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’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.
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.
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.