Posts Tagged 'anatomy'



Dorsal Dsungaripterus


You might think that by now I’d have done Dsugnaripterus to death given the number of times I’ve covered or mentioned this taxon. However there is one last think (at least for now) well worth mentioning, and that’s the shape of the skull in dorsal view.The vast majority of pterosaurs are preserved without a good skull, or one that’s only seen in lateral view, or is rather badly broken or crushed. There are few that are complete and in near perfect 3D but I’ve never shown them before at least.

Here then is a pterosaur skull in dorsal view. Perhaps rather unsurprisingly, it’s more or less triangular with a long and tapering rostrum and a relatively squared off posterior end. While this might fit with your mental picture of the skull, it’s nice to actually see that it’s the case. Even so, the rostrum anterior to the nasoantorbital fenestra is rather narrower than you might expect and the bulges of the tooth roots seem to stick out a little to the sides which is interesting and you can see just how thin the midline saggital crest is. All good really.

Jugal Boss

No, not a fashion designer (and one of the SS uniforms for that matter [exciting pub trivia]) but those delightful little cheek ‘horns’ that are seen on various ornithischians and especially the basal ceratopsians like Psittacosaurus shown here. While many derived ceratopsids still have significant jugal bosses, their frills and horns mean that these tend to be overlooked by the casual observer as a small part of a greater series of ornaments and bizarre structures, but in the basal genera, where the frills were barely developed and horns not yet in evidence, they are the dominant feature. Naturally these play an important taxonomic role and are used in part in separating and identifying a number of Psittacosaurus species. This IVPP specimen has especially nice long ones thought foolishly I didn’t note down which species it was, thought it makes for a good example.

Ornithischian pubis anterior process

Obviously one of the big differences between the saurischian and ornithischian dinosaurs is the reversed and backwards pointing pubis of the latter clade. But this doesn’t mean that none of the pelvis (aside from the ilium of course) extends anteriorly. While the ‘arm’ of the pubis might be pointed to the rear, in many taxa there is also an expansion – the anterior process – that extends forwards into the place where the pubis would otherwise lie. Given how tight the arm of the pubic can be to that of the ischium, it’s possible to confuse an ornithischian pelvis for a saurischian one at first glance. There is a big extension forwards where the pubis should be and no immediately obvious posteriorly pointing part, though a quick double check should clear things up.

Cartilage covered surfaces

Not too long ago I hurriedly finished off a piece on the cartilage cap of bones in order to be in time with the spate of blog posts accompanying new work by Casey Holliday. However, in doing so, I forgot to include this rather nice image of the end of a sauropod longbone. You are probably used to seeing bones with lovely smooth ends as, while they too have a cap of cartilage, this helps the bones move freely past one another. In big bones on big animals however, where there is a big load of cartilage, it’s important that this is held in place on the bone itself and here the surface of the bone is not smooth but carunculated to provide a larger surface are for the two to bond together.

I´m working in Germany right now hence the paucity of posts this week, but normal service should resume shortly.

Chevrons

I have somehow managed to all but avoid mentioning chevrons on the Musings before so it is about time I redressed the balance, though I don’t think even their strongest supporters (if indeed there are any) would claim that they are exceptionally exciting bones. These are the small bones that sit below the tail vertebrae in most archosaurs (they’re missing in birds and some pterosaurs) and reptiles in general. There’s a fair bit of variation in size, shape, and distribution and I’ll not be going into any great detail (as ever) but they are the kind of thing that often gets overlooked and so even covering the very basics will likely be of interest.

Chevrons are absent from the first few vertebrae since this is still part of the main body of the animal as can be seen by the position of the pelvic bones, and so they only arise where the tail starts to be free of the rest of the body (essentially, posterior to the cloaca). It’s been suggested that females might have one or two fewer chevrons than males since they would not want this to interfere with egg laying but there seems to be no correlation of this in extant reptiles and thus where differences occur it’s likely merely a result of intraspecific variation than sexual dimorphism.

Chevrons are paired one to one with the caudal vertebrae they hang beneath (though as noted above, they are usually absent from the first few caudals, and indeed often the last ones as well – this can be quite a number in the case of things like diplodocids). However, they can articulate with either a single vertebra, or at the junction between two vertebra. They typically have some form of Y or U shape when seen from the front or back since between them and the vertebrae runs the caudal artery and vein of the tail. Thus the chevrons provide some protection for these blood vessels rather than having them hang free under the tail (mammals have the ‘haemal arch’ which is effectively formed by the fusion of chevrons right onto the caudals).

The lower part of the chevron really shows the variation though with some having a slight anterior process, many having a long posteriorly facing one, other extend vertically, and on occasion you can get really unusual ones (like in dromaeosaurs tails). There can be quite a bit of variation along the row of even a single tail too, and obviously the chevrons get smaller towards the end and are typically absent for the very distal tail, and there is a general trend for the more distal ones to be less morphologically complex.

I’m pretty much out on chevrons, though I managed a few hundred words more than I expected. Coming next, oh, something on archosaurs I’d imagine.

Pathologies

Animals, somewhat inevitably, get ill. (As a minor aside, this is one of the great fallacies about life in the wild that animals live in some kind of paradise where everything lives free and happy and never gets eaten alive by a predator, or starves to death, or gets and infected wound and disintegrates from gangrene for example. Anyway, while of course most illnesses only affect soft tissues, some diseases and infections will leave a mark on the bones themselves and obviously as a palaeontologist these do turn up in the fossil record.

There are all kinds out there (I’ve mentioned one on a Mamenchisaurus before) but of course it is useful to see these things in modern animals to see what effects cause which pathologies and then try to track them on the fossils. There are fossils (including dinosaurs) diagnosed with arthritis, cancers and pathologies resulting from infected injuries. In this case, this is the jaw of a hippo which has clearly had something go quite wrong (though what I’m not sure, though my guess is a nasty infection). The bone is swollen and pitted at the base of the tooth is actually visible – this is clearly not natural and looks very different to the other side of the jaw. These kinds of thing at least can give you an idea if the feature you are looking at is a weird bit of anatomy or the result of a pathology – it’s not always easy to tell.

Bone ends & cartilage

I started this post quite literally a year ago but never got very far having moved onto other things I was more interested in and leaving it behind. However a recent new paper out of the Witmer lab as led by Casey Halliday has pushed this to the fore of my thoughts and prompted me to finish this off (though rather shorter than I had originally planned).

It is easy to overlook the fact that in palaeontology we generally only have bones to deal with. It’s a bit of a ‘wood for the trees’ issue. You intuitively know that the bones are not the start and end of an organism, but when that’s all you have it’s easy to forget the rest. All longbones of vertebrates have a cap of cartilage on then end. This acts as a sort of catch-all shock absorber and low friction surface, helping the joint move and protecting the bone from damage (imagine walking with the actual end of the femur grinding against the pelvis with every step, not a great idea).

We know they must have been there, and that they were important, even if we almost never see them. They do turn up occasionally, there are some dinosaur bones with preserved cartilage on the end and some pterosaurs have calcite crystals on their joints where the cartilage lay in life. However, in general this gets completely overlooked since it was hard to guess how much difference the cartilage would have made to the shape of the bones or the sizes of the joints. Hard because we didn’t really know how this looked in living archosaurs, so how should this be applied to dinosaurs.

Well Casey and his team have gone and done just that. Sampled a bunch of birds and crocs to determine just how much cartilage is there and what kind of effect does it have on the joints. Between them we can now have a good idea of just that and how it varies in different taxa and with increasing size. I won’t go into the details here as much has been written about this already and the paper is online at PLoS ONE so go and read it there. LINKS However, it’s worth noting that some of the sauropods might well gain a fair few inches in height thanks to some very big cartilage caps.

Birds in 3D

Leon Claessens directed me today towards his online archive of bird morphology based on various 3D scans of living and fossil taxa. It’s a great scientific resource and well worth some time looking around even if you are not big on your bird anatomy. Enjoy.

The dromaeosaur pubis – ornithischian saurischians

It would of course be silly to say that bird have anything but a bird-like pelvis, but of course birds descended from the saurischian dinosaurs (the lizard hips) and not the ornithischian (bird hipped) dinosaurs. Birds then and their closest relatives are therefore saurischians with an ornithischian condition. The differences between the pelves of the two groups we have covered before but as with much in evolution, it’s not like birds just popped into place with their pubic bones pointing backwards.

Here then are the pubes of (somewhat inevitably) Sinornithosaurus. While the pelvis of this animal are disarticulated, you should be able to see that the ‘arms’ of the bone while mostly straight, then curve posteriorly such that the spoon shaped ends finish under the ischia (the bones that sits behind the pubes). A more typical saurischian pubis would point forwards to a degree, so we can see that this droamaeosaur pubis (or rather pubes, both are there and fused together at the end) is somewhat transitional between a more normal anteriorly facing pubis of other saurischians and the very posteriorly facing one of birds.

Back to that pterosaur sacrum / pelvis

While hurridly ploughing through various things over the last week (including submitting my Palaeo Paper Challenge paper) I stuck up a very lame post which consisted entirely of a photo of a pterosaur sacrum and a label to say it was a pterosaur sacrum. Now I have a little more time, I’ll revisit this topic in at least a bit more detail.

Continue reading ‘Back to that pterosaur sacrum / pelvis’

Anatomical Order

Anyone who has read a bunch of vertebrate palaeontology papers will by now (hopefully) have noticed that the description of various anatomical bits tend to be in the same order, each bone has it’s place in the queue so to speak. It’s a simple enough concept as it makes it easy to access details quickly – if you want to know what the hand looks like you need to know where in the paper you are likely to find it and not have to read the whole thing to find the information you want. This is an established protocol that is, sadly, not always followed (most especially in character lists of cladistic characters), but it worth spelling out for the interested.

Continue reading ‘Anatomical Order’

To me – to you: directions and descriptions

Having covered the ongoing saga (though to be honest it’s less of a saga than a small novella) of the ‘cranial-caudal’ descriptions issue, it seemed an appropriate time to bring up the ideas of directions in anatomical descriptions. As I noted in the earlier post, the point about directions in anatomy are to be clear and concise and to provide an unambiguous definition of something to avoid confusion for the readers and researchers who want to follow what you have written accurately.

It should be easy to see how people can get confused without being careful about terms. You could describe a feature on a humerus say as being on the same side as the thumb, but rotate your wrist and suddenly that point of reference disappears. Most people would describe their chest as facing forwards, but in a quadruped that same structure (the ribcage) would be facing down towards the ground, and while the metatrsals in our feet face flat down on the ground in most other animals they would face backwards. If you used terms like this it would be easy to get lost or end up comparing the wrong things hence the general standardisation of terms. I say general of course as there are still issues here and there that crop up, like the cranial-caudal one, or those of the scapula.

So here as a brief guide / aid memoire, are a few of the basic directions used in anatomical descriptions for the orientation and position of bones and their features. Please excuse the very basic outline drawing, it was the best I could do* quickly:

Anterior (or cranial) – towards the head end of the animal.
Posterior (or caudal) – towards the tail end of the animal.

Dorsal – towards the back.
Ventral – towards the front.

Medial – towards the midline (something can also lie medially like vertebrae).
Lateral – away from the midline.

Proximal – towards the body.
Distal – away from the body and towards an extremity (so you can also talk about the distal tail for example).

Palmar (or plantar)> – towards the palm of the hand / sole of the foot.
Dorsal – towards the back of the hand / top of the foot.

Labial – towards the lips or outer part of the mouth.
Lingual – towards the tongue or inner part of the mouth.

These rather obviously all come in pairs of opposites but they can be used together or in combinations. So your spine is medially positioned and runs anterio-posteriorly (from front to back) and while your orbits face anteriorly (or rostrally if you prefer) you can swivel your eyes to face dorso-laterally (up and to the side) or medio-ventrally (down towards the middle) if you so choose.

The point is, and should be, that even as joints move and rotate and even whole body plans change and evolve (humans stand rather differently to monkeys, let alone cats or mice) the points remain relevant and accurate and directly comparable. Thus providing a secure set of references to make both descriptions and comparisons accurate.

*and by ‘do’ I mean ‘bother with’, but for this it should be more than sufficient.

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