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''Askeptosaurus'' 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 ...
of askeptosauroid, a
marine reptile Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. Only about 100 of the 12,000 extant reptile species and subspecies are classed as marine reptiles, including mari ...
from the extinct
order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...
Thalattosauria Thalattosauria ( Greek for "sea lizards") is an extinct order of marine reptiles that lived during the Triassic Period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea and Thalattosauroide ...
. ''Askeptosaurus'' is known from several well-preserved
fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
found in
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...
marine
strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...
in what is now
Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...
and
Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...
.


History of discovery

''Askeptosaurus,'' and its only known species ''Askeptosaurus italicus,'' were first named and described in 1925 by Hungarian
paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...
Franz Nopcsa von Felső-Szilvás Baron Franz Nopcsa von Felső-Szilvás (also Baron Nopcsa von Felső-Szilvás, Baron Nopcsa, Ferenc Nopcsa, báró felsőszilvási Nopcsa Ferenc, Baron Franz Nopcsa, and Franz Baron Nopcsa; May 3, 1877 – April 25, 1933) was a Hungarian Aristoc ...
. It was most recently redescribed by Dr. Johannes Müller in 2005. ''Askeptosaurus'' is known from several disarticulated and articulated skeletons preserved at the MSNM (
Museo Civico di Storia Naturale di Milano The Museo Civico di Storia Naturale di Milano (Milan Natural History Museum) is a museum in Milan, Italy. It was founded in 1838 when the naturalist Giuseppe de Cristoforis donated his collections to the city. Its first director was the taxono ...
) in
Milan, Italy Milan ( , , ; ) is a city in northern Italy, regional capital of Lombardy, the largest city in Italy by urban area and the List of cities in Italy, second-most-populous city proper in Italy after Rome. The city proper has a population of nea ...
, and the PIMUZ ('' Paläontologisches Institut und Museum der Universität Zürich'', Paleontological Institute and of the
University of Zurich The University of Zurich (UZH, ) is a public university, public research university in Zurich, Switzerland. It is the largest university in Switzerland, with its 28,000 enrolled students. It was founded in 1833 from the existing colleges of the ...
) in
Zurich, Switzerland Zurich (; ) is the largest city in Switzerland and the capital of the canton of Zurich. It is in north-central Switzerland, at the northwestern tip of Lake Zurich. , the municipality had 448,664 inhabitants. The urban area was home to 1.45 ...
. These specimens were discovered in the
Grenzbitumenzone The Besano Formation is a geological formation in the southern Alps of northwestern Italy and southern Switzerland. This formation, a thin but fossiliferous succession of dolomite and black shale, is famous for its preservation of Middle Triassic ...
of
Monte San Giorgio Monte San Giorgio is a Swiss mountain and UNESCO World Heritage Site near the border between Switzerland and Italy. It is part of the Lugano Prealps, overlooking Lake Lugano in the Swiss Canton of Ticino. Monte San Giorgio is a wooded mountai ...
, a
UNESCO World Heritage Site World Heritage Sites are landmarks and areas with legal protection under an treaty, international treaty administered by UNESCO for having cultural, historical, or scientific significance. The sites are judged to contain "cultural and natural ...
on the Swiss-Italian border. Also known as the
Besano Formation The Besano Formation is a geological formation in the southern Alps of northwestern Italy and southern Switzerland. This formation, a thin but fossiliferous succession of Dolomite (rock), dolomite and Shale, black shale, is famous for its preserva ...
in
Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...
, the Grenzbitumenzone has produced many well-preserved fossils from the
Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...
-
Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...
boundary within the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...
.


Description

''Askeptosaurus'' was a fairly large thalattosaur, with a skull in length, and a total length up to . Like other askeptosauroids, it had a long neck, a very long tail, and small but well-developed limbs with five independent digits.


Skull


Cranium

The skull was low and somewhat broad at the back, though the snout was long and slender. Nearly half of the snout is formed by the large
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 ...
e, which send back a long triangular projection into the frontals along the midline of the snout. Each premaxilla has up to 12 sharp and slightly curved teeth. They are implanted in a pleurothecodont manner, meaning that they lie in shallow sockets along a groove which has a lowered edge on the lingual (tongue) side of the tooth row. The premaxilla is followed by the low and smaller
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 ...
, and an elongated
naris A nostril (or naris , : nares ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbinates, ...
(nostril) is present at the border between the two bones. The maxilla has 16 teeth, which are similar to those of the premaxilla, albeit slightly smaller. Unlike thalattosauroids, the teeth have the same general shape and 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 ...
(gap) between the premaxillary and maxillary tooth rows. Like other thalattosaurs, the
nasals In phonetics, a nasal, also called a nasal occlusive or nasal stop in contrast with an oral stop or nasalized consonant, is an occlusive consonant produced with a lowered velum, allowing air to escape freely through the nose. The vast majorit ...
are separated from each other by midline projections of the premaxilla and frontal. The nasals have a small contribution to the border of the naris, and the sharp rear tip of each nasal projects into the frontal. Apart from the triangular notches incised by the premaxillae and nasals, the frontals have a simple and subrectangular form. Two small bones, the lacrimal and
prefrontal Prefrontal may refer to: *Prefrontal bone, a skull bone in some tetrapods *Prefrontal cortex, a region of the brain of a mammal *Prefrontal scales The prefrontal scales on snakes and other reptiles are the scales adjacent and anterior to the fr ...
, lie in front of the large
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
(eye socket). The presence of a separate lacrimal is a
plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, an ...
(ancestral) trait only found in ''Askeptosaurus'' among thalattosaurs. ''Askeptosaurus'' is also unique in how its lacrimal has a slight contact with the frontal between the nasal and prefrontal. The posterodorsal (rear-upper) edge of the orbit has two more bones, the large and multi-pronged
postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...
and the much smaller
postfrontal The postfrontal is a paired cranial bone found in many tetrapods. It occupies an area of the skull roof between and behind the orbits (eye sockets), lateral to the frontal and parietal bones, and anterior to the postorbital bone. The postfrontal ...
. Once again, ''Askeptosaurus'' retains the plesiomorphic condition, since in other thalattosaurs these bones fuse into a single postorbitofrontal. The parietals, which lie at the rear of the skull roof, send out sprawling projections over the braincase. A slit-like
upper temporal fenestra 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 (al ...
develops between the parietal and postorbital, while a circular
pineal foramen A parietal eye (third eye, pineal eye) is a part of the epithalamus in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with the pineal gland, which regulates circadian rhythmicity and hormone production ...
is positioned near the parietal's suture with the frontal. The rear branch of each parietal host two slender bones, the
squamosal 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 ancestra ...
and
supratemporal The supratemporal bone is a paired Skull, cranial bone present in many Tetrapod, tetrapods and Tetrapodomorpha, tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) re ...
. The supratemporal is slightly longer while the squamosal has a small but distinct downward projection at its rear tip. The lower rear edge of the skull has a large and rectangular lower temporal fenestra, which is not closed from below due to the absence of a
quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...
. Slender branches of the jugal form the entire lower edge of the orbit, and about a third of the lower edge of the lower temporal fenestra. The rear edge of the skull is formed by a large quadrate, which has broad upper and lower extents, and concave rear and lateral edges.


Palate, braincase, and lower jaw

Unlike thalattosauroids, the
palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sep ...
(roof of the mouth) was completely toothless. The
vomer The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...
is long and slender, forming the lower midline of the snout. The succeeding pterygoids are larger, with a pair of sharp muscle scars at their lateral extent. A T-shaped ectopterygoid forms a strut between the pterygoid and the jugal, while the less complete
palatine A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, Roman times.
connects the pterygoid to the maxilla. The upper part of the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
is poorly known, but the lower part is less obscure. The
parabasisphenoid The parasphenoid is a bone which can be found in the cranium of many vertebrates. It is an unpaired dermal bone which lies at the midline of the roof of the mouth. In many reptiles (including birds), it fuses to the endochondral (cartilage-derived) ...
(lower plate) sends forward an elongated cultriform process. Further back, each side has a bulbous basipterygoid process (which connects to the palate), deep grooves for arteries and nerves, and finally a sharp posterolateral (rear-outer) tip. The parabasisphenoid is followed by a broad and fairly simple
basioccipital 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 ...
(lower rear plate), which forms the
occipital condyle The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the Atlas (anatomy), atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape ...
. 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 ...
is flanked by distinct exoccipitals, which are connected (but not fused) to elongated and posterolaterally-directed opisthotics. The
supraoccipital 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 ...
, which roofs the foramen magnum, has a strong fan-shaped crest on it. The lower jaw is slender, with 20 teeth on the
dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...
which are similar to those of the upper jaw. The dentary sheaths over the entire
splenial The splenial is a small bone in the lower jaw of reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology ...
and much of the angular, making it by far the longest bone when the jaw is seen in lateral view. A sharp and subtriangular coronoid rises up in the rear half of the jaw, above the low but elongated
surangular The surangular or suprangular is a jaw bone found in most land vertebrates, except mammals. Usually in the back of the jaw, on the upper edge, it is connected to all other jaw bones: dentary, angular bone, angular, splenial and articular. It is o ...
. The surangular lacks a retroarticular process behind the jaw joint, but it does have sharp crest in its rear half. The splenial and prearticular are the most prominent parts of the jaw when seen in medial view.


Postcrania


Vertebrae

All of the vertebrae were amphicoelous, with concave front and rear faces of their respective centrum (main body). The neck was fairly elongated, with 13 cervical (neck) vertebrae. The components of 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 ...
(atlantal neural arch, intercentrum, and proatlases) were not connected, while 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 ...
had a low and long neural spine and a large single rib facet. The atlantal and axial
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 were small, slender, and single-headed. The other cervicals were slightly longer than tall and connected to small rectangular neural spines. Their double-headed cervical rib facets were originally misinterpreted, with the lower facet reported as connected to a wedge-shaped intercentrum rather than the centrum. Later investigation determined that intercentra were not present and that each side of the centrum had two facets. The cervical ribs in the latter half of the neck increased in length and acquired small additional projections between the joints with the vertebrae. The 25 dorsal (torso) vertebrae are similar to the cervicals but larger, and they only had single rib facets. Their neural spines are proportionally taller and vertically oriented, though still quite short by thalattosaur standards. The rib facets are directed at a steep diagonal angle towards the anteroventral (front-lower) end of the centrum. Their single-headed ribs were larger and had expanded heads towards the front of the torso, and were gradually smaller towards the hip.
Gastralia Gastralia (: gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In these reptil ...
are slender but poorly-studied. The two sacral (hip) vertebrae are also obscure, but their fan-shaped, single-headed ribs were known to connect to the lower part of the centrum. The tail was very long, with at least 60 caudal (tail) vertebrae which were more elongated towards the tip of the tail. The first five caudals had short, single-headed ribs. The first, fourth, and fifth caudal ribs have a curved, tapering form, and the other two have straight ribs with expanded heads. These first few caudals also had straight, vertical neural spines. The rest of the tail had prominent
chevrons Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * '' Eulithis testata'', a moth * Chevron (geology), a fold in rock la ...
and neural spines which were shorter and more sharply inclined backwards.


Limbs

The pectoral (shoulder) girdle includes an
interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals ( marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In ...
,
clavicle The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the scapula, shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavic ...
s,
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 ...
e, and
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 ...
s. The scapula and coracoid are broad and rounded, connecting to form a laterally-oriented
glenoid The glenoid fossa of the scapula or the glenoid cavity is a bone part of the shoulder. The word ''glenoid'' is pronounced or (both are common) and is from , "socket", reflecting the shoulder joint's ball-and-socket form. It is a shallow, pyrif ...
(shoulder socket). The interclavicle is arrow-shaped, with a very long rear shaft and small lateral projections overlapping the clavicles. The clavicles are curved and slender, connecting the interclavicle to the scapulae. 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 ...
(forearm) was large and twisted. Its joint surfaces were poorly-defined apart from an ectepicondylar groove near the elbow and a small deltopectoral crest near the shoulder. The
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 ...
and
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 ...
of the lower arm are smaller bones with a straight and simple form. The radius is slender and tubular, unlike the broader radius of thalattosauroids. The ulna lacks an
olecranon process The olecranon (, ), is a large, thick, curved bony process on the proximal, posterior end of the ulna. It forms the protruding part of the elbow and is opposite to the cubital fossa or elbow pit (trochlear notch). The olecranon serves as a lever ...
and has a distinct hourglass-shaped form with a constricted shaft. The wrist has six to seven
carpal bones The carpal bones are the eight small bones that make up the wrist (carpus) that connects the hand to the forearm. The terms "carpus" and "carpal" are derived from the Latin wikt:carpus#Latin, carpus and the Greek language, Greek wikt:καρπός ...
including an intermedium,
ulnare The triquetral bone (; also called triquetrum, pyramidal, three-faced, and formerly cuneiform bone) is located in the wrist on the medial side of the proximal row of the carpus between the lunate and pisiform bones. It is on the ulnar side of the ...
, and four small distal carpals. The middle three
metacarpals In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular skeleton, appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones (wrist, wris ...
of the manus (hand) are slender, metacarpal V is slightly thicker, and metacarpal I is short and massive. The digits are fairly short, with small claws. There is some intraspecific (within-species) variation in the manus, with specimen PIMUZ T 4846 having an extra carpal in the wrist and an extra joint in the third finger, relative to MSNM V456. The
pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...
(hip) contains an ilium, pubis, and
ischium The ischium (; : is ...
and is generally similar to other thalattosaurs. The ilium was deep around the
acetabulum The acetabulum (; : acetabula), also called the cotyloid cavity, is a wikt:concave, concave surface of the pelvis. The femur head, head of the femur meets with the pelvis at the acetabulum, forming the Hip#Articulation, hip joint. Structure The ...
(hip) socket, and sends back a long rectangular process at its posterodorsal (upper-rear) corner. The pubis is large and broad in the anteroposterior (front to back) direction and has a concave front edge. The ischium is similarly broad, with a concave posterodorsal edge and other edges which are straighter. There is some uncertainty over the presence of a thyroid fenestra (a gap between the pubes and ischia) based on specimen preservation, but it was probably small or absent. The
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
(thigh bone) is the largest limb bone in the skeleton and has a simple, slightly twisted form. 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 ...
is stout and semi-cylindrical while 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 fan-shaped and flattened, expanded near the ankle and narrow near the knee. There were six tarsals in the ankle. They include a large reniform (kidney-shaped)
astragalus Astragalus may refer to: * ''Astragalus'' (plant), a large genus of herbs and small shrubs *Astragalus (bone) The talus (; Latin for ankle or ankle bone; : tali), talus bone, astragalus (), or ankle bone is one of the group of foot bones known ...
, a subrectangular
calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is t ...
, and four rounded distal tarsals. The pes (foot) is similar to the manus, though the fifth digit has an additional phalange and a thickly curved metatarsal V.


Classification

Prior to 2005, fossils of ''Askeptosaurus'' were rare and the position of ''Askeptosaurus'' on the phylogenetic tree had been a heated debate between different research teams. Recent analyses corroborate Renesto's (1992) argument that ''Endennasaurus'' is more closely related to ''Askeptosaurus'' than to other thalattosaurs. It is now commonly agreed that a
monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...
group is formed by ''Anshunsaurus'' and ''Askeptosaurus''. To be more specific, ''Askeptosaurus'' belongs to the family
Askeptosauridae Askeptosauridae is a family of thalattosaurs within the superfamily Askeptosauroidea. Fossils have been found from Italy, Switzerland, and China. Askeptosaurids are distinguished from other thalattosaurs by their long necks and narrow skulls. C ...
, a division of the suborder
Askeptosauroidea Askeptosauroidea is a superfamily of thalattosaurs, a Triassic group of marine reptiles. Askeptosauroidea is one of two major subgroups of Thalattosauria, the other being Thalattosauroidea. It includes the family Askeptosauridae and a more basa ...
. Within
Askeptosauridae Askeptosauridae is a family of thalattosaurs within the superfamily Askeptosauroidea. Fossils have been found from Italy, Switzerland, and China. Askeptosaurids are distinguished from other thalattosaurs by their long necks and narrow skulls. C ...
, Askeptosaurus is considered to be the
sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
of ''
Anshunsaurus ''Anshunsaurus'' is a genus of thalattosaurs within the family Askeptosauridae. Fossils have been found from Middle Triassic deposits in Guizhou, China. Three species are known: the type species ''A. huangguoshuensis'' (named in 1999), the slight ...
'', from
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...
deposits in
Guizhou ) , image_skyline = , image_caption = , image_map = Guizhou in China (+all claims hatched).svg , mapsize = 275px , map_alt = Map showing the location of Guizhou Province , map_caption = Map s ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
. At present, the Askeptosauroidea is only known from the Alpine
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
and southern China, and ''Askeptosaurus'' represents the oldest record for this
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 ...
.


Paleoecology

''A. italicus'' was found in the Middle Triassic of Switzerland and Italy, with most specimens recovered from sites on
Monte San Giorgio Monte San Giorgio is a Swiss mountain and UNESCO World Heritage Site near the border between Switzerland and Italy. It is part of the Lugano Prealps, overlooking Lake Lugano in the Swiss Canton of Ticino. Monte San Giorgio is a wooded mountai ...
. It lived alongside many other fish and marine reptiles, including two other genera of
thalattosaurs Thalattosauria (Greek for "sea lizards") is an extinct order of marine reptiles that lived during the Triassic Period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea and Thalattosauroidea. ...
: ''
Clarazia ''Clarazia'' is an extinct genus of thalattosaur from the Middle Triassic of Monte San Giorgio in Switzerland. It is represented by a single type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type specie ...
'' and ''
Hescheleria ''Hescheleria'' is an extinct genus of thalattosaurian marine reptile from the Middle Triassic (247.2 to 235 Ma) of Monte San Giorgio in Switzerland. It is represented by a single type species, ''H. ruebeli'', which was named in 1936. Descript ...
.''


References

{{Taxonbar, from=Q602583 Middle Triassic reptiles of Europe Fossils of Italy Triassic reptiles Thalattosauria Prehistoric reptile genera Aquatic reptiles