Perisphinctoidea

Kepplerites vardekloeftensis, from Mitta (2008).

Belongs within: Stephanocerataceae.
Contains: Tulitidae, Perisphinctidae, Morphoceratidae, Neocomitidae.

The Perisphinctoidea are a group of ammonoids known from the middle Jurassic to the early Cretaceous, many of which exhibit evolute, planulate conches with sharp, branched ribs (Énay & Howarth 2019). The perisphinctoids are thought to include the ancestors of most later Cretaceous Ammonitida, except those included in the Haplocerataceae (Enseger & Kaup 2002).

The morphology of the aptychus is known for several families in the group, with the majority possessing a praestriaptychus with weak concentric striae. This aptychal morphology is probably ancestral for the Perisphinctoidea, with the granular granulaptychus of Perisphinctidae and smooth laevaptychus of Aspidoceratidae representing derived modifications (Enseger & Kaup 2002).

The age of the perisphinctoid
Published 11 December 2021

During the Mesozoic era, the world’s oceans were dominated by the ammonites. The coiled shells of these extinct cephalopods can be found preserved in rocks of this era around the planet, encompassing a bewildering array of species. During the latter half of the Jurassic, the most diverse ammonites were members of the superfamily Perisphinctoidea.

Likely Perisphinctes, copyright Spacebirdy.

Perisphinctoids first appear around the mid-point of the Jurassic, during what is known as the Bajocian epoch (Énay & Howarth 2019). As with other major ammonite groups, perisphinctoids are characterised by features of the folding around the edges of the septa that separate chambers of the shell. Perisphinctoids have basally five-lobed septa that differ from their ancestors in the Stephanoceratoidea in the loss of the UII lobe towards the outer edge of the whorl. The earliest perisphinctoids had more or less evolute shells (that is, later whorls did not significantly overlap the predecessors) with a rounded venter. Some later lineages would become more involute, with older whorls becoming partially hidden, and the venter might get sharper or flatter. Others would pretty much retain the original conformation to the end. The majority of perisphinctoids exhibited strong ribs on the outside of the shell, these ribs usually branching towards the outer rim of the whorl. Some forms developed further elaborations of the shells such as prominent nodules or spines.

Dimorphism was widespread in the perisphinctoids, if not universal. As with other dimorphic ammonites, populations included distinct microconches and macroconches (the majority interpretation is that macroconches were female and microconches male, but of course this is speculative). Macroconches usually had simple peristomes whereas microconches commonly had the mature shell aperture flanked by elongate lappets. The early Late Jurassic (Bathonian and Callovian) Tulitidae had a tendency in macroconches for the shell coiling to become eccentric in the outermost whorls, the peristome being distinctly skewed from the main plane of the shell.

Aspidoceras hirsutum, copyright Daderot.

Perisphinctoid faunas were often markedly provincial with many lineages being restricted to particular regions (such as the bipolar Perisphinctes or the western Eurasian Parkinsoniinae). They were mostly animals of shallower waters, perhaps foraging close to the bottom. This may go some way to explaining their high diversity but it can provide a challenge to their use in stratigraphy. Ammonites of the ‘perisphinctoid’ type would survive the end of the Jurassic but would fade from the fossil record not too long afterwards. Nevertheless, that would not be the end of their lineage: at the beginning of the Cretaceous, they would also spawn two derived descendants (Besnosov & Michailova 1991), the largely smooth-shelled Desmoceratoidea and the Ancyloceratoidea with four-lobed septa, that would continue to dominate the Mesozoic seas.

And while I’m on the subject of ammonites, I have another correction to make to an earlier post. However, while I was able to shift some of the blame for the correction in my last post onto my original source, in this case the blame is entirely mine. In a prior discussion of the live anatomy of ammonites, I discussed the evidence that the aptychus (a pair of calcified plates that probably functioned as an operculum) originated as a modification of the lower jaw. As such, I criticised reconstructions of ammonites that showed the aptychus articulating with the shell in the manner of a nautilus’ hood. Unfortunately, I had overlooked a significant difference between ammonites and nautiluses. The coiled shell of a nautilus is exogastric—that is, when they evolved from their straight-shelled ancestors, the shell coiled upwards so the original lower edge corresponded to the outside of the whorl. However, the shell of ammonites was endogastric, with the shell coiled downwards so the original venter was on the inside (in the absence of preserved soft anatomy, we can infer this from the position of the siphuncle within the shell). This means that, even though the lower ammonite aptychus was anatomically on the opposite side of the animal from the upper nautilus hood, functionally they would have appeared in life to occupy much the same position. Entirely my mistake, and a reminder to me that describing orientation in coiled animals can be confusing.

Systematics of Perisphinctoidea
<==Perisphinctoidea [Perisphinctaceae]
| i. s.: ‘Ancyloceras’ ischeri Favre 1876DCH81
|--+--TulitidaeDCH81
| |--PerisphinctidaeDCH81
| |--MorphoceratidaeDCH81
| `--NeocomitidaeDCH81
`--KosmoceratidaeDCH81
| i. s.: Toricellites approximatus Buckman 1933P87
| SeymouritesH79
| ZugokosmocerasH79
| Sigaloceras Hyatt 1900EK02
| `--S. callovienseP87
|--KosmocerasCDH04 [Gulielmiceratinae, KosmoceratinaeDCH81]
| |--K. ‘compressum’ (Quenstedt 1846) (see below for synonymy)CDH04
| |--K. ornatumCDH04
| |--K. ‘rotundum’ (Quenstedt 1856) (preoc.) [incl. *Granulaptychus calloviensis Trauth 1927]EK02
| `--K. (Kosmoceras) spinosumP93
|--GarantianinaeDCH81
| |--Garantiana Mascke 1907EK02
| |--PseudogarantianaDCH81
| |--Ermoceras Douvillé 1916DCH81
| |--Arkelloceras Frebold 1958DCH81
| | `--A. tozeri Frebold 1951P87
| `--StrenocerasDCH81
| |--S. ‘bifurcatum’ (Quenstedt 1846) (see below for synonymy)CDH04
| `--S. latisulcatumW81
`--Kepplerites Neumayr & Uhlig 1892P87 [GowericeratinaeDCH81, Keppleritinae]
|--K. costidersusP93
|--K. mclearniW81
|--K. rosenkrantzi Spath 1932P87
|--K. stephanoidesP87
|--K. svalbardensisP87
|--K. traillensisP87
|--K. tychonis [incl. K. tychonis var. fasciculata]P87
`--K. vardekloeftensisP87

Kosmoceras ‘compressum’ (Quenstedt 1846) [=Ammonites ornatus compressus nec A. compressus de Blainville 1840 nec A. radians compressus Quenstedt 1845 nec A. valdani compressus Quenstedt 1845]CDH04

Strenoceras ‘bifurcatum’ (Quenstedt 1846) [=Ammonites parkinsoni bifurcatus nec A. bifurcatus Bruguière 1789 nec de Roissy 1805 nec Schlotheim 1820]CDH04

*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.

[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.

[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.

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.

[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).

[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.

[P87] Poulton, T. P. 1987. Zonation and correlation of Middle Boreal Bathonian to Lower Callovian (Jurassic) ammonites, Salmon Cache Canyon, Porcupine River, northern Yukon. Geological Survey of Canada—Bulletin 358: 1–155.

[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.

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