Ammonitida

Aegoceras luridum, from Yorkshire Ammonites (and other fossils) revisited.

Belongs within: Neoammonoidea.
Contains: Ammonites, Arietitinae, Oxynoticeratidae, Eoderoceratidae, Polymorphitidae, Aptychophora, Dactylioceratidae.

The Ammonitida (here used for the Ammonitina of some authors) are a group of ammonoids known from the Jurassic and Cretaceous with five-lobed primary sutures. Early members of this lineage retained a single hemispherical anaptychus with which to close the body chamber; this became divided into two valves in the derived Aptychophora (Enseger & Keupp 2002).

The Early Jurassic (Hettangian to Sinemurian) Arietitidae had narrow-whorled evolute shells with strong straight simple ribs, a ventral keel and grooves (Donovan et al. 1981).

The arms of an ammonite
Published 20 February 2017

The ammonoids are one of the most characteristic animal groups of the late Palaeozoic and Mesozoic. During their time on this earth, they were one of the most diverse and abundant groups of mollusks around. But as with other mollusks, their fossil record is overwhelmingly dominated by the hard shells, with little direct evidence of the softer parts of the animal. So what did the rest of an ammonoid look like?

A typical ammonite Asteroceras obtusum, copyright Dlloyd.

Ammonoids belong to the cephalopods, and hence to the same group of mollusks as modern octopods, squids and nautilus. Indeed, it is generally accepted that ammonoids were more closely related to octopods and squid than nautilus. As such, we can safely take as a starting assumption that those features shared by modern cephalopods were also present in ammonoids, such as a muscular siphon for propelling the animal, and an array of arms or tentacles surrounding a central mouth. But how many tentacles did ammonoids have? Squid and octopods have eight or ten arms, but nautilus have many more, about ninety. Because nautilus bear a superficial resemblance to early cephalopods in retaining an external shell, it has been tempting to assume that they are more primitive than octopods and squid, but there are good reasons to believe that the supernumerary tentacles of nautilus are a derived peculiarity of that group. Arm development in cephalopod embryos begins from ten original buds in both nautilus and squid, with these buds becoming divided in nautilus (Klug & Lehmann 2015), suggesting that the lower number could be the more primitive. With ammonoids on the squid line rather than the nautilus line as mentioned above, it seems likely that they retained the primitive arm number like their sister group. In their review of preserved ammonoid soft-tissue remains, Klug & Lehmann (2015) noted that there is only a single known fossil ammonoid (going by the memorable name of GSUB [Geosciences Collection, University of Bremen] C5836) that might include preserved arm tissue, but the area in question shows little more than a tarry smear. Trace fossils have been used to argue for a low tentacle number in orthocerids, a group of Palaeozoic cephalopods commonly believed to include the ancestors of both ammonoids and squid, but again the evidence is not enough to be conclusive.

If we do presume that ammonoids had a squid- or octopus-like number of tentacles, can we then interpret ammonoids as basically a squid in a coiled shell? This may be the most common representation of such animals:

Unfortunately for Akane’s purposes, ammonites may not have provided much in the way of good eating. Whereas the fossil record of ammonoid tentacles themselves is next to nonexistent, we do have a bit more evidence about the arrangement of an ammonoid’s mouthparts. Living cephalopods usually have a hardened beak at the opening of the mouth, with the ribbon-like radula sitting directly behind it. The majority of tearing and crushing of food is done by the beak; the radula mostly functions to pull food particles back into the gullet. In basal ammonoids, the beak was more or less similar to that of a recent cephalopod, but in the derived ammonites* it became quite modified. Ammonites possessed a broad structure near the opening of the body chamber that is called an anaptychus or aptychus according to its configuration (though just to confuse matters, the term ‘aptychus’ seems to sometimes be used to cover both types). An ‘anaptychus’ was a single chitinous, semi-circular plate; an ‘aptychus’ was a calcified, bivalved arrangement. The aptychi were not directly attached to the main shell and may commonly be found as isolated fossils. Examination of aptychi that have been preserved still in their original body chamber has lead to the widely held conclusion that they represent a modification of the original lower jaw of the beak. Meanwhile, the upper jaw became reduced and weakened in ammonites with aptychi (Tanabe et al. 2015).

*A quick explanation about ‘ammonoid’ versus ‘ammonite’: ‘ammonoids’ are a particular group of shelled cephalopods that first appeared during the Devonian. ‘Ammonites’ are a particular clade within the ammonoids including most of the Mesozoic species (a small number of non-ammonite ammonoids survived into the Triassic). So all ammonites are ammonoids, but not all ammonoids are ammonites.

Specimen of Neochetoceras with aptychus in place, from here.

Because they often have a similar configuration to the opening of the ammonite’s shell, the aptychi have often been interpreted as functioning as an operculum for when the animal retracted itself into the body cavity, presenting a tough barrier to any would-be predator. Certainly the reduced upper jaw meant that they could not function as a beak to bite into food (though some Late Cretaceous ammonites did exhibit a re-enlargement of the upper jaw and may have regained their bite). However, if aptychi functioned as opercula then the tentacles of ammonites could not have sat in quite same arrangement as in modern cephalopods. They could not have completely surrounded the mouth because then they would have prevented the operculum from closing. Perhaps some of the lower tentacles were lost, or perhaps the base of the circle became divided. Some authors have argued that aptychi were jaw structures only, with no operculum function, but I confess I find it difficult to understand their purpose in that case.

That most ammonoids were not subjecting their food to strenuous chewing is also indicated by the structure of the radula: where known, the majority of ammonoids had radulae with high, slender teeth more suited to grasping than rasping (Keupp et al. 2016). The overall indication is that most ammonoids were probably micropredators, feeding on small plankton such as crustaceans; where possible stomach contents have been identified in ammonoid fossils, they have also supported this conclusion. The modern nautilus has a similar diet, and ammonoid arms possibly did resemble nautilus tentacles in being short and slender rather than long and muscular (though at least one author has discussed the possibility of ammonoid arms being expanded into broad fans for the capture of plankton). The Late Jurassic ammonite Aspidoceras had a much more robust, powerful radula than is known for other ammonoids but may provide something of an exception to prove the rule: its stomach contents are dominated by the pelagic crinoid Saccocoma, suggesting that it was still a planktivore even if it was tackling tougher prey than its relatives (Keupp et al. 2016).

A speculative reconstruction of an ammonite with filter-feeding arms, copyright sethd2725.

So to sum up, ammonoids probably had only a small number of tentacles, no more than ten at the most. They were probably slight affairs, suited for sweeping small or poorly motile food objects out of the water rather than grabbing and manipulating struggling prey. A planktivorous habit for ammonoids would also seem to fit with their predominance when they were around; after all, there’s no shortage of plankton in the sea.

Systematics of Ammonitida

Characters (from Besnosov & Michailova 1991): Jurassic and Cretaceous ammonoids with primary five-lobed primasuture (VUU1ID); unstable five-lobed primasuture established in some Cretaceous forms. New elements formed in various ways: by division of saddle U1/I and appearance of new umbilical lobes (U1, U2…), occasionally by division of saddle I/D and appearance of inner lateral lobes (I1, I2…) or as a result of subdivision of inner lateral lobe; combination of these modes sometimes seen, more rarely umbilical lobe dividing, and in extreme cases lateral lobe arising. Umbilical lobe predominantly tripartite. Dorsal lobe in last stages initially bipartite, then becoming tripartite; again becoming bipartite extremely rarely in Cretaceous forms. In course of ontogenesis saddles dividing somewhat later than lobes or at same time.

Ammonitida [Ammonitina]
| i. s.: AmmonitesCDH04
| Cymbites Neumayr 1978DCH81 [incl. Metacymbites Spath 1923DCH81; CymbitaceaeP93, Cymbitidae]
| |--C. centriglobusP93
| `--C. laevigatusP93
| ChondrocerasWK81
| |--C. oblatumW81
| `--C. tenueWK81
|--Arietitidae [Arietitaceae]EK02
| | i. s.: Pseudotropites Waehner 1894 [Pseudotropitinae]DCH81
| | Burckharticeras Flores Lopez 1967EH19
| | `--*B. fallaxoides (Erben 1956) [=Arnioceras fallaxoides]EH19
| |--AlsatitinaeDCH81
| | |--Canavarites Hyatt 1900DCH81
| | |--Pseudaetomoceras Spath 1923 [incl. Proarnioceras Blind 1963]DCH81
| | `--Alsatites Haug 1894 [incl. Gonioptychoceras Lange 1941, Proarietites Lange 1922]DCH81
| | `--A. laqueolus [=Proarietites laqueolus]P93
| `--+--ArietitinaeDCH81
| |--AgassiceratinaeDCH91
| | |--Agassiceras Hyatt 1875 [=Aetomoceras Hyatt 1900]DCH81
| | `--Euagassiceras Spath 1924 [incl. Paracoronites Buckman 1927]DCH81
| `--+--OxynoticeratidaeDCH81
| `--AsteroceratinaeDCH81
| |--Asteroceras Hyatt 1867EK02
| |--Aegasteroceras Spath 1925 [incl. Arctoasteroceras Frebold 1960, Ptycharietites Spath 1925]DCH81
| |--Caenisites Buckman 1925 [incl. Euasteroceras Donovan 1953]DCH81
| |--Pompeckioceras Spath 1925DCH81
| `--Eparietites Spath 1924DCH81
| `--E. denotatusP93
`--EoderocerataceaeEK02
|--EoderoceratidaeDCH81
|--+--PolymorphitidaeDCH81
| `--AptychophoraDCH81
|--PhricodoceratidaeDCH81
| |--Pseuduptonia Bremer 1965DCH81
| |--Epideroceras Spath 1923 [incl. Coeloderoceras Spath 1923, Villania Till 1911]DCH81
| | `--E. exhaeredatumP93
| `--Phricodoceras Hyatt 1900 [incl. Hemiparinodiceras Geczy 1959]DCH81
| |--P. bettoni Geczy 1976C81
| |--P. subtayloriP93
| |--P. taylori (Sowerby 1826)C81
| `--P. urcuticum Geczy 1976C81
`--+--+--DactylioceratidaeDCH81
| `--CoeloceratidaeDCH81
| |--Apoderoceras Buckman 1921DCH81
| |--Hyperderoceras Spath 1926DCH81
| |--Pimelites Fucini 1896DCH81
| |--Praesphaeroceras Levi 1896 [incl. Diaphorites Fucini 1896]DCH81
| |--Coeloceras Hyatt 1867DCH81
| | `--C. pettosP93
| `--Tetraspidoceras Spath 1926DCH81
| `--T. birchiadesP93
`--+--AmaltheidaeDCH81
| |--Amauroceras Buckman 1913DCH81
| | `--A. ferrugineumD01
| |--Pleuroceras Hyatt 1867 non Riess 1854 (ICBN)DCH81
| | |--P. gigas Howarth 1958C81
| | |--P. hawskerenseP93
| | `--P. spinatumC81
| `--Amaltheus de Montfort 1808CDH04
| | i. s.: A. gibbosus (Quenstedt 1885)C81
| | A. margaritatus (Quenstedt 1885)C81
| | A. stokesi (Howarth 1958)C81
| |--A. (Amaltheus) [incl. Nordamaltheus Repin 1968, Proamaltheus Lange 1932]DCH81
| `--A. (Pseudoamaltheus Frebold 1922)DCH81
`--LiparoceratidaeDCH81
|--Androgynoceras Hyatt 1867DCH81
|--Vicininodiceras Trueman 1918DCH81
| `--V. simplicicostaP93
|--Liparoceras Hyatt 1867DCH81
| `--L. (Becheiceras Trueman 1918) [=Becheoceras (l. c.); incl. Anisoloboceras Trueman 1918]DCH81
| `--L. (B.) nautiliformeP93
`--Aegoceras Waagen 1869DCH81
|--A. (Aegoceras) [incl. Amblycoceras Hyatt 1900]DCH81
|--A. (Beaniceras Buckman 1913)DCH81
| |--A. (B.) centaurusC81
| |--A. (B.) dundryi Donovan 1978C81
| `--A. (B.) luridumC81
`--A. (Oistoceras Buckman 1911)DCH81

*Type species of generic name indicated

References

Besnosov, N. V., & I. A. Michailova. 1991. Higher taxa of Jurassic and Cretaceous Ammonitida. Paleontological Journal 25 (4): 1–19.

[C81] Callomon, J. H. 1981. Dimorphism in ammonoids. 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. 257–273. 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.

[D01] Delsate, D. 2001. L’ichthyofaune du Pliensbachian (Jurassique inferieur) de Lorraine et des Ardennes (France): premiers resultats. Bulletin de l’Académie Lorraine des Sciences 40 (1–2): 47–69.

[DCH81] Donovan, D. T., J. H. Callomon & M. K. Howart. 1981. Classification of the Jurassic Ammonitina. 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. 101–155. Academic Press.

[EH19] Énay, R., & M. K. Howarth. 2019. Part L, revised, volume 3B, chapter 7: Systematic descriptions of the Perisphinctoidea. Treatise Online 120: 1–184.

[EK02] Engeser, T., & H. Keupp. 2002. Phylogeny of the aptychi-possessing Neoammonoidea (Aptychophora nov., Cephalopoda). Lethaia 35: 79–96.

Keupp, H., R. Hoffmann, K. Stevens & R. Albersdörfer. 2016. Key innovations in Mesozoic ammonoids: the multicuspidate radula and the calcified aptychus. Palaeontology 59 (6): 775–791.

Klug, C., & J. Lehmann. 2015. Soft part anatomy of ammonoids: reconstructing the animal based on exceptionally preserved specimens and actualistic comparisons. In: Klug, C., et al. (eds) Ammonoid Paleobiology: From Anatomy to Ecology pp. 507–529. Springer Science.

[P93] Page, K. N. 1993. Mollusca: Cephalopoda (Ammonoidea: Phylloceratina, Lytoceratina, Ammonitina and Ancyloceratina). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 213–227. Chapman & Hall: London.

Tanabe, K., I. Kruta & N. H. Landman. 2015. Ammonoid buccal mass and jaw apparatus. In: Klug, C., et al. (eds) Ammonoid Paleobiology: From Anatomy to Ecology pp. 429–484. Springer Science.

[W81] Westermann, G. E. G. 1981. Ammonite biochronology and biogeography of the circum-Pacific Middle Jurassic. 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. 459–498. Academic Press.

[WK81] Wiedmann, J., & J. Kullmann. 1981. Ammonoid sutures in ontogeny and phylogeny. 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. 215–255. Academic Press.

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