As the title has already given away, I’m going to talk about mammals a bit on here for once (well, OK, they have cropped up occasionally before) but as a set-up to a point about convergence and evaluating evidence so it will hopefully be instructive and not overly extant mammal-y for those who like their extinct archosaurs. So onto, as also already given away by the title, spines in mammals.
Continue reading ‘Spiny-ness in mammals and rampant convergence’
One might think that after developing as a science for the last two hundred years, palaeontology would demand some pretty rigorous proof of a concept before it enters the area of ideas that one could consider ‘general consensus’ or perhaps nowadays more accurately (though less inclusively) ‘passed peer review’. However there is still one holdover from the early days that crops up from time to time in the literature (though far more often in general ramblings of unpublished ideas and online discussions) that being the concept that an idea is convincing if you can produce some nice looking artwork to demonstrate it. Essentially, proof by illustration. Continue reading ‘Proof by illustration’
Possibly. Anyway, I’ve now been formally taken on as one of the associate editors of the palaeontological journal ‘Historical Biology’ (my thanks to Gareth Dyke for his invitation to join the board). Having preached much about peer review, reviewing and writing papers and even the editorial process I hope that I can put this all into practice. This might put more people off than it encourages, but those of you writing palaeontological papers, (perhaps as part of the PPC?) an at least consider HB as your target journal.
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It occurred to me the other evening that the human memory can serve as quite a good analogy for the fossil record. While I have before covered some of the issues of bias in the fossil record, this might serve as something a little easier to think about since it’s based on something we all experience. Anyway, I’ll launch in and see how far I get before the whole edifice breaks down…
Continue reading ‘Memory and the fossil record – laboured analogy time’
My colleagues Mike Taylor and Andy Farke among others have done an admirable job of promoting the concept of open access in palaeontology, both for data and for the actual research papers that academics produce. However, while this is on the whole a very good thing, it has I believe (in conjunction with other phenomena) produced problems from the frontline scientists whom it is supposed to help.
While what I am about to write may be seen as a complaint it should not be – it is an observation. It is for me currently problematic, but that does not mean that I do not support open access (I do) or that this is a huge issue (it isn’t) or that on balance open access is a bad thing (not true either). With change comes problems, some foreseen and others not, and most if not all ultimately overcome or sidestepped to the general satisfaction of most so this is not something I expect to be a long-term issue. Here I simply want to illustrate a couple of problems that I have not seen commented on or discussed before. So with this in mind, what’s the problem?
Continue reading ‘Keeping up with the literature’
A long time ago in the dim and distant past on here I wrote about fossil chimeras and mounting skeletons and have since written about fake fossils of various kinds. In these I rather breezed over some of the different ways that fossils can be produced for display and it seems worth going over in a little greater detail and roughly defining a few terms to make things easier for people to understand and distinguish between the various things out there.
Increasingly, genuine fossils of large animals are not on display in museums. These are expensive and valuable artefacts and scientists need to access them, and the museums need to protect them. Big dinosaur mounts that tower into the air made up of original fossils that are hundreds of millions of years old are therefore rare. They are hard to examine, and difficult to keep clean and if they ever fell over…. However, even the most complete of big dinosaur mounts are often not as they seem and can be completed using a number of different techniques.
Here then are the ways that you can complete your dinosaur fossil:
1. Original material. While these are becoming rarer, there are a significant number of mounted skeletons being produced composed mostly or entirely of original fossil material. Since there are pretty much no *totally* complete dinosaur skeletons in 3-D, the odd part of another specimen may be used to fill in the gaps, effectively creating a chimera.
2. Repaired material. Even if you do have a complete specimen, the odds are there are a few chunks missing – a humerus with the end gone, teeth lost from the jaws, or the neural spines broken from a few vertebrae. These can still be used with the missing parts repaired and completed from plaster or a similar material.
3. Casts. You can of curse simply make a direct physical copy of the bones of your specimen and mount them, or from another specimen to fill in the gaps and these are casts. Most big specimens nowadays are casts of real specimens supplemented by sculptures of missing bits.
4. Sculptures. Finally, you can simply model the missing pieces from scratch and make them to fit the gaps and what you know of the existing anatomy or from close relatives. Sculptured bones run the full length from inept plasticine-like creations that look only vaguely like bones right through to superb ones that can even look better (since they have no breaks or distortions) than the originals.
A selection of casts and sculptures of dinosaur claws and various teeth.
Telling these different ‘bones’ apart is not normally too difficult with a little practice (though across a darkened dinosaur hall it’s not always easy). Typically original material looks organic in a way that even casts do not – natural swells and breaks and just the texture of the bones will look ‘right’. Repairs to original material are often crude, but in any case the instant change in texture and colour between a sculpted piece of plaster and the bone itself should be clear. Sculptures (whether as repairs or as whole replacement bones) often have little texture on their surface beyond a few scratches or dimples and are often a give away as their surface is so smooth. Finally casts often loose a little of the detailed surface texture of the originals from which they are copied but can usually be distinguished by their colours. Real bones generally have a range of colours (if minor) to them when casts are typically made using coloured resin or are pained after production and so are a uniform colour.
That’s quite probably more than enough of casts and sculptures, but this should serve as a guide to what is, and is not, real in museums and how to tell them apart and why this can be important.
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While we are talking birds with odd beaks, skulls, ornaments and all that, it seemed most pertinent that I dig up this image of a cassowary from my collection. The crest at the top of the head is more properly called a casque and while studies show that it certain does have an ornamental / signaling function, it is used for a few other things as well including clearing foliage and detritus off the rainforest floor where the animals live.
To return then to yesterdays general theme, it is usually a bad idea to go looking for extra possible odd functions and features in fossil animals. If you have a good set of data that strongly supports a given function of a morphological feature – don’t try and then second guess yourself with a raft of extra odd (and untestable) features just because they crop up in one or two extant organisms.
Case in point being this one, I suppose it’s possible something like Monolophosaurus did use it’s crest in a similar way to a cassowary, but I wouldn’t want to argue that and nor is it common enough or tied to an obvious structural feature that you could realistically test it on the dinosaur.
However, when trying to work out the possible range of behaviours that an animal may have exhibited or when faced with something unusual, it can be well worth digging around (so to speak) in the literature on extant animals. There is such a raft of unusual things that living animals do and features they have that it would be impossible to consider them all when looking at a new fossil – to do so would be a waste of time and effort and a great many simply could not be assessed properly. They can provide a source of ideas and information that could easily be missed otherwise so as ever a balance must be struck, but stick to those that can reasonably be tested and avoid the extreme.
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A recent discovery of ‘giant’ sauropod tracks in France has got the media all of a flutter and it seemed a pertinent opportunity to return to the concept of ‘tricky tracks‘ and the misinterpretation of fossil footprints. The media are especially impressed that some of these impressions are nearly 2 m across and while I have not seen anyone *directly* claim that these match the feet that left them (and nor have I looked that hard), I rather imagine most people will jump (not at all surprisingly) to that conclusion. But is this really the case? Are there sauropods out there with a pes six feet across?
Well once again that rhetorical question at the end of the first paragraph has a pretty obvious answer – no, not really. While I have not seen any researchers quoted on the French tracks or indeed seen any decent close-ups, I find it hard to credit that there were sauropods with feet this big, since frankly they would have trouble getting their feet past each other when the walked, and scaling up from the bones of sauropod feet an animal leaving tracks that big would be getting on for a size that is hard to comprehend. Hundreds, even thousands of tons I imagine – in other words, beyond credible. So what’s going on here?
One explanation is that the tracks as preserved are showing the effects of the substrate they were made in. In short, a heavy animal walking across very soft mud will gunge and slop the stuff everywhere and will leave a wide area affected by its passing at each point that a foot hits the substrate. In other words, big feet and soft mud can make for even bigger tracks.
However I suspect the answer is another related but somewhat different artefact – underprinting. Imagine a nice heavy sauropod putting its foot down on some relatively soft, but still firm, sediment that lies in multiple layers (like lots of mud layers that have built up on a tidal flat over a few weeks for example). Now the actual print the animal leaves on the surface of the mud will likely be quite clear and deep, and will indeed match the foot that left it. But as we move down through the layers the force will dissipate and spread out. On the second layer the impression will be less deep, less clear and, crucially, rather bigger. Go down a few more layers experiencing the same effect and what you are left with might well be recognisable as a sauropod footprint, but this undertrack might also be several times bigger and not very distinct. You might well have a 2 m wide sauropod track, but not a 2 m sauropod foot. An incredibly important, and hardly subtle distinction, but one rarely, if ever, discussed in the media or even some palaeontology textbooks.
Just a quick point this time out on an obscure (as far as the literature goes at least) and unusual little fact about footprints. If you look at the palm of you hand it is pretty obvious the each joint of the fingers and even the base of each finger on the palm has a fleshy pad on top of it, such that if you were to place you hand on some nice soft mud, you would get both a good representation of your hand *and* this would also give you a pretty clear picture of where the joints are (the gaps) and the bones are (the depressions in the substrate). You might therefore think that this pattern is pretty much the same for other animals and that hand (or foot) prints give a clear picture of the actual bones involved.
A rhea foot bones superimposed on the pads and gaps of the foot. Modified from Milan, 2006.
Not so in fact (as you have probably already guessed) and for two big reasons. First off while obviously humans do at least have a nice ratio of pads-to-bones and gaps-to-joints this is not consistent. First of all many animals do not have this ratio and foot-pads can cover several bones, or several pads can cover one bone, and gaps can occur in the middle of bones as opposed to at joints. There is also inevitably an issue of natural variation here and not all individuals have the same pad structure on their feet as other members of the species and some are highly variable and can even be different on the left and right feet of an individual. As such the number and position of pads and gaps can be very different to the actual bones and joints and not much of an indicator of the anatomy of the foot.
Secondly, footprints themselves are enormously variable. Obviously it can make a huge difference whether you are making tracks on mud or sand or hard soil or whatever, and if you are walking or running you can end up leaving rather different prints. However it is perhaps not obvious just how variable these can be. You might think that if you maintained a steady pace and gait over a fairly uniform surface then the prints would be consistent. Not so – even here pad and gaps can appear and disappear from track to track and between left and right.
All this variation I should point out has been recorded in living animals and trackways from live animals including controlled experiments. As such we can be pretty confident that these effects are real and a result of variation from the animals themselves and the tracks being laid down in addition to of course the inevitable variation as a result of preservation and erosion of trackways before their discovery. The practical upshot of this is that tracks become even harder to identify and analyse since for some tetrapods at least (and much of this work has been done on ratites and thus is particularly relevant to theropods) the actual pattern of the pads and gaps in the footprint can have little to do with the foot bones that they enclosed. In short don’t trust those tricky tracks.
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I recently covered the issues of ontogeny for taxonomy but deliberately did not cover how one can spot the features of a skeleton which might indicate that it is not an adult saving it for here. As ever this discussion is based primarily around archosaurs, though astute observers will note that many of these are applicable to other clades as well, and that there are other features not covered here that can be of use (such as finding a new set of teeth in a mammal’s jaw strongly suggests that it is not yet an adult).
The following features vary in their useful ness and applicability and are best used in comparison to other animals in the same clade that are known to be juveniles or adults. Of course these are not always 100% accurate (or even close) as indicators of an animal being a juvenile or not, but nevertheless, taken in concert these can provide solid evidence that an animal was young, or an adolescent, or an adult. Continue reading ‘Spotting ontogeny’
My recent post on the media coveragea of my paper on theropod feeding generated a huge amount of interest. Of special note by some outside observers were the comments of Charles Q. Choi who had interviewed me for his article on my work, and later dropped into the comments thread to talk about communication between scientists and journalists. Now Charles has kindly accepted my invitation to return to the Musings and work up his comments into a guest post on advice for researchers writing a press release for the media. Obviously what you want to communicate as a scientist is not always what they want or need to hear, so knowing what the other side consider useful and what is not is incredibly important. Charles is a freelance reporter who has written about science for Science, Nature, Scientific American and The New York Times, among others.
Continue reading ‘Guest Post: Writing a press release – a guide for researchers’
The final of my posts on characters that vary in populations that can cause problems for taxonomists. I’ve already covered ontogeny (growth) and intraspecific variation and now for perhaps the most tricky aspect of them all, sexual dimorphism. For those who have not come across the term before this basically refers to differences between genders as exhibited (typically) by adult organisms. It should be easy to see how this complicates things by taking humans as an example. In general men are taller than women, with proportionally broader shoulders and a narrower pelvis but of course the range of intraspecific variation covers most if not all of this, and ontogeny can cloud the issue further (young teenage girls are often taller than their male counterparts as they hit their growth spurt earlier). If you only have part of a skeleton (like an arm) to work from it could be very hard to say if you have a tall woman or short man before you.
Even if we go outside of mankind to animals with more obvious sexual dimorphism like a peacock, it’s not entirely clear how much of the obvious male and female differences would remain if you stripped off the feathers. Some for sure (like the males’ fighting spurs) but it would not be as clear. Even with obvious bony differences (such as the antlers and horns of deer and antelope) these can be shed regularly or lost in some individuals. In some species male and females both have horns and even very similar horns to each others and in some (like reindeer) females retain their antlers at times when males lose theirs. Even if as a palaeontologist you had a fairly complete skeleton of a male and female antelope, it might be clear that they are similar enough to be the same species and different enough to be sexual morphs, but it may not be clear which was which.
As such palaeontologists (or at least the ones working on archosaurs) do not often deal with sexual dimorphism. Some archosaurs (like crocs for example) really don’t have much difference to detect. Others might be present, but to sort it out first one needs a good sample size of individuals to work with (even if you have ten specimens, they might all be males, or a variety of ages, or if the pieces are non-overlapping it will be hard to spot patterns of differences). Even then a pattern can be hard to interpret – you might find that specimens fall into two fairly distinct and separate size categories but is this male and female, or two different species (one big and one small)? If it *is* male and female, which is which? It is true that in general male animals are ornamented so one measured group may have horns and another lack them or have smaller horns but again this may be a species split (look at just how similar many antelope and gazelle are in the African savannah that live side by side).
In some cases there are clues available to the lucky few such as the structure of the bones, (recently used to diagnose a Tyrannosaurus specimen as female), eggs or embryos being found inside a female, an enlarged pelvis for egg-laying and so on that can be more fixed but one still has to be careful when identifying putative males. There are few dinosaurs or pterosaurs with enough of a group of specimens to be able to work on effectively, but a number have been suggested as being males and females.
The problem here for a taxonomist is of course that it is so hard to tell these things apart with the limited information we have. It is easy to think that two animals are rather different in size and form and name them as different species when in fact they are two different genders (especially if one has a crest and the other does not for example). Without the large numbers of specimens required to even being to piece together possible differences, it can be futile to try to separate them out. There are probably therefore at least a few males and females languishing as separate species in the records of taxonomists – of the few putative male and female dimorphs that have been suggested, most are hotly contested. Of course in many cases this is largely irrelevant – if you have only a few bones for your specimen, its gender is not much of an issue, but when you have things like the ceratopsians where much of the taxonomic difference lie in the frills and horns that you might also think could well denote male and female differences (as with many modern ungulates) then you can see how things rapidly get much trickier. This can get still more complex as these kinds of characters often only show up in adults and thus juveniles which lack ornaments or have smaller ones can also be thrown into the mix to mess up the efforts of taxonomists.
As a vague conclusion to the three pieces here, taxonomy is about more than just looking for similarities and differences and erecting new species based on differences, or synonymising others based on similarities. One must assess these characteristics themselves to ensure that what you are naming is a genuinely different organism and not just a large and robust adult, a juvenile or a female. There is an awful lot of variation out there in biology and when working with only half a 200 million year old skeleton it can be tricky to keep on top of things. However these aspects (variation, ontogeny and dimorphism) are important and should not be ignored or underestimated.
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