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Saturday, 26 April 2014

Palaeoartworks, the case studies, part 3: Ammonites and... extinct snails?

For the the final set of Palaeoart Case Studies produced for my Lyme Regis palaeoart gallery (running up until May 4th - catch it now before it's too late! Details here.), we're going to focus on molluscs. Yeah, that's right: palaeoart of squidy things, snails, clams and allies. Given that most palaeoart focuses on charismatic reptiles and mammals, this might be a hard sell. Let's see how the blog hits go with this one...

The general rarity of molluscan palaeoart occurs in spite of this group having a much better fossil record than virtually any vertebrate, as well as being fossils with uses beyond keeping socially awkward vertebrate palaeontologists off the streets. Molluscs do occur in palaeoart of course, but mostly as secondary or background animals, adding flavour to scenes dominated by larger, more charismatic species. This is a shame because molluscs are extremely interesting creatures in their own right, and especially so when we look at the bizarre forms that existed before our Recent molluscan fauna. Conchology is fun in the modern day, but becomes downright mind-bending when multiplied with deep time.

How confident can we be about the life appearance of these ancient shellfish? Because many molluscs have a fossil record quality which is basically opposite that of many palaeoart favourites (i.e. mountains of complete specimens), you might expect that we know a lot about their soft-tissues and life appearance. Do we? Read on to find out.

Erymnoceras: Ammonites - common fossils, artistic enigmas

Are there any more creatures more frustrating to palaeoartists than ammonites? These ancient cephalopods (the group of molluscs that comprising the shelled Nautilus, and the coeloids - squid, octopuses and cuttlefish) are superabundant in many Mesozoic marine deposits and, given this, we would expect at least a few extremely well-preserved specimens which reveal details of their soft-part anatomy and life appearance. This has certainly happened for fossil squid and belemnites, which are known from specimens showing their tentacle counts, ink sacs, body shapes and - sometimes - even the sizes of their eyes. Ammonite fossils are nearly as common as belemnites and certainly far more abundant than fossil squid, so there must be some fossils which inform palaeontologists and palaeoartists about their life appearance... right?

Male and female (respectively) Jurassic ammonites, Erymnoceras coronatum. The size difference between these genders is well constrained by fossil data, but the appearance of the actual animals is not.

Amazingly, no. The basic details of ammonite life appearance are far from clear, and exceptional preservation in the group is almost unheard of. Even the mineralised components of their radulae (rasping organs bearing numerous ‘teeth’, common to most molluscs) are incredibly rare, and good soft-tissue outlines of their bodies are unknown. While we can be certain that a squid-like organism lived in the last chamber of their shells (the ‘body chamber’) and was anchored in by muscles which left distinctive scars on the internal body chamber wall, little else can be said with certainty about their appearance. For instance, how many tentacles did they have? They very likely had some because they represent a grade of cephalopod evolution between Nautilus and coeloids, both of which bear tentacles. However, Nautilus has 90 small tentacles, and most coeloids have 10 large ones (octopuses, of course, have only eight). So how many did ammonites have? 10? 90? Another number entirely? And what of their eyes? Coeloids have large eyes and excellent vision on par with that of vertebrates, while Nautilus eyes are little more than organic pin-hole cameras. Which sorts, if either, did ammonites have?

And these are only immediate, cosmetic quandaries: much remains to be learned about ammonite floating postures, swimming abilities, and lifestyles. Given how elaborate some of their shell shapes are, and the unusual proportions of their body chambers, some ammonites must have had very unexpected appearances and floating mechanics indeed.

Despite being creatures which occur so commonly as fossils that it seems like we should know everything about them, ammonites are creatures fraught with uncertainty for artists and palaeontologists alike. Until new data comes to light, all life reconstructions of ammonites should be taken as extremely tentative, almost speculative renditions of their actual appearance.

Viviparus: a modern glimpse of the past

At first consideration, it may seem that accurately restoring ancient snails may be as hopeless as precisely restoring an ammonite. Like ammonites, their soft-parts are virtually unknown in the fossil record, the slug-like organisms inhabiting their coiled shells only represented by muscle scars left inside the shell.

The Creaceous Wealden mud snail, Viviparus cariniferus; probably the most accurately reconstructed extinct animal on this blog.
This is only sometimes the case, however. Unlike ammonites, snails - known formally as gastropods - are still alive in the modern day, and some types have extraordinarily long evolutionary histories. In some cases, members of modern genera evolved hundreds of millions of years ago, and remain largely unchanged in the present. This is so for members of the gastropod genus Viviparus, which first appear in the Middle Jurassic (c. 168 million years ago) and are still around today. For palaeoartists, these modern animals provide direct insights into the probable life appearance of their older cousins. For instance, modern Viviparus often have variably developed brown and ochre colour banding swirling around shells, so we may infer that their extinct relatives had the same patterning. The head and muscular foot (the name of the creeping gastropod propulsive organ) of modern Viviparus are also rather short and relatively broad, with two long tentacles emerging from the head and prominent eyes situated at their bases. We can’t know for certain that this is exactly what ancient Viviparus looked like, but it’s more parsimonious to assume that they resembled their modern counterparts than looking drastically different. The Early Cretaceous species shown here, Viviparus cariniferus, has been reconstructed with this logic in mind.

Modelling extinct animals on modern variants of the same species does not only apply to gastropods. The closer a fossil assemblage is to the present, the more likely it is to contain animals which have extremely close modern relatives, if not the exact same species. These instances provide palaeoartists with many models to essentially copy and paste into extinct scenes. If the biology of the modern variants is also well understood, they can also lend some compositional input to a palaeoartwork. A painting with Viviparus, for instance, would be most sensibly set around a relatively still or slowly moving water body, as this is where species of these gastropods occur in the modern day. Likewise, the salinity tolerances of modern Viviparus are low, so they only occur in freshwater: a reconstruction of these animals in this domain would therefore be logical. As with lots of palaeoartistic tricks, this technique is directly adapted from palaeontological science, where the biology of modern animals with fossil counterparts is frequently used to shed light on the depositional conditions and palaeoenvironmental settings of the rocks they occur in.

1 comment:

  1. Ammonites are among my favorites invertebrates ! I'm used to collect fossils of belemnites and ammonites's shells. I'm also wondering what their eyes looked like...Were they similar to squid or octopus ?

    Any news from a ''book project'' Dr. Witton ?