Classification of Reptilla

DINOSAURS VS LIZARDS It is a fact that many reptiles continue to grow throughout their life, so one may be tempted to simplistically postulate that a dinosaur (“terrible lizard”) is merely the result of having a lizard live much longer than today in the favorable environment of the early earth. After all, if a man lived to be 960 years old, what would a lizard that lived for centuries look like? The answer is that it would still be a lizard, not a dinosaur. First of all, there are not specific differences between the lizards and the dinosaurs, but basic differences between all lepidosaurs (e.g., living lizards, snakes and tuatara, plus many more fossil forms) and all archosaurs (e.g., extinct thecodonts, pterosaurs, and dinosaurians, plus living crocodiles). Below is a streamlined and simplified classification derived from Zoological Record Volume 138.

Reptilia (Class)

     Anapsida (Subclass)  [TURTLES]	
     
     Lepidosauria (Subclass)  	
         
         Eosuchia (Order)  [Extinct order of lizardlike reptiles.]
         Rhynchocephalia (Order) [Tuataras]
                 	
         Squamata (Order)   	
             Sauria (Suborder)  [Lizards]              	
             Amphisbaenia (Suborder)  [Worm Lizards]
             Serpentes (Suborder)  [Snakes]	
                 
     Archosauria (Subclass)  	
         Thecodontia (Order)  [“Primitive” Triassic Reptiles]  	
         Saurischia (Order)  [Lizard-Hipped Dinosaurs]	
         Ornithischia (Order)  [Bird-Hipped Dinosaurs]
             Ornithopoda (Suborder)  	
             Stegosauria (Suborder)  	
             Ankylosauria (Suborder)  	
             Ceratopsia (Suborder)  	
         Pterosauria (Order)  [Pterosaurs]
             Rhamphorhynchoidea (Suborder)  	
             Pterodactyloidea (Suborder)  	
         Crocodylia (Order)  [Crocodiles] 	
  	
     Euryapsida (Subclass)  [Plesiosaurs]	
           	
     Synapsida (Subclass)  [Mammal-like Reptiles]

Having understood that lizards and dinosaurs belong to different subclasses within the class Reptilia, we can move on to discuss some of the important distinctions that made zoologists classify them differently. Classification happens by observing the entirety of an organism’s morphological traits in character space. Even though dinosaurs (and other archosaurs) clearly appear to fit the bill as reptiles, there are significant structural differences between them and other classes of reptiles. The most striking characteristic of the Archosauria is the triradiate pelvis. In the Lepidosauria (including lizards) the ilium extends dorsally, articulating with two, unfused, sacral vertebrae. The ischium and pubis are parallel to the ground surface, and fused in the midline (A S Romer, 1956, Osteology of the Reptiles, Chicago: Univ. Chicago Press; fig 153, p. 320.). In the archosaurs (including dinosaurs), the ilium is expanded along its dorsal margin, and articulates with three or four fused vertebrae. The ischium extends posteroventrally and the pubis anteroventrally. The pubes and ischii are fused laterally for most of their length.

The teeth in lepidosaurs are pleurordont. That is, they are set in a long groove in the jaws with a high outer and low inner wall. The individual teeth are fused to the outer wall of the groove without roots. In the archosaurs the teeth are thecodont. They are set in deep individual sockets which enclose the long, cylindrical root of the tooth. The lower jaw of the lepidosaurs (Romer, fig. 108, p. 208.) has a well developed coronoid process which forms the upper margin of the mandible behind the dentary bone, and both the surangular and articular bones participate in the jaw articulation. In the archosaurs the coronoid is confined to the inner surface of the mandible, below the surangular, which forms the upper margin of the mandible behind the dentary bone, and the jaw articulation is formed entirely by the articular bone (Romer, fig. 109, p. 210.).

Skulls are another distinction between the lizards and dinosaurs. In the lepidosaur skull (Romer, fig 62, p. 114.) the maxilla is firmly joined with the lachrimal and jugal bones and there is no anteorbital fenestra (opening in the skull in front of the eye). In the archosaurians (Romer, fig 69, p 131.) the maxilla forms the anterior border and the lachrimal and jugal the posterior border of a large anteorbital fenestra. In the archosauria both the quadrate and quadratojugal bones form the jaw articulation, while in the lepidosaurs the quadrate forms the articulation. It is difficult to describe complex vertebrae without pictures. But look at the dorsal vertebrae of lepidosaurs (Romer, fig 129, p. 255.) vs archosaurs (Romer, fig. 130, p. 257.) and you will see they are strikingly different. Compare the single headed ribs of a large monitor lizard (Varanus) with the double-headed ribs of a superficially similar dinosaur, like Thecodontosaurus.

There are also substantial differences in terms of locomotion/posture. As a result of their skeletons, we believe dinosaurs had a pretty much upright stride, while a lizard’s limbs are sprawled out to the side. There is good evidence for differences in physiology as well. The dinosaurs appear to have a much more "hot blooded" metabolism than lizards. Lizards are unqualified ectotherms (without a way to make their own heat), growing slowly and taking years to reach sexual maturity. They do not deposit fibrolamellar bone, a dense, interwoven tissue indicative of fast growth and high basal metabolism. Lizards also do not form large amounts of Haversian canals (channels running through a bone in which blood vessels and nerves are located); which are another indicator, though not unambiguous, of higher metabolism and growth rates. Dinosaurs display them (especially evident in juvenile dinosaur bones).

Jackson's chameleon is not a triceratops; bearded dragon aren’t descendents of anklyosaurs; and iguanas are not the modern version of iguanadons. There are very distinctive differences in the skeleton. That, not size, is the distinguishing point of dinosaurs. They are not just bigger, on average, than most other animals. There were massive salamanders, lizards, and crocodiles in the same ecosystem fossilized along with the dinosaurs. But their bones are not confused with dinosaurs. If scientists found a monitor lizard in the fossil record (and did not know of them in modern history), they still would not call it a dinosaur. The monstrous sea-going mosasaur discovered in the fossil record is classified as a lizard; not a dinosaur.

Also, there were many small dinosaurs—some no larger than a turkey. So we have large lizards and small lizards. We have large dinosaurs and small dinosaurs. All might have grown bigger in the past (because of a pre-Flood environment and because of fewer degenerative genetic elements). But lepidosaurs, like snakes and lizards, are closer to each other (except in the characteristics that all reptiles share) than lizards and dinosaurs. At the same time archosaurs, like dinosaurs and crocodilians, share more similar characteristics than either of them do with lizards. Although taxonomical differences are not always hard and fast scientifically, they are useful for scientists to study organisms. We observe certain anatomical similarities and naturally divide animals into convenient groups for our consideration. This discipline goes back far past the origin of Darwinism. Indeed, Carl Linnaeus, known as the Father of Taxonomy lived from 1707-1778. As a Lutheran minister and avid gardener, Linnaeus was a creationist. “Linnaeus opposed the pre-Darwin evolutionary ideas of his day, pointing out that life was not a continuum, or a ‘great chain of being’, an ancient pagan Greek idea. He could classify things, usually into neat groups, because of the lack of transitional forms.” (Batten, Don, Creation Magazine, 22:3, June, 2000, pp. 28-33.)

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