Dracovenator is moving

For some unknown reason blogger pages just won't load via my university network. For the longest time I've had all sorts of problems with gmail related sites. For instance I've been unable to add attachments to email from my gmail account. Now blogger won't load any of the buttons needed for making a post. So I'm leaving blogger. You can find my new blog at dracovenator.wordpress.com
See! I can't even link to the new site.
Update: OK, this message is now linked to my new site. See you on the other side!

I'm so frightened

Not for myself, for my 6 month old daughter Anwen. Forgive me this break in 'Fish Fortnight' to bring you a very personal post. I don't know why major events happen every time I try to run a theme on Dracovenator.I will throw in a bit of science so that I can keep my uber-nerd status. Plus knowing about a problem always helps me come to terms with it.
When Anwen was born she had a tiny lump in front of her fontanelle (where the frontal and two parietal bones of the infant skull fail to meet). I paid it no mind as it was virtually invisible amongst the lumpy irregualrities of a new-born's head. However it didn't straighten itself out, instead it got bigger. By this time it was obvious that it was a soft tissue structure of some sort. We showed it to the doctor who thought it was a benign cyst of some sort and that it posed no real problem because it appeared to be in front of the fontanelle. We went about getting an appointment for a general surgeon to remove it. The surgeon was a little leery of operating so close to a babies fontanelle and ordered MRI images of the lump. And what a sensible move that was. The MRI images show a tumor (probably a benign dermoid cyst)that rests largely IN the fontanelle. And therin lies the big problem.
But first a little about dermoid cysts and why they are classically located on the midline of a person.
It all comes down to early development of the embryo. Anwen, like all chordates, has a dorsal nerve chord formed by a process called neurulation. In neurulation a stripe of cells from the outer layer (the ectoderm), that runs down the back of the embryo, thickens, curls over and eventually pinches off to form a tube. This tub is what becomes the brain and spinal chord. You can see the process nicely in this diagram (of an amphibian - but the process is more or less the same in humans) which I nicked from Pharyngula. Incidentally Pharyngula has a much more detailed post about neurulation if you want to know more.



What happened in Anwen's case is that a few cells thay were supposed to stay in the ectodermal layer above the neural tube and go on to form the skin (the white tissue in the diagram above) became trapped under the ectodermis when the neural tube closed over. Thus stuck out of place but already set on the path to skin-hood they grow into a little sac of skin-like cells that we call a dermoid cyst. Normally these present no problem at all and can be removed with the minimum of fuss. However in Anwen's case the position is a real bummer. The cyst is not surrounded by fatty tissue as they often are and as a result the lower surface is in contact with the structure underneath. This just happens to be the sagittal sinus. This sinus is a space bounded by the dural membranes that runs between the hemispheres of the brain and collects the venous blood and returns it back to the heart. Several veins that drain the brain enter into the sinus in the vicinity of Anwen's cyst (or at least if I understand the surgeon correctly). These veins are not robust structures like the ones you find in your limbs, they are delicate membranous structures that are easily torn. Such a tear can have catastrophic even fatal consequences.
So there you have it. Anwen has a tumor on her head that if left will grow larger, impede skull development and probably eventually constrict venous bloodflow from the brain. To remove it is a tricky operation with the risk of tearing a dural vein which may lead to her death. We have the services of a skilled neurosurgeon of high repute but we are still understandably frightened. Its not an easy decision for a parent to expose their child to risk, even if small and for her greater benefit in the long run. Nonetheless the operation must go ahead and will probably happen next week. I'll let you know how it goes.



Anwen and her pesky little bump

Where do spiny sharks go?

Hi and welcome to fish fortnight on Dracovenator. You may notice a slight change in style here, as a new year's resolution I'm going to try to write for a more general audience. Does this mean I'm dumbing Dracovenator down? No, I'm just going to try to stop assuming a lot of specialist knowledge on a part of my readers, and will throw in some more basic anatomy for the Form and Function students I'll be teaching later this year.
Anyway by pure coincidence a bunch of fish-related items have come up all at once so I'm running a kind of fish-festival over the course of the next two weeks.
The first of these is a new paper in Nature this week by Martin Brazeau, which desribes the braincase and jaws of an acanthodian (popularly called spiny sharks, although they are not sharks, at least not in the conventional sense). Why is this a big deal? I'll get to that, but first some background.
Most modern vertebrate animals have jaws (only lampreys and hagfish do not). Those that have jaws belong to the great clade Gnathostomata (literally 'jaw-mouths'). Modern gnathostomes can be further divided into two clades: The cartilaginous fish (Chondrichthyes) including sharks and rays and the bony vertebrates (Osteichthyes)including everything else from goldfish to humans.
Both groups are monophyletic, that is they include all descendants from a common ancestor, thus neither group was ancestral to the other, both having split from a common jawed ancestor somewhere back in the past (probably in the Late Ordovician or Early Silurian Period).
Now there are some fossil jawed vertebrates that don't fit into either of these two clades. These are traditionally placed into two groups: The placoderms (armoured fish) and the acanthodians (so-called spiny sharks).



Dunkleosteus, a giant placoderm. Painting by Charles R. Knight. From universe-review.ca

How do these groups relate to the surviving Chondrichthyes and Osteichthyes, or in other words, what is the shape of the first part of the evolutionary tree of jawed vertebrates?
That has been a big mystery indeed. Just about every possible arrangement has been suggested. Placoderms are either regarded as the sister group of all other gnathostomes (that is they branched off first and the others all share a more recent common ancestor) or are thought to share a closer relationship with Chondrichthyes than with Osteichthyes. Nevertheless the plates of dermal bone which encase the front end of these fish has been thought to be an evolutionary novelty (synapomorphy in technical parlance) which marks out the placoderms as a monophyletic group.
The acanthodians have proven even more difficult to place, partly because the internal skeletal anatomy of most species has not been preserved (because like hagfish, lampreys and sharks it was cartilaginous). The favoured hypothesis of more recent years was that acanthodians shared a close relationship with Osteichthyes, that is they split from the line leading to bony vertebrates after the placoderms and chondrichthyans had already split and started their own evolutionary journeys. This was the position taken by Phillippe Janvier in his book 'Early Vertebrates' (1996, Oxford University Press)- an excellent book by the way. Once again the group had been thought to form a clade largely on the characteristic of bony spines supporting the leading edge of their paired fins and some details of their scales. Recent discoveries have shown that both of these features are probably general features for early gnathostomes that were lost in the modern groups, rather than evolutionary novelties unique to acanthodians. It should be noted that even back in 1996, Janvier noted that acanthodian monophyly wasn't all that strong and presented the suggestion that some acanthodians might turn out to be more closely related to chondricthyans (what we would call stem group chondrichthyans) while others might be stem group osteichthyans.



A diversity of acanthodians by Stanton Fink. Image from wikipedia-commons.

Enter Brazeau's recent paper. It includes a cladistic analysis of early jawed vertebrate phylogeny (the branching pattern of evolution) that uses individual genera, rather than large monophyletic groups. Lo and Behold, Janvier was right, or at least on the right track. Indeed Brazeau's analysis finds former acanthodians falling out all over the tree. Some are stem-group gnathostomes that branched away before modern gnathostomes split into Chondrichthyes and Osteichthyes, others are stem-group chondrichthyans others are stem-group osteichthyans. In particular the former acanthodian that forms the centrepiece of the paper, Ptomacanthus, is found in some of his trees to be the closest sister group to the gnathostome crown-group (the crown-group is the clade including the extant species and all descendants of their most recent common ancestor - in this case the common ancestor of Chondrichthyes and Osteichthyes). Ptomacanthus is special because unlike most other acanthodians, including all the early, more relevant species, does preserve some of its internal skeleton, namely the braincase and the jaws. The braincase is remarkably primitive, actually looking more like that of a placoderm than a crown-group gnathostome. This unusual combination of characters (externally crown-group like but internally more primitive) helps to break up the acanthodian group and gives this new tree. There are other odd things in this tree, for instance placoderms are not found to be monophyletic either but that is a topic for a later post (and further investigation methinks).

UPDATE: Check out the ever-informative Catalogue of Organisms for a post that slipped past me before for a little bit more about acanthodians.

Martin D. Brazeau (2009). The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins Nature, 457 (7227), 305-308 DOI: 10.1038/nature07436

HAPPY NEW YEAR and I'll be Back real soon

Hi guys,

You've probably twigged that I don't have an internet connection at home, so I'm limited to posting when I'm at university. I've been enjoying a long break over the christmas - new year season, hence the long silence.
I'm just going to show a interesting little curio that crossed my desk late last year. A gem and mineral dealer bought it in thinking he'd got hold of a dinosaur skull. At the time the specimen was embedded in what looked like sandstone matrix, but it was clear that it did contain some vertebral material and wondered if it was a short section of articulated vertebrae. While I made it clear that I wouldn't publish on fossil material of dubious origins and support the market in vertebrate fossil material, Celeste offerred her skills as a preparator to clean it up for him (hey we've got a family to support!). As the 'sandstone' came off it became clear that this was a fake - not even a very good one. Lumps of real dinosaur bone had been cobbled together with glue, resin and plaster. The biggest bit is a vertebral centrum, probably belonging to a sauropod based on its camellate internal structure. Nonetheless the piece does have its charm, it was only after preparation that I saw the 'skull' that the forger was trying to make. It kind of looks like a pig-snouted crocodile.

Comments

Mea Culpa, I didn't realise the comments were only letting google users have their say. I'm sure when I set this up I allowed comments from everyone. Anyway the problem is fixed - I invite comments from anyone.

End of year blues

Hi guys,

This blog has become far more quiescent than I ever intended it to be. I am currently feeling exhausted and swamped by marking at the end of the year, including honours projects, exam papers and essays - urgh. Of all my duties I hate marking the most. Time is of the essence but I want to be scrupulously fair to all, a desire that used to compell me to read and re-read each essay two or three times before finally settling on a mark. When not marking I am tidying up various other bits of end of year business - and occasionally writing up some of my research for publication. I wish I could tell you guys about it (it is one of the most enjoyable aspects of my job) but sadly this work has to go through the long drawn out process of peer-review and publication first. Maybe I'll be able to say something in 2010!
Anyway I've dragged up a picture of an old field trip from my vaults. You can see that the weather was kind of .... damp. This happened a lot on my field trips (and not always in rainy season either) so much so that one of the dinosaurs we happened to find in between rainstorms received the nickname 'Rainmaker'.
Anyway the load on shoulders should be lifting soon. I'll have a visit from my mum soon and we'll be doing a few trips round Gauteng. Then I'll be off on a short field trip to retrieve a dinosaur we left behind earlier in the year. So by December my batteries will be recharged and blogging will begin with renewed vigour (I hope!)

Tonsilitis again!

My little guy, Matthew, who is one month shy of his second birthday, has come down with serious tonsilitis for the second time in as many months. It would appear to be a law of nature that kids always fall sick on a weekend, usually a Sunday evening when medical services tend to be a little thin on the ground (especially for those not wishing to sit in a casualty waiting room for hours). At the moment this is my main blog-writing time. So sadly I'm empty handed - once again. I don't feel to bad about this however because I managed to get hours of research writing in, before Matthew got sick.
In lieu of my own work I would like to point folks to Matt Wedel's thorough dissection of the recent Aerosteon paper.

Matthew in better health doing some computer work of his own.

An unexpected run of bad luck

Hi,

This isn't the third installment of my sauropodomorph trilogy. The latter half of last week was best with all sorts of happenings including server failures, medical emergencies, power outages and topped off with my computer crashing on Friday (this is being written from a student computer in a common room at Wits). I wasn't sufficiently backed up and I've temporarily lost all my blog related files. So there is precious little for me to bring you right now, not even a nice picture. Check back soon, I hope to be up and running again by the end of the week.