Temnodontosaurus Trigonodon

''Temnodontosaurus'' (Greek for "cutting-tooth lizard"temno, meaning "to cut", odont meaning "tooth" and sauros meaning "lizard") is an extinct genus of ichthyosaur from the Early Jurassic period. They lived between 200 and 175 million years ago ( Hettangian-Toarcian) in what is now Western Europe (England, France, Luxembourg, Germany and Belgium) and possibly Chile. It lived in the deeper areas of the open ocean.Motani R.(2000). “Rulers of the Jurassic seas”. Scientific American. 283 (6): 52-59 University of Bristol paleontologist Jeremy Martin described the genus ''Temnodontosaurus'' as "one of the most ecologically disparate genera of ichthyosaurs," although the number of valid ''Temnodontosaurus'' species has varied over the years.

''Temnodontosaurus'' was one of the largest ichthyosaurs, with the type species (''T. platyodon'') reaching up to in maximum body length. It is known for its incredibly large eyes which, at approximately in diameter, are believed to be the largest of any known animal. It possessed a tail bend that was characteristic of Jurassic ichthyosaurs and had many conical teeth filling its jaw that were set in a continuous groove.Sander,P.M.(2000). "Ichthyosauria: their diversity, distribution, and phylogeny", Paläontologische Zeitschrift 74: 1–35McGowan, C. (1992). Dinosaurs, Spitfires, and Sea Dragons. Harvard University Press

History of discovery

The first ichthyosaur skull ever discovered was that of ''Temnodontosaurus platyodon''. The specimen (BMNH 2149; now NHMUK PV R1158) was found in the Lias of Lyme Regis by Joseph Anning in 1811. The rest of the skeleton was recovered in 1812 by his sister, Mary Anning, but was later lost and never recovered. The ichthyosaur was subsequently described by the anatomist Sir Everard Home in what was the first scientific description of an ichthyosaur. ''T. platyodon'' is the most common species of ''Temnodontosaurus''. The type of skull is currently located at the Natural History Museum in London. The specimen was originally named ''Ichthyosaurus platyodon'' but then renamed ''Temnodontosaurus''. The genus ''Temnodontosaurus'' was named by Richard Lydekker in 1889.Davis, Larry E. (2009). "Mary Anning of Lyme Regis: 19th Century Pioneer in British Palaeontology". Headwaters: The Faculty Journal of the College of Saint Benedict and Saint John’s University 26: 96-126Pierce, P. (2006). ''Jurassic Mary: Mary Anning and the Primeval Monsters''. Sutton Publishing

The species ''Temnodontosaurus platyodon'' was named by William Conybeare in 1822 from the specimen BMNH 2003 from the Lyme Regis. The specimen is located at the British Museum of Natural History. ''T. platyodon'' is from the Upper Hettangian-Lower Sinemurian. It is the type species for ''Temnodontosaurus''.'' T. platyodon'' specimens have been found in England, Germany, Belgium and Luxembourg. This includes the Lyme Regis in Dorset England, Herlikofen in Germany, Arlon in Belgium and Cloche d'or in Luxembourg. Only one known complete skeleton of ''T. platyodon'' exists (BMNH 2003), and there is also a well-preserved skull (BMNH R1158).

In 1995, Christopher McGowan explained that the previously named ''T. risor'' species were really juvenile versions of ''T. platyodon''. The specimen he used to back up his argument was one collected by David Sole in 1987 from Black Ven (East of Lyme Regis). The previously known ''T. risor'' specimens (three skulls) were seen as different from the ''T. platyodon'' species because they had larger orbits, smaller maxillae and curved snouts. However, McGowan described them as juveniles because of the small size of the forefin relative to the skull. The ''T. risor'' skulls are thought to be juveniles because the skull is relatively long compared to the postcranial skeleton.

The species ''T. acutirostris'' was initially named by Richard Owen in 1840. This holotype (BMNH 14553) was from the Alum Shale Formation of Lower Toarcian in Whitby, Yorkshire, England. Michael Maisch, in 2000, described it as belonging in the genus ''Temnodontosaurus''. However, in 2010, Maisch published a paper stating that the specimen didn't belong in ''Temnodontosaurus'', as he had thought previously, and should probably be assigned to ''Ichthyosaurus'' instead. As of 2022, this species has been placed as species inquirenda.

''T. trigonodon'' was named by von Theodori in 1843. The type specimen for ''T. trigonodon'' is an almost complete skeleton from the Upper Liassic of Banz, Germany of the Lower Toarcian. The specimen is roughly long with a long skull. Other specimens have been found in Germany, as well as in France from the Lower Toarcian of Saint Colombe in Yonne. A ''T. trigonodon'' specimen from the Upper Toarcian, Aalen, Baden-Württemberg, Germany is at the Staatliches Museum für Naturkunde, Stuttgart in Germany.

The species '' T. eurycephalus'' has only one specimen: a holotype. The specimen (R 1157) is a skull and was named in 1974 by McGowan. It is from the Lower Sinemurian, Bucklandi Zone, and was found in Lyme Regis, Dorset, England, in a limestone bed called Broad Ledge. The ''T. eurycephalus'' specimen (R 1157) is currently located at the Natural History Museum in London.

The validity of the species ''T. burgundiae'' has been disputed. In 1995, McGowan proposed ''Leptopterygius burgundiae'' should be placed in ''Temnodontosaurus''.McGowan C. (1996). "Giant ichthyosaurs of the Early Jurassic". Canadian Journal of Earth Sciences 33(7): 1011-1021 The paleontologist Michael Maisch does not see ''T. burgundiae'' as belonging to ''Temnodontosaurus''. In 1998, Maisch identified this name as a junior synonym of ''T. trigonodon''. Martin Sander, in 2000, recognsized''T. burgundiae'' as a separate species, describing specimens from the Toarcian of the Holzmaden area of Germany and France as being ''Temnodontosaurus burgundiae''. However, this again has been met disagreement, for McGowan and Motani (2003) argued that all specimens of the Toarcian ''T. burgundiae'' belong to ''T. trigonodon'' and that although there are small osteological differences, they are not sufficent enough to keep this species valid.

The species ''T. azerguensis'' was described in 2012 by Jeremy Martin of the University of Bristol, based on a holotype that is almost a complete skeleton from the Bifrons ammonite zone of the Middle Toarcian. It was found in 1984 by M. Dejob and Ms. Laurent from the Lafarge Quarry in Belmont d’Azergues, Rhone, France. The specific name derives from the name of the river and valley near the Belmont quarry where it was found: 'Azergues'. It is currently located at the Musee des Amis de la Mine in Saint-Pierre La Palud, Rhone department, France. ''T. azerguensis'' is younger than other ichthyosaurs. It had a similar size and postcranial anatomy to other ''Temnodontosaurus'' species; however, its cranial morphology differed. The rostrum was more elongated and thin and had a reduced quadrate. Since ''T. azerguensis'' either had very small teeth or no teeth at all, it has been proposed that it was probably not effective at eating hard-shelled or bony prey and instead had a diet of smaller and softer prey compared to the other ''Temnodontosaurus'' species.

The holotype of ''T. crassimanus'' is on display at the Yorkshire Museum; however, it has remained understudied and the validity of the species has long been questioned. Swaby and Lomax (2020) highlighted several morphological features of the postcranial skeleton and determined that ''T. crassimanus'' is a valid species of the genus ''Temnodontosaurus'' and includes several distinct characters that can be used to distinguish it from ''T. trigonodon'', to which it was once assigned.

The largest temnodontosaurus known, the "Rutland Sea Dragon" about 10 metres long and 181 million years old, was discovered largely intact in 2021 in an English reservoir when water levels were lowered for winter maintenance.

Description

''Temnodontosaurus'' was a large ichthyosaur, with its tail as long as the body or longer. Initial length estimates for the specimen with a long skull (SMNS 50000) ranged between to even .McGowan, C. (1995). "Temnodontosaurus risor is a Juvenile of ''T. platyodon'' (Reptilia: Ichthyosauria)". Journal of Vertebrate Paleontology. 14 (4): 472–479 However, the 2015 study argued that the previous studies have overestimated the size of this specimen. Martin and his colleagues measured the preflexural length of different ichthyosaur species (including ''Temnodontosaurus'') which resulted in a preflexural length of for SMNS 50000 (assigned to ''T. trigonodon'' in the supplementary online material), indicating that the maximum body length of this specimen didn't exceed ; the other large European species, ''T. azerguensis'', had a preflexural length of , indicating that it would measure approximately in body length, and a ''T. platyodon'' with a long skull may have also reached .Supplementary Information
/ref> In 2022, Gregory S. Paul suggested new size estimates for ''T. platyodon'' at in length and in body mass, while ''T. trigonodon'' at and ; ''T. crassimanus'' and ''T. nuertingensis'' were estimated at in length and in body mass.

The forefins and hindfins of ''Temnodontosaurus'' were of roughly the same length and were rather narrow and elongated. This characteristic is unlike other post-Triassic ichthyosaurs such as the thunnosaurians, which had forefins at least twice the length of their hindfins. It was different from other post-Triassic ichthyosaurs like ''Ichthyosaurus'', possessing an unreduced, tripartite pelvic girdle and having only 3 primary digits with one postaxial accessory digit. Like other ichthyosaurs, the fins exhibited strong hyperphalangy, but the fins were not involved in body propulsion; only the tail was used as the main propulsive force for movement, although it had a weak tail bend at an angle of less than 35°. Its caudal fin has variously been described as either lunate or semi-lunate; it was made of two lobes, in which the lower lobe was skeletally supported whereas the upper lobe was unsupported. The proximal elements of the fin formed a mosaic pattern, while the more distal elements were relatively round. It also had a triangular dorsal fin and had 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. It had roughly less than 90 vertebrae, and the axis and atlas of the vertebrae were fused together, serving as stabilization during swimming. ''T. trigonodon'' possessed unicipital ribs near the sacral region and the bicipital ribs more anteriorly, which helped to increase flexibility while swimming.Emily A. Buchholtz (2000). ''Swimming styles in Jurassic Ichthyosaurs''. Journal of Vertebrate Paleontology, 21, 63-71

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.Motani R.(2005). ''Evolution of fish-shaped reptiles (Reptilia : Ichthyopterygia) in their physical environments and constraints.'' Annual Review of Earth and Planetary Sciences. 33: 395-420

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 long, while ''T. eurycephalus'' had a shorter rostrum and a deeper skull compared to other species, perhaps serving to help crush prey. ''T. platyodon'' and ''T. trigadon'' had a very long snout that was slightly curved on its dorsal side and ventrally curved, respectively. It also had many pointed conical teeth that were set in continuous grooves, rather than having individual sockets. This form of tooth implantation is known as aulacodonty. Its teeth typically had two or three carinae; notably, ''T. eurycephalus'' possessed bulbous roots, while ''T. nuertingensis'' had no canine or bulbous roots.

Palaeobiology

Feeding mechanisms and diet

''Temnodontosaurus'' was an apex predator in the Early Jurassic seas. Its diet likely consisted mainly of vertebrates such as fish, plesiosaurs and other ichthyosaurs. It may have also preyed on cephalopods. It is the only Jurassic ichthyosaur genus for which a mainly-vertebrates diet has been proposed. One ''T. trigonodon'' specimen (located at the Staatliches Museum für Naturkunde in Stuttgart) shows remains of '' Stenopterygius'', another ichthyosaur, in its abdominal cavity. Due to its more robust teeth and deeper jaw, ''T. eurycephalus'' probably ate large prey such as other ichthyosaurs, while species with pointed but more modest-sized teeth, such as ''T. platyodon'', perhaps preferred soft-bodied prey and smaller vertebrates such as fish. ''Temnodontosaurus'' likely utilised ram feeding methods of predation. The movements of its jaw were likely rapid and so it probably used snapping rather than chewing mechanisms to eat its prey.Thies, D. & Hauff, R.B. (2013). ''A Speiballen from the Lower Jurassic Posidonia Shale of South Germany”. – N. Jb. Geol. Paläont. Abh., 267: 117–124; StuttgartScheyer, Torsten M. et al. (2014). ''Early Triassic Marine Biotic Recovery: The Predators’ Perspective.'' PLoS ONE 9.3 (2014): e88987

Swimming and movement style

Like other ichthyosaurs, ''Temnodontosaurus'' was a fast cruiser or swimmer. Jurassic ichthyosaurs such as ''Temnodontosaurus'' swam via the lateral oscillation of their caudal fluke on a flexible tailstock. ''T. trigonodon'' had a highly flexible, long, thin body with a high vertebral count and modest regional differentiation. It used its large limbs as rudders. Its style of swimming was thunniform, unlike more basal ichthyosaurs whose swimming was anguilliform. This trait can be inferred in ''Temnodontosaurus'' and other Jurassic and post-Jurassic ichthyosaurs because of their semi-lunate tail fins and shortened bodies relative to the tail.

Classification

''Temnodontosaurus'' is the only genus in the family Temnodontosauridae.HIERARCHICAL TAXONOMY OF THE CLASS EODIAPSIDA
Retrieved January 16, 2009.McGowan, C. (1974). "A revision of the longipinnate ichthyosaurs of the Lower Jurassic of England, with descriptions of two new species (Reptilia, Ichthyosauria)". Life Sciences Contributions, Royal Ontario Museum 97: 1–37 The family Temnodontosauridae was described by C. McGowan and is from the Lower Liassic. Temnodontosauridae is part of the monophyletic group Neoichthyosauria, a clade named by Martin Sander in 2000 that includes the families Temnodontosauridae, Leptonectidae and Suevoleviathanidae. ''Temnodontosaurus'' is one of the most basal post-Triassic ichthyosaurs.

The cladogram below is based on Maisch and Matzke (2000) and Maisch and Matzke (2003) with clade names following Maisch (2010):

The 2022 study which revived ''T. zetlandicus'' created a new majority rule consensus cladogram based on "non-clock Bayesian analysis," which found ''T. azerguensis'' to be too distant in relation compared to other species of ''Temnodontosaurus''. The study also removed ''T. acutirostris'' from the genus, placing it as species inquirenda.

Palaeoecology

The habitat of ''Temnodontosaurus'' was the open ocean, away from the shoreline. It lived in the pelagic zone of the water column and didn't associate with the seafloor.

Fossils of ''Temnodontosaurus'' have been found in England, Germany and France from rocks associated with marine environments. Specimens have been found especially in the Lias of the Lyme Regis in Dorset, England. The Lias is made up of alternating units of limestone and mudstone and contains many ammonites.“The lithostratigraphy of the Blue Lias Formation (Late Rhaetian–Early Sinemurian) in the southern part of the English Midlands”. Proceedings of the Geologists' Association. 112(2): 97-110 The newly described species ''T. azerguensis'' was found in a belemnite rich marlstone bed in the Bifrons ammonites zone, Middle Toarcian, in Belmont d’Azergues, Rhone, France.

''Temnodontosaurus'' fossils have been found in the Posidonia Shale near Holzmaden, Germany. The Posidonia Shale is composed of black bituminous shales with intercalated bituminous limestone. The environment is known to have been marine because fossils of marine animals such as plesiosaurs, crocodylians and especially ammonites have been found there in abundance.Bottjer, Etter, Hagadorn, Tang, editors (2001). “Exceptional Fossil Preservation”. Columbia University Press

See also

* List of ichthyosaurs
* Timeline of ichthyosaur research

References

{{Taxonbar, from1=Q853721, from2=Q41220358, from3=Q20721625, from4=Q28952974, from5=Q105710721, from6=Q11145135

Early Jurassic ichthyosaurs
Ichthyosaurs of Europe
Ichthyosauromorph genera