Mauriciosaurus (meaning "Mauricio [Fernández Garza's] reptile") is a genus of polycotylid plesiosaur from the Late Cretaceous of Mexico. It contains a single species, M. fernandezi (also named after Fernández Garza), described in 2017 by Eberhard Frey and colleagues from a single well-preserved juvenile specimen about long. Morphologically, it is overall most similar to the polycotyline polycotylids Trinacromerum and Dolichorhynchops. However, several features separate Mauriciosaurus from all other polycotylids, warranting the naming of a new genus. These include the sophisticated pattern of ridges on the bottom of the parasphenoid bone on its palate; the narrow openings in the palate bordered by the pterygoid bones; the lack of perforations in the surface of the coracoid; and the highly unusual arrangement of gastralia, or belly ribs, which is only otherwise seen in the non-polycotylid Cryptoclidus.
Partially as a consequence of the oxygen-poor environment that the specimen was preserved in, the type specimen Mauriciosaurus is notable for possessing well-preserved soft-tissues. In life, each of its four flippers bore a flexible trailing edge such that they formed hydrofoils, and contour fat created a drop-like outline of the body by merging the torso and immobile tail into a single unit. Both of these soft-tissue traits would have improved the animal's hydrodynamic performance, making it a fast swimmer comparable to modern leatherback sea turtles. Additionally, a heavily-pigmented lining protected the organs of the body cavity from UV exposure. After death, the sediment allowed the contour fat to preserve into adipocere instead of putrefying, which preserved most of the epidermal soft tissue save for that of the belly.
The oxygen-poor deposits that Mauriciosaurus was preserved in, collectively known as the Vallecillo plattenkalk, would have represented the bottom of a continental shelf about offshore in the prehistoric Gulf of Mexico. Inoceramid bivalves represent the only endemic benthic fauna at the site; other fossil organisms known from Vallecillo include invertebrates such as ammonites and barnacles that grew on those ammonites, as well as vertebrates consisting of various fish, aigialosaurid mosasaurs, and turtles. The type specimen of Mauriciosaurus is associated with an ammonite, Mammites nodosoides, which is present only in the Turonian interval of the Vallecillo deposits, allowing it to be dated to that age.
Discovery and naming
The type and only known specimen of Mauriciosaurus was found in the summer of 2011 by workers at a laminated limestone quarry near the town of Vallecillo, in the state of Nuevo León, Mexico. It is labelled under the collection number INAH CPC RFG 2544 P.F.1 (INAH standing for the Instituto Nacional de Antropología e Historia, or National Institute of Anthropology and History) at the Museo del Desierto in Saltillo, Coahuila, Mexico; currently, however, it is on display currently at the Museo Papalote Verde in Monterrey, the capital of Nuevo León. The specimen has been broken into 28 slabs, which collectively preserve a nearly-complete and articulated skeleton, missing only the left coracoid, parts of the flippers, and the tip of the tail (which were lost during collection by the workers).
INAH CPC RFG 2544 P.F.1 was found in a section of laminated limestone mixed with marl, part of the plattenkalk Konservat-Lagerstätte (fine limestone deposits that exquisitely preserve animals) within the Cenomanian-Turonian Agua Nueva Formation.Vallecillo Mauriciosaurus type locality at Fossilworks.org The plattenkalk is quite widespread, being largely concentrated around Vallecillo but spanning an area of . Outside of these deposits, macroscopic fossils are relatively rare in the Agua Nueva. Detailed biostratigraphic zonation has been conducted for the Vallecillo plattenkalk; the presence of the vascoceratid ammonite Mammites nodosoides allows the slab containing Mauriciosaurus to be dated to the early Turonian portion of the plattenkalk.
Eberhard Frey, Eric Mulder, Wolfgang Stinnesbeck, Héctor Rivera Sylva, José Padilla Gutiérrez, and Arturo González González described the specimen in 2017, formally naming it as a new taxon in a paper submitted to the Boletín de la Sociedad Geológica Mexicana (Bulletin of the Geological Society of Mexico). The genus and species name, Mauriciosaurus fernandezi, is in honour of Mauricio Fernández Garza, who has provided the specimen for scientific study, in addition to providing funding for both research at Vallecillo and public science education in Nuevo León.
Description
Mauriciosaurus is a short-necked plesiosaur, bearing a drop-shaped body with a small tail. The type specimen of Mauriciosaurus would have measured long in total, wide at the flippers and weighed . However, this specimen is immature, judging by the lack of fusion in the cervical vertebrae; the coracoids bearing no forward projections that interlock with the clavicles; the thin bones of the shoulder and hip girdles; the lack of a ridge on the top surface of the scapula; the short ischia; and the presence of gaps (filled by extensive cartilage in life) between the long bones of the flippers. The palate is mineralized, suggesting that the specimen was still at least a subadult. Adult individuals would have grown up to twice the length of the type specimen.
Skull
The snout of Mauriciosaurus is round in cross-section and straight-sided. The bottom of the jaw is even, unlike Thililua where the jaw projects downwards at the level of the eye sockets. Each side of the upper jaw of Mauriciosaurus contain at least 42 teeth, including 4 in the premaxilla and 32 in the maxilla. All of the teeth are of the same shape, being four times taller than they are wide with conical and recurved tips; the smallest tooth, from the back of the mouth, is about a quarter the size of the largest. The gaps separating the teeth are a slightly wider than the teeth themselves. While it is hard to infer much about the precise arrangement of the teeth in life, the teeth of the upper and lower jaws probably would have been interlocking. Compared to other polycotylids, the teeth are smaller than those of Eopolycotylus, Pahasapasaurus, and Georgiasaurus, and more numerous than those of Trinacromerum (30-38) or Dolichorhynchops (26-30).
Most of the diagnostic characteristics in the skull of Mauriciosaurus are found on the palate. Like other polycotylids, the pterygoid bone of Mauriciosaurus bears a dish-like depression. Unlike both Trinacromerum and Dolichorhynchops, however, the depression does not span the entire bottom surface of the bone, and is instead restricted to the front portion. The back of the bottom surface of the pterygoid bears prominent ridges arranged uniquely in a W-shape; Polycotylus has a similar, albeit less complex, arrangement of ridges. Like Trinacromerum but unlike Dolichorhynchops, the tips of the processes extending out from the front of the pterygoids are significantly widened; it is three times as wide as the rest of the process in Mauriciosaurus, and twice as wide in Trinacromerum. Similarly, Mauriciosaurus and Trinacromerum both bear projections on the rear quarter of the pterygoid (a rounded tubercle in Mauriciosaurus, and a sharp ridge in Trinacromerum), whereas Dolichorhynchops lacks any such projection.
An additional trait shared between Mauriciosaurus and Trinacromerum involves the parasphenoid, which separates the pterygoid from the midline of the skull; the front of the parasphenoid is bifurcated in both, while it tapers to a single blunt point in Dolichorhynchops. The pterygoid and parasphenoid enclose three holes in the palate, the anterior and two posterior interpterygoid vacuities. The anterior interpterygoid vacuity is the largest of the holes in the palate, extending roughly from the 23rd to the 32nd tooth positions in the maxilla. Like other polyoctylids, the posterior interpterygoid vacuities are closed by the pterygoid. However, they are unusually narrow and oval-shaped in Mauriciosaurus, whereas they are wider and more rounded in Trinacromerum due to the concave inner margins of the pterygoid and parasphenoid. Meanwhile, on the lower jaw, the splenial contributes about 50% to the bottom surface of the jaw, which is more than Palmulasaurus and Eopolycotylus.
Vertebral column
Mauriciosaurus bore 21 cervical vertebrae, at least 23 thoracic vertebrae, and at least 25 in the sacrum and tail (the delineation between these two types is not clear, since it is obscured by the ischium). The centra of Mauriciosaurus are constricted on their outer surfaces, unlike the cylindrical centra of Trinacromerum. Both the cervical and thoracic centra have length-width ratios of about 2:3; the cervical centra are procoelous (concave in front and convex behind), whereas the thoracic centra are platycoelous (both surfaces are flat). The sacral and caudal centra are generally shorter, with length-width ratios of about 1:3, but they also become narrower such that the last several preserved caudal centra are twice as long as they are wide; the first caudal centrum appears to be opisthocoelous (convex in front and concave behind), while the rest are platycoelous (flat on both sides). Each caudal centra only articulated with one haemal arch at its back end, as opposed to one at each end in Dolichorhynchops.
Like other members of the Sauropterygia, the cervical vertebrae in the front half of the neck bore two pits on their bottom surfaces, separated by a median ridge; this ridge is abruptly replaced by a depression in the twelfth to eighteenth vertebrae, following which the ridge is present again. The same pits are present on the thoracic vertebrae, where they are separated by not one but two distinct ridges. An additional pit is present immediately beside each of the blunt keels that border the sides of the bottom face. The neural spines of the cervical vertebrae become taller and more inclined, with the seventeenth neural spine being as tall as the centrum and being inclined at an angle of 15°. Out of the thoracic ribs, the ninth is the longest, with rib size gradually decreasing on either side. All of the thoracic ribs bear only one articular facet; the third rib unusually has a facet that is strongly triangular in cross-section, which is probably an artifact of preservation.
A particularly unusual feature of Mauriciosaurus is its gastralia. There are six gastralia, each made up of a central V-shaped medial ossicle and various lateral ossicles attached to its sides. The second and third gastralia share a single medial ossicle, as do the fifth and sixth. The fourth medial ossicle is unusually massive, being about twice the thickness of the other medial ossicles. All of the gastralia converge to a single central point, which is a feature that is unprecedented among polycotylids, being only otherwise seen in Cryptoclidus. It is unlikely that this unusual arrangement is the product of taphonomic processes, since all of the gastralia seem to be preserved in alignment; the arrangement of the gastralia does not appear to be influenced by age either.
Limbs