The Age of the Perisphinctoid

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 U_II 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.

REFERENCES

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

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

The Age of Olcostephaninae

Ammonites are among the iconic fossils of the Mesozoic. These shelled cephalopods dominated the oceans during their heyday and diversified into a wide array of taxa. Many of these have become significant for recognising particular periods in the earth's history; among these are members of the Olcostephaninae of the Early Cretaceous.

Olcostephanus astierianus, copyright Hectonichus.

The Olcostephaninae, as recognised by Wright et al. (1996), are known from the Valanginian and Hauterivian epochs of the Early Cretaceous, disappearing from the fossil record some time during the earlier part of the latter. The Valanginian ran from about 140 to 133 million years ago; the Hauterivian lasted for about three and a half million years after that. A brief reminder here: the Cretaceous lasted for a bloody long time, with more time separating the beginning and end of the Cretaceous than separates the end of the Cretaceous and today. One genus described from Pakistan, Provalanginites, has been supposed to come from the latest Jurassic but, as this is at least five million years earlier than any known olcostephanine anywhere else, its age is regarded as questionable. Olcostephanines can be very abundant in formations of the right age. A mass occurrence in the latest Valanginian of northwestern Europe has long been recognised as a geological marker, dubbed the 'Astierien Schichten' (Astieria being a synonym of Olcostephanus; Lukeneder 2004).

Saynoceras verrucosum, from here.

Olcostephanines are small to moderate-sized ammonites. Lukeneder (2004) refers to macroconches* of Olcostephanus guebhardi up to about ten centimetres in diameter. The olcostephanines pictured in Wright et al. (1996) seem to indicate an average size smaller than this and the group also includes a number of dwarf genera that look to only be a bit over one centimetre in diameter. The shell of olcostephanines is usually characterised by a pattern of transverse ribs coalescing in bundles to meet tubercles on the inner margin of the whorl. One dwarf genus, Saynoceras, has a stronger ornamentation of two rows of tubercles near the midline and outer margins of the whorls.

*A common pattern in ammonoids is the co-occurrence within a formation of distinct forms, termed 'macroconches' and 'microconches', that are broadly similar except in size and the configuration of the aperture (generally simple in macroconches but with protruding lappets in microconches). The most popular interpretation of this phenomenon is that the forms represent sexual dimorphism. Obviously which sex is which can't be known at this time though comparison with living cephalopods suggests that the macroconches may be female.

Valanginites nucleus, from here.

Olcostephanines are very similar in external appearance to the earlier subfamily Spiticeratinae (known from the very earliest part of the Cretaceous) and are likely to be descended from among that group. Though the Olcostephaninae themselves as currently recognised disappeared during the Hauterivian epoch, this may not have been the actual end of the olcostephanine lineage. The slightly later Holcodiscidae are very similar to the olcostephanines and some have questioned whether they even warrant separation. There is also a strong similarity between early members of the superfamily Desmoceratoidea and species of Olcostephanus (Wright et al. 1996). If this similarity also indicates ancestry, then the family line of the olcostephanines would continue right until the final extinction of the ammonites at the end of the Cretaceous.

REFERENCES

Lukeneder, A. 2004. The Olcostephanus level: an Upper Valanginian ammonoid mass-occurrence (Lower Cretaceous, Northern Calcareous Alps, Austria). Acta Geologica Polonica 54 (1): 23–33.

Wright, C. W., J. H. Calloman & M. K. Howarth. 1996. Treatise on Invertebrate Paleontology pt L. Mollusca 4, revised vol. 4. Cretaceous Ammonoidea. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

The Age of the Ceratites

The ammonites are unquestionably one of the most famous groups of fossil mollusks, indeed of fossil invertebrates in general. Even those who have little consciousness of the fossil world might be expected to have a vague mental picture of a coiled shell housing a squid-like beast. But ammonites are far from being the only group of shelled cephalopod known from the fossil record. And though ammonites may have dominated the marine environment during the Jurassic and Cretaceous periods, during the preceding Triassic period they were overshadowed by another such group, the ceratites.

Reconstruction of Ceratites spinosus, from Klug et al. (2007).

The ceratites of the order Ceratitida (or suborder Ceratitina, depending on how you've tuned your rank-o-meter today) were close relatives of the ammonites, each deriving separately from an earlier cephalopod group known as the prolecanitids. The earliest forms regarded as ceratites appeared during the mid-Permian, though the exact dividing line between prolecanitid and ceratite seems to be somewhat arbitrary (as, indeed, is only to be expected with a well-known historical lineage). During the remainder of the Permian their diversity remained fairly subdued. When marine life was hit with the cataclysmic upheaval that was the end-Permian extinction, two lineages of ceratites managed to squeak through, together with a single other prolecanitid lineage that would give rise to the ammonites during the ensuing Triassic. With most of their competitors thus eliminated, ceratite diversity expanded rapidly.

Externally, the shells of ceratites and ammonites were very similar, and without knowing their evolutionary context one would be hard-pressed to tell one from the other. Most ceratite shells formed the typical flat spiral one associates with ammonoids, with different species being variously evolute (with successive coils lying alongside the previous one) to involute (outer coils overlapping and concealing the inner ones), and cross-sections varying from narrow and lenticular to broad and low (Arkell et al. 1957). One later Triassic family, the Choristoceratidae, had shells that began as an evolute coil but became uncoiled or straightened in later stages. Another Upper Triassic group, the Cochloceratidae, had turreted shells that might externally be mistaken for those of a gastropod.

Ceratites dorsoplanus, showing ceratitic sutures, copyright Hectonichus.

Internally, ceratites and ammonites often differed in the structure sutures, the lines formed by the join between the outer shell and the septa dividing the internal chambers. In ammonoids as a whole, the sutures are variously curved back and forth on the inside of the shell, with those parts of the suture going forwards (towards the shell opening) forming what are called saddles and those going backwards (away from the opening) forming lobes. In most ceratites, the sutures more or less form a pattern that is known (appropriately enough) as ceratitic: the saddles are simple and not future divided, but the lobes have multiple smaller digitations. In some later taxa, the sutures became goniatitic (with both saddles and lobes simple, secondarily similar to those found in earlier ammonoids) or ammonitic (with both saddles and lobes subdivided, the pattern more commonly associated with ammonites).

Our knowledge of the soft anatomy of ceratites remains limited. We know that they possessed an anaptychus (a leathery plate at the front of the body that may have functioned as an operculum, as I described in an earlier post). Known radulae have fairly simple, slender, undifferentiated teeth (Kruta et al. 2015) so they were probably micro-predators or planktivores in the manner of most ammonites. A black, bituminous layer sometimes preserved against the inside of the shell in the body cavity may represent the remains of the dorsal mantle. Similarity between this layer and the dorsal mantle of nautilids lead Klug et al. (2007) to infer the presence of a non-mineralised hood in ceratites, though I wonder how the presence of a hood would relate to an anaptychus. Conversely, Doguzhaeva et al. (2007) interpreted the black layer as the remains of ink from a ruptured ink sac.

Assemblage of Arcestes leiostracus, copyright Lubomír Klátil.

Ceratites were to remain the ecological upper hand throughout the course of the Triassic. Though ammonites (represented by the phylloceratidans) were not uncommon during this period, their diversity remained consistently lower. However, the end of the Triassic was marked by a spike in global temperatures and ocean acidification, generally regarded as connected to the volcanic rifting activity that marked the beginning of formation of the Atlantic Ocean (Arkhipkin & Laptikhovsky 2012). Of the two ammonoid lineages, only the ammonites survived into the Jurassic; the ceratites were wiped out. Whether some aspect of ammonite biology made them better suited to survive the stresses of global climate change, or whether their survival was a question of simple dumb luck, seems to be an open question. Nevertheless, with the ceratites out of the picture, the way was open for the ammonites to become the lords of the Mesozoic ocean.

REFERENCES

Arkell, W. J., B. Kummel & C. W. Wright. 1957. Mesozoic Ammonoidea. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt L. Mollusca 4. Cephalopoda: Ammonoidea pp. L80–L465. Geological Society of America, and University of Kansas Press.

Arkhipkin, A. I., & V. V. Laptikhovsky. 2012. Impact of ocean acidification on plankton larvae as a cause of mass extinctions in ammonites and belemnites. Neues Jahrbuch für Geologie und Paläontologie—Abhandlungen 266 (1): 39–50.

Doguzhaeva, L. A., R. H. Mapes, H. Summesberger & H. Mutvei. 2007. The preservation of body tissues, shell, and mandibles in the ceratitid ammonoid Austrotrachyceras (Late Triassic), Austria. In: Landman, N. H., R. A. Davis & R. H. Mapes (eds) Cephalopods Past and Present: New Insights and Fresh Perspectives pp. 221–238. Springer.

Klug, C., M. Montenari, H. Schulz & M. Urlichs. 2007. Soft-tissue attachment of Middle Triassic Ceratitida from Germany. In: Landman, N. H., R. A. Davis & R. H. Mapes (eds) Cephalopods Past and Present: New Insights and Fresh Perspectives pp. 205–220. Springer.

Kruta, I., N. H. Landman & K. Tanabe. 2015. Ammonoid radula. In: Klug, C., et al. (eds) Ammonoid Paleobiology: From Anatomy to Ecology pp. 485–505. Springer: Dordrecht.