I am a palaeontologist living and working in Alice Springs, in the red centre of Australia. I moved here with my wife and three kids from Johannesburg, South Africa. I used to focus my research on dinosaurs, and it is fair to say I am still a dino nut but these days I work on fossils from the NT, be they turtles, tassie tigers or anything else. In my spare time I like to watch birds, catch beetles, lizards and snakes and generally find out as much about the species around me as I can.
Thankyou for everyones kind support, it meant a lot to us. Anwen's operation was a complete success. The tumor was indeed attached to the underlying vein but the surgeon was able to sepparate them without any problems. The whole operation took just under an hour. After a short stay in hospital just to monitor her after the anaesthetic we took our smiling baby girl home. I'm now basking in a glow of relief and the knowledge that we made the right decision to have it removed.
OK on with Fish fortnight, Aachenosaurus is an infamous a case of misidentification where some pieces of petrified wood were mistaken for dinosaur bones, and a name was coined for them in the literature. The small pieces of bone in the photos above were also strikingly misidentified in the BPI catalogue as belonging to a dinosaur (though fortunately never published as such). They actually belong to a fish, a ray-finned fish (actinopterygian) to be a little more precise. As fish they are very interesting because the come from the upper Elliot Formation, and as far as I can tell are the first recognized ray-finned fish from this unit (lungfish are known from the odd small toothplate here and there). The upper Elliot Formation was deposited in arid conditions with most of the streams being small and ephemeral. Nonetheless ray-fins can’t cocoon themselves when their pond dries up the way some lungfish can and their presence indicates that some permanent water bodies, however small, existed on the upper Elliot floodplain. That’s all I can say about this fish right now - the fossil will be subjected to further prep and study.
Thankyou to everyone who wished us well and has shown support. I'm sure the risks, though scary, are not very likely. Anyway I hope you all have a wonderfull Australia Day. A couple of us expats thought about singing the national anthem at the tea table this morning - but couldn't remember the words.
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.
Last post we looked at the basics of ray-finned fish classification and some of the problems associated with them. Foremost among these are two rather dramatically different topologies. Morphology supports a clade called the Neopterygii which includes ginglymods, halecomorphs and teleosts whereas mitochondrial genetics support an ‘Ancient Fish Clade (AFC)’ that groups chondrosteans, ginglymods and halecomorphs to the exclusion of teleosts. Divergence dates based on the molecular clock are also dramatically older than minimal dates based on the fossil record. Hurley et al. (2007) tackle both problems with a two-pronged approach. Firstly they relook at the early ray-finned fossil record, scrutinizing it for the first appearance of derived characters diagnostic of these major groups and incorporating the data into a new cladistic analysis. Secondly they assembled a new, comprehensive molecular data set of four nuclear genes 29 species covering all the relevant clades (except the cladistians), thus is the first analysis capable of addressing the timing of the whole genome duplication event. The tree based on the morphological data found strong support for the Neopterygii and virtually no support for the ‘Ancient Fish Clade’ at all. Indeed when the AFC topology was enforced upon an analysis that included only the living taxa the tree length grew by 80 steps (125% of the number of steps in the shortest possible tree) and found just one character that could be interpreted as a synapomorphy of this clade. Clearly the morphology doesn’t just fail to support the molecular ‘AFC’ it is strongly contradicting it. Analysis of the nuclear gene data also strongly supports the neopterygian clade over the AFC. Thus the signal for the AFC is coming from the mitochondrial genes alone. Given that this data set is so at odds with morphology, nuclear genes and the fossil record it seems likely that the source of error is the mitochondrial data. Perhaps more interesting is the morphological analysis that includes the fossils. Neopterygii continues to be strongly supported but the divergence date estimates have changed. Two fossils in particular were found to be significant: Brachydegma and Discoserra. Brachydegma from the Early Permian (285 million years) of Texas was previously regarded as a basal actinopterygian that diverged before the chondrostean-neopterygian split. However Hurley et al. found that it had a number of characteristics of Halecomorpha (that is the bowfin and its fossil relatives), such as an enlarged gular plate, a medial shelf at the front end of the maxilla, and possibly a posteriorly indented maxilla. The latter character is less secure because it depends on the interpretation of a small elliptical patch of differing ornament on the rear edge of the maxilla. If this patch is interpreted as a fused-on scale, then the maxilla does have the classic halecomorph indented maxilla (see figure below).
Brachydegma (from Hurley et al. 2007) on the left and the modern bowfin (Amia) on the right (not to scale; from Grande and Bemis 1998). Two diagnostic features of the Halecomorphi are colourised – the posteriorly indented maxilla (red) and the very large gular plate (green).
Sure enough Brachydegma comes out as the basal most member of the Halecomorphi in their analysis. As an halecomorph, Brachydegma is part of the neopterygian crown-group and pushes the origin of this clade back the Palaeozoic Era, before the big extinction event at the end of the Permian Period. The previous oldest known crown-group neopterygians were the parasemionotids (also halecomorphs) from the Early Triassic of Madagascar and Greenland. Discoserra from the Early Carboniferous (320 million years) of Montana (the famous Bear Gulch Limestone fish deposits) is a far older fish, and is not apparently a member of the neopterygian crown group but it has many of the synapomorphies of the crown-group indicating that by this early stage the neopterygian bodyplan was mostly in place. Previously Discoserra was thought to be an early cladistian.
Discoserra, from Lund 2000.
The new fossil data places the origination of the neopterygian crown group into the Paleozoic Era, long before the big Permo-Triassic mass extinction event of 251 million years, and at least 40 million years earlier than the previous oldest crown-group neopterygian and somewhat closing the gap between the molecular and palaeontological dating of the Neopterygian crown-group origination. Furthermore molecular clock dating using the nuclear gene data, rather than the mitochondrial genes yields a more recent range of dates 271-371 million years that actually ecompasses the age of Brachydegma, thus the discrepancy is more or less resolved. Once again it appears that the mitochondrial genes are giving misleading results, but why this is so is not immediately clear. No particularly ancient crown-group teleosts were recognized in this study so the fossil based minimum age for this clade is unchanged. However the new nuclear genetic data were used to estimate the divergence of the crown group. Like the estimates for the age of the neopterygian crown-group the estimates for the teleost crown-group based on nuclear genes are considerably younger than the estimates based on mitochondrial data. Thus using these new age estimates the discrepancy between molecular and palaeontological dates closes to a minimum of 30 million years. Lastly Hurley et al. look at the timing of the whole genome duplication and how it relates to the explosive radiation of teleost fishes. The duplication event is indeed shown to be a feature of the teleost stem, as its products appear to be present in all living teleosts but are not found in the other surviving actinopterygian groups. This suggests that the duplication event most likely happened sometime in the Permian or Triassic, with the most recent possible occurrence being the Mid Jurassic, before the first appearance of crown-group teleosts in the fossil record. However the explosion in teleost diversity does not get underway until the Late Cretaceous, demonstrating a considerable lag between the duplication event and the rapid diversification event. This falsifies the hypothesis that the duplication was the direct causative agent of diversification. In conclusion, this is a case where the morphological signal appears to have given a more reliable estimate of phylogeny than mitochondrial genes, whereas the huge discrepancy in divergence date estimates were a product of both overlooked fossil data and misleading signal based on the same unreliable genetic data.
Grande, L. and Bemis, W.E. (1998) A comprehensive phylogenetic study of amiid fishes (Amiidae) based on comparative skeletal anatomy. An empirical search for interconnected patterns of natural history. Society of Vertebrate Paleontology Memoir 4: 1-690.
Hurley, I.A., Lockridge Mueller, R, Dunn, K.A.,Schmidt, Friedman, M., Ho1, R.K., Prince, V.E., Yang, Z., Thomas, M.G. and Coates, M.I. (2007)A new timescale for ray finned fish evolution. Proc. R. Soc. B 274, 489–498
Lund, R. (2000) The new actinopterygian order Guildayichthyiformes from the Lower Carboniferous of Montana (USA). Geodiversitas 22, 171-206.
I’ve touched on the tension between genetic and morphologic data in this post and the comments it attracted. As I stated then both sets of data are the products of a single history and should therefore be more or less in accord. When they are not, well then it is our job as scientists to find out why. Gone are the days of simply giving a hurumph and declaring one’s personally preferred set of data to be correct and the other erroneous. I want to feature a 2007 paper by Hurley et al. as an excellent model for the unification of both sources of data. Although a few years old now, it remains one of my favourite bits of palaeontological research. And it doesn’t even feature dinosaurs. No, it is about the phylogeny of ray-finned fish. But before I can get to the paper in question I will need to bring my readers up to speed with the relevant issues in this post before turning my attention to the paper itself in my next post. For those not aquainted with fish taxonomy, I’ll first give a quick rundown of the groups involved. All vertebrates that replace their cartilaginous inner skeletons (endoskeleton) with bone are known as Osteichthyes (literally “bony fish”). Note that not all fish with bone are osteichthyans. Several extinct groups like placoderms and osteostracans covered their bodies in large plates of bone. However this bone is derived from skin (ectodermal) tissue and remains superficial to the endoskeleton which remains cartilaginous. Also note that we tetrapods also replace our endoskeletons with bone: we are bony fish (albeit highly terrestrially adapted bony fish). Anyway the bony fish clade divides into two great clades: Sarcopterygii (including tetrapods) and Actinopterygii. It is the Actinopterygii, or ray-finned fish that concern us here. Actinopterygians are a clade and can be diagnosed by a bunch of characteristics, the most obvious one being that there is just one dorsal fin. Of course the path of evolution is rarely simple and we find that many modern actinopterygians divide their single dorsal fin into an anterior spiny portion and a posterior portion with soft rays. Sometimes the dorsal fin is bilobed with a dip in the profile between the spiny and soft sections but more often there is a finless gap, thus two dorsal fins are secondarily acquired. The evolutionary success of ray finned fish cannot be overstated. In terms of diversity they outnumber all other vertebrate clades put together 2:1 and exhibit a jaw-dropping array of specializations, some of them nightmarishly bizarre to our eyes. Living ray-fins can be divided into five clades, the vast majority of ray-fins belong to just one of these the Teleostei. The other four groups are the cladistians (bichirs), chondrosteans (sturgeons and paddlefish, not to be confused with the chondrichthyans which are the sharks, rays and ratfish), ginglymods (gars*) and halecomorphs (bowfin).
Polypterus, the bichir, a living cladistian from Central Africa. Image from wildwoods.co.uk
Pallid sturgeon, a chondrostean. Image from wikipedia.
Lepisosteus, gar, a living ginglymod from North America. Image from wikipedia.
Amia calva, the bowfin. The sole surviving halecomorph. From North America. Image from wikipedia.
The staggering diversity of modern ray-fins is actually a relatively recent phenomenon compared to the venerable age of the total actinopterygian group. The first ray-fins appeared no later than the Late Silurian, when we find the first fossils of their sister group, the Sarcopterygii (e.g. Psarolepis in China) whereas the steep rise in diversity and disparity didn’t really start until the Late Cretaceous and is restricted to the teleost clade. One intriguing feature of living teleosts is that their entire genome has been duplicated. Some have suggested that this duplication event may have been the impetus for the teleost diversification. With two copies of every gene it is easy to imagine that duplicates of essential genes that have little freedom to vary without compromising the organism would be free to vary, perhaps allowing a more rapid exploration of morphospace. It is universally agreed on both morphological and molecular data that the cladistians are basal to all other rayfins, but the relationships of the other clades remain contentious. Morphological analyses support a ladder like arrangement with chondrosteans, ginglymods and halecomorphs forming serially closer outgroups to the teleosts. The clade including ginglymods, halecomorphs and teleosts is known as the Neopterygii and is characterized by, amongst other features, loss of the clavicle bone, a vertical hyomandibula (jaw suspensorium) and a mobile maxilla (but not in modern ginglymods). Molecular data based on sequences of mitochondrial genes however gathers these three groups into an ‘ancient fish clade’ that is the sister group of teleosts. The following diagram from Hurley et al. (2007) illustrates these two different arrangements well.
The shape of the tree is not the only source of molecular vs. morphological tension in ray finned fish evolution. Estimates of the timing of the divergences between these clades is also dramatically different when using the different data sources. For example the oldest crown-group teleost (reminder: the crown-group is the group descended from the most recent common ancestor of all extant species) in the fossil record dates from the Late Jurassic (151 million years), whereas mitochondrial data estimates the divergence date at either the Early Permian (285 million years) or Early Carboniferous (334 million years) depending on the method used. That’s a difference of 134 to 183 million years. Now we can always expect that a clade will not show up immediately after its divergence from its sister group, due to low diversity and low abundance but this difference is extraordinary. Other molecular divergence dates are also strongly discordant with the fossil data, for instance the bowfin-teleost split (which would provide a minimum age of crown-group Neopterygii) is estimated to range from 417-390 million years (Late Silurian to Mid Devonian) using mitochondrial data whereas the palaeontological date of neopterygian crown-group is 245 million years (Early Triassic). However the picture isn't quite as bad as it once seemed and we shall look at how improvements in our understanding of both sets of data is straightening out this problem in the next post.
*Note to Australian fisherpeople, the ‘garfish’ you will be familiar with are part of the vast teleost radiation and are not ginglymods of any sort.
Hurley, I.A., Lockridge Mueller, R, Dunn, K.A.,Schmidt, Friedman, M., Ho1, R.K., Prince, V.E., Yang, Z., Thomas, M.G. and Coates, M.I. (2007)A new timescale for ray finned fish evolution. Proc. R. Soc. B 274, 489–498
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
We're just under a fortnight into the new year and already the new dino papers are stacking up. The DML brings news of Ceratonykus a newly named alvarezsaur. And PNAS have published a short paper on a stunning new specimen of the therizinosaur Beipiaosaurus. This specimen is much less badly fragmented, than the holotype but sadly still only consists of the front end of the skeleton. It gives us all sorts of new details to mull over. First and foremost are the weird large single filament feathers that line the neck, back and tail (present in the holotype but not the new specimen). This fossil combined with last year's Epidexipteryx is showing us that a diversity of feather forms, now extinct, evolved before the pennaceous feathers that dominate modern birds plumage. In this case the feathers take the form of stiff, single filaments, that are about 2mm in diameter. The authors call them EBFFs (Elongate Broad Filamentous Feathers) but I will simply call them 'quills' and I really wonder whether they had some spiny defensive function. The apparent lack of any modern style pennaceous feather, with a central rachis and rows of barbs on either side, does suggest that therizinosaurs are not so closely related to oviraptorosaurs (which have pennaceous feathers in spades) as once thought. This does back up recent analyses based on skeletal anatomy which have not been returning an oviraptorosaur-therizinosaur clade of late. Furthermore these quills bear more than a pasing resemblance to the structures of the tail of the ceratopsian Psittacosaurus. The authors suggest what I guess a great number of us have wondered: did feathers evolve much earlier in archosaur history than we currently recognise? Of course the complete skull alone is reason enough to make theropodophiles drool. Intriguingly it appears quite derived (it acyually looks like a little Iguanodon, oops that should be Dollodon skull) which indicates that cranial modification occured earlier than some of the postcranial modifications that therizinosaurs are famous for, e.g. the re-enlarged hallux, or big toe. Other cool details include the outlines of a throat pouch, a feature that seems to have been reasonably widespread amongst dinosaurs. 2009 is shaping up to be a good year!
X. Xu, X. Zheng, H. You (2009). A new feather type in a nonavian theropod and the early evolution of feathers Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0810055106
The known remains of Thecodontosaurus minor, the scale bar was added by me but the rest is taken directly from Haughton 1924
What better way to kick off the new year but with a post about basal sauropodomorphs from South Africa? In 1918 Sidney Haughton named a small collection of tiny sauropodomorph bones from the Elliot Formation, near the town of Maclear, Eastern Cape, as Thecodontosaurus minor. There isn’t much to the specimen, just a cervical vertebra, a tibia and a fragmentary ischium. This little guy has remained in almost complete obscurity ever since it was named. All recent treatments that even give it a mention regard it as either a nomen dubium, or as a synonym of Massospondylus carinatus. However there are some odd things about these remains that suggest this taxon deserves a closer look. Firstly it is very small, with the tibia just over 10 cm, a length not much greater than that of Microraptor zhaoianus which is widely cited as the smallest non-avian dinosaur. Usually T. minor is dismissed as a juvenile but this is not necessarily the case because it appears that the neurocentral suture of the cervical vertebra is closed. I freely admit that I’ve only ever given the specimen a cursory look, and that this needs to be checked more closely. If it is closed then it would indicate the individual was approaching maturity. One does have to be careful when using the closure of neurocentral sutures to age a dinosaur but in this case there is abundant evidence that basal sauropodomorphs did not close their neurocentral sutures until close to maturity, or even after adult size had been reached. Indeed the majority of all presacral vertebrae preserved in the basal sauropodomorph record have separated along their neurocentral sutures. Now if T. minor really is a mature, or near mature individual, then it would have the smallest known adult size of any sauropodomorph. Just for fun here is the silhouette of a small sauropodomorph scaled to a tibia length of 10 cm, next to the hand of Brachiosaurus brancai. So what else is odd about this taxon? Well the ischium is a little odd. Although incomplete there is a short stretch of the ischial shaft preserved behind the proximal obturator region. This shaft is unusually flattened, whereas most basal sauropodomorphs, have an ischial shaft that has equilaterally triangular cross-section. Amongst basal sauropodomorphs I’ve only seen this type of flattened ischial shaft in Thecodontosaurus, Anchisaurus and Mussaurus. By itself this isn’t enough to hang a taxon on but it is enough to reject synonymy with Massospondylus (unless of course there is postmortem crushing at play, or I've misjudged the amount of ischial shaft that is present). However there is one other aspect of this little fossil that has me kicking myself for not looking more closely at it when I had the chance. It would appear that this little fossil comes from the lower Elliot Formation. This became apparent after I read the original (unillustrated) description, rather than Haughton’s later 1924 illustrated monograph on the fossils of the Stormberg group. Quoting the 1918 paper the horizon in which this specimen was discovered was the “Red beds, just below halfway from base”. As I’ve mentioned before on this blog ‘red beds’ is the old name for the Elliot Formation and this Formation comes has two members, of different age, depositional style and fauna. At the southern end of the outcrop area (where Maclear is located) the lower Elliot makes up about two thirds of the stratigraphic thickness of the Elliot Formation, placing T. minor in the lower Elliot and puts paid to any notion that the specimen is a juvenile Massospondylus. This is remarkable for the sauropodomorph fauna of the lower Elliot consists entirely of large robust forms. Indeed small vertebrate fossils of any sort are exceedingly rare. Indeed there are only two named taxa that would have massed less than 50 kg as adults: the ornithischian Eocursor and the trithelodontid cynodont, Elliotherium, both of which are based on unique specimens. A minute sauropodomorph from the Triassic of South Africa would be an interesting and welcome addition. So is T. minor a valid taxon? That is a difficult question. The known remains do not present any obvious autapomorphies other than its tiny size. If it can be confirmed either by histological sampling of the tibia, or micro ct scanning of the cervical vertebrae, that these are indeed the remains of a mature individual then, yes I think it would be a valid taxon, though just barely (on the basis that it can be excluded from all other known Triassic sauropodomorphs on the basis of size and the mix of characters it displays). Sadly the known remains are utterly inadequate for narrowing down its phylogenetic placement among basal sauropodomorphs, it could easily fall anywhere between Thecodontosaurus and Melanorosaurus. So there is little justification for keeping it in Thecodontosaurus, a new name will have to be coined for it but I think that that action would be best held off until such time as better remains come to light. I just hope that I might find some on my next field excursion.
Haughton, SH (1918) On a new dinosaur from the Stormberg beds of South Africa. Ann. Mag. nat. Hist. 2: 468-469.
Haughton, SH (1924) The fauna and stratigraphy of the Stormberg series. Ann. S. Afr. Mus. 12: 323-497.
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.