Chimeras in palaeontology

I have always been a fan of Roman and especially Greek mythology and especially the mythical monsters. I suppose in a way it is no different to my interest in palaeontology or zoology – all animals (and yes, even plants on occasion) are interesting in a bizarre way. Even human have enough features about them to make them truly ‘bizarre’, it just depends on how you look at things really.

I’d like to make the transition a little more smooth, but I started without thinking it through, and since the title is a bit of a give away, I’ll talk about chimeras now. For those that don’t know the chimera is a beast of Greek mythology which (depending on exactly which tale / translation you are reading) had the body of a lion with a second head of a goat attached behind the lion’s own head, and a snake for a tail. If you are lucky, you might get eagle feet or bat’s wings thrown in for a small surcharge. In other words, it is basically one animal made up of several others, all mixed together. Of which leads me onto fossil chimeras – specimens made up of more than one animal (individuals of a species or multiple species depending on the context), and depending on exactly how they came about and what they are, they can be very handy or a complete nightmare.

Chimeras can be created for a number of reasons accidentally or deliberately. Perhaps the most famous recent example was ‘Archaeoraptor’, a chimera made of bird and dromaeosaur bits that was put together by a fossil dealer and sold on to unsuspecting researchers. The fraud was discovered and no hard was done, and parts of the specimen turned out to be something valuable in its own right. I recently saw a pterosaur that on close examination was made of at least four individuals (it does make it easier to spot when the ‘tibia’ has three metacarpals attached to it!), and despite this obviousness of this example, sadly at this kind of level the frauds are getting harder to spot.

Accidental chimeras were common in the past when palaeontology was still becoming a formal science. Digs conducted in large quarries would often not catalogue the bones collected and thus when reassembly occurred months, or even years later, by people who had not even been present at the original dig would unsurprisingly mix up and match parts of skeletons unaware of what was happening. Even in well organised excavations, arranged by top professionals of the time like Marsh could still weld two halves from different species together without realising. Even today this kind of error still happens with careless collecting and more are identified in existing collections and descriptions from odd pieces that don’t appear to fit. Often a bit of detective work can hunt don the culprit and the offending bone removed, though on occasion it can change the diagnosis of the taxon, causing yet more problems.

Another famous chimera (though you would not necessarily realise looking at it) of a rather different kind is the Berlin mount of Brachiosaurus brancai. It is composed of at least three and probably more like five individuals (the skull was found on its own for starters) plus a few bits of cast and sculpted material (image shown dismounted ribs from this ‘specimen’). Although of course I would prefer the thing to made of casts rather than original bones (there is more to come on this subject), one can hardly complain about the magnificent result, but of course it is made this way because the Tendaguru material is rather capricious in its preservation – whole long bones are preserved in great condition, but perhaps just a few bones were actully found of any given animal – quite remarkable. As a result the mount is a chimera, with various bits and pieces cobbled together to make a single individual, look closely and you will see that some paired bones (like the femora if I remember rightly) are rather different sizes as a result.
Various ribs of Brachiosaurs

The term chimera is really only used for these kind of ‘real’ specimens where several bits have been put together, but it could also be applied to chimeras when writing a description or in a cladistic analysis, and here they can actually be very useful. More commonly called an Operational Taxonomic Unit (or OTU, at least in cladistics) a chimera here represents a combination of individuals that can confidently be assigned to a single species, but with no one individual being very complete. A good example is Dysolotosaurus (keeping with the Tendaguru theme, also called Dryosaurus) where beds of thousands of disarticulated bones were found together. The obvious assumption is that a whole herd died together and left their remains scattered, with individuals being all mixed together, but one would hardly described each new bone as a new taxon of ‘Ornithischian incertae sedis’, when all the bits can be cobbled together to form a very good picture of the whole animal. In these cases a chimera can massively add to your knowledge, even if getting some exact figures (such as limb ratios) remains impossible.

In cladistics of course, this can be especially important. Cladisitc analyses on palaeontological datasets are plagued by missing data and this can really impair your ability to get some meaningful information out of what might appear to be a well founded analysis. The opportunity to fill in much of that by creating an OTU from more than one specimen (or even species – such as having a Brachiosaurus OTU based on B. brancai and B. altithorax) can make a big difference to an analysis and provide additional data. In a group like sauropods where there are lots of specimens with no skull material at all, being able to include a Brachiosaurs OTU adds loads of data and can help resolve some tricky areas of sauropod systematics. It’s not without its problems, however – it is all too easy to make mistakes. If you think a large 4th cervical vertebrae is a small 5th, the you could give your chimera two 4th cervicals in a row and code them as something else, or confuse a wrist with an ankle or worse – it is much easier than you might think when the bones are from a mixture of shapes and sizes and in different conditions.

On the whole, I don’t have a problem with well documented chimeras. The ones that have entered the scientific literature or palaeontological collections, by accident or design, need to be sorted out (a time consuming job with little reward) but many of these have not be fixed, and new ones are increasingly rare. Making chimeras for display is hardly a crime if it makes a nice mount as the public will probably not notice and researchers will be aware of what went where from which specimen. The only real issue for me, is over chimeras in cladistic analyses, if only for how they are used.

It is not uncommon to find early published cladistic analyses with just a tree provided as a figure. No list of characters, no data matrix, not statement of which computer program was used, with what options or optimisations – it was almost a joke. As ever, over time scientific practices become better documented and thus early problems are eliminated and these kind of things became registered, but some gaps remain and one of them is the specimens used to make up the initial codings. Too often explicit references are not made to individual specimens and chimeras (well, OTUs mostly, but also chimeras too – you will see B. brancai listed as a ‘specimen’ when it is anything but) usually just a given taxon name (genus or species). This is a criticism of cladistic practice rather than the use of OTUs as a whole, but of course it is a contributing problem – if you don’t know what the coding is based on, how can you check what was done, and how can you use those codings in confidence that you would come to the same conclusions?

Much like the intro, I have not really got nice ending to this piece. I just thought it was worthwhile bringing up chimeras and their effects at various levels in scientific practice. They are oft overlooked, especially by the general public who think that every mount of a dinosaur in a museum is made from a complete skeleton – if only that were the case. Still, it is worth giving them some though, and keep an eye out for them when you next hit a big exhibit, or are playing in the dustier parts of a large collection, you don’t know what you might spot…

This is a revised Mk.1 post, to see the original with comments etc., go here.

@Dave_Hone on Twitter


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