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''Askeptosaurus'' is an extinct genus of askeptosauroid, a marine reptile from the extinct
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
Thalattosauria. ''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 in ...
found in
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma an ...
marine strata in what is now Italy and 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 palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of foss ...
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 naturalist Giuseppe de Cristoforis donated his collections to the city. Its first director was Giorgio Jan. ...
) in Milan, Italy, and the PIMUZ ( Paläontologisches Institut und Museum der Universität Zürich) in Zürich, Switzerland. These specimens were discovered in the Grenzbitumenzone of Monte San Giorgio, a UNESCO World Heritage Site 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 short but fossiliferous succession of dolomite and black shale, is famous for its preservation of Middle Triass ...
in Italy, the Grenzbitumenzone has produced many well-preserved fossils from the
Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic ...
-
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 epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma an ...
.


Description

''Askeptosaurus'' was a fairly large thalattosaur, with a skull 26 cm (~10 inches) in length and a total length up to 2.5 meters (8.2 feet). 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 mammal has ...
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, and an elongated
naris A nostril (or naris , plural ''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 tu ...
(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 (plural diastemata, from Greek διάστημα, 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 ...
(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 major ...
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, lie in front of the large orbit (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, and ...
(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 skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
. 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 develops between the parietal and postorbital, while a circular
pineal foramen A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhythm ...
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 ancestral co ...
and
supratemporal The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) relative to the squamosal and later ...
. 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 sepa ...
(roof of the mouth) was completely toothless. The
vomer The vomer (; lat, vomer, lit=ploughshare) 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 maxil ...
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 connects the pterygoid to the maxilla. The upper part of the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan 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 calvaria or sku ...
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 t ...
(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 vertebra. The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...
. The
foramen magnum The foramen magnum ( la, great hole) 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 oblonga ...
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 overlies 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 anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
which are similar to those of the upper jaw. The dentary sheaths over the entire splenial 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 suprangular or surangular 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, splenial and articular The articular bone ...
. 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 maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographi ...
(atlantal neural arch, intercentrum, and proatlases) were not connected, while the
axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis *Axis (mathematics), a designator for a Cartesian-coordinate ...
had a low and long neural spine and a large single rib facet. The atlantal and axial
cervical rib A cervical rib in humans is an extra rib which arises from the seventh cervical vertebra. Their presence is a congenital abnormality located above the normal first rib. A cervical rib is estimated to occur in 0.2% to 0.5% (1 in 200 to 500) of the ...
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 (singular 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 ...
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 l ...
and neural spines which were shorter and more sharply inclined backwards.


Limbs

The pectoral (shoulder) girdle includes an interclavicle,
clavicle The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the right ...
s,
scapula The scapula (plural 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 eithe ...
e, and
coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) 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 prese ...
s. The scapula and coracoid are broad and rounded, connecting to form a laterally-oriented glenoid (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 (; ) 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 and ulna, and consists of three sections. The humeral upper extremity consists of a round ...
(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 and
ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
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 eminence of the ulna, a long bone in the forearm that projects behind the elbow. It forms the most pointed portion of the elbow and is opposite to the cubital fossa or elbow pit. The olecranon ...
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 (or carpus) that connects the hand to the forearm. The term "carpus" is derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In human anatomy, the ...
including an intermedium, ulnare, and four small distal carpals. The middle three
metacarpals In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal bones ar ...
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 (hip) contains an ilium, pubis, and ischium and is generally similar to other thalattosaurs. The ilium was deep around the
acetabulum The acetabulum (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure There are three bones of the ''os coxae'' (hip bone) that c ...
(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 (thigh bone) is the largest limb bone in the skeleton and has a simple, slightly twisted form. The tibia is stout and semi-cylindrical while the fibula 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'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
, a subrectangular
calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. S ...
, 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 group is formed by ''Anshunsaurus'' and ''Askeptosaurus''. To be more specific, ''Askeptosaurus'' belongs to the family Askeptosauridae, 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 bas ...
. Within Askeptosauridae, 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'', from
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma an ...
deposits in Guizhou, China. At present, the Askeptosauroidea is only known from the Alpine
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
and southern China, and ''Askeptosaurus'' represents the oldest record for this clade.


Paleoecology

''A. italicus'' was found in the Middle Triassic of Switzerland and Italy, with most specimens recovered from sites on Monte San Giorgio. It lived alongside many other fish and marine reptiles, including two other genera of thalattosaurs: '' Clarazia'' and '' Hescheleria.''


References

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