1. LON-CAPA Logo
  2. Help
  3. Log In


Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)

2/29/00 Text (7th ed.): Ch. 29

possible web site: http://netvet.wustl.edu/reptiles.htm (with links to many reptilian sites)

possible HVCC audio/visual material: VT 3383,"Eyewitness: Reptile" (to be seen in lab this week)

VT 1185"Life on Earth, Part 14: Victors of the Dry Land" (Galápagos islands, marine iguanas, tortoises)

VT 1186:"Life on Earth, Part 15: Dinosaurs and Their Descendants (dinosaurs, fossils, crocodiles)


The amniotes are those organisms united by the egg type described last week. This includes all of the non-amphibian land dwellers, and as seen below, their lineages are characterized by different skull types. Organisms that we call reptiles (cold-blooded, scaly, egg-laying) represent a variety of organisms that share a common"approach" to structure and function.

FEATURES OF REPTILES: As with the scaly skin and (leather) shelled egg described last week, these features allow the reptiles to be more effective land-dwellers than the amphibians.

Circulation. Two atria, right and left. Incompletely divided ventricle, and no more of the spiral valves seen in amphibians. A quick recap of reptilian blood flow: systemic veins > right atrium > right ventricle > pulmonary artery > lung > pulmonary vein > left atrium > left ventricle > left and right systemic arteries. The systemic arteries unite to form the dorsal aorta (compare to our situation, where we have only one systemic arch or aorta, which arches to the left).

Respiration. Lungs work by negative pressure in reptiles, in contrast to amphibians. Movement of the ribs and the body wall (or movement of internal organs) lowers the pressure in the cavities surrounding the lungs, and so draws air in. The diaphragm as a muscle of respiration is seen only in mammals.

Senses. Sense of smell in reptiles includes snakes"tasting" and"smelling" the air. Most reptiles have Jacobson's organs as accessory olfactory organs. Rattlesnakes have heat-sensitive pit organs to help night-time hunting.

Water balance. The adult kidney of reptiles is metanephric (i.e., advanced), containing the nephron units for filtration, and drained by a ureter. Reptiles excrete uric acid, which is semisolid and nontoxic. Many reptiles use salt glands to excrete excess salt ("crocodile tears").3


Reptiles are well represented by the fossil record. A major way of distinguishing the major groups of reptiles is through skull modifications for jaw muscle attachment. Early enlargement of reptilian jaw musculature, e.g, the temporalis muscle for crunching. To deal with this, openings in the head shield (skull roof) allow for the muscle to become much bigger, and to allow the temporalis to bulge while the mouth is closed.1 Three different types of reptile skulls arose independently over time: anapsid, diapsid, synapsid.

We use this as the major basis for classification, although recently some zoologists have viewed turtles as more in common with diapsids and not as primitive anapsids.2

Subclass Anapsida ("without arch") lacking openings in the temporal region of the skull. Several fossil reptiles; the turtles today.

Order Testudines ("tortoise") (Chelonia). The turtles. 225 species. No teeth, but a horny beak. Short, broad body, with a shell made of dorsal carapace and ventral plastron. The carapace is formed from fusion of vertebrae, ribs, and dermal bones, and is covered with keratin in the form of epidermal scale scutes. The pectoral girdle and other dermal bones form the plastron, which is also covered with keratinized scutes. Cervical (neck) vertebrae are flexible to allow head withdrawl. All turtles are oviparous (egg-laying). There is no hole or fossa in an anapsid skull, so a temporal notch allows for jaw muscle access.4

Subclass Diapsida ("double arch"), with upper and lower openings in the temporal region of the skull. Superorder Lepidosauria, with scaly skins. Modern snakes, lizards, and tuataras.

Order Squamata ("scaly"). Three suborders:

Suborder Sauria, the lizards. 3,300 species. Two pairs of legs and external ear opening behind the eye. Geckos, iguanas, chameleons, etc.

Suborder Serpentes, the snakes. 2,300 species, 300 of which venomous. Snakes are elongate and limbless, although vestigial pelvis often present, and external ear absent. Many ribs, one functional (right) lung. Snake jaws can be loosely hinged (via attachment to quadrate and angular bones) to allow the swallowing of a large meal. Venom glands are modified salivary glands, with a mixture of toxins. Movement can either be rectilinear creeping, using the ventral scales like that of a tank, serpentine movement (lateral undulation), or concertina movement (accordian -like).5

Suborder Amphisbaenia, the worm lizards. 135 species of specialized burrowers.

Order Sphenodonta ("wedge tooth"). The living fossil tuatara from New Zealand. Well-developed pineal eye gland. Individually long-lived and bites with ferocity. Not a lizard: its diapsid skull is different.

Superorder Archosauria. Additional openings in skull between eyes and nostrils, and in lower jaw (for more muscle expansion).4 Dinosaurs, pterosaurs, crocodiles, and bird ancestors.

Order Crocodilia, 21 species. As with dinosaurs, with skull openings anterior to the eye, triangular eye orbits, and laterally compressed teeth. Alligators, crocodiles, caimans, and gavials. Teeth are for seizing prey, which is chunked by shaking. Egg laying, but show parental care of eggs.

Order Ornithischia, duck-billed dinosaurs, etc., and Order Saurischia, the meat-eaters and the brontosaurs. These orders are so named because of the relative position of the ischium in their hip bones. Birds are descended from small meat-eating dinosaurs (e.g., the raptors of Jurassic Park fame).

Subclass Synapsida ("together arch") with one lateral opening in the lateral region of the skull, below the squamosal bone (portion of our temporal bone). Includes the pelycosaurs (sail-bearing reptiles) and therapsids (mammal-like reptiles). If you check your zygomatic arch ("cheekbones"), you can recognize that we're descended from these"one archers."

  1. M Cartmill et al., Human Structure (Harvard U. Press, Cambridge, MA, 1987), pp. 366-367, 409.
  2. E Garcia,"Where do Turtles Go?" http://coloherp.org/cb-news/cbn-9809/TurtleTax.html, accessed 2/28/00
  3. CP Hickman Jr et al., Biology of Animals, 7th ed., (WCB McGraw-Hill, Boston, 1998), p. 626
  4. NM Jessop, Schaum's Outline of Theory and Problems of Zoology (McGraw-Hill, New York, 1988), p. 314-315, 318, 319, 323.
  5. FH Pough et al., Vertebrate Life, 4th ed. (Prentice Hall, Upper Saddle River, NJ, 1996), pp. 466-467.


|main page| |background| |03028: Physiology| |03048: Anatomy|

|03050: Invertebrate Zoology| |03051: Vertebrate Zoology| |03074: Economic Botany|

Please send comments and questions to: cronewil@hvcc.edu

HVCC home page

Copyright 1999, 2000 by Wilson Crone

External and unofficial links are not endorsed by Hudson Valley Community College

This web page last updated on February 29, 2000