Palæos:		Unit 220: Lepidosauromorpha
The Vertebrates		200: Pachypleurosauridae

Lepidosauromorpha: Pachypleurosauridae

EUREPTILIA
|--ARCHOSAUROMORPHA
|
Lepidosauromorpha
|--Sauropterygia
|  |--Pachypleurosauridae
|  |  |--Hanosaurus  
|  |  `--+--+--Dactylosaurus 
|  |     |  `--Anarosaurus  
|  |     `--+--Serpianosaurus  
|  |        `--Neusticosaurus  
|  `--Eusauropterygia  
|     |--Nothosauridae
|     `--Plesiosauria
|        |--Pliosauroidea
|        `--Plesiosauroidea
|           |--Elasmosauridae
|           `--Cryptocleidoidea
`--Lepidosauriformes
   |--RHYNCHOCEPHALIA
   `--SQUAMATA                

1. Hanosaurus X

2. Pachypleurosauridae X

Neusticosaurus edwardsii.  Middle Triassic - Tethys Sea; length about 60 cm.

The Pachypleurosaurs

The pachypleurosaurs were small, lightly built lizard-like reptiles, generally less than a meter (the smallest about 20 cm).  They were clearly related to the nothosaurs, with which they were contemporary.  For a long time the pachypleurosaurs were included among the nothosaurs, and originally even placed in the same family Nothosauridae.  It is now coming to be realized that they represent a distinct group that was only related via a primitive sauropterygian common ancestor.

The neck is long but, unlike the Nothosauridae, the head is very small (like Claudiosaurus), so pachypleurosaurs probably fed on very small fish and Crustacea.  Relatively small dorsal temporal openings in the skull also clearly distinguish the pachypleurosaurs from the nothosaur genera (and their close relatives) such as Lariosaurus, Ceresiosaurus, Corosaurus, Paranothosaurus and Simosaurus, in all of which the upper temporal openings are much larger than the orbits.  The tail is deep and would have been used as a swimming organ.  The legs were semi-flattened to serve as paddles, but the hips and shoulders were still powerful and functional enough to provide support on land.  It is likely that these animals were able to crawl about on land like modern seals, dragging themselves by their strong forelimbs.  As with the nothosaurids they were semi-aquatic forms, mostly lagoonal but also venturing out to sea, perhaps not unlike the modern marine iguana in that respect.  Their small size meant however they probably did not venture out far, being primarily near-shore forms. 

MAK991211.

Descriptions

Pachypleurosauridae: Nopsca 1928.  Neusticosaurus Seeley, 1882; Anarosaurus Dames, 1882; Dactylosaurus Bardet, 1992; Hanosaurus Lee, 1958; Serpianosaurus Rieppel 1989. Pachypleurosaurus and Phrygosaurus are considered nomina dubia, and have been sunk into Neusticosaurus and Serepianosaurus, respectively. 

Range: Middle Triassic (possibly Early Triassic in China) (possibly to Late Triassic [CG85] [RK95] in Europe) of Central Europe & China. [R95]. 

Phylogeny: Nothosauria : Eusauropterygia + * : Hanosaurus + ((Dactylosaurus + Anarosaurus) + (Serpianosaurus + Neusticosaurus)). 

Characters: S-M size (<150 cm) [CG85]; similar to nothosaurs; skull unusually small [CG85]; skull not depressed as in Nothosauridae [R89]; prefrontal broad & domed [RK95]; frontals with concave lateral margin participating in orbit [RK95]; postorbital skull shorter than preorbital [R89] [RK95]; upper fenestra smaller than large orbit [CG85]; strong upper bar; no lower temporal bar [CG85]; UTF usually small [R89]; postorbital primitively enters margin of UTF [R89]; quadrate embayed (impedence-matching ear for airborne vibrations & thus amphibious?) (primitive) [R89] [RK95]; jaw joint not behind occipital condyle [R89] [RK95]; pterygoids expanded to completely cover braincase ventrally [CG85]; ectopterygoid absent [R89] [RK95] (contra [CG85]); jaws not elongated; jaw with retroarticular process bearing dorsal trough [RK95] [3]; anterior teeth nor strongly procumbent [RK95]; vertebrae amphicoelous throughout [R89]; long (18 cervicals) neck; dorsal vertebrae have low neural spines, thickened and dense (pachyostosis -- see Note); pachyostosis extends to ribs in more derived species; 3 sacral vertebrae; dorsoventral extension of vertebral spines near base of tail; girdles poorly ossified (thus obligate marine?); limbs not highly derived for aquatic life; sexual dimorphism in forelimb [ORS99]; interclavicle, posterior process rudimentary or absent [RK95]; radius slightly longer than ulna [RK95]; juveniles with entepicondylar foramen distal edge of ossified portion of humerus [R95]; iliac blade reduced to narrow dorsal process [RK95]; restricted to epicontinental & coastal areas. Appear to have originated in China and migrated to Europe via the northern border of Tethys Sea [RK95].

Notes: [1] Pachyostosis, the thickening of bones, is common in recently aquatic tetrapods. Examples include mammalian Sirenia (e.g. manatees) and an unusual plesiosaur, Pachycostasaurus. deRicqlès (1989). But see Gaudant & Meunier (1996). This may have more to do with ballast for a benthic habit.  

[2] The current belief is that the pachypleurosaur line forms part of the bridge between the Sauropterygian stem (e.g. placodonts) and the Eusauropterygia (e.g. plesiosaurs). The connection is an admittedly difficult one to make, but the fossil record gives few options. To make this phylogenetic region yet stranger, a prolonged stare at the pachypleurosaurs tends to bring turtles to mind -- yet another taxon of still-uncertain affinities.  Look, for example, at the way the dorsal and caudal ribs seem to rise up above the level of the sacrum (the fused area of bone at the intersection of the pelvis and the spine) in some photographs of the nothosaur, Keichousaurus.  Turtles are the only group of vertebrates with both sets of limb girdles inside the rib cage.  Yet it would take little modification to give this unique character to a pachypleurosaur.  The pachyostosis of the ribs, from which the group derived its name, is similar to the broad, heavy ribs of turtles.  Although pachypleurosaurs themselves are unarmored, one group of placodonts has strikingly turtle-like armor.  This relationship between sauropterygians and turtles was widely defended at the end of the last century.  It seemed to fall apart, on details of skull anatomy among other things.  However, the concept has recently been revived by Olivier Rieppel and others.  The idea has also gained some support from DNA cladistics which tends to place the turtles up above the diapsid split, although their placement is sometimes (perhaps embarrassingly) closer to archosaurs than to lepidosaurs.  However, given the total absence of (other?) living sauropterygians, a little uncertainty is only to be expected. 

[3] The "trough" may be the same as what Sander (1989) identifies as a pit to receive the posterior process of the quadrate in Neusticosaurus.  See Some Jaw Notes. 

Links: Fossil Room; The Pleasiosaur Site; Pachypleurosauridae (Mikko's Phylogeny); Denver Gem, Mineral, Fossil Show - Sept 2004

References: Carroll & Gaskill (1985) [CG85]; O'Keefe et al. (1999) [ORS99]; Rieppel (1989) [R89]; Rieppel (1995) [R95]; Rieppel & Hagdorn (1997); Rieppel & Kebang (1995) [RK95].  ATW070113.

Hanosaurus:

Range: China

Phylogeny: Pachypleurosauridae : ((Dactylosaurus + Anarosaurus) + (Serpianosaurus + Neusticosaurus)) + *.

Characters: 

Links: the journal of vertebrate paleontology (abstract of Rieppel's authoritative1998 article); -- Professor Paul's Guide to Reptiles - Sauropterygia; plesiosaurs ....   

checked ATW050109