Euposaurus: basal squamate/basal iguanid

Among the many lizards found in Late Jurassic  (155 mya) European lithographic limestones that have no living counterparts, there is one, Euposaurus (Fig. 1) that is basal to all members of the clade Iguania, which includes Iguana, the iguana; Phyronsoma, the horned lizard; Trioceros, the chameleon; and Draco, the rib-gliding lizard.

Euposaurus cirrensis, a basal squamate and basal  member of the clade Iguania.

Figure 1. Euposaurus cirrensis, (not the generic holotype) a basal squamate and basal member of the clade Iguania. The large orbit and less than fused ankles are primitive, not juvenile, traits.

Euposaurus cirinensis ( Lortet 1892, MHNL 15681, Late Jurassic, Kimmeridgian, 155 mya, 3.5cm snout vent length) nests as the basalmost member of the Iguania (Cocude-Michel 1963) and is a basal squamate. Evans (1994) assigned it to Squamata incerta sedis. The large skull and large orbit might seem to be juvenile traits, but all sister taxa share these traits. Liushusaurus and Calanguban are sister taxa at the base of the Scleroglossa.

Figure 2. Euposaurus insitu.

Figure 2. Euposaurus insitu.

Evans 1994 reexamined the three specimens attributed to Euposaurus and reported they “belong to different genera. Euposaurus thiolleri, the type species, is a juvenile pleurodont lepidosaur which is probably, but not certainly, a lizard. It has no characters which suggest that it is an iguanian and is here designated Lepidosauria incertae sedis. The remaining two specimens have an acrodont dentition and are juvenile rhynchocephalians. One is referable to Homoeosaurus; the other appears to belong to the group currently represented by Sapheosaurus, Kallimodon, Piocormus (aka Sapheosaurus) and Leptosaurus although the latter two may not be valid genera.”

Figure 2. Basal squamates. Here Euposaurus is a basal Iguania. Liushusaurus and Calanguban are basal Scleroglossa. Scandensia is presently their last common ancestor.

Figure 3. Basal squamates. Here Euposaurus is a basal Iguania. Liushusaurus and Calanguban are basal Scleroglossa. Scandensia is presently their last common ancestor.

Derived from ScandensiaEuposaurus is larger overall and has a larger skull with a robust palate. The tail is longer and more robust. The limbs are more robust. Scandensia is much smaller than its predecessor, the mis-named “Langobardisaurus” rossii, so the origin of lepidosaurs is one more case of miniaturization, as in mammals, birds and reptiles.

Figure 4. Langobardisaurus? rossii compared to tiny Scandensia.

Figure 4. Langobardisaurus? rossii compared to tiny Scandensia.

References
Cocude-Michel M 1963. Les rhynchocephaJes et les sauriens de calcaires
lithographiques (Jurassique supérieur) d’Europe occidentale. Nouvelles Archives
du Muséum d’Histoire Naturelle de Lyon 7: 1-187.
Evans SE 1994. A re-evaluation of the Late Jurassic (Kimmeridgian) reptile Euposaurus (Reptilia: Lepidosauria) from Cerin, France. Geobios 27(5):621-631.
Lortet M 1892. Les reptiles fossiles du Bassin du Rhone. Archives du Musee de
Histoire Naturelle, Lyon 5: 1-139.

image from planet-terre

Euposaurus corninesss 

Musée des confluences, Lyon / Pierre Thomas 

http://planet-terre.ens-lyon.fr/planetterre/objets/Images/Img307/307-rhynchocephales-Cerin-06.jpg

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