MOSASAURS AND PLIOSAURS

["Darren Naish" <dwn194@soton.ac.uk>]

Stan replied to a post asking about mosasaurs and pliosaurs, and a few of the
points he made cause me to de-cloak from temporary lurk mode. 

MOSASAUR RELATIONSHIPS

Mosasauroids (mosasaurids and all of their close relatives) are platynotan
lizards that, in the most recent analyses, share 40 shared derived characters
with snakes (Ophidia) - therefore they share a single ancestor and form the
clade Pythonomorpha Cope, 1869. Varanids and lanthanotids (Bornean earless
monitor and extinct relatives) form a clade and out-group to pythonomorphs: the
varanid-lanthanotid clade is not named AFAIK, but (varanids + lanthanotids) +
pythonomorphs are the Thecoglossa. Helodermids/helodermatids (gila monsters etc)
out-group to thecoglossans. 

So... mosasaurs are not actually as close to monitors as used to be implied -
 the case constructed by palaeoherpetologists Mike Lee and John Scanlon is
pretty much concrete, and most of the workers I've spoken to that are aware of
this work support it virtually 100%. Mosasaurs are most closely related to
snakes. As you may note by the fact that Cope named Pythonomorpha for this very
clade way way back in 1869, the idea is not new and striking mosasaur-snake
similarities (e.g. zygosphene-zygantrum articulations) have been generally
overlooked because few workers ever had the time or incentive to include
mosasaurs and snakes in the same analysis. 

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!).

PLIOSAURS

'Pliosaur' is an ambiguous term that, in the past, has been applied to any
plesiosaur with a shortish neck and large head. One group of such animals, the
mid-->Upper Cretaceous polycotylids, are very distinct from other 'pliosaurs'
and are almost certainly related to elasmosaurids (Bakker 1993 and Carpenter
1995, 1997). True pliosaurids, in contrast, are primitive in the plesiosaur
family tree as testified by their complete set of mandibular bones, and out-
group to all other plesiosaurs (clade Plesiosauroidea). Pliosaurids first appear
in the very earliest Jurassic (Hettangian) at Dorset with _Eurycleidus_ and
other indeterminate taxa but did not (as far as is presently known) make it as
far as the late Upper Cretaceous (polycotylids made it to the end). The term
'pliosaur' should not be used as it refers to an ecotype and not a clade.

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. 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. 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.

These reptiles are modified diapsids and may be neodiapsids up there in the
younginiform-sauria crown group. I'm unaware of any derived characters that link
sauropterygians (incl. placodonts) with either younginiforms or lepidosaurian
saurians (cases have been made for both alternatives). OTOH, perhaps the nearest
relatives to sauropterygians are araescelidians (incl. _Petrolacosaurus_) in
which case they are basal diapsids and not neodiapsids. The description in
recent years of a (probable) marine araescelidian with swimming adaptations,
_Spinoequalis_, implies that this relationship is a possibility. But it would
be a good idea to learn the character distributions before making suggestions
like this wouldn't it..

I wrote this without the refs to hand so any errors are mine.

"The possibilities of existence run so deeply into the extravagant that there
is scarcely any concept too extraordinary for Nature to realise"
                                                     - Louis Agassiz

DARREN NAISH