Temnodontosaurus

Temnodontosaurus (Greek for “cutting-tooth lizard” - temno, meaning “to cut”, odont meaning “tooth” and sauros meaning “lizard”) is an extinct genus of ichthyosaurs from the Early Jurassic, ranging between 200 and 175 million years ago (Hettangian - Toarcian), and known from Europe (England, France, Germany and Belgium). They lived in the deeper areas of the open ocean. University of Bristol paleontologist Jeremy Martin described the genus Temnodontosaurus as “one of the most ecologically disparate genera of ichthyosaurs”. Temnodontosaurus was one of the largest ichthyosaurs. Estimates of the maximum length of Temnodontosaurus have ranged from 9 m (29.5 ft) to 12 m (39 ft). The largest length estimates rival those of another giant ichthyosaur, Shonisaurus popularis, which was previously considered the largest ichthyosaur. Temnodontosaurus is known for its incredibly large eyes which, at approximately 20 cm (8 in) in diameter, are believed to be the largest of any known animal. It possessed a tail bend which was characteristic of Jurassic ichthyosaurs and had many conical teeth filling its jaw that were set in a continuous groove. The number of valid Temnodontosaurus species has varied over the years. Christopher McGowan in 1992 believed there to be around thirteen species in the genus Temnodontosaurus. Michael Maisch in 2000 listed T. platyodon, T. trigonodon, T. acutirostris, T. nuertingensis and T. eurychephalus as the valid species of Temnodontosaurus. Temnodontosaurus had a thunniform, or 'fish-shaped' body form. Its body was long, robust and slender. The tail was either as long as the body or longer. The vertebral count was roughly less than 90. The axis and atlas of the vertebrae were fused together, serving as stabilization during swimming. T. trigonodon possessed unicipital ribs near the sacral region while it had bicipital ribs more anteriorly. This helped to increase flexibility while swimming. Temnodontosaurus did not have gastralia. The forefins and hindfins of Temnodontosaurus were of roughly the same length and were rather narrow and elongate. This is unlike other post-Triassic ichthyosaurs such as the thunnosaurians, which had forefins at least twice the length of their hindfins. The pelvic girdle was also not reduced, unlike post-Triassic ichthyosaurs. Like other ichthyosaurs, the fins exhibited strong hyperphalangy.Temnodontosaurus only had 3 primary digits compared to Ichthyosaurus which had 6 to 7 digits. It also had one postaxial accessory digit. The proximal elements of the fin formed a mosaic pattern while the more distal elements were relatively round. There were two notches on the fin’s anterior margin. The paired fins were used to steer and stabilize the animal while swimming instead of paddling or propulsion devices. The pelvic girdle was tripartite. It had a triangular shaped dorsal fin. Like other ichthyosaurs, Temnodontosaurus likely had high visual capacity and used vision as its primary sense while hunting. Temnodontosaurus had the largest eyes of any ichthyosaur, and of any animal measured. The largest eyes measured belonged to the species T. platyodon. Despite the enormous size of its eyes, Temnodontosaurus had blind spots directly above its head due to the angle at which its eyes were pointed. The eyes of Temnodontosaurus had sclerotic rings, hypothesized to have provided the eyes with rigidity. The sclerotic rings of T. platyodon were at least 25 cm in diameter. The head of Temnodontosaurus had a long robust snout with an antorbital constriction. It also had an elongated maxilla, a long cheek region and a long postorbital segment. The carotid foramen in the basisphenoid in the skull was paired and was separated by the parasphenoid. Also, the parasphenoid had a processus cultriformis. The skull of T. platyodon measured about 1 m to 1.5 m long.