DIAPSIDS   \                         

   Diapsid reptiles evolved from the first anapsid reptiles.  The openings in the temporal region of the skull allowed the development of more powerful jaw muscles.  From the first primitive species evolved two groups of diapsid reptiles: archosaurs (dinosaurs, birds, crocodiles, and pterosaurs) and lepidosaurs (the tuatara, lizards, snakes, and a number of extinct groups).  Petrolacosaurus is the oldest known diapsid, dating from the Upper Pennsylvanian.    It is intermediate between the earlier and more primitive romeriid anapsids (such as Hylonomas) and later diapsids (Reisz, 1977)

skulls 2 skulls 3

1) Primitive Diapsids

      Primitive diapsids first appeared in the Carboniferous and some survived into the Upper Permian and Triassic.  Coelurosauravus was the first flying (although gliding) vertebrate.  Interestingly, the bones which supported the flying membranes were not attached to either the vertebrae or the ribs—these bones developed independently in the skin.  The gliding membrane did not include the arms (Frey, 1997; Wuethrich, 1997).

     Thalattosaurs were elongated aquatic forms whose limbs were not specialized as paddles.  Askeptosaurus had an extremely long neck and could reach 2 m in length.

2) Archosauromorphs

    Archosauromorphs included the ancestors of thecodonts, dinosaurs, crocodiles, birds, and pterosaurs that will be discussed later.

3) Lepidosauromorphs

   In comparison to archosauromorphs, lepidosauromorphs were sprawling and their movement involved lateral undulation of the vertebral column (moving their backs from side to side).  Their limbs were similar to those of the primitive diapsids.  One reptile from the Lower Triassic (Uatchitodon, known from incomplete remains) might have been poisonous since it had teeth with grooves similar to those found in modern lizards and snakes that are venomous (Sues, 1991).

      Eosuchians were a basal group of lepidosauromorphs and many fossils formerly classified as eosuchians are now classified as basal members of other groups. Youngina and Thadeosaurus had skulls that were equivalent to the earliest diapsids.  The scapula and coracoid began as two separate bones but later in life fused together.  Intercentra were still maintained in the vertebral column.  They still retained a cleithrum in their shoulder that was lost in later lepidosaurs.   All of the characteristics of the Eosuchians (with the possible exception of the hand) indicate that they could have served as the ancestors of all other lepidosaur groups.  Some eosuchians were aquatic and apparently swallowed stones for ballast.





      Lepidosaurs are the most diverse and successful modern reptiles with 6000 species.  The Sphenodontida were a diverse group in the early Mesozoic but dwindled in Cretaceous.  No fossils are known from the Cenozoic but one modern species Sphenodon (the tuatara) still survives today but only in New Zealand.  Its skull is similar to Jurassic species and still has a pineal foramen.  Pleurosaurs were elongated aquatic forms that evolved from sphenodonts early in Mesozoic.




     The first lizards were small, less than 10 cm in length, and were probably insectivorous.  Upper Permian and Lower Triassic lizards are more primitive than later forms in that they possessed unfused proximal tarsal bones in the foot, poorly developed fenestration of limb girdles, and primitive vertebrae.  One early group reduced their limbs, like modern skinks. A fossil lizard from the Early Cretaceous was specialized for gliding (Li, 2007).

Most major groups of lizards were present by the end of Jurassic.  Many groups developed a firmer tooth attachment and one extinct group possessed deep sockets for its teeth.  Geckoes are the earliest known branch of modern lizards and are the most different from the rest of modern groups.  Varanoid lizards include the modern Komodo dragon, which reaches 3 meters, and the extinct Megalania that reached twice this size.  Some aquatic varanoids, such as the mosasaurs, reached 10 meters (Morwood, 2004).

A number of modern lizards can run on their hind legs including Basiliscus, Hydrosaurus, and Chlamydosaurus (Czerkas, 1990).


Mosasaurs seem to have evolved from smaller aquatic lizards known as aigialosaurs.


legless lizard



Anatomical evidence indicates that the sister group of snakes is the fossil Mosasauroidea (composed of the mosasaurs, aigialosaurs, and the basal form Coniasaurus). In 1869, Cope suggested that mosasaurs were more closely related to snakes than to other lizards. The common ancestor of this group would have been an aquatic lizard with limbs. The limbs were reduced (especially the forelimb) in coniasaurs and dolichosurs, modified into paddles in mosasaurs, and reduced and eventually lost in snakes. Some extinct aquatic lizards possessed an elongated body and reduced limbs; some even had snakelike bodies (Caldwell, 1999; Lee, 2005).

Mosasaurs and snakes also share a reduced ossification of the hips, large teeth on the pterygoid on the roof of the mouth, and increased jaw mobility because of a hinge between the two halves of the lower jaw. In the earliest snakes additional teeth appeared on the palatine bones. While the earliest snakes also possessed a hinge between the halves of the lower jaw, advanced snakes lost this hinge and the two halves of the lower jaw are no longer joined (Fraser, 1997; Coates, 2000).









Lizards are a paraphyletic group given that snakes are derived lizards and that many lizards are more closely related to snakes than to other types of lizards. The oldest branch of the lizard lineage produced the group which includes iguanas, chameleons, and agamids. In the remaining group of lizard lineages (referred to as Scleroglossa), limblessness evolved separately in the lineages leading to snakes, amphisbaenans, and several lineages of legless lizards. Limb reduction and body elongation are beneficial in a number of ecological circumstances as evidenced in lizards which are adapted for burrowing and others which are adapted for swimming (Lee, 2005). Several modern groups of lizards have greatly reduced or lost their limbs independently of the lineage that led to snakes.

garter snake water snake
     The number of vertebrae in snakes varies from 120 to 454 and most snakes have highly mobile jaw elements.  Snakes are similar to lizards in a number of features; for example, the possession of a pair of penises known as hemipenes is a characteristic that both lizards and snakes share.  The first known snakes were aquatic(Pachyophis, Mesophis, Archaeophis, Paleophis, and dolichosaurs) as were the aquatic varanoid lizards from which they seem to have evolved.  The most primitive snakes (Ophiomorphus, Haasiophis,  and Pachyrachis) share a number of skeletal features which the varanoid lizards and some even possessed remnants of hind legs.  Pachyrachis was a marine predator with a body which was laterally compressed rather than round in cross section.  In Pachyrachis the leg and pelvic girdle were tiny vestigial structures and included a femur, tibia, fibula, and two tarsal bones.  Tarsal bones of Pachyrachis similar to mososaurs (Fraser, 1997).  Haasiophis also possessed a vestigial leg which included 3 tarsal bones, 4 metatarsals, and even 2 phalanges on its foot (Scanlon, 1999; Tchernov, 2000; Caldwell, 1997).
pachy leg

The Cretaceous snake Podophis possesses snakes which have migrated to the posterior of the body (Coates, 2000).


Dinilysia was a boa from the Upper Cretaceous that seems to be the type from which all snakes evolved.  Modern boas and pythons still retain small skeletal remnants of their hind limbs (pythons still possess an ilium, ischium, pubis, and short femur).  One giant aquatic snake is known from the fossil record of Mali (Paleophis colossaeus) which could reach 33 feet (Bauchot, 1994).   No snake, modern or fossil, is known to have any vestige of arms or pectoral girdles.  Vestigial pelvic girdles exist in the modern snake families Typhlopidae, Leptotyphlopidae, Aniliidae, Boidae, and the subfamily Cylindrophinae of family Uropeltidae (Mattison, 1995).  Males may use these for intermale combat (Greene, 1997).  Some primitive snakes have nerves in pelvic region suggesting their ancestors had legs (Mattison, 1995).  Snake embryonic cells do send the signal for limb formation but snake cells apparently do not respond.  Some signals for limb formation, such as FGF2, can promote growth of python limb vestiges (Pennisi, 1999).

     Smaller snakes diversified quickly in the Age of Mammals and the spread of small mammals provided an abundant food source.  The advanced groups of snakes which compose the majority of living snakes are known since the Miocene. (Greene, 1997). 

     Snake skull is lighter than those of other reptiles and the bones are not as tightly secured to each other.  Snakes can’t chew—that would restrict the size of the prey they could eat.  Their upper jaws not fused to each other or to the braincase.  The mandibles not fused but instead are joined by an elastic ligament.  Teeth exist on the maxillary, palatine, and pterygoid bones of the upper jaw and the teeth of most snakes similar in size and shape (Mattison, 1995).

snake skull snake skeleton
     The number of vertebrae varies from under 100 in vipers to over 300 in most colubrids, more than 400 (with almost 300 ribs) in pythons, and almost 600 in a fossil snake (Bauchot, 1994).  Most snakes have short tails compared to lizards.  The vertebral column allows for a great deal of lateral flexion but little movement in the ventrodorsal plane (Bauchot, 1994).  The muscles between the ribs, costal muscles, alone are responsible for all movement in snakes (Bauchot, 1994).
snake skel 2 snake ribs
The giant snake Gigantophisto existed in Egypt 40 million years ago, could reach almost 11 meters in length, and may have preyed on ancestral elephants (A giant, 2004).
garter water snake 2