Jurassic

Published on February 1st, 2021 | by David Marshall

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Episode 120: Naked Ammonite

It wouldn’t be outlandish to state that many a fossil collection has started with the acquisition of an ammonite. Their planispiral shells (termed a conch) are instantly recognisable and since that conch was originally composed of the relatively hard mineral aragonite, they better lend themselves to the fossilisation process.

But how much do we actually know about the animal that produces the conch? We might be able to make superficial inferences based on comparisons with the modern Nautilus, but ammonites are actually closer related to squid and octopuses.

So could you recognise an ammonite without its shell?

Prof. Christian Klug of the University of Zurich has recently described just that: a naked ammonite. In this episode, we learn about ammonite soft body anatomy and sink our teeth into the mystery of how this ammonite lost its shell.

Read the original open access paper here: Klug, C., Schweigert, G., Tischlinger, H. et al. 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 J Palaeontol 140, 3 (2021).

The planispiral conch of an ammonite is unmistakable.
Big perisphinctid ammonite (Late Jurassic) from the ‘Solnhofener Plattenkalke’ in the Museum Solnhofen.
Image: Christian Klug
A simplified family tree of the cephalopds showing the relation between nautiloids, orthocerids, ammonoids and coleoids (e.g. squid and octopuses) through time.
Modified after Kröger et al. (2011: Cephalopod origin and evolution. In Bioessays).
Image: Christian Klug
Within a ammonite’s conch are internal walls called septa. If the outer conch is lost, the contact between the septa and conch can be traced as ‘suture lines’. The evolution of suture lines is a classic topic of study in any palaeontology course.
Zigzag-sutures of the Frasnian (Late Devonian) ammonoid Beloceras from the eastern Anti-Atlas of Morocco.
Image: Christian Klug
Suture lines can only be observed with the loss of the conch. Here, the shelly material has been dissolved away (leaving behind its negative impression in sediment), hence the gaps where the simple septa of this ammonoid should have been. Interestingly, the large holes you can see where chains of pearls would have been.
Internal mould of the late Emsian (Early Devonian) ammonoid Sellanarcestes neglectus from the eastern Anti-Atlas (Morocco). Diameter 55 mm.
Image: Christian Klug
Where the shell is still present, the pearls can be sectioned and studied.
Cross section through a fixed pearl surrounding a small tube (tube about 1 mm long) in the ammonoid Sellanarcestes from the late Emsian (Early Devonian) of Oufrane (Morocco).
Image: Christian Klug
Everything that happens between the death of an ammonite until its discovery is termed its ‘taphonomic history’. Taphonomy is the study of these processes including decay, transportation and chemical alteration. Here, conches have been transported in the sea, leaving tyre-print impressions on the sea floor sediments.
Roll marks of ammonites from the “Solnhofen Plattenkalk” of Painten.
Image: Christian Klug
In the taphonomic process, varying conditions can affect what and how much of a carcass is preserved. Here, only the siphuncle of the ammonite has been preserved. The rest of the shell and soft body has been lost.
Jurassic ammonite in the Kimmeridgian Plattenkalk of Painten in siphuncle-preservation (flattened otherwise).
Image: Christian Klug
Günter Schweigert, one of the study’s co-authors, in a quarry near Nusplingen, where platy lithographic limestones of Kimmeridgian age (Jurassic) can be found. This small quarry yielded remains of over 300 species.
Image: Christian Klug
The discovery of a remarkable new specimen has now redefined what we know about ammonite soft body anatomy. It’s an ammonite found without the shell!
Soft parts of Subplanites sp. with a Strigogranulaptychus sp. from the early Tithonian of Wintershof near Eichstätt (Germany); SMNS 70,610. a Photo taken under white light. b Line drawing of the structures visible in the white light photo (a) with possible interpretations.
Original Illustration from Klug et al. (2021: Fig. 1)
Photography using intense UV light was used to help discover detail invisible to the naked eye.
Soft parts of Subplanites sp. with a Strigogranulaptychus sp. from the early Tithonian of Wintershof near Eichstätt (Germany); SMNS 70610. a Photo taken under UV light. b Line drawing of the structures visible in the UV photo (a) with possible interpretations.
Original Illustration from Klug et al. (2021: Fig. 2)
Reconstruction of the internal anatomy of Subplanites as it came to rest on the sediment. Note that the interpretations of some organs such as the reproductive organ, the central nervous system, the hyponome and the gills are a matter of debate and represent only one possible interpretation out of several. The coiling of the soft parts corresponds to the coiled body chamber (ca. 300°), which was probably altered when these soft parts sank onto the sediment and came to a rest. a Reconstructed as in the fossil. b Organs arranged according to the conch.
Original Illustration from Klug et al. (2021: Fig. 6)
So how did this ammonite lose its shell? There are several hypotheses that have been suggested, but only one of these was considered likely enough to have an image drawn.
Reconstruction of the possible events that might have led to the extraction of the ammonite soft parts. A coleoid (belemnite or octopus ancestor) bit a hole in the rear of the body chamber and pulled out the soft parts; subsequently, it might have dropped them.
Original Illustration from Klug et al. (2021: Fig. 7)
When an ammonite loses its shell, what kind of anatomical information can be found lying beneath? With Prof. Christian Klug.
Prof. Christian Klug during a fieldtrip to the Moroccan Tafilalt. Here standing on Eiflian (Devonian) limestones with Zoophycos on Jebel Amessoui.
Image: Laura Heck

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