Squamata

From Palaeos.org

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

[edit] Introduction

[edit] Information

Squamata: .

Range: from early jurassic.

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

[edit] 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.

[edit] Links

[edit] Credits

Palaeos com