Askeptosaurus: Difference between revisions
order template |
navbox |
||
| (30 intermediate revisions by 17 users not shown) | |||
| Line 1: | Line 1: | ||
{{Short description|Extinct genus of reptiles}} |
|||
{{Italic title}} |
|||
{{Speciesbox |
|||
{{Taxobox |
|||
| name = ''Askeptosaurus'' |
|||
| fossil_range = [[Middle Triassic]], {{fossilrange|247|225}} |
| fossil_range = [[Middle Triassic]], {{fossilrange|247|225}} |
||
| image = Askeptosaurus.jpg |
| image = Askeptosaurus Deep Time.jpg |
||
| image_caption = |
| image_caption = A cast of specimen MSNM V456 |
||
| |
| genus = Askeptosaurus |
||
| ⚫ | |||
| phylum = [[Chordata]] |
|||
| |
| species = italicus |
||
| ⚫ | |||
| ordo = [[Thalattosauria]] |
|||
| familia = [[Askeptosauridae]] |
|||
| genus = '''''Askeptosaurus''''' |
|||
| ⚫ | |||
| species = '''''A. italicus''''' |
|||
| binomial = ''Askeptosaurus italicus'' |
|||
| ⚫ | |||
}} |
}} |
||
'''''Askeptosaurus''''' |
'''''Askeptosaurus''''' is an [[extinction|extinct]] [[genus]] of [[Askeptosauroidea|askeptosauroid]], a [[marine reptile]] from the extinct [[Order (biology)|order]] [[Thalattosauria]]. ''Askeptosaurus'' is known from several well-preserved [[fossils]] found in [[Middle Triassic]] marine [[Stratum|strata]] in what is now [[Italy]] and [[Switzerland]].<ref name=":0">{{Cite journal|last=Müller|first=Johannes|date=2005|title=The anatomy of ''Askeptosaurus italicus'' from the Middle Triassic of Monte San Giorgio and the interrelationships of thalattosaurs (Reptilia, Diapsida)|url=https://www.researchgate.net/publication/237169287|journal=Canadian Journal of Earth Sciences|volume=42|issue=7|pages=1347–1367|doi=10.1139/e05-030|bibcode=2005CaJES..42.1347M}}</ref> |
||
| ⚫ | |||
''Askeptosaurus,'' and its only known species ''Askeptosaurus italicus,'' were first named and described in 1925 by [[Hungary|Hungarian]] [[paleontologist]] [[Franz Nopcsa von Felső-Szilvás]].<ref>{{cite book|title=Askeptosaurus, ein neues reptil der Trias von Besano: Centralblatt für Mineralogie, Geologie und Paläontologie|year=1925|editor=Nopcsa, F|pages=265–2}}</ref> It was most recently redescribed by Dr. [[Johannes Müller (paleontologist)|Johannes Müller]] in 2005. ''Askeptosaurus'' is known from several disarticulated and articulated skeletons preserved at the MSNM ([[Museo Civico di Storia Naturale di Milano]]) in [[Milan|Milan, Italy]], and the PIMUZ (''[[Paläontologisches Institut und Museum der Universität Zürich]]'', Paleontological Institute and {{ill|Paleontological Museum of the University of Zurich|lt=Museum|de|Paläontologisches Museum Zürich|fr|Musée paléontologique de Zurich}} of the [[University of Zurich]]) in [[Zurich, Switzerland]]. These specimens were discovered in the [[Grenzbitumenzone]] of [[Monte San Giorgio]], a [[World Heritage Site|UNESCO World Heritage Site]] on the Swiss-Italian border. Also known as the [[Besano Formation]] in [[Italy]], the Grenzbitumenzone has produced many well-preserved fossils from the [[Anisian]]-[[Ladinian]] boundary within the [[Middle Triassic]].<ref name=":0" /> |
|||
==Description== |
==Description== |
||
''Askeptosaurus'' was a fairly large thalattosaur, with a skull {{Convert|26|cm|in|abbr=off}} in length, and a total length up to {{Convert|3|m|ft|abbr=off}}.<ref name=":0" /> Like other [[Askeptosauroidea|askeptosauroids]], it had a long neck, a very long tail, and small but well-developed limbs with five independent digits.<ref name=":0" /><ref>{{Cite journal|last1=Rieppel|first1=Olivier|last2=Müller|first2=Johannes|last3=Liu|first3=Jun|year=2005|title=Rostral structure in Thalattosauria (Reptilia, Diapsida)|url=https://www.researchgate.net/publication/237169317|journal=Canadian Journal of Earth Sciences|volume=42|issue=12|pages=2081–2086|doi=10.1139/e05-076|bibcode=2005CaJES..42.2081R}}</ref> |
|||
| ⚫ | |||
''Askeptosaurus'' was a thin, elongated creature. Fossil records show that ''Askeptosaurus'' averaged around 2 to 3 meters (6.6 to 9.8 feet) in length.<ref>http://www.reptileevolution.com/askeptosaurus.htm</ref> Its tail was very long, accounting for almost half of the animal's total length. |
|||
=== Skull === |
|||
Morphological analysis on its short limbs and the expanded [[zeugopodia]] suggested the species to be adapted to near-shore [[Marine (ocean)|marine]] environment.<ref>http://www.ingentaconnect.com/content/nrc/cjes/2005/00000042/00000007/art00004</ref> Its webbed feet helped for steering through the water. Its slender body and long tail are believed to assist with [[aquatic locomotion]]. Judging from its elongated [[jaw]]s and the many teeth-covered [[palate]], its primary food source was fish.<ref name=EoDP>{{cite book |editor=Palmer, D.|year=1999 |title= The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals|publisher= Marshall Editions|location=London|page= 83|isbn= 1-84028-152-9}}</ref><ref name=EoD>{{cite book |editor=Müller, J.|year=2005 |title= The anatomy of Askeptosaurus italicus from the Middle Triassic of Monte San Giorgio and the interrelationships of thalattosaurs (Reptilia, Diapsida)|publisher= Canadian Journal of Earth Science|location=Canada|pages= 1347–1367}}</ref> |
|||
==== Cranium ==== |
|||
Typical features of thalattosaurs are the elongated [[premaxilla]] and the relatively large [[snout]], and a small or absent upper [[temporal fenestra]].<ref>http://www.nrcresearchpress.com/doi/pdf/10.1139/e05-076</ref> The [[cranium|cranial]] reconstruction of ''Askeptosaurus'' is consistent with the features previously listed: ''Askeptosaurus'' had a very slender and flattened skull. The snout was significantly elongated, the [[orbit]]s were comparatively large, and the posterior skull table was deeply emarginated. The [[premaxilla]] of ''Askeptosaurus'' is almost half as long as the skull (maximum length is 26 cm (10.2 inches) among the investigated specimens) and forms more than one-third of the tooth row in the upper jaw.<ref name="EoD"/> However, ''Askeptosaurus'' also had the following traits differ from the non-askeptosauroid thalattosaurs: |
|||
[[File:Askeptosaurus italicus head.JPG|thumb|The skull and a small part of the neck of specimen PIMUZ T 4831|left]]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]]e, which send back a long triangular projection into the [[Frontal bone|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]] (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]] (gap) between the premaxillary and maxillary tooth rows. Like other thalattosaurs, the [[Nasal bone|nasals]] 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.<ref name=":0" /> |
|||
Two small bones, the [[Lacrimal bone|lacrimal]] and [[Prefrontal bone|prefrontal]], lie in front of the large [[Orbit (anatomy)|orbit]] (eye socket). The presence of a separate lacrimal is a [[plesiomorphic]] (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 bone|postorbital]] and the much smaller [[Postfrontal bone|postfrontal]]. Once again, ''Askeptosaurus'' retains the plesiomorphic condition, since in other thalattosaurs these bones fuse into a single postorbitofrontal. The [[Parietal bone|parietals]], which lie at the rear of the skull roof, send out sprawling projections over the braincase. A slit-like [[Supratemporal fenestra|upper temporal fenestra]] develops between the parietal and postorbital, while a circular [[Parietal eye|pineal foramen]] is positioned near the parietal's suture with the frontal.<ref name=":0" /> |
|||
'''1.''' Lack of a fusion between postorbital and postfrontal. |
|||
The rear branch of each parietal host two slender bones, the [[Squamosal bone|squamosal]] and [[Supratemporal bone|supratemporal]]. 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 [[Infratemporal fenestra|lower temporal fenestra]], which is not closed from below due to the absence of a [[Quadratojugal bone|quadratojugal]]. 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 bone|quadrate]], which has broad upper and lower extents, and concave rear and lateral edges.<ref name=":0" /> |
|||
'''2.''' Presence of a homogenous dentition. |
|||
==== Palate, braincase, and lower jaw ==== |
|||
'''3.''' Absence of a [[diastema]] and palatal dentition. |
|||
[[File:Askeptosaurus.jpg|thumb|Specimen MSNM V456, one of the most complete fossils of ''Askeptosaurus italicus'']]Unlike [[Thalattosauroidea|thalattosauroids]], the [[palate]] (roof of the mouth) was completely toothless. The [[vomer]] is long and slender, forming the lower midline of the snout. The succeeding [[Pterygoid bone|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 bone|palatine]] connects the pterygoid to the maxilla.<ref name=":0" /> |
|||
The upper part of the [[Neurocranium|braincase]] is poorly known, but the lower part is less obscure. The [[parabasisphenoid]] (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]] (lower rear plate), which forms the [[Occipital condyles|occipital condyle]]. The [[foramen magnum]] is flanked by distinct [[exoccipitals]], which are connected (but not fused) to elongated and posterolaterally-directed [[opisthotics]]. The [[supraoccipital]], which roofs the foramen magnum, has a strong fan-shaped crest on it.<ref name=":0" /> |
|||
'''4.''' Presence of low neural spines. |
|||
The lower jaw is slender, with 20 teeth on the [[dentary]] which are similar to those of the upper jaw. The dentary sheaths over the entire [[splenial]] and much of the [[Angular bone|angular]], making it by far the longest bone when the jaw is seen in lateral view. A sharp and subtriangular [[Coronoid process of the mandible|coronoid]] rises up in the rear half of the jaw, above the low but elongated [[surangular]]. 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.<ref name=":0" /> |
|||
'''5.''' Presence of a slender radius.<ref name=EoD /><ref>{{cite journal |author=Cheng, L. |year=2011 |title=New Study of Anshunsaurus huangnihensis Cheng, 2007 (Reptilia: Thalattosauria): Revealing its Transitional Position in Askeptosauridae |journal=Journal of the Geological Society of China|volume=85|pages=1231–1237 |issue=6|doi=10.1111/j.1755-6724.2011.00584.x|url=http://onlinelibrary.wiley.com/doi/10.1111/j.1755-6724.2011.00584.x/full}}</ref> |
|||
=== Postcrania === |
|||
According to the research, the vertebrae of ''Askeptosaurus'' were amphicoelous with no sign of pachyostosis. Based on observations, ''Askeptosaurus'' possessed at least 38 presacral vertebrae, two sacral vertebrae, and more than 60 caudal vertebrae. There is double-headed rib articulation in the cervical region and single-headed articulation in the dorsal area, which gave the estimated number of cervical vertebrae at least 13. Sacral and caudal ribs are all significantly shorter than the dorsal ribs and have holocephalous heads.<ref name=EoD /> |
|||
==== Vertebrae ==== |
|||
The maximum length within the investigated specimens on femur is 13.8 cm (5.4 inches). The distal tarsals have different sizes. The largest element is confirmed to be distal tarsal 4. The latter articulates with metatarsals 4 and 5 distally, while the remaining distal tarsals meet the associated metatarsal.<ref name=EoD /> The phalangeal formula of the pes is 2-3-4-4-4. |
|||
| ⚫ | |||
[[File:Askeptosaurus italicus.JPG|thumb|Specimen PIMUZ T 4846|left|276x276px]]All of the vertebrae were amphicoelous, with concave front and rear faces of their respective [[Vertebral centrum|centrum]] (main body). The neck was fairly elongated, with 13 [[Cervical vertebrae|cervical (neck) vertebrae]]. The components of the [[Atlas (anatomy)|atlas]] (atlantal neural arch, intercentrum, and proatlases) were not connected, while the [[Axis (anatomy)|axis]] had a low and long neural spine and a large single rib facet. The atlantal and axial [[cervical rib]]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.<ref name=":0" /> |
|||
The 25 [[Thoracic vertebrae|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. [[Gastralium|Gastralia]] are slender but poorly-studied. The two [[Sacrum|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 [[Chevron (anatomy)|chevrons]] and neural spines which were shorter and more sharply inclined backwards.<ref name=":0" /> |
|||
| ⚫ | |||
''Askeptosaurus'' and the species ''Askeptosaurus italicus'' were first used and described in a 1925 research done by [[Hungary|Hungarian]] [[paleontologist]] [[Franz Nopcsa von Felső-Szilvás]].<ref>{{cite book |editor=Nopcsa, F|year=1925 |title= Askeptosaurus, ein neues reptil der Trias von Besano: Centralblatt für Mineralogie, Geologie und Paläontologie|page= 265–2}}</ref> [[Specific epithet (zoology)|Specific epithet]] "italicus" is a scientific adjective used to describe "Italian" <ref>https://en.wiktionary.org/wiki/italicus</ref> |
|||
==== Limbs ==== |
|||
[[File:Askeptosaurus italicus forelimb.JPG|thumb|left|Forelimb of specimen MSNM V456|239x239px]]The [[Pectoral girdle|pectoral (shoulder) girdle]] includes an [[interclavicle]], [[clavicle]]s, [[scapula]]e, and [[coracoid]]s. The scapula and coracoid are broad and rounded, connecting to form a laterally-oriented [[Glenoid cavity|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]] (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 (bone)|radius]] and [[ulna]] 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]] and has a distinct hourglass-shaped form with a constricted shaft. The wrist has six to seven [[carpal bones]] including an [[intermedium]], [[ulnare]], and four small distal carpals. The middle three [[Metacarpal bones|metacarpals]] of the manus (hand) are slender, [[Fifth metacarpal bone|metacarpal V]] is slightly thicker, and [[First metacarpal bone|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.<ref name=":0" /> |
|||
The [[pelvis]] (hip) contains an [[Ilium (bone)|ilium]], [[Pubis (bone)|pubis]], and [[ischium]] and is generally similar to other thalattosaurs. The ilium was deep around the [[acetabulum]] (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 [[Tarsus (skeleton)|tarsals]] in the ankle. They include a large reniform (kidney-shaped) [[Astragalus bone|astragalus]], a subrectangular [[Calcaneus|calcaneum]], 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 [[Fifth metatarsal bone|metatarsal V]].<ref name=":0" /> |
|||
The most recent research on ''A. italicus'' is done by Dr. [[Johannes Müller (paleontologist)|Johannes Müller]] in 2005. He used the fossils preserved by MSNM (Museo di Scienze Naturali Milano, Italy) and PIMUZ (Paläontologisches Institut und Museum der Universität Zürich, Switzerland) to distinguish ''A. italicus'' from its sister taxa ''[[Anshunsaurus]]''. |
|||
==Classification== |
==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)<ref>{{cite journal |author=Renesto, S. |year=1992 |title=The anatomy and relationships of Endennasaurus acutirostris (Reptilia, Neodiapsida) from the Norian (Late Triassic) of Lombardy. |journal=Rivista Italiana di Paleontologia e Stratigrafia|volume=97 |pages=409–430}}</ref> 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]]. Within [[Askeptosauridae]], Askeptosaurus is considered to be the [[Sister group|sister taxon]] of ''[[Anshunsaurus]]'', from [[Middle Triassic]] deposits in [[Guizhou]], [[China]].<ref>{{cite journal |author=Liu, J. |year=1999 |title=Sauropterygian from Triassic of Guizhou, China |journal=Chinese Science Bulletin |volume=44 |pages=1312–1316 |issue=14|doi=10.1007/BF02885852 |bibcode=1999ChSBu..44.1312L |s2cid=129934751 }}</ref> At present, the Askeptosauroidea is only known from the Alpine [[Triassic]] and southern China, and ''Askeptosaurus'' represents the oldest record for this [[clade]].<ref>{{cite journal |author=Cheng, L. |year=2011 |title=New Study of Anshunsaurus huangnihensis Cheng, 2007 (Reptilia: Thalattosauria): Revealing its Transitional Position in Askeptosauridae |journal=Journal of the Geological Society of China|volume=85|pages=1231–1237 |issue=6|doi=10.1111/j.1755-6724.2011.00584.x|bibcode=2011AcGlS..85.1231C |s2cid=129819570 }}</ref> |
||
[[File:Askeptosaurus italicus.JPG|thumb|Fossil]] |
|||
| ⚫ | Prior to 2005, |
||
{{clade| style=font-size:100%;line-height:80% |
{{clade| style=font-size:100%;line-height:80% |
||
| Line 63: | Line 63: | ||
|1='''''Askeptosaurus italicus''''' |
|1='''''Askeptosaurus italicus''''' |
||
}} }} }} }} |
}} }} }} }} |
||
==Paleobiology== |
|||
[[File:Askeptosaurus italicus head.JPG|thumb|right|Skull and a small part of neck]] |
|||
''Askeptosaurus'' probably hunted in deep waters, because it had large eyes suited to conditions of low light. However, similar to [[ichthyosaur]]s it also had a protective bony ring around the eyes, which would have prevented them from collapsing under the immense [[water pressure]] of great depths.<ref name=EoDP/> |
|||
==Paleoecology== |
==Paleoecology== |
||
''A. italicus'' was |
''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 [[Thalattosauria|thalattosaurs]]: ''[[Clarazia]]'' and ''[[Hescheleria]].''<ref name=":0" /> |
||
==References== |
==References== |
||
{{ |
{{Reflist}} |
||
| ⚫ | |||
{{Taxonbar|from=Q602583}} |
|||
==External links== |
|||
{{portal|Paleontology}} |
|||
*The anatomy of ''Askeptosaurus italicus'' from the Middle Triassic of Monte San Giorgio and the interrelationships of thalattosaurs (Reptilia, Diapsida) [http://www.ingentaconnect.com/content/nrc/cjes/2005/00000042/00000007/art00004] |
|||
* The reconstructed palatine image of ''Askeptosaurus'' [https://pterosaurheresies.files.wordpress.com/2012/09/askepto-muller-palate888.jpg] |
|||
| ⚫ | |||
| ⚫ | |||
[[Category:Middle Triassic reptiles of Europe]] |
[[Category:Middle Triassic reptiles of Europe]] |
||
[[Category:Fossils of Italy]] |
|||
| ⚫ | |||
| ⚫ | |||
[[Category:Prehistoric reptile genera]] |
[[Category:Prehistoric reptile genera]] |
||
[[Category:Aquatic reptiles]] |
[[Category:Aquatic reptiles]] |
||
[[Category:Neodiapsids]] |
|||
[[Category:Saurians]] |
|||
| ⚫ | |||
Latest revision as of 10:47, 7 June 2024
| Askeptosaurus Temporal range: Middle Triassic,
| |
|---|---|
| |
| A cast of specimen MSNM V456 | |
Scientific classification 
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Thalattosauria |
| Family: | †Askeptosauridae |
| Genus: | †Askeptosaurus Nopcsa, 1925 |
| Species: | †A. italicus
|
| Binomial name | |
| †Askeptosaurus italicus Nopcsa, 1925
| |
Askeptosaurus is an extinct genus of askeptosauroid, a marine reptile from the extinct order Thalattosauria. Askeptosaurus is known from several well-preserved fossils found in Middle Triassic marine strata in what is now Italy and Switzerland.[1]
History of discovery
[edit]Askeptosaurus, and its only known species Askeptosaurus italicus, were first named and described in 1925 by Hungarian paleontologist Franz Nopcsa von Felső-Szilvás.[2] 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) in Milan, Italy, and the PIMUZ (Paläontologisches Institut und Museum der Universität Zürich, Paleontological Institute and Museum of the University of Zurich) in Zurich, 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 in Italy, the Grenzbitumenzone has produced many well-preserved fossils from the Anisian-Ladinian boundary within the Middle Triassic.[1]
Description
[edit]Askeptosaurus was a fairly large thalattosaur, with a skull 26 centimetres (10 inches) in length, and a total length up to 3 metres (9.8 feet).[1] Like other askeptosauroids, it had a long neck, a very long tail, and small but well-developed limbs with five independent digits.[1][3]
Skull
[edit]Cranium
[edit]
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 premaxillae, 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 (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 (gap) between the premaxillary and maxillary tooth rows. Like other thalattosaurs, the nasals 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.[1]
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 (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 and the much smaller 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 develops between the parietal and postorbital, while a circular pineal foramen is positioned near the parietal's suture with the frontal.[1]
The rear branch of each parietal host two slender bones, the squamosal and supratemporal. 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. 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.[1]
Palate, braincase, and lower jaw
[edit]
Unlike thalattosauroids, the palate (roof of the mouth) was completely toothless. The vomer 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.[1]
The upper part of the braincase is poorly known, but the lower part is less obscure. The parabasisphenoid (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 (lower rear plate), which forms the occipital condyle. The foramen magnum is flanked by distinct exoccipitals, which are connected (but not fused) to elongated and posterolaterally-directed opisthotics. The supraoccipital, which roofs the foramen magnum, has a strong fan-shaped crest on it.[1]
The lower jaw is slender, with 20 teeth on the dentary 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 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.[1]
Postcrania
[edit]Vertebrae
[edit]
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 (atlantal neural arch, intercentrum, and proatlases) were not connected, while the axis had a low and long neural spine and a large single rib facet. The atlantal and axial cervical ribs 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.[1]
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 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 and neural spines which were shorter and more sharply inclined backwards.[1]
Limbs
[edit]
The pectoral (shoulder) girdle includes an interclavicle, clavicles, scapulae, and coracoids. 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 (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 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 and has a distinct hourglass-shaped form with a constricted shaft. The wrist has six to seven carpal bones including an intermedium, ulnare, and four small distal carpals. The middle three metacarpals 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.[1]
The pelvis (hip) contains an ilium, pubis, and ischium and is generally similar to other thalattosaurs. The ilium was deep around the acetabulum (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, a subrectangular calcaneum, 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.[1]
Classification
[edit]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)[4] 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. Within Askeptosauridae, Askeptosaurus is considered to be the sister taxon of Anshunsaurus, from Middle Triassic deposits in Guizhou, China.[5] At present, the Askeptosauroidea is only known from the Alpine Triassic and southern China, and Askeptosaurus represents the oldest record for this clade.[6]
| Askeptosauroidea |
| |||||||||||||||
Paleoecology
[edit]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.[1]
References
[edit]- ^ a b c d e f g h i j k l m n o Müller, Johannes (2005). "The anatomy of Askeptosaurus italicus from the Middle Triassic of Monte San Giorgio and the interrelationships of thalattosaurs (Reptilia, Diapsida)". Canadian Journal of Earth Sciences. 42 (7): 1347–1367. Bibcode:2005CaJES..42.1347M. doi:10.1139/e05-030.
- ^ Nopcsa, F, ed. (1925). Askeptosaurus, ein neues reptil der Trias von Besano: Centralblatt für Mineralogie, Geologie und Paläontologie. pp. 265–2.
- ^ Rieppel, Olivier; Müller, Johannes; Liu, Jun (2005). "Rostral structure in Thalattosauria (Reptilia, Diapsida)". Canadian Journal of Earth Sciences. 42 (12): 2081–2086. Bibcode:2005CaJES..42.2081R. doi:10.1139/e05-076.
- ^ Renesto, S. (1992). "The anatomy and relationships of Endennasaurus acutirostris (Reptilia, Neodiapsida) from the Norian (Late Triassic) of Lombardy". Rivista Italiana di Paleontologia e Stratigrafia. 97: 409–430.
- ^ Liu, J. (1999). "Sauropterygian from Triassic of Guizhou, China". Chinese Science Bulletin. 44 (14): 1312–1316. Bibcode:1999ChSBu..44.1312L. doi:10.1007/BF02885852. S2CID 129934751.
- ^ Cheng, L. (2011). "New Study of Anshunsaurus huangnihensis Cheng, 2007 (Reptilia: Thalattosauria): Revealing its Transitional Position in Askeptosauridae". Journal of the Geological Society of China. 85 (6): 1231–1237. Bibcode:2011AcGlS..85.1231C. doi:10.1111/j.1755-6724.2011.00584.x. S2CID 129819570.
| |||||||||||||
| |||||||||||||
