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Life Science: Session 6

Classifying Reptiles

What is a reptile?

Snake and lizard skulls
Snake and lizard skulls

In the video, Dr. Jim Hanken shared part of the collection of reptiles stored at the Museum of Comparative Zoology (MCZ) at Harvard University. The specimens he showed us are part of a vast collection of reptiles and amphibians, the study of which is called herpetology, from the Greek word “herpeton,” meaning “creeping thing.” Historically, reptiles and amphibians have been grouped together. Fossil and other evidence conclusively shows that reptiles evolved from an amphibian-like ancestor. However, the features that distinguish reptiles from amphibians show that they are very different. These features reflect a transition from an aquatic life to life on land.

Amphibians Reptiles
  • Smooth skin that is not watertight
  • Dependence on water for reproduction
  • External fertilization
  • Jelly-like egg that is not watertight
  • Breathe through gills, skin, and lungs
  • Chest does not expand, muscles force in air
  • Legs positioned to the side of body
  • Watertight skin with horny scales
  • Able to reproduce on land
  • Internal fertilization
  • Watertight egg with leathery shell
  • Breathe through lungs
  • Chest expands, air is sucked in
  • Legs positioned under body

How are reptiles classified?

Reptiles are part of the domain Eukarya, which includes all organisms that have cells with a nucleus. They are included in the kingdom Animalia, with other organisms that are multicellular, ingest food, and have cells that lack cell walls. Further classification places them in the phylum Chordata. Chordates have a nerve cord running along the lengths of their backs (e.g., our spinal cord). As chordates with backbones, they are placed in the subphylum Vertebrata. Reptiles form the class Reptilia.

There are four existing orders of reptiles, including turtles, crocodiles and alligators, lizards and snakes, and tuataras. Dr. Hanken focused on comparisons between members in the class that includes lizards and snakes, showing evidence that snakes evolved from lizards. Recent advances in classification methods have determined that birds arose from a group of dinosaurs — as a result, they will likely be re-classified as an order within the reptiles.

Order Distinguishing Features
Testudines Turtles. 250+ species. Body encased in shell made of bone with backbone fused to shell; sharp, horny jaw without teeth; land turtles (tortoises) and sea turtles.
Crocodylia Crocodiles, alligators, and gharials. 22 species. Four-chambered heart (other reptiles have three-chambered hearts); extended jaw with teeth in sockets; five digits on forelimbs and four on hindlimbs.
Squamata Lizards (Suborder Sauria) and snakes (Suborder Serpentes). 3,750+ species and 3,000+ species respectively. Skin with horny scales that is periodically shed; teeth in sockets; lizards have five digits on fore- and hindlimbs; snakes are limbless; snakes have hinged jaws.
Rhyncocephalia Tuatara. 1 species. Wedge-like skull; socketless teeth; primitive eyes under skin of forehead. Sole survivor of ancient branch of reptiles.
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