Maragaia filed block. Image courtesy Octavio Mateus
Another guest post for you concerning a recent palaeo paper by the authors who produced the original work. Susie Maidment returns to the Musings (her previous post is here) to bring some extra information on the new stegosaur that has got dinosaur researchers so excited.
In 2005, Octavio Mateus and colleagues at the Museu Lourinha discovered two stegosaur specimens lying close together in a road cut between the villages of Mirigaia and Sobral, close to Lourinha, Portugal. Preparation of the specimens showed the larger of the two to be an extremely interesting find. Elements from the front of the skull were preserved and represent the first skull remains from any stegosaur found in Europe. The articulated neck was almost completely preserved and remarkably 15 cervical vertebrae were identified. Since the axis and atlas were not preserved, the specimen had 17 cervical vertebrae in total: more cervical vertebrae than any other non-avian archosaur except the Chinese sauropods Omeisaurus, Euhelopus and Mamenchisaurus, which also had 17 cervical vertebrae. In a paper published online in Proceedings of the Royal Society B this week, we name the specimen Miagaia longicollum, and discuss the implications of the find.
Miragaia excavation. Imgae courtesy Octavio Mateus.
Miragaia’s long neck. In mammals, the number of vertebrae in the neck hardly varies, despite the great morphological diversity of the clade: almost all mammals (including giraffes) have seven cervical vertebrae. In contrast, cervical vertebra number in diapsids is much more variable, and neck elongation is a trend that is observed during the evolution of the sauropod dinosaurs. This trend has not previously been observed in Ornithischia, however. The most primitive ornithischians, animals like Heterodontosaurus from southern Africa and Scutellosaurus from North America had nine cervical vertebrae. The basal stegosaur Huayangosaurus retained this primitive number, but the discovery of Miragaia suggests that during the evolution of the stegosaurs, there was an evolutionary trend for neck length increase. Stegosaurus, the iconic stegosaur from North America, had 13 cervical vertebrae, and its close relative Miragaia had 17 cervical vertebrae.
What are the evolutionary driving forces that could have led to this trend through stegosaur evolution? One possibility is that a longer neck allowed the stegosaurs to browse for foliage at a height that other ornithischians in the fauna could not reach. Unfortunately, the ornithischian fauna of the Upper Jurassic Lourinha Formation is too poorly known to let us rigorously test that hypothesis, but from some simple browse height estimations, it seems possible that Miragaia might have been able to browse higher than most of the taxa currently known in the formation. Dental micro- and macrowear studies would allow us to get a better handle on what different elements in the fauna were eating, and perhaps help to tell whether there was any ecological partitioning. Another possibility, and one that has been recently proposed for sauropods as well as for giraffes, is that the long neck was the product of sexual selection. In biology, behavioural observation and identification of the sex of specimens is important for identifying features that evolved under the influence of sexual selection. Often features that are sexually selected for are dimorphic – larger or more developed in one sex when compared with the other. Obviously we cannot observe the behaviour of an animal that has been extinct for 150 million years. Methods for identifying sex in various dinosaur genera have been proposed but none have yet been identified that are universally applicable, and with a sample size of one it is impossible to look for evidence of sexual dimorphism. So we cannot test the hypothesis that the long neck of Miragaia evolved due to sexual selection based on the material we currently have. We will have to wait for Octavio and the folks at the Museu Lourinha to discover a herd of Miragaia to properly test these theories. Miragaia’s long neck does tell us that the traditional view of stegosaurs as a small, morphologically conservative clade of low browsing feeders is incorrect, and that stegosaurs and potentially thyreophorans (armoured dinosaurs) in general were far more ecologically and morphologically diverse than was previously understood.
Miragaia skeleton. Imgae courtesy Octavio Mateus.
Stegosaurian relationships. The inclusion of Miragaia in my cladistic analysis of Stegosauria led to some surprising results. Miragaia is resolved as the sister taxon to Dacentrurus. This is not particularly surprising since Dacentrurus is also known from Portugal, and we noticed a number of similarities and features that the two taxa shared when we studied the specimen. What was more surprising was that this clade, Dacentrurus + Miragaia, is found to be closely related to Stegosaurus. Previously, my analysis had suggested that Dacentrurus was a relatively primitive stegosaur, but the addition of more anatomical information from Miragaia shows that this is probably incorrect, and actually the European stegosaurs are relatively derived. In hind sight, this is perhaps not so surprising, since Stegosaurus was recently described from Portugal. Miragaia is an important specimen for a number of reasons. It includes the first skull material from any European stegosaur. Since the skull contains many phylogenetically informative characters it allows us to better understand the systematic position of Miragaia and Dacentrurus, and the systematics of the whole stegosaurian clade. The long-necked specimen shows that stegosaurs were far from being a small, morphologically conservative clade, and it challenges our understanding of Upper Jurassic ecosystems as a whole.
Miragaia reconstructions. Imgae courtesy Octavio Mateus.
The enlightened folks at the Royal Society have made all papers published in Royal Society Journals freely available online. A copy of the paper can be downloaded here, and be sure to download the supplementary information as well: it contains many more details. Alternatively, contact me at susie_maidment AT hotmail.com for a .pdf copy that includes supplementary info.
Dave Hone's Archosaur Musings 
Thanks for the rundown Susie!
Darren Naish has suggested that Miragaia is potentially synonymous with Dacentrurus (over on SV-POW). Is there any basis for that possibility? How much overlap is there between the known remains for Dacentrurus and Miragaia?
Wonderful stegosaur, though, and a great paper, too. When I first read it, I was immediately reminded of Brachytrachelopan, the sauropod who wanted to be a stegosaur!
I think it was actually Adam Yates suggested it was synonymous, and Darren that it was not synonymous, but I’ll let Susie answer the crux of the question.
It was indeed Adam who was mooting possible synonymy, not me.
Anyway: I have a question, and I hope someone (hopefully Susie) can answer it. Do we know exactly where the parascapular spines belong? The Miragaia reconstruction shown above puts the spine very high, way up near the vertebral border of the scapula (will people please stop using ‘proximal’ and ‘distal’ on scapulae, it doesn’t make any sense). Ken Carpenter did this (in Dong 1990), and I’ve seen it elsewhere here and there. However, most reconstructions (e.g., the many pics by Ford in Olshevsky’s article, Greg Paul’s reconstructions) show it lower down – nearer the so-called acromion part of the scapula. Gao et al. (1986) [which I haven’t seen] reported in-situ spines of Tuojiangosaurus to be positioned over the ‘proximal end’ of the scapula according to Sereno & Dong (1992): however, Sereno & Dong’s use of the term ‘proximal’ seems to be different from my understanding as, while they cite Dong (1990) as showing the spines in a ‘proximal’ location, Carpenter’s figure in Dong (1990) actually shows the spine near the vertebral border – a position that people typically call ‘distal’ on a scapula!
Any thoughts? Where do the spines really go? I should have made a Tet Zoo post of this…
Congrats on a wonderful dinosaur. For an ornithischian, that is…
About the parascapular spine:
Part of a spine was found near the scapula in the type specimen, but was impossible to determine the exact position or even confirm if that was a parascapular or tail spine.
Actually, that reconstruction with the scapular spines was a first attempt for drawing the skeleton but end up to not the one in the paper and should not be used for osteology purposes.
We do not depict the parascapular spine in figure of the article, also seen in my Lusodinos blog here: http://lusodinos.blogspot.com/search/label/Miragaia.
Darren Naish wrote- “will people please stop using ‘proximal’ and ‘distal’ on scapulae, it doesn’t make any sense”
Wouldn’t the “vertebral border” be the medial/costal border, though? It certainly seems to be facing the vertebrae more than the anterodorsal edge in that pic. What’s your term for the oppposite side? Sternal border? I don’t see how using proximal and distal for scapulae and coracoids is any less sensical than for ischia and pubes. They’re both paired elements with a primarily sagittal orientation whose long axes have varying angles to the vertical but do join at a certain center point that can be called proximal.
Thanks for the comments! I’ll take the questions one at a time:
Synonymy with Dacentrurus
There are many similarities between Miragaia and Dacentrurus, and they are sister taxa in my analysis. So whether they are syonymous or not just comes down to whether you are a splitter or a grouper. The names don’t really matter: the relationships do. Having said that, finding more Miragaia specimens (or Dacentrurus specimens) will help us to understand these two taxa better. Right now, Dacentrurus is known mainly from the posterior portion of the skeleton, while Miragaia is known from the anterior portion, and there are not many elements that overlap between the two (there are no cervical vertebrae preserved in the Dacentrurus holotype, or skull material). Secondly, there are several fragmentary stegosaur specimens known from Portugal that have been assigned to Dacentrurus sp. although they are too incomplete to warrant a specific referral. They are known mainly from ilio-sacral blocks and femora (neither of which we have for Miragaia). I examined the specimens in 2004 and they are definitely different from the Dacentrurus holotype (which is from the UK). So knowning more about the Portuguese Dacentrurus species will help us to understand the exact relationship between all of these taxa. So in conclusion: we need more data to confidently refer Miragaia to Dacentrurus, which is the main reason we didn’t. However, as with everything in palaeontology, our paper isn’t ‘the answer’, just our best hypothesis given the data we currently have, and is likely to change in the future.
Parascapular spines
The short answer to the question of the exact orientation of the spines Darren, is I don’t know. As you correctly mentioned, Gao et al. described a specimen of Tuojiangosaurus with parascapular spines preserved in place. I think that the specimen discussed in that paper is actually the specimen now referred to as Gigantspinosaurus, given the photograph in the paper and reconstruction of how Gigantspinosaurus was found in the Zigong dinosaur museum. Anyway, this specimen has parascapular spines lying alongside the scapulae. But of course the specimen was flattened in burial, and the exact location and elevation of the spines is not known. To be honest I’m not sure we will ever know exactly where the spines were located, or how they were elevated.. not unless we find a three-dimensionally preserved stegosaur, that is. To my knowledge this is the only specimen ever found with parascapular spines in anything similar to a life position.
Response to Mickey on ‘proximal’ vs ‘distal’. Unfortunately, people now use these terms to mean ‘nearer the centre’ and ‘further from the centre’ with respect to the bone itself (whatever ‘centre’ is). I know that many disagree with me, but this is flat-out contradictory to what should be the correct use of these terms: they should only be used for appendages (like limbs, horns, antlers, feathers, the tail), and they originally described the position of the structure relative to the heart (or, if you like, centre of the body). So, using it for pubes or ischia is just as bad as using it for scapulae. We have lots of other directional terms that are not ambiguous: for pubes and ischia, you should be talking about dorsal and ventral, for skull elements anterior (or rostral) and posterior are appropriate. I will avoid the debate over use of ‘Baumelese’ (NAA and NAV nomenclature).
Scapulae are problematical in particular: if you’re talking of the ‘proximal’ part of the scapula, you should actually be talking about the part closest to the centre of the BODY, >not< the part of the scapula closest to ‘the centre’ of the scapula itself. So.. which part of the scapula is this? Do you mean the acromion, or somewhere closer to the vertebral border (and ‘vertebral border’ = dorsalmost edge close to the vertebral column, not the medial or costal side. Sometimes called the ‘medial border’ in humans). In most tetrapods, scapulae are arranged in an approximate dorsoventral orientation: therefore, we should be talking about the ventral or dorsal end of the scapula in this case (with other qualifiers used for clarity when appropriate).
No doubt some will think that this approach is dumb and contradictory to widely adopted usage. I disagree: talking of the ‘proximal part of the scapula’ (for example) is flat-out confusing and we should stop doing it.
Interesting. I never knew proximal and distal were originally used on reference to the heart’s position. My response would be that words change meaning over time, and there’s no point in sticking to original usage if the entire field has modified their usage. Especially when the element shifts orientation often like scapulae do. Just look at a lizard and a bird. In the first the distal end is vertebral and dorsal, while in the second the distal end is posterior and the former anterior end is vertebral and dorsal. So your choices are to use different terms for different groups (which complicates matters and hides homology) or use orientation terms which don’t refer to the real orientation in many taxa. Why not just ignore orientation and enable yourself to refer to homologous areas by using proximal as toward the base and distal as away from the base?
What about _”Wuerhosaurus” ordosensis_? Could the presence of only 11 dorsals in this specimen indicate that anterior dorsals were incorporated (‘cervicalized’) in the neck, and therefore that _W. ordosensis_ had a long neck too? (Though, in this case, it would be maybe 15 cervicals, rather than the 17 seen in _Miragaia_).
🙂