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FW: Varanids and their relatives



>From Darren Naish:
----------------------
David Peters asked about ancestral varanid morphology.
The following may, or may not, be helpful.

Varanoidea Camp, 1923 was defined by Gao & Norell
(1998) as _Telmasaurus grangeri_ and Varanidae, and all
the descendants of their MRCA. The modern phylogenetic
definition of Varanidae includes only _Varanus_ and
_Lanthanotus_. The latter, the poorly known Bornean
earless monitor, is a rather odd-looking lizard with a
blunter, rounder snout than _Varanus_. Because (by
definition) it's a varanid, remember that varanid is not
synonymous with 'monitor'. _Varanus_ has of course been
split by some workers into multiple subgenera (traditionally
six: _Odatria_, _Varanus_, _Indovaranus_,
_Psammosaurus_, _Eupagusia_ and _Polydaedalus_), and
whether or not _Megalania_ deserves to be regarded as a
separate genus remains somewhat uncertain.

Several fossil taxa, including most notably_Cherminotus
longifrons_ Borsuk-Bialynicka, 1984 and _Aiolosaurus
oriens_ Gao & Norell, 2000, fall within Varanoidea.
Though the detailed anatomy is of course distinct, what is
known of these taxa looks superficially monitor-like.
_Cherminotus_ has been suggested to be particularly close
to _Lanthanotus_ (e.g., Lee 1997) but this hasn't been
supported by more recent data. _Palaeosaniwa_ has been
suggested to be a varanoid (e.g., Gao & Norell 1998, 2000)
or even varanid (Gao & Fox 1996) but Balsai (2001, phd
thesis) has shown that it was a (possibly venomous) stem-
group monstersaurian. _Saniwa_ has also been included
within Varanoidea by some authors but Caldwell has shown
that the type material is referable to the xenosaurid
_Restes_. Several other taxa at times suggested to be
varanoids or varanids, including _Parasaniwa_,
_Proplatynotia_, _Saniwides_ and _Eosaniwa_, have
proven difficult to place within Platynota but seem to
mostly lie near the base of this radiation (Gao & Norell
1998). Platynota = Varanidae + Monstersauria, as defined
by Gao & Norell 1998). Platynota Camp, 1928 was
originally coined to include Varanoidea and Mosasauroidea,
but later more widely employed as a synonym for Camp's
Varanoidea. As discussed below, Platynota probably does
include Mosasauroidea, but Gao & Norell have yet to
include the latter clade within their analyses (to my
knowledge).

So (in view of _Cherminotus_ and _Aiolosaurus_ at least),
fossil varanoids seem to have been pretty much similar to
_Varanus_. What about members of the varanid sister-
group, Monstersauria? This clade was defined by Norell
& Gao (1997) as _Gobiderma_, _Heloderma_ and all the
descendants of their MRCA (for a different interpretation
see Lee (1997) where no such clade is recognised, and
_Gobiderma_ and _Parviderma_ are allied in the new basal
varanoid clade Gobidermatidae [argued to be an artificial
grouping by Norell & Gao 1997]. In fact do note that the
phylogeny offered by Lee differs markedly from that of Gao
& Norell). Helodermatids have of course a highly
apomorphic morphology (e.g., steep nasal process of
maxilla, venom grooves on teeth, short tail, osteoderms
covering head and body) but stem-group monstersaurians
seem to have been rather more like varanoids. _Estesia_
Norell et al., 1992 for example, for which phenomenally
well preserved 3-D skulls and other material is known,
seems to have looked, superficially at least, like a living
monitor. Judging from published photos of its skull (see,
e.g., Gao & Norell, 2000, Fig. 31), the most basal
monstersaurian, _Gobiderma pulchrum_ Borsuk-Bialynicka,
1984 looked rather more like an intermediate between a
monitor and a helodermatid, with a muzzle about half as
wide as the back of the skull, and a covering of osteoderms.

Helodermatidae, incidentally, was phylogenetically defined
by Norell & Gao (1997) as all descendants of MRCA of
_Heloderma suspectum_ and _H. horridum_. As with so
many PT definitions, the problem with this is that it then
excludes a number of similar taxa that have historically
been regarded as helodermatids, including _Lowesaurus_
and _Eurheloderma_.

Finally, what of mosasauroids? The evidence including
them within Platynota seems pretty good, and as argued by
Lee, Caldwell and colleagues they are most likely the sister-
taxon to Varanoidea (if Mosasauroidea is sister to
Monstersauria + Varanidae, then by definition it is outside
of Platynota sensu Gao & Norell). Again, basal
mosasauroids (traditionally termed aigialosaurids) seem to
have looked rather monitor-like, though apparently with
longer bodies and proportionally smaller limbs. As is
reasonably well known even to those unfamiliar with
squamate systematics, it remains controversial whether
snakes are the sister-taxon to Mosasauroidea, forming with
them the platynotan clade Pythonomorpha. Such has been
argued by Lee (1997), Lee & Caldwell (1998, 2000), Lee et
al. (1999) etc. Most of the evidence marshalled by these
workers has been seriously challenged by Rieppel and co-
workers (Zaher 1998, Zaher & Rieppel 1999, 2000, 2002,
Rieppel & Zaher 2000a, b, 2001, Rieppel et al. 2003 etc.).
At the moment I would say that the phylogeny proposed by
Lee et al. is looking increasingly doubtful and snakes and
mosasauroids probably aren't much to do with one another
after all. As for what snakes really are, Rieppel and
colleagues appear to favour an affinity of snakes with
amphisbaenians and dibamids, in which case snakes are not
even anything to do with anguimorphs.

Successively more distant outgroups to Platynota within
Anguimorpha are anguids and xenosaurids, though studies
differ on which is closer to Platynota.

Refs - -

Gao, K. & Fox, R. C. 1996. Taxonomy and evolution of
Late Cretaceous lizards from western Canada. _Bulletin of
the Carnegie Museum of Natural History_ 33, 1-107.

- . & Norell, M. A. 1998. Taxonomic revision of _Carusia_
(Reptilia: Squamata) from the Late Cretaceous of the Gobi
Desert and phylogenetic relationships of anguimorphan
lizards. _American Museum Novitates_ 3230, 1-51.

_ . & Norell, M. A. 2000. Taxonomic composition and
systematics of Late Cretaceous lizard assemblages from
Ukhaa Tolgod and adjacent localities, Mongolian Gobi
Desert. _Bulletin of the American Museum of Natural
History_ 249, 1-118.

Lee, M. S. Y. 1997. The phylogeny of varanoid lizards and
the affinities of snakes. _Philosophical Transations of the
Royal Society of London B_ 352, 53-91.

- . & Caldwell, M. W. 1998. Anatomy and relationships of
_Pachyrhachis problematicus_, a primitive snake with
hindlimbs. _Philosophical Transations of the Royal Society
of London B_ 353, 1521-1552.

- . & Caldwell, M. W. 2000. _Adriosaurus_ and the
affinities of mosasaurs, dolichosaurs, and snakes. _Journal
of Paleontology_ 74, 915-937.

- ., Caldwell, M. W. & Scanlon, J. D. 1999. A second
primitive marine snake: _Pachyophis woodwardi_ from the
Cretaceous of Bosnia-Herzegovina. _Journal of Zoology_
248, 509-520.

Norell, M. A. & Gao, K. 1997. Braincase and phylogenetic
relationships of _Estesia mongoliensis_ from the Late
Cretaceous of the Gobi Desert and the recognition of a new
clade of lizards. _American Museum Novitates_ 3211, 1-25.

Rieppel, O. & Zaher, H. 2000a. The braincases of
mosasaurs and _Varanus_, and the relationships of snakes.
_Zoological Journal of the Linnean Society_ 129, 489-514.

- . & Zaher, H. 2000b. The intramandibular joint in
squamates, and the phylogenetic relationships of the fossil
snake _Pachyrhachis problematicus_ Haas. _Fieldiana,
Geology, New Series_ 43, 1-69.

- . & Zaher, H. 2001. Re-building the bridge between
mosasaurs and snakes. _Neues Jahrbuch fur Geologie und
Paläontologie, Abhandlungen_ 221, 111-132.

- ., Zaher, H., Tchernov, E. & Polcyn, M. J. 2003. The
anatomy and relationships of _Haasiophis terrasanctus_, a
fossil snake with well-developed hind limbs from the mid-
Cretaceous of the Middle East. _Journal of Paleontology_
77, 536-558.

Zaher, H. 1998. The phylogenetic position of
_Pachyrhachis_ within snakes (Squamata, Lepidosauria).
_Journal of Vertebrate Paleontology_ 18, 1-3.

- . & Rieppel, O. 1999. Tooth implantation and replacement
in squamates, with special reference to mosasaur lizards and
snakes. _American Museum Novitates_ 3271, 1-19.

- . & Rieppel, O. 2000. A brief history of snakes.
_Herpetological Review_ 31, 73-76.

- . & Rieppel, O. 2002. On the phylogenetic relationships of
the Cretaceous snakes with legs, with special reference to
_Pachyrhachis problematicus_ (Squamata, Serpentes).
_Journal of Vertebrate Paleontology_ 22, 104-109.

--
Darren Naish
School of Earth & Environmental Sciences
University of Portsmouth UK, PO1 3QL

http://web.port.ac.uk/departments/sees/staff/NaishD.htm
email: darren.naish@port.ac.uk
tel: 023 92846045
Darren Naish
School of Earth & Environmental Sciences
University of Portsmouth
Burnaby Building
Burnaby Road                           email:
darren.naish@port.ac.uk
Portsmouth UK                          tel: 023 92846045
PO1 3QL                                www.palaeobiology.co.uk


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