some additions to Re: MOSASAURS AND PLIOSAURS

[Pieter.Depuydt@rug.ac.be]

The following comments are meant as additions, not critique, to an otherwise 
very 
up-to-date and comprehensive review made by Darren.

> MOSASAUR RELATIONSHIPS

> The description or re-description in recent years of basal snakes
> (_Pachyrhachis_ in particular, and also the madtsoiids) - animals that turn 
> out
> to be so 'lizard'-like that workers have had to keep looking further back in 
> the
> squamate tree - has contributed to this approach. Basal snakes have 
> articulated
> pelves and hindlimbs and were marine. That mosasauroids are all marine argues
> that the first pythonomorph was too. Madtsoiids (most primitive snakes after
> _Pachyrhachis_ etc.) were still 'wet' and I reckon they were amphibious, while
> post-madtsoiid snakes adapted to full terrestriality and underwent a fossorial
> stage (hence those funny eyes!).

According to a study of Caldwell and Carroll of well preserved 
postcranial remains of the mosasauroid  aigialosaur Carsosaurus, the 
aigialosaur 
body was not very different from that of the anguimorph cousins in 
the proportion of the limbs to the body and the structure of the 
carpus. Carsosaurus also possessed a strong shoulder joint 
(humero-scapulocoracoid joint) and a well developed iliac blade for 
muscle attachment. So aigialosaurs were probably terrestrial animals 
that were only  facultative aquatic, and present an intermediate 
organisation between the fully terrestrial anguid lizards and the 
fully aquatic mosasaurs.
 
> PLIOSAURS

> Plesiosaurs represent a derived clade in the Sauropterygia Owen, 1860. This
> group contains a Triassic radiation of amphibious-marine forms long lumped
> together as a paraphyletic 'Nothosauria' but now consisting of discrete clades
> that became less land-dependent and more derived in style of paraxial 
> locomotion
> (i.e. they grade up the flight of plesiosaurs). Glen Storrs recognises the 
> most
> derived 'nothosaurs' (_Nothosaurus_, _Pistosaurus_ and relatives) as forming 
> the
> clade Nothosauriformes with the plesiosaurs.

According to a study of nothosaur braincases by Rieppel, a 
monophyletic family Nothosauridae can be defined (Nothosaurus, 
Paranothosaurus, Lariosaurus and Ceresiosaurus). Sister-group of 
Nothosauridae is Cymatosaurus, together they form the clade 
Nothosauria.

> Basal- and non-nothosauriforms are
> predominantly Tethyan in distribution and the presence of the 
> pachypleurosaurs -
>  a clade of lizard-shaped amphibious, small-bodied reptiles - here too 
> suggests
> that this is the sauropterygian home. Pachypleurosaurs have traditionally been
> regarded as 'nothosaurs' too but they are increasing regarded as a
> sauropterygian out-group.

Pachypleurosauroidea is the sister group of Eusauropterygia, within 
Sauropterygia,  according to Rieppel (Eusauropterygia including 
Simosaurus, Corosaurus, Nothosauria and Plesiosauria). 

> Whatever, they are primitive members of the group that
> includes plesiosaurs. Placodonts are a problem. Rieppel and Storrs have them 
> as
> true sauropterygians, but this is mildly controversial. The most comprehensive
> and current view of placodont relationships is a paper by Mazin in _Geobios_.
> It's in French and I haven't understood it all yet. Cladistic analysis of
> sauropterygian and placodont characters by Rieppel did nest Placodontia within
> Sauropterygia. If this view does not become accepted, perhaps resurrection of
> a Euryapsida (Placodontia + Sauropterygia) will do.

Rieppel also considers turtles and placodonts as sister groups if 
memory serves, so in his view, turtles are extant Sauropterygia...
Right?


Pieter Depuydt