Tylosaurus (; "knob lizard") is a genus of russellosaurine mosasaur (an extinct group of predatory marine lizards) that lived about 92 to 66 million years ago during the Turonian to Maastrichtian stages of the Late Cretaceous. The genus contains at least ten validly named species, the largest of which, T. rex, measuring over . Its fossils have been found primarily around North Atlantic Ocean including in North America, Europe, and Africa.
The first named species, T. proriger, was originally described in 1869 by Edward Drinker Cope as a member of a separate mosasaur genus, Macrosaurus which is now synonymous with Halisaurus, and subsequently redescribed as the type species of the genus Tylosaurus named in 1872 by Othniel Charles Marsh. Fossil evidence shows that Tylosaurus regularly engaged in intraspecific combat and consumed various prey as an apex predator of their ecosystem, including predatory fish, ornithuran birds, sea turtles, plesiosaurs and other mosasaurs. It had an endothermic metabolism, which is unlike nearly all modern squamates.
Research history
Possible first finds The earliest Tylosaurus fossils were likely discovered by various Native American peoples and may have been the source of much of their folklore, with the earliest known ones dating back to well before the arrival of European settlers, around the 1500s. More recent accounts from peoples living in the Great Plains even speak of an ancient era ruled by massive aquatic creatures that were in constant combat with thunderbirds and were petrified by them. The considerable presence of fossils of large mosasaurs such as Tylosaurus and pterosaurs such as Pteranodon in this region may have been the origins of these myths.
In 1804, the Lewis and Clark Expedition discovered a now-lost fossil skeleton alongside the Missouri River, which was identified as a long fish. In 2003, Richard Ellis speculated that the remains may have belonged to Mosasaurus missouriensis. Alternatively, a 2007 study led by Robert W. Meredith and colleagues suggested that the fossils would possibly come from a tylosaurine mosasaur based on the measurements cited by Clark and Gass and the evidence of Tylosaurus fossils that have been found in the Missouri River. However, the authors also mentioned the possibility that the remains would also come from an elasmosaurid plesiosaur, which are also known from the river, although being rarer.
First formal discoveries
Tylosaurus was the third new genus of mosasaur to be described from North America behind Clidastes and Platecarpus and the first in Kansas. The early history of the genus as a taxon was subject to complications spurred by the infamous rivalry between Edward Drinker Cope and Othniel Charles Marsh during the Bone Wars. The type specimen was discovered in the late 1860s by U.S. Army colonel John Butler Conyngham and a certain Mr. Minor in a deposit of the Niobrara Formation located near Monument Rocks, close to the Union Pacific Railroad line at Fort Hays, Kansas. The discovery consisted of a partial skull measuring nearly in length associated with thirteen vertebrae. Acquired by Louis Agassiz during his visit to the Great Plains in August 1868, the specimen has since been catalogued as MCZ 4374. After noticing photographs of it at a meeting of the Academy of Natural Sciences of Philadelphia, Cope borrowed the specimen from Agassiz and presented a preliminary description of it in 1869. His first publication of the fossil was very brief and was named Macrosaurus proriger, the genus being a preexisting European mosasaur taxon. The specific epithet proriger means "prow-bearing", which is in reference to the specimen's unique prow-like elongated rostrum and is derived from the Latin word prōra (prow) and suffix -gero (I bear). In 1870, Cope published a more thorough description of the type specimen. Without explanation, he moved the species into another European genus Liodon and declared his original M. proriger a synonym.
In 1872, Marsh argued that L. proriger is taxonomically distinct from the European genus and must be assigned a new one. For this, he erected the genus Rhinosaurus, which means "nose lizard" and is a portmanteau derived from the Ancient Greek words (, meaning "nose") and (, meaning "lizard"). Cope responded by arguing that Rhinosaurus was already a preoccupied synonym of Liodon. He disagreed with Marsh's arguments but proposed that in case Marsh was indeed correct, the genus name Rhamphosaurus should be used. Marsh later discovered that the taxon Rhamphosaurus was preoccupied as a genus of lizard named in 1843. As a result, he suggested a move to a newly erected genus named Tylosaurus. This name means "knob lizard" in another reference to the elongated rostrum characteristic of the genus. It is derived from the Latin tylos (knob) and Ancient Greek . Despite coining the new genus, Marsh never formally transferred this Rhinosaurus species to Tylosaurus; this was first done in 1873 by Joseph Leidy. Tylosaurus subsequently became the almost universally accepted genus to include this species, the exception to this adoption being Cope, who refused to accept Marsh's new genus and continued to refer the species as Liodon. Cope's persistence can be seen in his 1874 description of another species which he named L. nepaeolicus. The type specimen of this species was discovered by Benjamin Franklin Mudge near the Solomon River, and consists of several cranial fragments and a dorsal vertebra now catalogued as AMNH 1565. This species, whose specific epithet refers to Nepaholla, the Native American name for the Salomon River is formally transferred to the genus Tylosaurus in 1894 by John Campbell Merriam. In 2026, Amelia R. Zietlow, Michael J. Everhart, and Michael J. Polcyn redescribed the holotype specimens of T. proriger and T. nepaeolicus, 150 years after their original descriptions by Cope, revising the diagnoses of both species.
Later discoveries and other species
In his major work published in 1967, Dale A. Russell recognized only two valid species in Tylosaurus, namely T. proriger and T. nepaeolicus. However, throughout the 19th and 20th centuries, many species of mosasaurs coming from around the world, originally described as being from separate genera, were now recognized as belonging to Tylosaurus.
In 1885, Louis Dollo described the genus and species Hainosaurus bernardi from an almost complete but poorly preserved skeleton discovered in a phosphate quarry in the Ciply Basin of Mesvin, Belgium, since numbered as IRSNB R23. The genus name Hainosaurus means "lizard from the Haine", in reference to the eponymous river located nearby the Ciply Basin. The specific epithet bernardi is named in honor of Leopold Bernard, who made the excavation and exhumation of the specimen possible. In 1988, a second species historically pertained to Hainosaurus was described by Elizabeth Nicholls based on a partial skeleton catalogued as MT 2 and having been discovered in Manitoba, Canada. The specific epithet refers to the type locality of the taxon, namely the Pembina Member of the Pierre Shale. The attribution of H. pembinensis to Hainosaurus is first discussed by Johan Lindgren in 2005, but it was in a revision published in 2010 that the species was moved to Tylosaurus by Timon Bullard and Michael Caldwell, being then renamed as T. pembinensis. In this same revision, the authors suggested that a redescription of the type species H. bernardi would be necessary in order to know if Hainosaurus should be maintained as a distinct genus. This redescription was finally carried out by Paulina Jimenez-Huidobro and Caldwell in 2016, in which they transferred the species to Tylosaurus, being then renamed as T. bernardi. Although this new combination has been widely recognized since, some authors nevertheless suggest continuing to maintain the genus Hainosaurus as distinct, justified in particular on the basis of dental traits not detailed in the 2016 revision.
In 1896, Armand Thevenin described a new mosasaurid on the basis of a partial skull discovered at Éclusier-Vaux, in Somme, France. In his description, Thevenin thinks that this specimen, since catalogued as MNHN 1896–15, would represent a new species of Mosasaurus, naming it Mosasaurus gaudryi. The specific epithet is named in honor of his mentor Jean Albert Gaudry, the latter having previously studied the skull and thinking that it would come from a species of Liodon. In 1992, Theagarten Lingham-Soliar reassigned the species to Hainosaurus, having previously interpreted the holotype as an additional specimen of H. bernardi. In 2005, Lindgren moved this species to Tylosaurus, notably due to its dental characteristics being closer to other members of the genus.
In 1963, Per Ove Persson identified a new mosasaurid on the basis of isolated teeth discovered in a deposit located in Ivö Klack, near Ivö Lake in the Kristianstad Basin in Scania, Sweden. Fossils from this same taxon have been documented in this area since 1836, but it is from that year onwards that they are described as coming from a subspecies of Mosasaurus hoffmannii, named M. hoffmannii ivoensis in reference to Ivö Klack. In 1967, Russell elevated the taxon to a separate species within the genus, and assigned to it fossils from the Niobrara Formation of Kansas, including a partial skull. When the taxon was significantly revised in a in 2002 study, being reassigned to Tylosaurus, Lindgren and Mikael Siverson referred additional fossils to this latter that had been discovered at Ivö Klack, including cranial and vertebral remains. In their study, the authors also found that Russell's attributions of the Kansas fossils to this species were erroneous, the remains coming from a distinct taxon. The study furthermore noted similarities between these teeth and those of Taniwhasaurus, but considered such an attribution unlikely because of the limited material known for this related genus and its geographic distance (Taniwhasaurus then being known exclusively from New Zealand). However, in 2008, Caldwell and colleagues reported the presence of Taniwhasaurus in northern Japan and reaffirmed the dental similarities with T. ivoensis. This discovery revived the hypothesis of a possible reassignment of the species to Taniwhasaurus, but the authors see this as a subject for another study.
In 1964, Miguel Telles Antunes described the species Mosasaurus iembeensis from a partial skull excavated from the Itombe Formation near the town of Iembe (hence the name), Angola. In 1992, Lingham-Soliar argued that the cranial features were not consistent with those of Mosasaurus and were more characteristic of Tylosaurus, the species being renamed as T. iembeensis. However, the author did not identify the holotype skull, which he considered to reside in the collections of the NOVA University Lisbon without a catalogue number, and it is since 2006 reported as being destroyed in a fire. In 2012, Octávio Mateus and colleagues reported that an additional specimen of T. iembeensis consisting of fragmentary cranial elements was recovered during an expedition to the locality of the since-destroyed holotype, although it was not figured or formally described.