Askeptosaurus is an extinct genus of askeptosauroid, a marine reptile from the extinct order Thalattosauria. Askeptosaurus is known from several well-preserved fossils found in Middle Triassic marine strata in what is now Italy and Switzerland.[1]
Askeptosaurus, and its only known species Askeptosaurus italicus, were first named and described in 1925 by Hungarian paleontologist Franz Nopcsa von Felső-Szilvás.[2] It was most recently redescribed by Dr. Johannes Müller in 2005. Askeptosaurus is known from several disarticulated and articulated skeletons preserved at the MSNM (Museo Civico di Storia Naturale di Milano) in Milan, Italy, and the PIMUZ (Paläontologisches Institut und Museum der Universität Zürich) in Zürich, Switzerland. These specimens were discovered in the Grenzbitumenzone of Monte San Giorgio, a UNESCO World Heritage Site on the Swiss-Italian border. Also known as the Besano Formation in Italy, the Grenzbitumenzone has produced many well-preserved fossils from the Anisian-Ladinian boundary within the Middle Triassic.[1]
Askeptosaurus was a fairly large thalattosaur, with a skull 26 cm (~10 inches) in length and a total length up to 2.5 meters (8.2 feet). Like other askeptosauroids, it had a long neck, a very long tail, and small but well-developed limbs with five independent digits.[1][3]
The skull was low and somewhat broad at the back, though the snout was long and slender. Nearly half of the snout is formed by the large premaxillae, which send back a long triangular projection into the frontals along the midline of the snout. Each premaxilla has up to 12 sharp and slightly curved teeth. They are implanted in a pleurothecodont manner, meaning that they lie in shallow sockets along a groove which has a lowered edge on the lingual (tongue) side of the tooth row. The premaxilla is followed by the low and smaller maxilla, and an elongated naris (nostril) is present at the border between the two bones. The maxilla has 16 teeth, which are similar to those of the premaxilla, albeit slightly smaller. Unlike thalattosauroids, the teeth have the same general shape and there is no diastema (gap) between the premaxillary and maxillary tooth rows. Like other thalattosaurs, the nasals are separated from each other by midline projections of the premaxilla and frontal. The nasals have a small contribution to the border of the naris, and the sharp rear tip of each nasal projects into the frontal. Apart from the triangular notches incised by the premaxillae and nasals, the frontals have a simple and subrectangular form.[1]
Two small bones, the lacrimal and prefrontal, lie in front of the large orbit (eye socket). The presence of a separate lacrimal is a plesiomorphic (ancestral) trait only found in Askeptosaurus among thalattosaurs. Askeptosaurus is also unique in how its lacrimal has a slight contact with the frontal between the nasal and prefrontal. The posterodorsal (rear-upper) edge of the orbit has two more bones, the large and multi-pronged postorbital and the much smaller postfrontal. Once again, Askeptosaurus retains the plesiomorphic condition, since in other thalattosaurs these bones fuse into a single postorbitofrontal. The parietals, which lie at the rear of the skull roof, send out sprawling projections over the braincase. A slit-like upper temporal fenestra develops between the parietal and postorbital, while a circular pineal foramen is positioned near the parietal's suture with the frontal.[1]