Taniwhasaurus is an extinct genus of mosasaurs (a group of extinct marine lizards) that lived during the Campanian stage of the Late Cretaceous. It is a member of the subfamily Tylosaurinae, a lineage of mosasaurs characterized by a long toothless conical rostrum. Two valid species are attached to the genus, T. oweni and T. antarcticus, known respectively from the fossil record of present-day New Zealand and Antarctica. T. capensis from present-day South Africa represents a chimera of two different mosasaur genera, potentially Prognathodon and Taniwhasaurus, but not identifiable at the species level. The other formerly assigned species, T. mikasaensis from present-day Japan, remains problematic due to the fragmentary state of the attributed fossils. The generic name literally means "taniwha lizard", referring to a supernatural aquatic creature from Māori mythology.
Taniwhasaurus is a medium-sized mosasaurid, with maximum size estimates putting it at around in length. The rare fossils of the axial skeleton indicate that the animal would have had great mobility in the vertebral column, but the tail would generate the main propulsive movement, a method of swimming proposed for other mosasaurids. The constitution of the forelimb of Taniwhasaurus indicates that it would have had powerful paddles for swimming. CT scans performed on the snout foramina of T. antarcticus show that Taniwhasaurus, like various aquatic predators today, would likely have had an electro-sensitive organ capable of detecting the movements of prey underwater.
The fossil record shows that both officially recognized species of Taniwhasaurus were endemic to the seas of the ancient supercontinent Gondwana, nevertheless living in different types of bodies of waterbodies. Indeterminate specimens including the holotype of T. mikasaensis have also been found in Eurasia, but they are not diagnostic to the species level. The concerned geological formations shows that the genus shared its habitat with invertebrates, bony fishes, cartilaginous fishes, and other marine reptiles, including plesiosaurs and other mosasaurs.
Research history Recognized species T. oweni
The first known species, Taniwhasaurus oweni, was discovered in the 1860s in the cliffs of Haumuri Bluff, located in the Conway Formation, eastern New Zealand. This formation is dated from the Upper Cretaceous, more precisely from the lower and middle Campanian stage. The first fossils formally attributed to this taxon were described by the Scottish naturalist James Hector in 1874. The skeletal material of T. oweni consisted of a skull, vertebrae and paddles, divided into three distinct sections. In 1888, noting that the fossils are incomplete, Richard Lydekker uncertainly placed T. oweni within the genus Platecarpus, being renamed Platecarpus oweni. In 1897, in his revision of the distribution of mosasaurs, Samuel Wendell Williston put Taniwhasaurus back as a separate genus, but considered it to still be close to Platecarpus. As Hector did not designate a holotype fossil for this taxona, Samuel Paul Welles and D. R. Gregg designate specimen NMNZ R1536, a fragmented skull consisting of frontal and parietal bone accompanied by partial dentary bone, as the lectotype of T. oweni in 1971. The genus name Taniwhasaurus is made up of the Māori word Taniwha, and the Ancient Greek word σαῦρος (saûros, "lizard"), all literally meaning "lizard of Taniwha", in reference to a supernatural aquatic creature from Māori mythology. The specific epithet oweni is named in honor of the famous English paleontologist Richard Owen, who was the first to describe the Mesozoic marine reptiles of New Zealand.
In his article, Hector describes several skeletal remains which he attributes to another mosasaur, which he names Leiodon haumuriensis. In 1897, Williston suggested to transfer this taxon within the genus Tylosaurus, a proposal that was carried out in 1971, being renamed Tylosaurus haumuriensis. Welles and Gregg also referred to specimen NMNZ R1532 as the lectotype of Tylosaurus haumuriensis in the article. Although most of these remains have been lost since the 1890s, it's in 1999 that new cranial and postcranial material was discovered in the cliffs of Haumuri Bluff and that these findings were formalized by Michael W. Caldwell and his colleagues in 2005. Based on extensive analyzes of these fossils, researchers found that there are in fact few morphological differences between the two mosasaur taxa from this locality, the differences being mainly due to the larger size of specimen NMNZ R1532, making Tylosaurus haumuriensis a junior synonym of T. oweni.
T. antarcticus
In January 2000, paleontologist Juan M. Lirio discovered a remarkably well-preserved specimen of a mosasaur in the Gamma Member of the Snow Hill Island Formation, located on James Ross Island in Antarctica. This geological member was originally misidentified as belonging to the neighboring Santa Marta Formation. The Gamma Member of the Snow Hill Island Formation is dated in the late Campanian to late Maastrichtian stages of the Upper Cretaceous. This discovery concerns a tylosaurine specimen which heve been discovered in the Upper Campanian fossil record, cataloged IAA 2000-JR-FSM-1, containing a skull measuring long, teeth, some vertebrae and rib fragments. Unlike the majority of other Antarctic mosasaurs, which are primarily known from teeth and postcranial remains, the skull of this specimen is almost complete and articulated. After analyzing the material, Fernando E. Novas and his colleagues named it Lakumasaurus antarcticus. The genus name Lakumasaurus comes from the Lakuma, a sea spirit from the mythology of the Yahgan people, and from the Ancient Greek term σαῦρος (saûros, "lizard"), to literally give "lizard of Lakuma". The specific epithet antarcticus refers to Antarctica, where the animal lived.
From 2006, James E. Martin questioned the validity of Lakumasaurus as a separate genus, noting that the cranial features are small enough to justify such a proposal. However, he state that there are enough differences to classify Lakumasaurus antarcticus as the second species in the genus Taniwhasaurus, being renamed T. antarcticus, a proposal that he would confirm the following year with his colleague Marta Fernández. The same year, Martin and his colleagues announced the discovery of a juvenile skull considered to belong to the same species and dating from the Maastrichtian, but subsequent studies are skeptical of this claim. Less than two years later, in 2009, the same authors published an article that described in more detail the fossil material and the phylogenetic relationships between the species T. antarcticus and T. oweni, a relationship that happens to be still recognized today.
Formerly assigned species T. capensis At the beginning of the 20th century, several fossils began to be collected in the region of Pondoland, in South Africa. These fossils turn out to belong to squamates and sea turtles dating from the Santonian stage of the Upper Cretaceous. In 1901, one of the sets of fossils discovered (catalogued as SAM-PK-5265), being a few fragmentary pieces of a jawbone, was referred as belonging to a reptile considered close to the genus Mosasaurus. This collection of fossils was later given to the Scottish paleontologist Robert Broom, who published in 1912 an article describing the same bones, along with a vertebra attributed to this specimen. He concludes that the fossils would belong to a large South African representative of the genus Tylosaurus, naming it Tylosaurus capensis.
Throughout the remainder of the 20th century, Tylosaurus capensis was generally viewed as a valid species within the genus, being identified primarily by the size of the parietal foramen and the suture between the frontal and parietal bones. However, both characteristics are highly variable within the genus Tylosaurus and are not considered diagnostic at the species level. In 2016, Paulina Jimenez-Huidobro published a thesis which analyzes the deep relationships between the various tylosaurines. Based on observations of the specimen SAM-PK-5265, she proposes moving this species to Taniwhasaurus, claiming that the characteristics found there are closer to this latter than to Tylosaurus. In 2019, Jimenez-Huidobro and Caldwell reaffirm this proposition, but found that the fossils were too poorly preserved to identify definitively to the genus. In 2022, an anatomical review of South African mosasaurs analyzed SAM-PK-5265 and found the specimen to be a chimera of two different mosasaur genera, but not identifiable at the species level: the frontoparietal complex and the posterior dentary fragment without teeth represent cf. Prognathodon, while the anterior dentary fragment with replacement teeth represents cf. Taniwhasaurus.
T. mikasaensis In June 1976, a large front part of a mosasaur skull was discovered on a bank of the Ikushumbetsu River in Hokkaidō, Japan. This specimen was found in a floating concretion, and its formation of origin was identified with the Kashima Formation, in the Yezo Group, the locality being the exposed area of this same place. Like the previously mentioned sites, the formation from which the animal was found is dated to the Santonian-Campanian stage. The specimen, cataloged MCM.M0009, was informally named "Yezosaurus mikasaensis" in a press release issued by Kikuwo Muramoto and Ikuwo Obata on November 30, 1976, before being erroneously classified as a tyrannosauroid dinosaur in an article published by Muramoto in December of the same year. The genus name "Yezosaurus" comes from Yezo, the group containing the Kashima Formation from which the taxon was discovered, and from the Ancient Greek σαῦρος (saûros, "lizard"), all literally meaning "Yezo lizard". The specific epithet "mikasaensis" is named after the city of Mikasa, a place near the site of discovery. In the same year before the informal description, the Japanese Ministry of Education decided to consider the fossil as the country's national treasure. The specimen would later be known as .
In 2008, Caldwell and colleagues formally redescribed the specimen as a new species of Taniwhasaurus, T. mikasaensis. In the 2016 thesis by Jimenez-Huidobro, three sets of fossils discovered in the original locality were listed and attributed to this proposed species. These consist of additional cranial parts (MCM.A600), two dorsal vertebrae (MCM.M10) and caudal vertebrae elements associated with an isolated dorsal vertebra (MCM.A1008). In 2019, the phylogenetic revision of tylosaurines conducted by Jimenez-Huidobro and Calwell still considers the specimen to be a representative of the genus Taniwhasaurus, but the authors argued that the fossils are insufficient for a diagnosis to classify it as a distinct species, so they reclassified it as an indeterminate species of Taniwhasaurus. In 2020, 3D scans were performed on replicas of the specimen, with the real fossil requiring special permission from the Japanese Ministry of Education.
Description Size Although fossils of Taniwhasaurus are incomplete, existing remains suggest the genus was a medium-sized mosasaur shorter than its relative Tylosaurus. The largest known specimen of T. oweni is the partial skull specimen SMU NMNZ R1532, which was estimated to have measured long, resulting in a total body length estimate of .Table S1 T. antarcticus represents a smaller species, with the holotype having a skull length of .
Skull