Cleidoic eggs: There are two main layers to the shell of a cleidoic egg, an outer and inner organic layer. The calcium carbonate outer layer is made up of crystalline, columnar shell units that are separated by pores. These pores allow gas exchange to take place. The shell membrane is an inner organic layer made up of protein fibers. Cleidoic eggs also contain an external, middle, and internal albumin, as well as a yolk, chalaza, and an air cell.
Cleidoic eggs are variable in flexibility and thickness. The egg of Chelydra serpentina, the common snapping turtle, is hard and thick (traits that are often found in birds and many turtles). The eggs of Elaphe, as well as the eggs of all snakes, lizards, crocodilians, dinosaurs, and some turtles, are soft and leathery.
Embryos and Development: Reptiles exhibit oviparity or ovovivipairy during development. Oviparous reptiles hatch from eggs laid by the mother, and ovoviviparous reptiles emerge from eggs retained within the mother (a form of live birth). All reptiles exhibit direct development, where embryos form into miniature versions of the adult. Cleared and double stained specimens of Trachemys scripta embryos were examined. Cartilage is stained blue, bone is stained red, and soft tissues are cleared in these specimens.
Hatching: An egg tooth is commonly used by reptiles to assist them in the process of hatching. An eggtooth attached to a late stage embryo of Geochelone emys was observed.
Skin: The skin of reptiles is usually covered in scales or feathers, and has few glands when compared to amphibians. The three layers found in reptilian skin are the stratum corneum, the stratum germinativum, and the dermis. The stratum corneum is the outer layer made up of dead cells, the stratum germinativum is the deep epidermis made up of living cells, and the dermis is made up of the nerves, blood vessels, and sometimes osteoderms.
Scale Morphology: One of the diagnostic features that can be used to identify reptile species is external scale morphology. There are number of different scale types, including, but not limited to: granular, rectangular, keeled, cycloid, and juxtaposed. Many species of reptile exhibit variation in scalation, with different scale types present on different areas of the body, such as the ventral and dorsal surfaces. Many lizards and snakes also have distinct patterns of enlarged scales on their heads that are useful in identification.
Shed snake skin (cycloid)
Glands: Many lizards have glandular pores present anterior to the cloaca (precloacal pores) or on the underside on the hind leg (femoral pores). These pores are typically found on male lizards, and, if present, will smaller in females than males of a species.
General Skeletal Anatomy: We also examined the basic skeletal anatomy of reptiles. We examined the general body regions of a reptileon a dried alligator skeleton. The vertebral column is broken up into the cervical region (the neck vertebrae anterior to the pectoral girdle), the trunk (vertebrae between the pectoral girdle and the sacrum), and the caudal region (vertebrae posterior to the sacrum). The forelimb and hind limb of reptiles are anchored to the body by the pectoral and pelvic girdles, respectively.
Skull Morphology and Temporal Fenestration: We also examined crocodilian skulls, which demonstrate the diapsid condition present in all extant reptiles, excluding turtles. This means that there are two temporal fenestrae, which are openings in the temporal region of the skull that allow for muscle attachment. Diapsid skulls have both a supratemporal fenestra and a lower subtemporal fenestra.
This is in contrast to the anapsid condition seen in turtles, in which there are no temporal fenestrae, but rather deeply inscribed temporal emarginations at the posterior of the skull to allow space for muscle attachment. Mammals and their ancestors exhibit the synapsid condition, in which there is a single subtemporal fenestra. Although all extant reptiles except for turtles possess a phylogenetically diapsid skull, in many cases the skull is highly derived, such that these fenestrae have been variously modified or lost.
Dan Paluh & Sean Harrington