The soft side of ammonites

Updated: Jun 22, 2021

Recent research in ammonites led to major discoveries in the field: the organic content of the famous shells known worldwide and valuable data on tentacles. Albeit it might seem not exceptional at first glance, it definitely is given the rarity of such preservation in those extinct cephalopods. Prepare your fork and knife, and let's cut the shell out to see what's yummy inside!



The organs and bauplan of ammonites (Klug et al., 2021)


Nusplingen and Wintershof, Late Jurassic of Germany. This is where the specimens of the studied ammonite (genus Subplanites) were discovered, whose one was found isolated from its shell and the other found associated to its aptychi (ammonite scuttles, located at the entry of the shell), anchoring its identification. The soft tissues most probably sank from the water column, without any traceable predation or scavenging activites (Fig. 1). It is worth noting that conchs (shells) and jaws are well-known in ammonites, however, soft parts are excessively rare, or doubtful in nature. Even if some of the organic part of the organism were to be preserved, it would go unnoticed most of the time, emprisoned inside the shell. Jaws, the radula (the tongue of molluscs somehow, that is a chitinous ribbon able to scrap and to cut food before ingestion into the oesophagus) and the oesophagus are the best candidates for preservation after the shell given that they are sclerotised, which yields to a high number of organs that have low preservation probabilities.


Figure 1. One of the studied specimen of Subplanites with a mollusc from the Upper Jurassic of Wintershof, Germany. a: picture taken under white light. b: line drawing of visible structures from the white light picture, with interpretations. Modified, from Klug et al. (2021).

From the few previous studies on inner ammonite anatomy as well as early coleoid's (group including among others the octopus), a simple anatomy comparable to the modern nautiloid's is expected. Thus, the main assumption for this study is that the typical organ arrangement of cephalopods applies to ammonites as well, allowing homologic comparisons (i.e., comparing and determining undetermined organs from the ammonite to e.g. the extant nautiloid, from their location inside the shell, etc.). Thanks to that approach, the buccal region, jaw or salivary glands, oesophagus, crop or cephalic retractor muscle, stomach, intestines, possibly the caecum, could be traced up. Regions corresponding to the eyes and the brain could be spotted, along male sexual organs (spermatophoric gland and sacs, spermatophores, vas deferens - the canal exporting mature sperm - and the penial appendage). Finally, the mantle and paired gills are also recognised, but the heart, retractor muscles and the arms (tentacles) could not identified, and there are still imprints that might be part of the digestive track, the sexual apparatus as well as the mantle... (Fig. 2). That's a lot to take, huh? Imagine now the researchers doing that job.


Figure 2. Reconstitution of Subplanites as it sank onto the sediment. a: reconstructed as in the fossil. b: reconstructed organs adapted to the shell. Modified, from Klug et al. (2021).

Two likely hypotheses arise: (i) post-mortem detachment of the soft body or (ii) soft body actively extruded by predators (Fig. 2). The second hypothesis is reinforced by the recurrent observation of a hole a the posterior end of the body chamber that might have been a blind spot of the shell, i.e. the breach in their defense that would have allowed predators to undertake the ammonites. But in this case, and for an unexplained reason, the predator probably missed its prey, which ultimately became fossilised for our eyes only. Among probable predators (pycnodont fishes, turtles, ichtyosaurus, and other cephalopods), a coleoid identity is the strongest possibility since they are very common in the same deposits.


In conclusion, this study suggests that the anatomic plan of ammonites was not significantly different from other cephalopods including extant species like the nautilus. The identification of the specimens as males points towards the male affinity for microconch ammonites (basically, the small version of an ammonite given species) whereas the macroconchs would be the females (the opposite of the microconch). The most likely hypothesis retained for the shell - soft body separation is a failed hunt on the ammonite, with an attack on the posterior side of the shell to pull out the organism in the open. The arm crown could be absent of the rare preserved soft tissues due to ripping off, or just eaten by the predator.


Figure 3. Reconstruction of the hunting hypothesis. a: a belemnite attacks the ammonite and breaks the shell at the so-called 'blid spot' located at the posterior side of the conch. b: the belemnite pulls out the soft body. From Klug et al. (2021).

The tackle of tentacles (Smith et al., 2021)


Kinda complementing Klug et al. (2021), Smith and coauthors just brought the missing piece for the ammonite brachial crown in a scaphitid ammonite species (Rhaeboceras halli). It has never been described before in ammonites, even though fossil coleoid tentacles are already known from a couple of specimens (Fig. 3). Those tentacles are actually the mirror of what could be preserved out of it, namely the hooks.


Figure 3. Examples of extinct belemnoid hooks. a: hook-bearing belemnite. b: zoom of picture (a), focused on the hooks and brachial crown. c: scheme of a hook and associated terminology. d: examples of different hook morphologies. From Smith et al. (2021).

Hook-like structures are know since the last decades, but their actual affinity remained controversial. Such hooks were associated with ammonites, and thanks to X-ray imagery, it has been easier to study those structures in more details because they are difficult to access in any other condition. Given that most of them are embedded within sedimentary infillings of the body chamber of ammonites (thus, virtually inaccessible), their 3D geometry was only resolved in 2020 and classed into several morphotypes (see Kruta et al., 2020) (Fig. 4). Both size and shape of the hook-like structures definitely ruled out the radular interpretation presented previously for these fossils.


Figure 4. 3D rendering of some ammonite hook morphotypes. Modified, from Smith et al. (2021).

In the best specimens of the ammonite species R. halli, hooks reflect the arm crown, (two paired axes, i.e., implying two tentacles (Fig. 5). Along those tentacles, hooks are distributed longitudinally following their morphotype group, and single elements are chiral, meaning that one could place a mirror between the two tentacles, and so images of corresponding hooks would not overlap.

Figure 5. Hypothetical reconstruction of the tentacles of Rhaeboceras halli. The longitudinal distribution of the morphotypes follows 3D observations of the fossils studied in Smith et al. (2021), and from comparisons with modern coleoids. 3D reconstruction by © A. Lethiers. From Smith et al. (2021).

The average number of morphotypes of fossil forms is way above modern species. Some arms with more morphotypes than usual are actually modified for reproduction, and subsequently called hectocotyli; those modified arms are so that a trench is created between more laterally disposed morphotypes, driving the spermatophores into the female mantle. As some species are able to self-amputate the hectocotylus at play, leaving it inside the female, it might be possible that inner hooks retrieved from ammonites would be the result of a reproductive act. However, the high number of hooks per arm (up to 60-70 single hooks) and their shape is not consistent with only one tentacle, so reinforcing the hypothesis of a pair of traditional cephalopod arms.


To sum up, the ammonite hooks resemble modern ones, and were most probably designed for prey grasping as well. Ammonites would have practiced ambush hunting strategies to hook slower swimmers. The virtual absence of ammonite hooks in the fossil record would be due to the almost systematic retraction of the tentacles inside the body chamber due to stress or death of the animal. Indeed, modern cephalopods are able to retract their arms in some kind of tentacle pocket beneath their jaw, and this might explain the absence of arm crown preservation that has sticked to ammonites for a long time. Yes, even the main subjects that palaeontologists do love so much do not help them at all, leaving them bamboozled for a tremendous part of their carreer!



Where are we now? What are the next steps?


Those studies definitely pushed forward the science of ammonites: we went from 'I can't even imagine what it looks like inside the shell' to 'oh now I wanna taste it' in a time lapse corresponding to some papers only. However, there are still a lot to discover. Even though the internal anatomy of Subplanites might be easily applied to other ammonites as a general rule, this is not the case for the hooks of Rhaeboceras halli; other ammonite species might display other morphotypes, a different amount of morphotypes and elements on a single arm, maybe more tentacles, etc. In other words, other specimens from diverse species and genus throughout the ammonite phylogeny need to be analysed to have a better idea of the overall tentacle diversity in ammonites, from their origins to their extinction.


One thing is definitely sure now: in the end of the day, every one is softer inside.



References


  1. Klug C., Schweigert G., Tischlinger H. & Pochmann H. (2021). Failed prey or peculiar necrolysis? Isolated ammonite soft body from the Late Jurassic of Eichstätt (Germany) with complete digestive tract and male reproductive organs, Swiss Journal of Palaeontology 140. doi: 10.1186/s13358-020-00215-7

  2. Smith C.P.A., Landman N.H., Bardin J. & Kruta I. (2021). New evidence from exceptionally "well-preserved" specimens sheds light on the structure of the ammonite brachial crown, Nature 11, 11862. doi: 10.1038/s41598-021-89998-4

  3. Additional reference: Kruta I., Bardin J., Smith C. P. A., Tafforeau P. & Landman N. H. (2020). Enigmatic hook-like structures in Cretaceous ammonites (Scaphitidae), Palaeontology 63, 301–312. doi: 10.1111/pala.12457














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