Archive for September, 2009

Spotting ontogeny

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’

Guest Post: Writing a press release – a guide for researchers

CharlesQChoi_350x233My 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’

Sexual dimorphism and taxonomy

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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Ontogeny and taxonomy

Having covered intraspecific variation, it seemed only appropriate to nail down another oft missed (or misunderstood) aspect of change that relates to taxonomy: ontogeny. This is the term we give to the changes that an individual undergoes as it grows and develops (in humans at least from a single cell to an adult, though much of that early stuff would be of little interest to a vertebrate palaeontologist while there are no bones involved). Obviously this has an incredibly important impact on taxonomy (and again, especially of fossil vertebrates) as juveniles can look quite different to adults and change markedly during their development.

I’ll deal separately with indictors of ontogeny in a future post and try to deal here specifically with what this might mean for taxonomy (for vertebrate palaeontologists at least). To take one obvious pattern as an example though, throughout vertebrates most newborn animals generally have proportionally large heads that get progressively smaller as the animal matures to become an adult. Human babies are an excellent example of this – they are born with heads close to the size of their torso and this of course is far from the case in adults.

If we then take this example over to theropod dinosaurs you might know that tyrannosaurs have proportionally large heads compared to their bodies (and in comparisons to allosaurs for example). If therefore you were a bad taxonomist just looking at a few gross characters of body size proportions you might note that your new theropod had a big head and consider it to be a tyrannosaur, when in fact it was just a young allosaur that had yet to grow out of its ‘baby’ shape. Worse you might think that it was a dwarf species given how small it was and even miss that it belonged to an already described and well known species because you were sticking to sized-based characters or those that changed markedly during ontogeny.

Again, this is something that has happened a lot in the past and causes lots of work for modern taxonomists. While this is certainly something that is less problematic that it once was, it is something that does still crop up. I should mention at this point that there is not necessarily anything wrong with naming a new taxon based on obviously juvenile material *provided* that the characters you are using to define it are not subjected to ontogenetic variation. As with intraspecific variation we can get quite a good idea of this from looking at living taxa, but even in the fossil record we have some good examples of growth series from juvenile to adult for some taxa which gives us a great idea of how some characteristics change (or remain the same) during ontogeny, so it’s not guesswork or intuition but based on observation and deduction. Taxonomy remains inevitably somewhat subjective, but much of that operates at defining exact species as opposed to what can and cannot be used to separate species, an important distinction.

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