Palaeos		Sauropterygia
Vertebrates		Pistosauridae

Sauropterygia: Pistosauridae

DIAPSIDA
|--ARCHOSAUROMORPHA
|
LEPIDOSAUROMORPHA
|--SAUROPTERYGIA
|  |--Placodontia
|  |--Wumengosaurus
|  |==Pachypleurosauridae
|  `--+--Nothosauridae
|     `--+--Corosaurus     PISTOSAUROIDEA 
|        `--+--Cymatosaurus
|           `--+--Pistosaurus 
|              `--+--Yunguisaurus  
|                 `--+--Augustasaurus  
|                    `--+--Pliosauroidea
|                       `--Plesiosauroidea
|                          |--Elasmosauridae
|                          `--Cryptocleidoidea
`--Lepidosauriformes
   |--SPHENODONTIA
   `--SQUAMATA                

1. Augustasaurus X
2. Corosaurus X
3. Cymatosaurus X
4. Pistosauroidea X
5. Pistosaurus X
6. Yunguisaurus X

Life reconstruction of Augustasaurus hagdorni, a pistosaur from the Middle Triassic of Nevada (West Pangaea). Artwork by Nobu Tamura, 2007. Wikipedia, GNU Free Documentation/Creative Commons Attribution

The Pistosaurs are morphologically intermediate between nothosaurs and the Plesiosauria proper.  See image at right by J. Aragón from PLESIOSAURIOS. (The head may be proportionally too small in this picture; compare with skeletal recosntruction below). Pistosaurus, from the Middle Triassic (Ladinian epoch).  Until recently, Pistosaurus was until recently the only genus in its family, but now we add the American Augustasaurus, the Chinese Yunguisaurus , and a number of other forms. At first interpreted cladistically as a monophyletictaxon (Rieppel et al 2002), the group is now considered a paraphyletic grade (Cheng et al 2006). We have retained the family ranked term Pistosauridae here for this assemblage because we don't believe anything can be gained by forcing Linnaean ranks to conform to Phylogenetic nomenclature (or vice versa). However, the (unoffical?) clade Pistosauroidea is used in this context.

Pistosaurus, as a typical (and eponymous) represetative of the pistosaurs, had a nothosaur-like body with a plesiosaur-like head.  In Evolutionary systematics, it is variously classified under the Nothosauridae and the Plesiosauria, depending on what characteristics one uses to define membership of each group.  Even if it is a nothosaur it is acknowledged to be closely related to the ancestors of plesiosaurs.

These may have been ocean going creatures.  They are known from both sides of Pangea (Europe/Tethyan province and China, and North America) so they seem to have had a wide distribution.  Nobody has as yet done an analysis on the way these animals moved so we do not know whether it swam like a crocodile (typical nothosaurid) or paddled like a turtle or sea lion (plesiosaurian).  In view of the nothosaurid body it is likely that with Pistosaurus the characteristic plesiosaur locomotor pattern had not yet evolved.

Pistosaurs had fibrolamellar bone in contrast to the standard reptilian lamellar-zonal bone type of Nothosaurus. They may or may not have been descended directly from pachypleurosaurs like Anarosaurus; there is a contradiction here between histology and gross physiology, similar to the sorts of contradictions one encounters between morphology-blased cladistic studies and molecular sequenced phylogenies. It has been suggested that this indicates a superior metabolism which would have given them an advantage (faster growth, cold temeperture tolerance). In this way the group could became globally distributed, spreading to colder waters, and finally giving rise to the pelagic plesiosauria that dominated the post-Triassic oceans (Klein 2010 p.23). This does not have to have been the case though, because during the Triassic and indeed most of the Mesozoic, temperatures were warm and tropical worldwide, and global distribution is easily explained in terms of changes from pachypleurosaur and nothosaur to plesiosaur morphotype, sea-turtles for example, who are ectothermic also have a global distribution in warmer waters (the Quaternary environment being rather colder than the Mesozoic one). Fibrolamellar bone in sauropterygians is better explained in terms of rapid growth when young. Because small animals are more at risk from preditors, the less time spent small the greater the likelihood of overall survival. So it may have been their growth regime rather than overall metrabolism that gave sauropterygia the edge over their Triassic competitors. MAK991210, revised MAK111103

Descriptions

Pistosauroidea

Range: Middle Triassic to Late Cretaceous

Phylogeny: Eusauropterygia : (Simosaurus + Nothosauridae) + * : Corosaurus + (Pistosaurus + (Pliosauroidea + Plesiosauroidea)

Comments: Inclusive clade for Corosaurus, Cymatosaurus, Augustasaurus, Pistosaurus and Plesiosauria (Plesiosaur Directory) MAK111101

Links: Wikipedia

Corosaurus: C. alcovensis Case 1936a. 

Range: Early Triassic to Late Triassic of North America

Phylogeny: Pistosauroidea : * + (Cymatosaurus + (Pistosaurus + (Yunguisaurus + (Augustasaurus + Plesiosauria))))

Links: The Journal of Vertebrate Paleontology; The Plesiosaur Site - Species.

Comment: Most primitive (basal) pistosaur (Rieppel 1997b)

References: Holmes et al. (2008) [HCW08]. ATW020627, rev'd ATW080330, MAK111125.

Cymatosaurus.

Range: Uppermost Buntsandstein and lower Muschelkalk (Early Middle Triassic - Early Anisian of Germany, Austria and Poland, possibly also Netherlands and Israel?

Phylogeny: Pistosauroidea : Corosaurus + ((Pistosaurus + (Yunguisaurus + (Augustasaurus + Plesiosauria)))+ *)

Comments: Small to large pistosaurs with moderately flattened skull, in cladistic analaysis comes out between Corosaurus and Pistosaurus (Rieppel 1997b). Known from cranial material, limb bones attributed to this genus may not belong with this taxon (Klein 2010) Ketchum and Benson, 2011 place this form in the Pistosauroidea but outside outside the Pistosauria

Links: Plesiosaur directory. MAK111103.

References: Rieppel 1997b

Pistosaurus:  P. grandaevus von Meyer 1855, P. longaevus von Meyer 1839

Range:  Middle Triassic of Europe & North America, Ladinian, Upper Muschelkalk

Phylogeny: Pistosauroidea : Corosaurus + (Cymatosaurus + ((Yunguisaurus + (Augustasaurus + Plesiosauria)) + *))

Characters: ~3m; vomeronasal organ present; very long neck; body form streamlined; more rigid spine as in plesiosaurs; gastralia present; limb girdles not strongly bound together as in Plesiosauria; limbs formed as paddle-shaped flippers.

Note: Pistosaurus is now the flagship genus of the Pistosauridae, the other member of the family being Augustasaurus hagodorni Sander, Rieppel & Bucher 1997.  See The Journal of Vertebrate Paleontology.   Since then additional genera have been added.

Links: Nothosaurus- Enchanted Learning Software; A Manual of the Anatomy of Vertebrated Animals (1881); Dinosaurios: Pistosaurus; PLESIOSAURIOS (Spanish J. Aragón); The Plesiosaur Site - Species; The Journal of Vertebrate Paleontology; pistosaurus. 

References: Holmes et al. (2008) [HCW08]. ATW020630, rev'd ATW080330, MAK111101.

Image: David Peters - Reptile Evolution

Yunguisaurus:  Y. liae Cheng et al 2006

Range:  Late Triassic(Carnian) of southern China

Phylogeny: Pistosauroidea : Corosaurus + (Cymatosaurus + (Pistosaurus + ((Augustasaurus + Plesiosauria) + *)))

Size: preserved skeleton measures about 1.7 m from snout-tip to end of the incomplete tail; skull: length 12.6 cm, maximum width 6.3 cm

Comments: combination of primitive and derived traits, cladistic analysis places it between Pistosaurus and Augustasaurus

References: Cheng et al 2006 MAK111101

Augustasaurus:  A. hagdorni Sander et al., 1997

Range: Middle Triassic (Late Anisian) of Nevada,

Phylogeny: Pistosauroidea : Corosaurus + (Cymatosaurus + (Pistosaurus + (Yunguisaurus + ( (Pliosauroidea + Plesiosauroidea) + *))))

Comments: from Wikipedia "Augustasaurus' skull shares many general characteristics with its relative, Pistosaurus, such as tall, blade-like upper temporal arches.[1] The skull's elongated rostrum tapers to a dull point, the anterior premaxillary and maxillary teeth have been described as "fang-like", and the squamosal makes a box-like suspensorium. The dorsal neural spines of Augustasaurus are low with rugose tops. Its coracoids are large plates similar to those in other plesiosaurs. However, the coracoid foramen are missing from Agustasaurus, in a way similar to those in the plesiosaur Corosaurus.[5] Its cervical ribs have anterior process, and like most plesiosaurs, Augustasaurus' vertebrae have "thickened transverse processes" "

Comments: Originally included with Pistosaurus in a monophyletic Pistosauridae (Rieppel et al 2002); Cheng et al 2006 makes the Pistosauridae and places Augustasaurus as the sister group to all higher Plesiosauria

References: Sander et al 1997, Rieppel et al 2002 O'Keefe 2001) MAK111101

Image: David Peters - Reptile Evolution (redrawn from Rieppel et al 2002 fig.3)