parent_taxon aigialosauridae
Snakes are elongated , legless , carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears . Like all squamates , snakes are ectothermic , amniote vertebrates covered in overlapping scales . Many species of snakes have skulls with several more joints than their ___MASK69___ ancestors , enabling them to swallow prey much larger than their heads with their highly mobile jaws . To accommodate their narrow bodies , snakes ' paired organs ( such as kidneys ) appear one in front of the other instead of side by side , and most have only one functional lung . Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca . The ___MASK31___ ( from Greek meaning " scaled lizards ") are reptiles with overlapping scales . This subclass includes ___MASK54___ and Rhynchocephalia . It is a monophyletic group and therefore contains all descendents of a common ancestor . ___MASK54___ includes snakes , lizards , and amphisbaenia . Rhynchocephalia was a widespread and diverse group 220 - 100 million years ago ; however , it is now represented only by the genus " Sphenodon ", which contains two species of tuatara , native to New Zealand . ___MASK31___ is the sister taxon to Archosauria , which includes Aves and Crocodilia . Lizards and snakes are the most speciose group of Lepidosaurs and , combined , contain over 9 , 000 species . There are many noticeable distinguishing morphological differences between lizards , tuataras , and snakes . Lizards are a widespread group of squamate reptiles , with over 6 , 000 species , ranging across all continents except Antarctica , as well as most oceanic island chains . The group , traditionally recognized as the suborder Lacertilia , is defined as all extant members of the ___MASK31___ ( reptiles with overlapping scales ) that are neither sphenodonts ( i . e ., tuatara ) nor snakes  they form an evolutionary grade . While the snakes are recognized as falling phylogenetically within the ___MASK18___ clade from which they evolved , the sphenodonts are the sister group to the squamates , the larger monophyletic group , which includes both the lizards and the snakes . Tuatara are reptiles endemic to New Zealand . Although resembling most lizards , they are part of a distinct lineage , the order ___MASK21___ . Their name derives from the Mori language , and means " peaks on the back ". The single species of tuatara is the only surviving member of its order , which flourished around 200 million years ago . Their most recent common ancestor with any other extant group is with the squamates ( lizards and snakes ). For this reason , tuatara are of great interest in the study of the evolution of lizards and snakes , and for the reconstruction of the appearance and habits of the earliest diapsids , a group of amniote tetrapods that also includes dinosaurs , birds , and crocodilians . Reptiles are tetrapod ( four - limbed vertebrate ) animals in the class Reptilia , comprising today ' s turtles , crocodilians , snakes , amphisbaenians , lizards , tuatara , and their extinct relatives . The study of these traditional ___MASK34___ orders , historically combined with that of modern amphibians , is called herpetology . The Late Cretaceous ( 100 . 5  66 Ma ) is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale . Rock strata from this epoch form the Upper Cretaceous series . The Cretaceous is named after the white limestone known as chalk which occurs widely in northern France and is seen in the white cliffs of south - eastern England , and which dates from this time . ___MASK18___ ( Greek for " those who ___MASK83___ toxins "), is a proposed clade of scaled reptiles ( squamates ) that includes the Serpentes ( snakes ), ___MASK19___ ( monitor lizards , gila monster , and alligator lizards ) and Iguania ( iguanas , agamas , and chameleons ). ___MASK18___ contains about 4600 species , ( nearly 60 %) of extant ___MASK54___ . It encompasses all venomous reptile species , as well as numerous related non - venomous species . There is little morphological evidence to support this grouping , however it has been recovered by all recent molecular analyses . The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period million years ago ( ___MASK70___ ) to the beginning of the Paleogene Period ___MASK70___ . It is the last period of the Mesozoic Era . The Cretaceous Period is usually abbreviated K , for its German translation " Kreide " ( chalk ). Aigialosauridae ( Greek , aigialos = `` seashore '' + sauros = ___MASK69___ ) is a family of Late Cretaceous semi - aquatic varanoid lizards , regarded by some paleontologists as a distinct monophyletic group and by others as an adaptive grade within the basal mosasauroids .