea [Anarcestida, Bactritacea, Bactritaceae, Bactritida, Bactritina, Bactritoidea, Cloiochoanites, Ctenobactritidae, Lobobactritidae, Macrochoanites]HK93

*Lobobactrites ellipticus (Frech 1897) [=Bactrites ellipticus; incl. B. carinatus Sandberger & Sandberger 1852 non Orthoceratites carinatus Münster 1840]E64

*Type species of generic name indicated


[B81] Birkelund, T. 1981. Ammonoid shell structure. In: House, M. R., & J. R. Senior (eds) The Ammonoidea: The evolution, classification, mode of life and geological usefulness of a major fossil group pp. 177–214. Academic Press.

[CDH04] Callomon, J. H., D. T. Donovan & M. K. Howarth. 2004. F. A. Quenstedt’s trinomial nomenclature of Jurassic ammonites. Palaeontology 47 (4): 1063–1073.

Engeser, T., & H. Keupp. 2002. Phylogeny of the aptychi-possessing Neoammonoidea (Aptychophora nov., Cephalopoda). Lethaia 34: 79–96.

[E64] Erben, H. K. 1964. Bactritoidea. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K491–K505. The Geological Society of America and the University of Kansas Press.

[F71] Fletcher, H. O. 1971. Catalogue of type specimens of fossils in the Australian Museum, Sydney. Australian Museum Memoir 13: 1–167.

[FF13] Fröbisch, N. B., J. Fröbisch, P. M. Sander, L. Schmitz & O. Rieppel. 2013. Macropredatory ichthyosaur from the Middle Triassic and the origin of modern trophic networks. Proceedings of the National Academy of Sciences of the USA 110 (4): 1393–1397.

[HK93] Hewitt, R. A., J. Kullmann, M. R. House, B. F. Glenister & Wang Y.-G. 1993. Mollusca: Cephalopoda (pre-Jurassic Ammonoidea). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 189–211. Chapman & Hall: London.

Hewitt, R. A., & G. E. G. Westermann. 2003. Recurrences of hypotheses about ammonites and Argonauta. Journal of Paleontology 77 (4): 792–795.

[H79] Hölder, H. 1979. Jurassic. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A390–A417. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[H81] House, M. R. 1981. On the origin, classification and evolution of the early Ammonoidea. In: House, M. R., & J. R. Senior (eds) The Ammonoidea: The evolution, classification, mode of life and geological usefulness of a major fossil group pp. 3–36. Academic Press.

[H84] Hyatt, A. 1883–1884. Genera of fossil cephalopods. Boston Soc. Nat. History, Proc. 22: 253–338.

Jacobs, D. K., & N. H. Landman. 1993. Nautilus—a poor model for the function and behavior of ammonoids? Lethaia 26: 101–111.

[KK08] Kennedy, W. J. & H. C. Klinger. 2008. Cretaceous faunas from Zululand and Natal, South Africa. The ammonite subfamily Lyelliceratinae Spath, 1921. African Natural History 4: 57–111.

[KK02] Klug, C., & D. Korn. 2002. Occluded umbilicus in the Pinacitinae (Devonian) and its palaeoecological implications. Palaeontology 45 (5): 917–931.

[KK12] Korn, D., & C. Klug. 2012. Palaeozoic ammonoids—diversity and development of conch morphology. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 491–534. Springer.

[KVF11] Kröger, B., J. Vinther & D. Fuchs. 2011. Cephalopod origin and evolution: a congruent picture emerging from fossils, development and molecules. Bioessays 33: 602–613.

Münster, G. de. 1834. Mémoire sur les clymènes et les goniatites du calcaire de transition du Fichtelgebirge Annales des Sciences Naturelles, seconde série, Zoologie 1: 65–99, pls 1–6.

[M17] Mutvei, H. 2017. The new order Mixosiphonata (Cephalopoda: Nautiloidea) and related taxa; estimations of habitat depth based on shell structure. GFF 139 (3): 219–232.

[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.

[R81] Ramsbottom, W. H. C. 1981. Eustatic control in Carboniferous ammonoid biostratigraphy. In: House, M. R., & J. R. Senior (eds) The Ammonoidea: The evolution, classification, mode of life and geological usefulness of a major fossil group pp. 369–388. Academic Press.

[SG89] Spinosa, C., D. M. Gallegos, D. Wang, M. Norris-Willing & S. A. Ward. 1989. Lower Permian ammonoid and conodont biostratigraphy of the Western Marginal Facies of the Dry Mountain Trough, northeast Nevada. Abstracts with Programs—Geological Society of America 21: 148.

[TMNZ64] Teichert, C., R. C. Moore & D. E. Nodine Zeller. 1964. Rhyncholites. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K467–K484. The Geological Society of America and the University of Kansas Press.

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