Triassic Ichthyosaurs Research Articles

The skull of a large Lower Triassic ichthyosaur from Spitzbergen and its implications for the origin of the Ichthyosauria

The skull of a large Lower Triassic ichthyosaur from Spitzbergen and its implications for the origin of the Ichthyosauria

The skull of a large Lower Triassic ichthyosaur from Spitzbergen and its implications for the origin of the Ichthyosauria

An incomplete ichthyosaur skull from the Lower Triassic (Spathian) Sticky Keep Formation of Spitzbergen is one of the earliest ichthyosaurs known and by far the largest of the most basal ichthyosaurs. The specimen shows an extremely plesiomorphic cheek region of the skull and allows for the first time a confident reconstruction of the grundplan condition of the lower cheek region of ichthyosaurs. It unequivocally demonstrates the presence of a deep emargination of the lower cheek, which is bordered dorsally by the quadratojugal, squamosal and jugal, and therefore a plesiomorphic lack of contact between jugal and quadratojugal. The specimen also allows an evaluation of recent skull reconstructions of several Lower Triassic taxa which probably contain severe misinterpretations. The structure of the temporal and cheek region of the specimen can be most parsimoniously explained by the initial presence of a lower temporal fenestra. This provides important new evidence that the ancestry of ichthyosaurs might have lain within the Diapsida or their stem-group.

The Cranial Osteology Of The Middle Triassic Ichthyosaur Contectopalatus From Germany

A new partial skull of the Middle Triassic ichthyosaur Contectopalatus atavus allows many new osteological observations and makes the referral of several additional specimens, including lower jaw material, possible. This aids very much in the understanding of the anatomy of this highly derived mixosaurid species. The lectotype of Ichthyosaurus atavus is shown to be diagnostic and the recent proposal of a neotype is unnecessary. Contectopalatus shows thecodont dentition in all parts of the jaws. The teeth are labyrinthodont at their bases. Some specimens, which are interpreted as very late ontogenetic stages of C. atavus, indicate that Contectopalatus grew very large, reaching four to five times the size of other mixosaurids. Mixosaurid taxonomy is reviewed. Three valid genera can be included in a monophyletic Mixosauridae: Mixosaurus (including M. cornalianus and M. kuhnschnyderi), PhalarodonContectopalatus (monotypic). The mixosaurids are the sister group to the remaining ichthyosaurs, apart from the Lower Triassic forms. The status of the Omphalosauridae remains unresolved, but they are not closely related to the mixosaurids, because the durophagous dentition was acquired convergently in the two groups.

Observations on Triassic ichthyosaurs. Part VI. On the cranial osteology of Shastasaurus alexandrae MERRIAM, 1902 from the Hosselkus Limestone (Carnian, Late Triassic) of Northern California with a revision of the genus

Observations on Triassic ichthyosaurs. Part VI. On the cranial osteology of Shastasaurus alexandrae MERRIAM, 1902 from the Hosselkus Limestone (Carnian, Late Triassic) of Northern California with a revision of the genus

The mixosaurid ichthyosaur Contectopalatus from the Middle Triassic of the German Basin

The skull of the mixosaurid species Contectopalatus atavus (Quenstedt, 1851–52) is the most bizarre of any known ichthyosaur. It possesses a very high sagittal crest formed by the nasal, frontal and parietal bones which grows higher during ontogeny. This skull structure - found to a lesser extent in the other mixosaurid genera Mixosaurus and Phalarodon - is a synapomorphy of the family Mixosauridae. It is here interpreted as correlated with a unique arrangement of the jaw adductor musculature among tetrapods, with the internal jaw adductors extending over most of the skull roof up to the external narial opening. This reconstruction would increase the biting force considerably and the hypothesis is supported by peculiarities of the dentition and jaws of Contectopalatus. Contectopalatus probably reached a length of about 5 meters. It is therefore the largest known mixosaurid and one of the largest Triassic ichthyosaurs. The general text-book picture of mixosaurs as small, rather unspecialized, primitive ichthyosaurs is incorrect. Mixosaurs were a highly specialized, uniquely adapted and very diverse ichthyosaur family, some members of which rank among the marine top predators of their time.

Taxonomy and limb ontogeny of Chaohusaurus geishanensis (Ichthyosauria), with a note on the allometric equation

ABSTRACT Three of the six earliest ichthyosaurs, namely, Chaohusaurus geishanensis, Chensaurus chaoxianensis, and Chensaurus faciles, occur in the Lower Triassic (Spathian) of the Chaohu area, Anhui Province, China. A reexamination of the three holotypes and two referred specimens indicates that the three species share derived features; however, the characters originally used to distinguish among the three are either growth-related or absent. Measurements of the specimens suggest that they represent a growth series of a single species, Chaohusaurus geishanensis Young and Dong 1972. The forefin of this species shows a strongly positive allometry, leading to the unusually large forefin in the largest specimen (the holotype). The use of the standard allometric equation may be problematic when two structures compared start their developments asynchronically. However, the alternative equation of Schmalhausen is also problematic, and therefore more scrutiny is required. The lunate fin elements, which commonly occur in the first and fifth digits of Early Triassic ichthyosaurs, first become ossified as biconcave elements, as in other metacarpals and phalanges.

New finds ofOmphalosaurus and a review of Triassic ichthyosaur paleobiogeography

Triassic ichthyosaurs are very widespread and diverse but most taxa are poorly known. New discoveries, such as described in this paper, underscore the first statement but only slowy invalidate the second.

The forefin of Chensaurus chaoxianensis (Ichthyosauria) shows delayed mesopodial ossification

The hitherto poorly known forefin of Chensaurus chaoxianensis (Ichthyosauria) from the Lower Triassic (Spathian) is redescribed on the basis of the holotype and two new specimens. The humerus resembles that of Utatsusaurus hataii but is distinctive in having an emarginated anterior margin. The anteroproximal prominence of the radius is well developed, unlike that of other ichthyosaurs. All three specimens have five metacarpals and many phalanges, but only three carpals, which are identified as the ulnare, intermedium, and fourth distal carpal. These specimens show that delayed mesopodial ossification occurred in ichthyosaurs, at least in an early evolutionary stage. Because delayed mesopodial ossification is common among diapsids and is unknown in Jurassic ichthyosaurs, it was lost during the evolution of the Ichthyosauria. The osteogenic developmental axis appears to have continued into the fourth digit, as in other amniotes. The ossification pattern provides conclusive evidence to support the suggestion that the basal element of the fifth digit in Early Triassic ichthyosaurs is a metacarpal, rather than a carpal.

Modified perichondral ossification and the evolution of paddle-like limbs in ichthyosaurs and plesiosaurs

ABSTRACT Evolution of paddle-like limbs in ichthyosaurs and plesiosaurs is correlated with loss of perichondral bone from the shafts of long bones. Among ichthyosaurs, loss of perichondral bone is first observed on the shafts of digit bones of Early Triassic taxa. Late Triassic ichthyosaurs show perichondral bone loss on the postaxial margins of the ulna and fibula. Among plesiosaurs, loss of perichondral bone is first observed on the postaxial margins of the ulna and fibula of Lower Jurassic taxa. In geologically later species of both groups, perichondral bone is progressively lost on all margins of the ulna and radius, and fibula and tibia. Late Triassic and Jurassic ichthyosaurs show an absence of perichondral ossifications on all limb bones distal to the humerus and femur. Delayed ossification of the mesopodium is not observed in ichthyosaurs. Evolutionary changes to the ossification of perichondral tissues appear to affect the sequence of limb ossification as long bones lose perichondral bone. Limb bones of ichthyosaurs and plesiosaurs resemble mesopodial elements through loss of perichondral bone. The proportion of endochondral bone found on these transformed bones is increased, resulting in an increased articular surface area and increased articular complexity.

Mikadocephalus gracilirostris n. gen., n. sp., a new ichthyosaur from the Grenzbitumenzone (Anisian-Ladinian) of Monte San Giorgio (Switzerland)

An almost complete skull of a new ichthyosaur from the Middle Triassic Grenzbitumenzone Beds of Monte San Giorgio (Kanton Tessin, Switzerland) represents one of the most complete and best preserved finds of a large Triassic ichthyosaur cranium. Its affinites with other Triassic ichthyosaur taxa are discussed and it is demonstrated to represent a new genus and species,Mikadocephalus gracilirostris, which does not fit into any of the currently recognized families of Triassic ichthyosaurs. Remarkable similarities in cranial structure exit to postTriassic ichthyosaurs, with which a number of important apomorphies are shared.

New information on the forefin of Utatsusaurus hataii (Ichthyosauria)

Utatsusaurus hataii, an Early Triassic ichthyosaur from Japan, is represented by exceptionally well preserved materials but is poorly known because of incomplete preparation. Further preparation of the holotype and one of the paratype specimens has revealed nearly complete forefins that are pentadactyl, with no more than five phalanges in any of the digits. The first digit is not well developed, and the fifth distal carpal appears to be absent. The phalanges along the fin margin tend to be lunate; others are flattened cylinders. These findings indicate that the relationship of Utatsusaurus with other Early Triassic ichthyosaurs must be reconsidered.

Redescription of the dental features of an Early Triassic ichthyosaur,Utatsusaurus hataii

ABSTRACT Utatsusaurus hataii is represented by exceptionally well preserved material for an Early Triassic ichthyosaur, but the preparation was incomplete at the time of the original description. Preparation of the holotype reveals new information concerning dental features. A principal diagnostic character of U. hataii is its isodont dentition, originally described as comprising very slender, needle-sharp teeth set in distinct alveoli. That description was based largely on an equivocal radiograph of a poorly prepared referred specimen, whose specific assignation to U. hataii is not well established. The teeth of the holotype are shown to be robust, with blunt points, and are set in a dental groove, not as previously described.

A new and typically Jurassic ichthyosaur from the Upper Triassic of British Columbia

A new ichthyosaur species is described from the Upper Triassic (middle Norian) of Williston Lake, northeastern British Columbia. Aside from the foramen enclosed between the radius and ulna–a characteristic of Triassic ichthyosaurs–the new specimen is typical of Lower Jurassic forms. Indeed, if the specimen had been collected from the English lower Liassic, there would have been no hesitation in referring it to the common English genus Ichthyosaurus, a taxonomic course I follow here. Referring the new species to Ichthyosaurus extends the geological range of the genus by approximately 9 Ma, to the middle Norian. Ichthyosaurus janiceps sp. nov. has an abbreviated snout, like that of Ichthyosaurus breviceps, but it is a much larger species, and has a distinctly different forefin.

A remarkable small ichthyosaur from the Upper Triassic of British Columbia, representing a new genus and species

A small, nearly complete ichthyosaur skeleton is described from the Upper Triassic of Williston Lake, in northeastern British Columbia. The age of the material, based on conodonts, is early Norian. Although the length of the entire skeleton would probably not have exceeded 1 m, there is no evidence of immaturity–quite the contrary. Named Hudsonelpidia brevirostris, the new taxon shares some features with Triassic taxa, as exemplified by Mixosaurus from the European Middle Triassic, and with post-Triassic ichthyosaurs like Ichthyosaurus, from the English Lower Jurassic. Mixosaurian characters include an elongate tibia with emarginated pre- and postaxial margins, and phalanges in the hindfin with pre- and postaxial notches. Like Ichthyosaurus, the humérus is elongate rather than broad, so too is the pubis and ischium. Mixosaurus is unusual among Triassic ichthyosaurs for having a relatively large orbit, but the orbit is even more prominent in Hudsonelpidia, probably because of the shortness of the snout. Hudsonelpidia has an unusually large femur that approaches the length of the humérus, dwarfing the rest of the hindfin. The rostrum is unique in being perforated by foramina, but this could possibly be an abnormality.

New marine vertebrate fauna from the Middle Triassic of Nevada

The marine Triassic rocks of Nevada are well known for their ichthyosaur finds, and a number of new genera and species were described from this state. Based on a few vertebrae, Leidy (1868) erected a new Middle Triassic ichthyosaur genus,Cymbospondylys, with two species,C. petrinusandC. piscosus.The type material ofC. petrinuscame from the Prida Formation of the southern Humboldt Range, and that ofC. piscosusfrom the Favret Formation of the New Pass Range.

Cymbospondylus(Shastasauridae: Ichthyosauria) from the Middle Triassic of Spitsbergen: filling a paleobiogeographic gap

The occurrence of the large ichthyosaurCymbospondylusin the Middle Triassic of Spitsbergen is recognized based on two different specimens. One consists of a semiarticulated string of 17 dorsal vertebrae with associated ribs and the other of an anterior dorsal vertebral centrum and possibly a caudal vertebral centrum. Both specimens can only be identified asCymbospondylussp. This record fills a gap in the distribution ofCymbospondylus, establishing this genus as the second most widespread Triassic ichthyosaur afterMixosaurus.

The oldest South American ichthyosaur from the late Triassic of northern Chile

AbstractIchthyosaur remains found in late Triassic shallow marine limestones in Quebrada Doña Inés Chica (latitude 26° 07′ S; longitude 69° 20′ W), northern Chile, are the oldest known ichthyosaur fossils from Central and South America. The previous oldest South American ichthyosaur was recorded in Hettangian rocks, also from northern Chile. The fossils comprise five worn teeth, one paddle bone and one scapula or humerus fragment, all collected from a single bed at the same locality. Although the material has not been identified to a generic or specific level, its presence alone broadens the knowledge of the distribution and habitat of Triassic ichthyosaurs. Late Triassic ammonites and brachiopods in the same stratum provide the age control.

The affinities and ecology of Triassic ichthyosaurs

The resolution of the problem of ichthyosaurian affinities has been complicated and prolonged by an emphasis on the morphology of Jurassic ichthyosaurs, some 30 to 40 m.y. younger than the earliest known species. Morphology of the skull and axial skeleton of Triassic ichthyosaurs is the key to resolving the problem. The ichthyosaurs are included here in the Subclass Diapsida on the basis of skull roof and palatal features. Within the Diapsida, ichthyosaurs appear to be most closely related to the Younginiformes of the Infraclass Lepidosauromorpha, with which they may have shared a common ancestor. Triassic ichthyosaurs were considerably different ecologically from their Jurassic descendants. Many Triassic ichthyosaurs were longer bodied and had long, broad tails in contrast to the deep, streamlined bodies and lunate tails of Jurassic species. As a result, Triassic ichthyosaurs probably tended to be ambush predators, whereas Jurassic ichthyosaurs were mainly pursuit predators. Early ichthyosaurs may have been more generalized in their prey preference, as suggested by heterodonty in many Early and Middle Triassic species.

Shastasaurus altispinus(Ichthyosauria, Shastasauridae) from the Upper Triassic of the El Antimonio district, northwestern Sonora, Mexico

New specimens ofShastasaurus altispinusMerriam are recognized from the Upper Triassic Antimonio Formation in the El Antimonio district of northwestern Sonora, Mexico. This represents the first occurrence of the species outside of its type locality in Shasta County, California, and the first known occurrence of Triassic ichthyosaurs from Mexico. Some remains are well preserved, but in general they are fragmented, structurally distorted, and incomplete; however, they enhance the current understanding of the morphology of a species that has been known hitherto only from an even more incomplete holotype. The material is the first from anyShastasaurusthat preserves the anterior portions of the skull and lower jaw and implanted teeth. Revised diagnoses for the genus and species are given on the basis of the new material. Biostratigraphic zonation furnishes an early Late Carnian age for the specimens, which is in accord with that for the holotype.

A new interpretation of the fore-fin ofutatsusaurus hataii (reptilia, ichthyopterygia)

A re-examination of the fore-fin ofUtatsusaurus hataii leads to conclude that this Lower Triassic ichthyosaur possesses a particular tetradactyl limb. This type of limb is new among the Ichthyopterygia and can be interpreted as another attempt in the evolution of the ichthyosaurian paddle. av}Abbreviations IGPS Institute of Geology and Paleontology, Tohoku University, Sendai KUJM Department of Geology and Mineralogy, Kyoto University, Kyoto