Macroplata
Macroplata is an extinct genus of Early Jurassic rhomaleosaurid plesiosaur which grew up to in length. Like other plesiosaurs, Macroplata probably lived on a diet of fish, using its sharp needle-like teeth to catch prey. Its shoulder bones were fairly large, indicating a powerful forward stroke for fast swimming. Macroplata also had a relatively long neck, twice the length of the skull, in contrast to pliosaurs. It is known from a nearly complete skeleton NHMUK PV R 5488 from the Blue Lias Formation of
Harbury, Warwickshire, UK.
A different species, Macroplata longirostris, which lived somewhat later, during the Toarcian stage, was also included in the genus; however, in 2011, Benson et al. reclassified it as a pliosaurid in the genus Hauffiosaurus, H. longirostris.
Description
Macroplata bore an elongated skull, with more than half of its cranial length taken up by a roughly triangular snout. The premaxillae bear six teeth each, with the first being the smallest and the second, third, and fourth the largest. Together, they form the rounded region at the front of the jaw, and have a rough outer surface bearing foramina, ridges, and grooves. The snout is constricted where the premaxillae border the maxillae. These bones continue to contact each other until they reach the level of the external nares. While the premaxillae become narrow beyond this point, they extend far back on the skull, forming a ridge rearward of the external nares. The top side of each maxilla is ornamented like that of the premaxillae. The front edges of the orbits are formed by the maxillae. The maxillae also seem to make up the front and outer rims of the external nares, although poor preservation makes it difficult to verify this. The better-preserved maxilla of the holotype bears 18 alveoli, with the smallest at the back. The maxillary teeth are separated from the premaxillary teeth by a gap. The teeth of Macroplata have circular cross-sections and possess vertical ridges that all at least come close to the tooth's tip.The frontals of Macroplata are long, spanning from external nares to temporal fenestrae, and widen towards the back of the skull. They bear foramina and probably formed the rear edges of the external nares. The frontals are separated from each other by the premaxillae in the front and the parietals at the rear, with a small area of contact between. The frontals articulate with the prefrontals above the orbits. The prefrontals form part the orbits' upper edge. The postfrontals form the border between the orbits and the temporal fenestrae, and each bone bearing a ridge running perpendicularly to the skull's length. Upon the discovery of Macroplata, its pineal foramen was claimed to be both large and small by different sources. As it was later obscured by plaster, its true size is unknown.
The parasphenoid is smooth and approximately diamond-shaped, with a wide surface at its front that articulates with the pterygoids. The posterior interpterygoid vacuities are roughly elliptical. Unlike in other plesiosaurs, their hind ends are angled inwards, making this trait an autapomorphy of Macroplata. It is unclear where the parasphenoid meets the basioccipital, although a possible suture between these bones is present roughly at the level of the hind end of the posterior interpterygoid vacuities. This suggests that the rearmost part of the palate was composed of the basioccipital. The occipital condyle juts out beyond the palate when viewed from below. The occipital condyle is formed solely by the basioccipital, with the exoccipital-opisthoics not participating. The paraoccipital processes of the latter bones bear wide upper surfaces and slope downwards and forwards. The foramen magnum is constricted into an 8-shape. The supraoccipital is wider than tall.
The mandibular symphysis bears seven or eight tooth pairs and is covered by foramina on its sides. The suture between the dentary and angular takes the shape of a straight line sloping upwards and backwards. The splenial might form part of the mandibular symphysis. It continues to contact the bottom of the dentary until the angular separates them further rearwards. The angular forms the entire lower surface of the rear mandible, widening below the jaw joint before narrowing to form the lower part of the retroarticular process. The articular forms the top part of the retroarticular process. The retroarticular process is broad and weakly angled towards the skull's midline.
Macroplata bears 26 cervical (neck) vertebrae. The parts of the atlas and axis, the first two cervical vertebrae, are not entirely fused together. The centrum of the atlas is surrounded by the crescent-shaped atlantal intercentrum below and the atlantal neural arches on the sides, and is not visible from below. The lower surface of the intercentrum of the axis tapers to a point facing forwards. Beyond the axis, the undersides the cervical centra bear keels, with a prominent pit on each side in which a small subcentral foramen is located. The moderately long cervical centra are amphicoelous and slightly wider than tall. The fronts, backs, and sides of the centra are rough. The zygapophyses span about as wide as the centra, and the joints they articulate with are directed upwards and inwards. The lower parts of the neural spines are narrow from side to side. The cervical vertebrae increase in size towards the back of the neck, and the articulations between the neural arches and centra become increasingly pointed. The cervical ribs are double-headed and their heads are roughly circular.