Fossil range: Late Jurassic
Miragaia longicollum, Portugal
| Scientific classification
Miragaia (named after Miragaia, the area in Portugal and geologic unit where its remains were found) is an extinct genus of stegosaurid dinosaur. Its fossils have been found in Upper Jurassic rocks in Portugal. Miragaia is notable for its long neck, which included at least 17 vertebrae.
Miragaia is based on ML 433, a nearly complete anterior half of a skeleton with partial skull (the first cranial material for a European stegosaurid). Among the recovered bones were most of the snout, fifteen neck vertebrae (the first two, which articulated with the skull, were absent), the shoulder bones, most of the forelimbs, and thirteen bony plates. ML 433 was found in the Miragaia Unit of the Sobral Formation, Lourinhã Group, which dates to the late Kimmeridgian-early Tithonian (Late Jurassic, approximately 150 million years ago). Octávio Mateus and colleagues described Miragaia in 2009. The type species is M. longicollum ("long neck"). A partial hip and partial vertebrae from a juvenile individual (ML 433-A) were found at the same location, and were also assigned to M. longicollum. Mateus and colleagues performed a phylogenetic analysis and found Miragaia to group with Dacentrurus in a clade Dacentrurinae, the sister group to Stegosaurus.
Size and diagnosisEdit
The total length of Miragaia has been estimated at 5.5 – 6 metres (18-20 ft). In 2010, Gregory S. Paul estimated the length at 6.5 metres, the weight at two tonnes.
The describers established six distinguishing traits. At their very midline, the praemaxillae meet in a small sharp point, set within a larger notch in the snout tip as a whole. The front lower side edge of the praemaxilla protrudes to below. At least seventeen cervical vertebrae are present. The neural spines of the middle cervical vertebrae have a notch at their lower front edge with immediately above it a process directed to the front. The vertebrae of the middle neck, rear neck and front back possess neural spines that have a transversely expanded upper end. On the neck two rows of triangular bony plates are present that have a lightly convex outer side and a notch at the upper front edge creating a hook.
The most notable feature of Miragaia is its long neck, which was composed of at least 17 vertebrae. This represents the culmination of a trend of longer necks seen in stegosaurians. Additionally, Miragaia had more neck vertebrae than most sauropods, dinosaurs known for their long necks, which contrasts with the traditional view of stegosaurians as low browsers with short necks. Only the Chinese sauropods Euhelopus, Mamenchisaurus, and Omeisaurus had as many neck vertebrae as Miragaia, with most sauropods of the Late Jurassic possessing only 12 to 15. Mateus and colleagues suggested that the long neck either allowed Miragaia to browse at a level that other herbivores were not exploiting, or that the neck arose due to sexual selection.
In sauropods, great neck length was achieved by a combination of three processes: incorporation of back vertebrae into the neck; addition of new vertebrae; and lengthening of the individual neck vertebrae. The long neck of Miragaia appears to have resulted mostly from back vertebrae becoming incorporated into the neck, based on vertebral counts of other stegosaurians. There is currently no evidence that new vertebrae contributed to the neck; instead, the distribution of existing vertebrae in the back and neck changed. There is some evidence for increasing vertebral length in Miragaia and Stegosaurus, but this is equivocal and could be due to post-mortem distortion; this mechanism was seen as a minor factor in neck elongation.
Apart from the neck length, the known osteology of Miragaia resembles that of other stegosaurids, differing in small details. The tip of the beak was toothless, as in Stegosaurus. The upper beak, formed by the praemaxilla, was pendant. The notch in the snout tip was, seen from above, shaped like a W, whereas in Stegosaurus the notch is U-shaped, with a little bulbous projection in the middle. The upper surface of the nasal bone was ornamented. A ridge formed the contact with the maxilla. The maxilla had sixteen teeth. The postorbital was a small and triradiate element.
The cervical vertebrae had well-developed cervical ribs, fused to the vertebral body. The ribs were elongated, with a forward-pointing process on the capitulum, the main rib head. The neural spines of the rear cervicals and front dorsals are transversely expanded at their upper ends due to rugosities serving as an attachment for tendons. This expanded sector projects to the front also, creating a notch on the lower front edge. Additionally the neural spine base is transversely constricted. Ridges extend to the rear from the sides of the neural spine base, over the upper sides of the rear joint processes, the postzygapophyses. These postzygapophyses themselves project far beyond the rear facet of the vertebral body, a derived trait. The prezygapophyses to the contrary, are much shorter; they have a notch at the front upper edge.
The scapula had a large rectangular acromion, with a sharp upper corner, on the lower front edge. The more narrow coracoid had a rounded lower edge. Both the upper arm and ulna and radius (lower arm bones) are also comparable to those of Stegosaurus. The ribs of the neck vertebrae were fused to the vertebrae. The tuberosity of the rear humerus serving as attachment for the musculus triceps brachii is well-developed but the vertical ridge running from it to below is not. In the pelvis, the pubic bone had an expanded tip, as seen in Dacentrurus; the rear shaft had a lightly expanded tip.
Miragaia, like all known stegosaurians, showed an array of plates and spikes, consisting of skin ossifications or osteoderms. Paired triangular plates ran down the midline of the neck, reconstructed as eight pairs. They were asymmetrical with a convex outer side and a concave inner side. Their bases were not very expanded with the exception of a possible last pair, located on the front back. They were obtuse but lightly hooked at the front. A rather long, narrow and straight preserved spike was at first considered to have been a shoulder spine, but was later seen as part of some tail arrangement.
Miragaia was placed in the Stegosauridae in 2009. Mateus and colleagues performed a phylogenetic analysis and found Miragaia to group with Dacentrurus in a clade Dacentrurinae, newly named for the occasion, the sister group to Stegosaurus (the latter genus was in the cladistic analysis considered to include Hesperosaurus and Wuerhosaurus).
The position of Miragaia in the stegosaurid evolutionary tree is shown by this cladogram:
- ^ a b c d Mateus, Octávio; Maidment, Susannah C.R.; and Christiansen, Nicolai A. (2009). "A new long-necked 'sauropod-mimic' stegosaur and the evolution of the plated dinosaurs" (pdf). Proceedings of the Royal Society B: Biological Sciences online. doi:10.1098/rspb.2008.1909. http://rspb.royalsocietypublishing.org/content/early/2009/02/21/rspb.2008.1909.full.pdf+html.