Biology of the Reptilia Lab 1:
Authors: Matt Knestrick & Megan Thornhill

Purpose: We examined various features of amniotes (reptiles, specifically) to see what distinguished them from their evolutionary ancestors (fish and amphibians).

Eggs & Development
One of major features separating reptiles from their ancestors is the development of a cledoic egg. An egg is cledoic if it has developed a hard, calcium-carbonate outer shell, as opposed to a soft, membranous outer layer. This hard, outer shell is composed of shell units, which are small, teeth-like units that together make up the egg shell. This is essentially the difference between soft, gooey frog eggs, and the hard-shelled, Cledoic chicken eggs today.
Inside the eggshell there is an egg membrane, and an air sack, both of which are attached to the shell. Within the membrane is the outer, middle, and inner albumin, which are the gooey inners of the egg. In a chicken egg, these layers compose the egg white. Further inside the egg is the yellow yolk, and the yolk membrane that holds it together. Growing out of the yolk membrane is the chalaza. The chalaza are two membranous structures that anchor the yolk to the top and bottom of the egg .

Embryo development is also an important feature that separates amniotes from their predecessors. In amniotes, the embryos develop within the eggs into “miniature adults”. For example, a puppy looks like a miniature adult dog, and a baby human looks like a miniature grown-up. This is different from more ancestral animals, like amphibians, whose offspring first hatch as larvae, which exist on their own until they develop further into adults. The best example of this are frogs and toads, who offspring larval stage (tadpoles) look very different from the adults, and live very different lifestyles.

Skin & Glands
Reptile skin contains several different layers. The outermost layer is composed of dead cells this layer is called the stratum corneum, the next inner layer is composed of living cells of the deep epidermis, the innermost layer is the dermis, which contains nerves and blood vessels. One characteristic of reptiles is Ecdysis, or shedding. Reptiles will shed their outermost skin layers in order for growth of the organism.

Folding of the dermis and epidermis in these organisms produce specific irregularities which yield scales. Different combinations of these scale types can occur on the same organism. The types of scales that are expressed by each organism differ on different parts of the body. Some scale types include, smooth scales; granular scales, which appear bead-like and do not overlap; keeled scales, which have a distinguishable ridge on the scale; and scales with pits. Also, scales can be attached to the body in different ways, forming different characteristics. Cycloid scales overlap one another whereas juxtaposed scales do not overlap but simply sit close to one another.

Few glands are visible to the naked eye on most reptiles. Femoral glands are used for chemical communication between sexes as well as in marking territory. Usually only males possess these glands. If females do possess these glands then they are notably smaller than those on males of the same species.

Skeletal Structure
We also looked at reptilian skeleton to see the anatomical features that distinguish them from fishes.
The major skeletal feature that separates the amniotes from the fishes is the presence of skeletal girdles. These two girdles are known as the pectoral girdles (shoulders) that attach the forelimbs, and the pelvic girdle (hip-are) that attaches the hindlimbs. By having these girdles, the vertebral column was also noticeable sectioned off into the cervical, trunk, sacral, and caudelregions. These regions are used in almost all amniotes to describe body areas.
The skulls of amniotes are also different from ancetrals animals, in that they have a reduced number of skull bones Fish have a large number of skull bones, while amniotes have fewer. Amniotes’ skulls are also classified based on the number of finestra, or non-eye or nose holes, present. The ancestral condition, shared with fish, is anapsid, where that are no holes. Humans and some reptiles are synapsid, where only one hole is present. Some other reptiles are diapsid, where two holes are present. While technically, the term “apsid” refers to the number of arches found in the skull, this number is the same as the number of finestra (anapisids have no arches, diapsids have two), so the terminology works the same. These finestra act as channels for muscles to run through. In humans, these holes are underneath the check bone, and contain the muscle that operates the jaw.