Palæos:		Unit 240: Squamata
The Vertebrates		100: Squamata

Squamata

LEPIDOSAUROMORPHA
|--RHYNCHOCEPHALIA
`--Squamata
   |--Iguania
   |  |--Tropiduridae
   |  |--Acrodonta
   |  |  |--Priscagamidae
   |  |  `--+--Agamidae
   |  |     |  |--Amphibolurinae
   |  |     |  `--Draconinae
   |  |     `--Chamaeleonidae
   |  `--Iguanids
   `--SCLEROGLOSSA

Taxa on This Page

1. Iguania

2. Squamata

3. Tropiduridae

Squamata: lizards, mosasaurs and snakes. 

Range: from the Early Jurassic. 

Phylogeny: Lepidosauria : Rhynchocephalia + * : Scleroglossa + Iguania. 

Characters: premaxillae fused [C&L]; sutured mandibular symphysis [LBC]; anterior & posterior lower jaw elements immobilized relative to each other [LBC]; $ angular short, not reaching level of mandibular condyle [C&L]; $ coronoid process large, formed only by coronoid bone (not surangular) [C&L]; $ surangular bears transverse shelf [d&R]; maxilla, palatine & pterygoid sutured to other elements and are akinetic [LBC]; $ palatines with choanal groove [C&L]; $ transverse flanges of pterygoid point anterolaterally & are at or above level of marginal teeth [d&R]; $ pterygoids do not meet at midline [C&L]; $ pterygoids broadly enter suborbital fenestrae [C&L]; nasals reduced; $ anteroventral border of orbit formed by jugal rather than maxilla (frequent reversals) [C&L]; $ posterior process of jugal absent [C&L]; $ frontal-parietal suture simple & transverse [d&R][C&L]; flexible frontal-parietal articulation (mesokinetic joint) allows some raising of snout [LBC]; frontals and/or parietals [$ per C&L] may be fused at midline; $ parietal table short, with posterior exposure of braincase [C&L]; metakinetic hinge between parietals and occiput [LBC]; $ loss of lower temporal bar [d&R]; $ loss of quadratojugal [d&R][C&L]; $ flexible hinge at articulation of quadrate and squamosal (streptostyly: rotation of quadrate around squamosal allows pterogoideus muscle to exert maximum force near closure) [d&R] [LBC]; quadrate embayed to support stapes for hearing; squamosal reduced; $ squamosal rod-like, without ventral process [C&L], & squamosal does not wrap around posterior of skull [d&R]; $ stapes slender [C&L]; $ proatlas absent [C&L]; vertebral centra amphicoelous in basal forms, derived forms are often procoelous ($ of [C&L]); $ >7 cervical vertebrae [C&L]; $ intercentra absent [C&L]; $ gastralia absent [C&L]; primitively, scapulocoracoid sometimes fenestrated, $ thyroid fenestration of pelvis [C&L]; ends of humerus not twisted >20 deg. relative to each other; primitively, proximal tarsals not fused; strong tendency to reduction or loss of limbs; determinate growth (related to insectivory?). 

Note: Remarkably little overlap between synapomorphies of C&L and d&R. Note how many of the C&L synapomorphies are either absences of a feature or quantitative differences (e.g. parietal short). These are not necessarily a bad thing, but do raise questions about homology. Recall that the issue is whether these are derived features inherited from a common ancestor. This is less likely to be true of absences and quantitative differences. 

Image: Lacerta vivipara © Bioimages (UK), used by permission.

Links: Rattlesnake... from Skulls Unlimited; The Boa Constrictor Subspecies; Literature - Squamata; SQUAMATA; EMBL; Wild Herps; Order Squamata; Discover Canada's Aquatic Reptiles - Snakes and Lizards: Order Squamata; Squamata Verlag (English & German); Palaeoscincosaurus middletoni; Encyclopedia.com - Results for snake; temp; Squamata (ToL); Squamata (Mikko's Phylogeny); SQUAMATA; Squamata (Snakes & Lizards). 

References: Caldwell & Lee (1997) [C&L]; deBraga & Rieppel (1997) [d&R]; Lee et al. (1999) [LBC].  ATW040220.

Iguania: Euposaurus  

Range: from the Late Cretaceous.

Phylogeny: Squamata: Scleroglossa + * : Tropiduridae + Acrodonta + Iguanidae. 

Characters: Skull tall; temporal arch is present; fleshy tongue; teeth pleurodont or acrodont; $ frontals fused; $ parietal foramen on frontal - parietal suture; eyelids well developed; pupils round; most have dorsal crest and extensible dewlap; often ornamented with crests, spines, frills, and throat fans; six cervical vertebrae; usually robust bodies, short necks, distinct heads; breaking-point septa present in caudals and some forms have rather fragile tails; overlapping non-iridescent scales; four or more transverse belly scales per body segment; frequently brightly colored. Smaller, insectivorous forms primitive, but larger herbivores where less competition, including marine forms. Largely ambush predators; diurnal; either arboreal, terrestrial or semi-marine. 

Links: Higher Reptile Taxa (EMBL database); Iguania; herplab9.pdf; herplab3.pdf; Eric Pianka's Ctenotus Research; home2; Iguanidae; Foraging and Trophic Ecology; Iguania (Best on the Web).  

The ToL page still has only a vague polyotomy for a cladogram, and a list of references a decade old.  As for the cladogram, perhaps that's all there is.  Mikko's Phylogeny has the same, but Mikko includes some more recent references.  ADW has some really excellent material on the individual families, but nothing worth visiting at this level.  The EMBL site is its usual authoritative and encyclopedic self, but is really designed for use at the genus level and below.  Even the Swiss-Prot folks have shown only limited interest in the gene sequences of this taxon.  Fortunately, true afficionados of lizard phylogeny on the web may consult Frost et al. (2001).  This is one of the combined morphological - molecular studies the AMNH has pioneered in recent years.  Other articles of interest include Delheusy & Bels (1999) on iguanid feeding kinematics. The only pages we located with a list of shared characters are brief lists on a VMNH page, a lab manual from the University of Texas and Anthony Herrel's site at the University of Antwerp. However, we have saved the best for last:  Vitt & Pianka (2005) combine squamate ecological and phylogenetic data to create a really convincing explanation of the phylogeny based on diversification of feeding strategies.

Note: in addition to the listed groups the Euposauridae and Arretosauridae may be daughter or sister groups. No attempt has been made to describe their characteristics or position at this point. ATW050806.

Tropiduridae: Leiocephalus, Liolemus, Microlophus, Stenocercus, Tropidurus (= Plica?)

Phylogeny: Iguania : Acrodonta + Iguanidae + *.

Range: South America, Indonesia.

Characters: small- to moderate-sized; distinct head and neck; $ angular reduced; splenial extended posteriorly; teeth pleurodont; tails typically at least as long as body; body scales usually coarse ("spiny"); sternum with posterior extension with enlarged, median fontanelle; limbs & digits well developed; some have tail spines, dorsal crests or similar ornamentation; color typically cryptic; most insectivores or omnivores; diurnal; males may exhibit territorial and sexual displays involving stereotypic head and body movements.  

Links: Amphibians and Reptiles checklist - Project Amazonas field sites in the Peruvian Amazon; Tropiduridae (Best on the Web); A SEMI-ANNOTATED BIBLIOGRAPHY AND TAXONOMY OF THE; ADW: Tropiduridae: Information. 

Image: Tropidurus itambere.  Photo by Prof. Guarino R Colli.  ATW040220.

checked ATW050806