Miragaia

Miragaia (named after Miragaia, the parish in Portugal and geologic unit where its remains were found) is a genus of herbivorous stegosaurid dinosaur. Its fossils have been found in Upper Jurassic rocks in Portugal. Miragaia has the longest neck known for any stegosaurian, which included at least seventeen vertebrae.

Miragaia by Prehistoric Wildlife

Discovery

Miragaia is based on holotype ML 433, a nearly complete anterior half of a skeleton with partial skull (the first cranial material for a European stegosaurid). The remains were found after the construction of a road between the villages of Miragaia and Sobral. The rear half of the skeleton was probably destroyed by the roadcut. The fossils were dug up in August 1999 and August 2001. Among the recovered bones were most of the snout, a right postorbital, both angulars of the lower jaws, fifteen neck vertebrae (the first two, which articulated with the skull, were absent), two anterior dorsal vertebrae, twelve ribs, a chevron, the shoulder bones, most of the forelimbs including a possible os carpi intermedium, a right first metacarpal and three first phalanges; and thirteen bony plates plus a spike. The bones were not articulated but dispersed over a surface of about five to seven metres, though there was a partial concentration of fossils that could be salvaged within a single block. ML 433 was found in the Miragaia Unit of the Sobral Unit, Lourinhã Formation, which dates to the late Kimmeridgian-early Tithonian (Late Jurassic, approximately 150 million years ago).

Miragaia_longicollum_(fossil). Photo by Ghedoghedo

Octávio Mateus, Susannah Maidment and Nicolai Christiansen named and shortly described Miragaia in 2009. The type species is Miragaia longicollum. The generic name refers to the village of Miragaia but also is an allusion to mira, “wonderful” in Latin, and Gaia, the Earth Goddess. The specific name means “long neck” from the Latin longus, “long” and collum, “neck”. A partial pelvis (ilium and pubic bone) and two partial dorsal vertebrae from a juvenile individual (specimen ML 433-A) were found at the same location, intermingled with the bones of the holotype, and were also assigned, as a paratype, to M. longicollum.

Casts were made of the holotype bones and partially based on these a life-size skeletal model was constructed from polyurethane and polyester resin parts.

Alberto Cobos et alii in 2010 noted that all the diagnostic characters of Miragaia longicollum are based on skeletal elements that are absent in the Dacentrurus holotype found in England in layers of about the same age, while all traits that can be compared are shared by both genera. Cobos et alii therefore proposed that Miragaia is a junior synonym of Dacentrurus, meaning that it is the same dinosaur, because it is not possible to differentiate the two taxa through their holotypes.

Description

Size and diagnosis

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. Histology shows that the holotype specimen was agout 21 years old.

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.

Miragaia is from Mateus et al. (2009), and Brachytrachelopan from Rauhut et al. (2005). Both critters come with the 1 meter scale bars from their respective figures. Sauroposeidon looms in the background, just to keep things in perspective. The entire neck of Miragaia might have been about as long as one of the middle cervicals of Sauroposeidon or Supersaurus.

Neck elongation

The most notable feature of Miragaia is its long neck, which was composed of at least seventeen vertebrae. According to the authors, this represents the culmination of a trend towards longer necks seen in stegosaurians. The Thyreophora, the larger group they belong to, originally seem to have had nine neck vertebrae and this is also the number shown by the basal stegosaurian Huayangosaurus. Later forms like Stegosaurus or Hesperosaurus had twelve or thirteen. This is the more remarkable that Miragaia had more neck vertebrae than most sauropods, a different group of dinosaurs and one that is famous for its 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 twelve to fifteen. 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. The possible food gathering function of the neck makes sexual selection a less plausible explanation, but is not in itself entirely convincing: though the contemporary Iberian sauropods Lusotitan, Dinheirosaurus and Turiasaurus were all very large and might not have competed with a medium-height browser, the niche partitioning is still problematic because in Iberia stegosaurian remains have been referred to Dacentrurus and Stegosaurus, which would have possessed a feeding envelope or feeding height stratification overlapping that of Miragaia.

Skeleton

 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.
One of the seventeen neck vertebrae, seen from the right upper side

Osteoderms

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.

Phylogeny

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:

Stegosaurid evolutionary tree cladogram

The authors stressed that the only synapomorphy, shared derived trait, supporting the DacentrurusStegosaurus clade was the possession of the long cervical postzygapophyses, and that these are in fact unknown for Decentrurus itself, so that its close position to Stegosaurus was merely based on the new data provided by the description of Miragaia.

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