''Luskhan'' (meaning "water spirit chief") is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

brachauchenine Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages). The family is more inclusive than the archetypal short-necked large headed species that are placed in ...

pliosaur Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toot ...

from the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

. The

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...

and only species is ''Luskhan itilensis'', named by Valentin Fischer and colleagues in 2017 from a well-preserved and nearly complete skeleton. As an early-diverging brachauchenine, ''Luskhan'' consequently exhibits an intermediate combination of traits seen in more basal (less specialized) and more derived (more specialized) pliosaurs. However, ''Luskhan'' departs significantly from other pliosaurs in that it exhibits a lack of adaptations in its skull to feeding on large prey; its slender snout, small teeth, and short tooth rows instead indicate a skull adapted for feeding on small, soft prey. With these features, it is the pliosaur that approaches closest to the distantly-related

piscivorous A piscivore () is a carnivorous animal that primarily eats fish. Fish were the diet of early tetrapod evolution (via water-bound amphibians during the Devonian period); insectivory came next; then in time, the more terrestrially adapted rept ...

polycotylids Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. They are known as false pliosaurs. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widesprea ...

, having

convergently evolved Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

these traits more than 10 million years apart.

Discovery and naming

A nearly complete fossil skeleton of a

, preserved in three dimensions, was found by Gleb N. Uspensky in 2002 on the eastern bank of the

Volga River The Volga (, ) is the longest river in Europe and the longest endorheic basin river in the world. Situated in Russia, it flows through Central Russia to Southern Russia and into the Caspian Sea. The Volga has a length of , and a catchment ...

, north of the village of Slantsevy Rudnik in the

Ulyanovsk Ulyanovsk,, , known as Simbirsk until 1924, is a types of inhabited localities in Russia, city and the administrative center of Ulyanovsk Oblast, Russia, located on the Volga River east of Moscow. Ulyanovsk has been the only Russian UNESCO Ci ...

region of western

. Deposits in the region consist of dark grey and slightly

sandy Sandy may refer to: People and fictional characters *Sandy (given name), including a list of people and fictional characters with the given name or nickname * Sandy (surname), a list of people * Sandy (Iranian music band), Iranian singer, comp ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

layers and siltite layers, in which

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

nodules Nodule may refer to: *Nodule (geology), a small rock or mineral cluster *Manganese nodule, a metallic concretion found on the seafloor *Nodule (medicine), a small aggregation of cells *Root nodule Root nodules are found on the roots of plants, p ...

are embedded. The skeleton was found in the g-5

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

(layer). After its discovery, the specimen was stored in the I.A. Goncharov Ulyanovsk Regional Museum of Local Lore (YKM), under the specimen number YKM 68344/1_262. Uspensky co-authored a research paper describing YKM 68344/1_262 that was published in the journal ''

Current Biology ''Current Biology'' is a biweekly peer-reviewed scientific journal that covers all areas of biology, especially molecular biology, cell biology, genetics, neurobiology, ecology, and evolutionary biology. The journal includes research artic ...

'' on June 5, 2017. Other authors were Valentin Fischer, Roger Benson, Nikolay Zverkov, Laura Soul, Maxim Arkhangelsky, Olivier Lambert, Ilya Stenshin, and Patrick Druckenmiller. They named the specimen as a new genus and species, ''Luskhan itilensis''. The genus name, ''Luskhan'', is derived from ''luuses'', spirits and masters of water in

Mongolian Mongolian may refer to: * Something of, from, or related to Mongolia, a country in Asia * Mongolian people, or Mongols * Bogd Khanate of Mongolia, the government of Mongolia, 1911–1919 and 1921–1924 * Mongolian language * Mongolian alphabet * ...

and

Turkic mythology Turkic mythology refers to myths and legends told by the Turkic people. It features Tengrism, Tengrist and Shamanism in Central Asia, Shamanist strata of belief along with many other social and cultural constructs related to the nomadic and wa ...

, plus the suffix ''khan'', meaning "chief". ''Itil'' is the ancient Turkic name for the Volga River, hence the species epithet ''itilensis'' means "of the Volga River".

Description

The holotype of ''Luskhan'' measures long. At the end of a relatively short neck, the long skull tapers to an elongate snout, which is longer than the region of the skull behind the eyes, as in other members of the

Brachaucheninae Pliosauridae is a family (biology), family of plesiosaurian marine reptiles from the Late Triassic, Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages). The family is more inclusive than the archetypal short-necked large he ...

. It has a lower jaw long. Additional characteristics typical of brachauchenines include the

parietal bone The parietal bones ( ) are two bones in the skull which, when joined at a fibrous joint known as a cranial suture, form the sides and roof of the neurocranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four bord ...

extending forward to the position of the nostrils; the snout being unconstricted, but bearing an expansion on the bottom surface like '' Megacephalosaurus''; the retroarticular process at the back of the lower jaw being inturned; the teeth in the upper jaw being equally-sized; the facets that articulate with the

cervical rib Cervical ribs are the ribs of the neck in many tetrapods. In most mammals, including humans, cervical ribs are not normally present as separate structures. They can, however, occur as a pathology. In humans, pathological cervical ribs are usually no ...

s on the

cervical vertebrae In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...

being placed relatively high, similar to ''

Kronosaurus ''Kronosaurus'' ( ) is an extinct genus of large short-necked pliosaur that lived during the Aptian to Albian Stage (stratigraphy), stages of the Early Cretaceous in what is now Australia. The first known specimen was received in 1899 and consis ...

'' and ''

Brachauchenius ''Brachauchenius'' (meaning 'short neck') is an extinct genus of pliosaurs that lived during the Late Cretaceous in what are now North America and North Africa. Only one species is known, ''B. lucasi'', initially described by Samuel Wendell Willis ...

'' but unlike

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

pliosaurs and other plesiosaurs; the

transverse processes Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

attaching to the

dorsal vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...

above the level of the neural canals; and the long

coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...

measuring 2.3 times the length of the

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

(2.5 in ''Kronosaurus''). ''Luskhan'' also exhibits some more "primitive" traits which resemble non-brachauchenine

thalassophonea Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages). The family is more inclusive than the archetypal short-necked large headed species that are placed in ...

ns, consistent with it being among the earliest brachauchenines. Like ''

Pliosaurus ''Pliosaurus'' (meaning 'more lizard') is an extinct genus of thalassophonean pliosaurid known from the Late Jurassic (Kimmeridgian and Tithonian stages) of Europe and South America. This genus has contained many species in the past but recent ...

'', the

squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral ...

s overlap the rear processes of the

jugal bone The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic bone, zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by spe ...

s; it also excludes them, as in most plesiosaurs, from the border of the

temporal fenestrae Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly ( ...

. There is also a bulb on each squamosal, as well as a ridge extending upwards from the rear surface. As in ''Pliosaurus westburyensis'', the processes of the

pterygoid bone The pterygoid is a paired bone forming part of the palate of many vertebrates, behind the palatine bone In anatomy, the palatine bones (; derived from the Latin ''palatum'') are two irregular bones of the facial skeleton in many animal specie ...

s that articulate with the

quadrate bone The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids ( reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms up ...

are thick, and there is a U-shaped notch on the bottom of the

supraoccipital bone The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lobes of the cere ...

. The flanges on the bottom of the pterygoids come into contact with each other at the midline of the skull. On the bottom of the cervicals, there are large foramina, or pits. The number of cervical vertebrae in ''Luskhan'' (14) can be seen as intermediate between ''Pliosaurus'' (18) and ''Brachauchenius'' (12). Unusually, ''Luskhan'' also lacks many of the adaptations for hunting large prey seen in other brachauchenines: the snout is very thin; there is no keel on the bottom of the fused

symphysis A symphysis (, : symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing together o ...

of the lower jaw; there is no

diastema A diastema (: diastemata, from Greek , 'space') is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition may be referred to ...

(or gap in the tooth row); the bones of the upper jaw (the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

and

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

) are expanded outwards; and there are no

caniniform In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, eye teeth, vampire teeth, or fangs, are the relatively long, pointed tooth, teeth. In the context of the upper jaw, they are also known as ''fangs''. They can appear mo ...

("canine-like") teeth. The length of the symphysis as a proportion of the overall length of the lower jaw (34%) is also longer than brachauchenines and thalassophoneans, but instead is within the range of the symphyses of the distantly-related

Polycotylidae Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. They are known as false pliosaurs. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widesprea ...

. Also like polycotylids, the teeth are more widely spaced and terminate further forward on the jaw, below the midpoint of the eye socket. As in ''

Dolichorhynchops ''Dolichorhynchops'' is an extinct genus of polycotylid plesiosaur from the Late Cretaceous of North America, containing the species ''D. osborni'' and ''D. herschelensis'', with two previous species having been assigned to new genera. Definitiv ...

'', the arch of the squamosal is angled further forwards than thalassophoneans.

Autapomorphies

A number of unique characters, or

autapomorphies In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to ...

, set ''Luskhan'' apart from all other plesiosaurs. Among thalassophoneans, ''Luskhan'' is unique for having seven teeth in the premaxilla. The first of these is procumbent (angled forwards) such that it is nearly horizontal, and the space between it and the subsequent tooth is also widened. A roughened, hook-like projection develops on the squamosal from its suture with the quadrate. At the back of the skull, the exoccipital bones enclose the bottom of the

foramen magnum The foramen magnum () is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes thro ...

, thereby excluding the

basioccipital bone The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull, this area is rough and uneven, and is joined ...

. The plate-like lamellae of the pterygoids bear deep grooves on their outer surfaces. On the

atlas An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...

(the first cervical), the intercentrum is swollen and has a ridge underneath; meanwhile, the bottom of the

axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...

(the second cervical) is covered by a tongue-like projection from the third cervical. In the

shoulder girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists o ...

, the projection at the front of the coracoid that points forwards in other plesiosaurs is instead pointed downwards in ''Luskhan'', such that it is perpendicular to the rest of the bone. Unlike other thalassophoneans, but like the

Elasmosauridae Elasmosauridae, often called elasmosaurs or elasmosaurids, is an extinct family of plesiosaurs that lived from the Hauterivian stage of the Early Cretaceous to the Maastrichtian stage of the Late Cretaceous period (c. 130 to 66 mya). The taxo ...

, the blade of the scapula is relatively short, being only as tall vertically as the longitudinal distance from its base to the articulation with the coracoid. On the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

, the humeral tuberosity is located above the expansion of the

capitulum capitulum (plural capitula) may refer to: *the Latin word for chapter ** an index or list of chapters at the head of a gospel manuscript ** a short reading in the Liturgy of the Hours *** derived from which, it is the Latin for the assembly known ...

at the bottom end. The

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

and

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

of the front flippers are very small, being only about the same size as the tarsus of the hind flippers; the former of these is longer. Unlike all other pliosaurids, there is no opening (epipodial foramen) where the two bones meet. The

fibula The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...

is much longer than the

tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

; the intermedium of the tarsus contacts only the fibula, and lacks an articulation for the tibia unlike other thalassophoneans and ''

Marmornectes ''Marmornectes'' is a genus of pliosaurid known from the Middle Jurassic of Bedfordshire, United Kingdom. Etymology ''Marmornectes'' was first named by Hilary F. Ketchum and Roger B. J. Benson in 2011 and the type species is ''Marmornectes can ...

''.

Classification

The following

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

follows an analysis by Fischer and colleagues in 2017, based on a dataset published by Benson and Druckenmiller in 2014 that was previously modified for the description of ''

Makhaira The makhaira is a type of Ancient Greek bladed weapon and tool, generally a large knife or sword, similar in appearance to the modern day machete, with a single cutting edge. Terminology The Greek word μάχαιρα (''mákhaira'', plural ' ...

'' in 2015. Due to the completeness of ''Luskhan'', its position is based on scorings for 74% of the characteristics listed in the dataset. In the strict consensus of the 20,000 most

parsimonious In philosophy, Occam's razor (also spelled Ockham's razor or Ocham's razor; ) is the problem-solving principle that recommends searching for explanations constructed with the smallest possible set of elements. It is also known as the principle o ...

phylogenetic tree A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...

s recovered, ''Makhaira'' forms a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...

with ''Pliosaurus'' species, instead of forming a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

with other brachauchenines; however, in the consensus of the 24 most parsimonious trees, it is the most basal brachauchenine, with ''Luskhan'' being the next most basal brachauchenine. The 2023 study recovered ''P. patagonicus'' as a sister taxon of ''Luskhan'' within

, the phylogenetic analysis of which is reproduced below:

Evolutionary context

Along with ''Makhaira'' and ''

Stenorhynchosaurus ''Stenorhynchosaurus'' is an extinct genus of pliosaurid plesiosaurs which lived in the Early Cretaceous of South America. The type species and only known is ''Stenorhynchosaurus munozi''. Etymology The genus name is derived from the Greek w ...

'', ''Luskhan'' forms an

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Phylogenetics The concept of evolutionary grades ...

that fills a critical gap of 40 million years - from the

Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...

to the

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 125.77 Ma (million years ago) and 121.4 ± 1.0 Ma (Historically, this stage was placed at 129.4 million to approximately 125 million years ago) It is a ...

epochs of the Early Cretaceous - in the evolution of brachauchenines. While María Páramo-Fonseca and colleagues, the describers of ''Stenorhynchosaurus'', indicated that including it in the Brachaucheninae would necessitate a re-definition of the clade, Fischer and colleagues did not see this as necessary.

Paleobiology

Most thalassophonean

pliosaurids Pliosauridae is a family (biology), family of plesiosaurian marine reptiles from the Late Triassic, Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages). The family is more inclusive than the archetypal short-necked large he ...

, with their robust skulls and short necks, were well-adapted to

apex predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...

niches. Although serrated teeth, spatula-shaped snouts, and strongly-developed crests on the skull roof were lost in brachauchenines, their large size and teeth still indicate predatory lifestyles. Meanwhile, with their slimmer body proportions, polycotylids were likely fast-swimming

piscivore A piscivore () is a carnivorous animal that primarily eats fish. Fish were the diet of early tetrapod evolution (via water-bound amphibians during the Devonian period); insectivory came next; then in time, the more terrestrially adapted repti ...

Principal component Principal component analysis (PCA) is a linear dimensionality reduction technique with applications in exploratory data analysis, visualization and data preprocessing. The data is linearly transformed onto a new coordinate system such that th ...

and

ecomorphological Ecomorphology or ecological morphology is the study of the relationship between the ecological role of an individual and its morphological adaptations. The term "morphological" here is in the anatomical context. Both the morphology and ecology ex ...

analyses place ''Luskhan'' as being significantly closer to polycotylids than other thalassophoneans in terms of skull traits, but not in terms of the rest of the body; thus, ''Luskhan'' is the most polycotylid-like pliosaurid. The early thalassophonean ''

Peloneustes ''Peloneustes'' (meaning ) is a genus of pliosaurid plesiosaur from the Middle Jurassic of England. Its remains are known from the Peterborough Member of the Oxford Clay Formation, which is Callovian in age. It was originally described as a sp ...

'' is also polycotylid-like, but less so than ''Luskhan''. Ecologically speaking, ''Luskhan'' probably preyed on small, soft animals. This is suggested by the slender snout, the long symphysis, and the relatively short tooth row compared to other thalassophoneans. A few traits in ''Luskhan'' are shared with typical thalassophoneans, such as the presence of serrations in the rearmost teeth and the presence of weakly trihedral (triangular in cross-section) teeth; however, these are probably

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

traits retained from ''Pliosaurus''-like ancestors. Thus, ''Luskhan'' departed from the typical apex predator niche of thalassophoneans in order to colonize a lower

trophic level The trophic level of an organism is the position it occupies in a food web. Within a food web, a food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the ...

, having done so independently 10 million years after polycotylids. ''Makhaira'', by contrast, retained a bauplan that is better-suited to being an apex predator; for instance, its teeth are larger than ''Luskhan''.

Paleoecology

Based on the presence of the

ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

'' Speetoniceras versicolor'' several meters north of the site where ''Luskhan'' was discovered, the g-5 horizon can be

correlated In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...

with the ''S. versicolor''

zone Zone, Zones or The Zone may refer to: Places Military zones * Zone, any of the divisions of France during the World War II German occupation * Zone, any of the divisions of Germany during the post-World War II Allied occupation * Korean Demilit ...

Magnetostratigraphy Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their ' ...

indicates that this zone dates to the

Hauterivian The Hauterivian is, in the geologic timescale, an age in the Early Cretaceous Epoch or a stage in the Lower Cretaceous Series. It spans the time between 132.6 ± 2 Ma and 125.77 (million years ago). The Hauterivian is preceded by the Valangi ...

epoch of the early

Period, approximately 128 million years ago. ''Makhaira'' originates from the ''S. versicolor'' zone, and it was found nearby ''Luskhan'' as well - north of Slantsevy Rudnik. Also from the ''S. versicolor'' zone is the

cryptoclidid Cryptoclididae is a family of medium-sized plesiosaurs that existed from the Middle Jurassic to the Early Cretaceous. They had long necks, broad and short skulls and densely packed teeth. They fed on small soft-bodied preys such as small fish and ...

plesiosaurid ''

Abyssosaurus ''Abyssosaurus'' ("bottomless lizard") is an extinct genus of cryptoclidid plesiosaur known from the Early Cretaceous of Chuvash Republic, western Russia. The type specimen, consisting of a fairly complete postcranial skeleton and parts of a sku ...

'', which was found within the region of

Chuvashia Chuvashia, officially known as Chuvash Republic — Chuvashia, is a republics of Russia, republic of Russia located in Eastern Europe. It is the homeland of the Chuvash people, a Turkic languages, Turkic ethnic group. Its capital city, capital i ...

near a

tributary A tributary, or an ''affluent'', is a stream or river that flows into a larger stream (''main stem'' or ''"parent"''), river, or a lake. A tributary does not flow directly into a sea or ocean. Tributaries, and the main stem river into which they ...

of the

Sura River The Sura (, , ''Săr'') is a river in Russia, a north-flowing right tributary of the Volga. Its mouth on the Volga is about halfway between Nizhny Novgorod and Kazan. It flows through Penza Oblast, Mordovia, Ulyanovsk Oblast, Chuvashia and Nizhny ...

(itself a tributary of the Volga). Additionally, vertebrae from a

brachiosaurid The Brachiosauridae ("arm lizards", from Greek ''brachion'' (βραχίων) = "arm" and ''sauros'' = "lizard") are a family or clade of herbivorous, quadrupedal sauropod dinosaurs. Brachiosaurids had long necks that enabled them to access the l ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

have been found. In terms of invertebrates, the ''S. versicolor'' zone is additionally characterized by the ammonites ''S. coronatiforme'', ''S. pavlovae'', ''S. intermedium'', and ''S. polivnense''; the

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

'' Inoceramus aucella'', '' Prochinnites substuderi'', '' Astarte porrecta'', and '' Thracia creditica''; and the

belemnite Belemnitida (or belemnites) is an extinct order (biology), order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous (And possibly the Eocene). Unlike squid, belemnites had an internal skeleton that made up the cone ...

s '' Acroteuthis pseudopanderi'', '' Praeoxyteuthis jasikofiana'', '' Aulacoteuthis absolutiformis'', and ''A. speetonensis''. The

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

'' Ampullina sp.'', ''

Avellana hauteriviensis __NOTOC__ Avellana may refer to: People * Andy Avellana, Filipino paratriathlete * Daisy Avellana (1917–2013), Filipino stage actress and theater director * Lamberto V. Avellana (1915–1991), Filipino film and stage director Species * ''Corylus ...

'', '' Claviscala antiqua'', '' Cretadmete neglecta'', '' Eucyclus sp.'', '' Hudlestonella pusilla'', '' Khetella glasunovae'', '' Sulcoactaeon sp.'', '' Tornatellaea kabanovi'', '' Trilemma russiense'', and '' Turbinopsis multicostulata'' have also been found within the ''S. versicolor'' zone around Ulyanovsk.

References

{{Taxonbar, from=Q30088293 Sauropterygian genera Pliosauridae Early Cretaceous plesiosaurs Fossil taxa described in 2017