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Classifying Reptiles
What is a reptile?
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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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Classifying Reptiles
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