Temnodontosaurus

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***Temnodontosaurus*** was an ichthyosaur from the Early Jurassic, some 198 and 185 million years ago (Hettangian - Toarcian), in Europe (England & Germany). It was a large genus, exceeded 12 meters (30 ft) in length.

## History of discovery

The first ichthyosaur ever found was a *T. platyodon*, BMNH 2149, from the Lias of Lyme Regis by Joseph Anning (1811). in 1812, Mary Anning discovered the remaining postcrania, but was lost. It was then described by Sir Everard Home in the first ichthyosaur description. *T. platyodon* is the most common species of *Temnodontosaurus*. The type skull is now located at the Natural History Museum, London, and was originally named *Ichthyosaurus platyodon*, but moved into a new genus (*Temnodontosaurus*) by Richard Lydekker (1889). *T. platyodon* itself was named by William Conybeare (1822) from BMNH 2003 of Lyme Regis, now located at the British Museum of Natural History as the type species. *T. platyodon* have been found in England, Germany, Belgium and Luxembourg from Upper Hettangian-Lower Sinemurian-aged rock. BMNH 2003 is the only complete specimen known, alongside well-preserved skull BMNH R1158.

Christopher McGowan (1995) found *T. risor* was a juvenile *T. platyodon*. David Sole (1987) had collected the main specimen east of Lyme Regis, but three other skulls seen as different (due to larger orbits, smaller maxillae and curved rostra). However, McGowan described them as juvenile die to the small forefin compared to the skull. Today, *T. risor* material are seen as juveniles due to their elongate nature when compared to the postcrania.

*T. trigonodon*, named by von Theodori (1843), is known from near-complete postcrania from the Upper Liassic of Banz, Germany and the Lower Toarcian of Saint Colombe, Yonne. One *T. trigonodon* is known from Upper Toarcian-aged, Aalen, Baden-Württemberg, Germany in The Staatliches Museum für Naturkunde, Stuttgart in Germany under the specimen code 15950. In 2022, a 10-meter-long specimen was found from Rutland. It was excavated by relatively few people due to coronavirus limitations and led by Dean Lomax.

*"Ichthyosaurus" zetlandicus* was named by Seeley from CAMSM J35176, a 3D skull without mandible and anterior 3rd of the snout from the Whitby Mudstone Formation. It was later attributed to *I. acutirostris* before being combined as *Temnodontosaurus zetlandicus* in 2022. Though it bears many defining characters, phylogenetic analysis suggests that it should be included in the genus. MNHNL TU885, a very partial skull, was assigned to *T.* cf. *zetlandicus* in the same paper from the Schistes Carton Formation.

### Validity

The name *Proteosaurus* was once used to house *Temnodontosaurus* species. Since, however, this name does not qualify as a nomen oblitum and *Temnodontosaurus* does not qualify as a nomen protectum, it has been brought up that *Temnodontosaurus* may be suppressed by the genus name *Proteosaurus*. Thus, it is currently unknown which name will be decided to keep.

## Description

*Temnodontosaurus* could dive to very deep depths and returned to the open ocean to hunt just off the coast of the land. *Temnodontosaurus*' largest rival those of *Shonisaurus* and could have easily eaten anything smaller than it was. *Temnodontosaurus* has a large body, with five fins and one broad tail fluke, like any other ichthyosaur. *Temnodontosaurus*' jaws are slim, and lined with barbed teeth. The head broadens near the atlas of the head. Bony rings in the orbit support the large eyes, and prevent them from exploding from water pressure. Its tail was as long as its body, or longer. Less than 90 vertebrae are known to exist. The axis and the atlas fuse to form a stabilization mechanism. *Temnodontosaurus trigonodon* bore unicipital ribs near its sacral region, and bicipital ribs anteriorly. This increased flexibility. *Temnodontosaurus* did not bear gastralia. Both the fore and hind limbs are nearly the same length, and are elongate. This is unlike other post-Triassic ichthyosaurs that bore hind and fore limbs that differed in size by half. *Temnodontosaurus*' pelvic girdle was not reduced, another feature unlike post-Triassic ichthyosaurs. *Temnodontosaurus* has 3 primary digits while other ichthyosaurs have 6-7 primary digits. However, like other ichthyosaurs, *Temnodontosaurus*' fins bear strong hyperphalangy. *Temnodontosaurus* also bore 1 postaxial digit. Fin material proximal to the body form a sort of mosaic pattern while distal material is rounded-off. Two notches end the anterior margin. The pelvic girdle is notably tripartite and the dorsal fin is triangular in shape. All four fins were used as stabilizing and steering devices, rather than a mode of propulsion or paddling through water.

Evidence suggests that *Temnodontosaurus* had a very strong visual capacity, using it as its main method of hunting. *Temnodontosaurus* had the largest eyes of any ichthyosaur, and of any animal. Despite its massive eyes, *Temnodontosaurus* had a blind spot above its eyes due to the angle of its eyes. The largest eyes measured belonged to *T. platyodon*. *Temnodontosaurus*' eyes bore sclerotic rings, which provided rigidity and structure for its eyes when descending into the deep depths of the ocean. T. platyodon's sclerotic rings are about 25 centimeters long in diameter. *Temnodontosaurus*' skull bears an elongated and robust snout, constricting at the antorbital region. The maxilla lengthens, as well as the cheeks and the postorbital segment. Located in the basisphenoid in the skull, the carotid foramen is paired and separates by the parasphenoid, which itself possesses a processus cultriformis. Temnodontosaurus platyodon's skull measures about 1-1.5 meters long. *T. eurycephalus* possesses a shorter snout and a deeper snout compared to other species, possibly to help it crush prey. Unlike any other species, *T. platyodon*'s snout slightly curves to its dorsal side. *Temnodontosaurus trigadon*'s snout is elongated and ventrally curving. *T. acutirotris*' snout is considerably slender and bears a pointed tip. Rather than having individual tooth sockets, the many pointed conical teeth of *Temnodontosaurus* set into continuous depressions. This feature of tooth implantation is termed aulacodonty. *Temnodontosaurus* teeth are characterized from 2-3 carinae, however, *T. nuertingensis* bore none. *T. eurycephalus*' teeth bore bulbous roots. *Temnodontosaurus*' tail bent weakly at an angle of 35°. The caudal fin is often described as either lunate, or semi-lunate. The caudal fin is supported by two lobes, the uppermost unsupported and the lowermost supported. *Temnodontosaurus*' tail was the main method of propulsion, and fins were not used as a propulsive device.

## Classification

*Temnodontosaurus* belongs to Temnodontosauridae, named by McGowan. It is the only genus in the family Temnodontosauridae. T. risor was found to be a juvenile of *T. platyodon*. To rule out invalid taxa, Laboury et al. (2022) find:

## Paleobiology

*Temnodontosaurus* eyes were approximately 20 cm (8 in) in diameter making them the largest of any known vertebrate. Despite the impressive size of their eyes, temnodontosaurus had blind spots directly above their head due to the angle at which their eyes were pointed. They were powerful swimmers with great stamina and could descend into the deepest areas in the Jurassic sea to catch their prey.

### Diet

*Temnodontosaurus* is an apex predator, likely consuming fish, plesiosaurs, ichthyosaurs and cephalopods. It is the only Jurassic ichthyosaur from a main diet of vertebrates have been proposed. A *T. trigonodon* shows remains of *Stenopterygius* in its abdomen. *T. eurycephalus* ate large prey, such as ichthyosaurs, due to robust teeth, with the modest-sized teeth in *T. platyodon* is adapted to soft prey and small vertebrates, such as fish. It likely also used ram feeding methods. Its jaw moved quick, and was probably adapted to snapping rather than chewing.

### Locomotion

Like other ichthyosaurs, it was a fast cruiser and swimmer. It, with other Jurassic ichthyosaurs, used lateral oscillation in their caudal fluke and flexible tailstock to swim. *T. trigonodon* had a very flexible, long, thin body with a high vertebrae count and modest regional differentiation. Its large limbs were utilized as rudders. It swam thunniform, unlike more basal forms, due to it's semi-lunate tail fins and proportionally shortened body compared with the tail.

## Paleoecology

*Temnodontosaurus* lived in the open ocean, away from the shore in a pelagic zone, not associating with the seafloor. *Temnodontosaurus* have been found in England, France and Germany in marine deposits. Specimens are especially found in the Lias of Lyme Regis of Dorset. Lias is made of alternating limestone and mudstone units, containing many ammonites. *Temnodontosaurus* are also known from the Posidonia Shale of Holzmaden, Germany, composed of black bituminous with intercalated bituminous limestone. This is known to have been marine, containing plesiosaurs, crocodilians and (abundant) ammonites.

## Appearance in other media

### *Jurassic Park*

*Temnodontosaurus* is an ichthyosaur that appears in *Jurassic World: The Game*. It is the third one, the first being the *Ophthalmosaurus* and the second being the *Platypterygius*. *Temnodontosaurus* was added to the game on September 23rd, 2022.

## References

1. ↑ Maisch MW, Matzke AT. 2000. The Ichthyosauria. *Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie)* **298**: 1-159