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Reptilia Lab

Some basic instructions:

1. The goal of this lab is to prepare you to be able to identify reptile species by their key characteristics, and to learn some natural history about the major groups of reptiles.  You will be tested on this material in a quiz one week from today.

2. Readings are available in labeled file folders at the side table.  DO NOT remove these folders from the lab.

3. Be gentle with the specimens in the lab.  Always return reptile specimens to the jar that you got them from, and DO NOT leave them out to dry – donut return a specimen to the wrong jar!


1. What is a reptile? (Read pages 6- 12 in Ballinger and Lynch)

The early labyrinthodont amphibians, in addition to fostering various lines of amphibians, also gave rise to the reptiles, which first appear in the Carboniferous period.  The reptilian stage of the vertebrate organization carries forward to a greater degree than the amphibians adaptive trends conducive to a successful terrestrial existence.  These involve the development of mechanisms to reduce water loss, modifications of the respiratory and circulatory systems to provide for a greater reliance on lung breathing, and changes in the skeletal, muscular, nervous, and other systems to meet the demands of a more fully terrestrial life.

 A particularly significant advance in vertebrate organization appearing at the reptile level is the amniotic egg.  The special membranes associated with this egg eliminate the necessity of depositing the eggs in an aquatic medium as is the general rule among the lower vertebrates.  The presence or absence of these embryonic membranes provides a basis for the recognition of the two main groups of vertebrates - the Anamniota (fish-like vertebrates and amphibians) and the Amniota (reptiles, birds and mammals).  Other improvements in reproductive processes also appear at the reptilian level.

 Order Testudina (Chelonia)  - Turtles

Read pages 95 – 97 in Ballinger and Lynch and pages 1-16 in Carr.

External Morphology of Turtles

Examine a specimen of a turtle of the genus Chrysemys or Pseudemys.  The body is largely encased in a dorsal and ventral shell - the carapace and plastron, respectively.  These may be variously reduced in some forms.  The portion connecting the upper and lower parts of the shell is the bridge.  The carapace and plastron are actually composed of two parts, and underlying series of bony (dermal) plates covered with horny (epidermal) shields or laminae.

The number and arrangement of the epidermal shields of the carapace and plastron are of taxonomic significance and their terminology should be learned.  Identify the following:  nuchal (precentral), marginals, vertebrals (centrals), coastals (laterals), gular, humeral, pectoral, abdominal, femoral, anal, axillary, inguinal.
Examine the head of the turtle and note the terminal position of the nares.  Significance?  The eyes are protected with well developed lids and the indistinct tympanic membrane is located behind the angle of the jaws.  Teeth are absent in all recent chelonians and in their place is a horny beak, whose cutting edge may be termed the tomium.

The dermal plates of the carapace, to which vertebrae and expanded ribs are fused, are apparently unique developments in turtles.  Some of the bony elements of the plastron also seem to be of this type although parts of the pectoral girdle and gastralia also are believed to be included in its formation.

Read descriptions of the six families if turtles found in North America in pages 98 – 119 of Ballinger and Lynch.

Examine the limbs and note the adaptations for aquatic existence.  This trend toward a flipper-like condition is carried still further by the marine turtles.  On the other hand, highly terrestrial forms like the gopher turtles have elephantine limbs.

Sexual dimorphism in turtles.

Sexual dimorphism is variously developed among different species of turtles. Often the anus of the male is relatively farther from the rear edge of the plastron than that of the female.  In some species, size differences are apparent between the sexes, and in others differences in eye color, shape of plastron, or length of fingernails may distinguish males from females.

Examine a turtle skeleton and note the general modifications of the skeletal elements in connection with the development of the carapace and plastron.  Note particularly the absence of temporal fossae in the shell.

Super Order Squamata

 Order Lacertilia - Lizards

External Morphology of a Lizard

 Obtain a specimen of the fence lizard, Sceloporus.  The genus Sceloporus is the largest genus of lizards in the United States, including some 15 species and numerous  subspecies.

External Morphology.  The body of lizards exhibits greater regional differentiation than that of the fishes or amphibians, head, neck, trunk, and tail regions being recognizable.  The presence of a distinct neck is correlated with the more fully terrestrial habitat of the reptiles generally.

 The fore and hind limbs are well developed in the fence lizard, although they may be reduced or absent entirely in other forms.  The fore limb is composed of upper arm, forearm, wrist or carpus, and hand.  The corresponding divisions of the posterior limb include the thigh, shank, ankle or tarsus, and foot.  The primitive number of five digits is present on both hands and feet, and well developed claws terminate each digit.  Are the reptiles more advanced or primitive with respect to the number of digits than the amphibians?  The hind limbs are larger than the fore limbs, and both pairs still extend out laterally from the body in much the same way as in the urodele amphibians.  There are some differences in the posture, however, for the body is carried somewhat higher off the ground in the lizard and both fore and hind limbs have undergone some degree of torsion and thus departed from the primitive orientation as seen in the salamander.

 The head is less flattened than in the Recent amphibians.  What is the significance of the difference?  Locate the nostrils and eyes.  The eyes are protected by three lids, and upper and lower lid plus the third eyelid or nictitating membrane  located in the anterior corner of the eye.  What differences are apparent between the lids of the amphibian and reptile eye?  A distinct light spot in the middle of the most posterior large scale on the top of the head marks the position of the pineal eye.  It has the histological structure of an eye and may play a role in temperature regulation in some forms.  The ear in the fence lizard is a conspicuous opening toward the rear of the head about half way between the eye and the origin of the forelimb.  In some lizards the ear opening is less apparent, being partly concealed by scales, or hidden entirely beneath the skin.
In examining the ear, note that the tympanic membrane or ear drum with the attached columella is sunken within a slight depression.. The ear drum marks the outer boundary of the middle ear, already present in anurans, while the depression represents a new structure in the reptiles - the external ear.  This division of the auditory apparatus if destined to become more fully elaborated in the mammals.  What is the significance of this trend?

 The anus is in the form of a transverse slit, a characteristic feature of lizards and snakes.  Along the inner surface of the thighs will be seen a series of femoral pores.  The presence absence of these structures and their number, if present, are of value for taxonomic purposes.  These pores represent integumental glands which are particularly active during the breeding season.  The secretion forms elevated processes which apparently function in stimulating the female during courtship and mating activities.

 The tail is relatively long and is easily broken off (autotomy).  When this occurs the missing portion is regenerated, although a cartilaginous rod replaces the original vertebrae in the regenerated portion.  The latter is also distinguishable in that it lacks the color pattern typical of the normal tail.  Individuals with regenerated tails will probably be seen among the specimens being studied.

 An important characteristic of reptiles is the presence of a dry, usually scaly skin.  Scales when present are formed principally by the epidermis, although dermal thickenings may also be involved, and are not homologous to the scales of fishes.  In some reptiles, bony ossicles (osteoderms) may be associated with the scales.  Molting (ecdysis) of the epidermal covering takes place at more or less regular intervals during the period of the year in which the animals are active.  Lizards generally shed the “skin” in fragments, whereas snakes normally molt the epidermis in a single piece.  A trenchant difference between the skin of reptiles and amphibians is the absence of glands in the former.

 The kinds and arrangements of the scales are important in the classification and identification of lizards, and a complex terminology has been developed to describe the various patterns of scalation encountered.  Note in the fence lizard that the head is covered with large scales which are quite different from those in the remainder of the body.   Identify the following scales of the head:  rostral, postrostrals, nasal, canthals, superciliaries, frontal, frontalparietal, nuchals, interparietal, temporals, parietal, circumorbitals, supraoculars, prefrontals, internasals, median frontonasals, lateral frontonasals, postoculars, auriculars, gulars, subocular, lower labials, upper labials, preocular, loreals, mental, postmentals, and chin shields.  Also identify the supraocular ridge and canthus rostralis.  The scales in the above list that are underlined are to be learned.

 Note the scales on the remainder of the body.  The dorsal scales are keeled and in this species are arranged in oblique rows.  The ventral scales are not keeled.  The scales on the underside of the toes and fingers have several keels, a feature characteristic of the family Iguanidae to which the fence lizard belongs.

 Examine the skeleton of an iguana as an example of a typical reptilian form.

The reptilian skull in addition to its higher profile, differs from that of amphibians such as Ambystoma in greater ossification, more enlarged and better protected cranium, and reduction in the number of dermal bones.  The latter is a trend in vertebrate evolution that culminates in the mammals.

Note the presence of a single occipital concyle and its relation to the foramen magnum.  How does one reconcile the presence of a single condyle in reptiles with the double condition in recent amphibians?  Identify the orbital fossae and behind them, at the rear of the skull, the two temporal fossae.  The upper and lower temporal fossae are separated by a bridge of bone involving the postorbital and squamosal.  This is the diapsid condition and defined one of the large subclasses of reptiles.  In lizards, the lower border of the lower fossa is incomplete, whereas in snakes the bar separating the two fossae is also gone.  This represents a specialization for increasing the size of the gape.  Other parts of the skull reflect this condition also.  What is the function of the temporal fossae?  Note the external and internal nares.  Compare the position of the latter with that in amphibians.
As in amphibians and fish-like gnathostomes, the lower jaw of reptiles is formed of a series of bones, of which the dentary bears teeth. The articular serves to join the lower jaw to the quadrate of the upper skull.

 The vertebral column exhibits greater differentiation than that of amphibians.  Note the presence of cervical, thoracic, lumbar, sacral, and caudal vertebrae.  The first two cervicals are differentiated into an atlas and axis.  The sacrum consists of two vertebrae which provide a firm articulation for the pelvic girdle.  Contrast this condition with the amphibians.  Note the presence of free ribs on some of the cervical, thoracic and lumbar vertebrate.  Most of the thoracic ribs articulate by means of additional segment with the sternum.  Note the development of intelocking mechanisms between the vertebrae, the pre- and postzygopophyses.

Sexual dimorphism
 Sexual dimorphism is pronounced in the fence lizard.  The males are generally smaller that the females, exhibit a less distinct dorsal color pattern, and have well developed bluish -colored patches (dark in preservative) on the throat and belly.  What do you think are the adaptive significance of these differences?  The femoral pores are more prominent in the male, and the area immediately behind the anal opening is swollen due to the presence of the intermittent organs, the hemipenes.  Males also have a pair of large scales, the postanal scales, just behind the anus;  these are absent in the females.  Determine the sex of your specimen and examine an individual of the opposite sex by trading specimens with another student.

Read pages 31-34 in Smith.

Order Serpentes - Snakes

External Morphology of a Snake

Obtain a specimen of the common garter snake, Thamnophis sirtalis.

Head, body, and tail regions may be recognized.  The latter includes that portion of the animal posterior to the transverse anus.  The form of the body of snakes is quite variable and is closely associated with habits.

Examine the head and note the external nares and eyes.  Although the eyes appear to lack lids, they are in fact covered by the transparent, fused upper and lower lids.  This arrangement has apparently evolved through several stages, involving a progressive development of a window or clear area in the lower eyelid and then the eventual fusion of the lower lid with the upper.  What is the significance of this arrangement?  The pupil of the eye may be round or vertically or horizontally elliptic.  What is its shape in the garter snake?  There is no external evidence of ears in snakes, the tympanum, middle ear cavity, and Eustacian tube being absent.  The columella is attached to the quadrate bone.  Snaked apparently hear through conduction through the substrate and not by airborne vibrations.

Note the extreme length of the gape, a feature associated with the feeding habits of snakes.  Relate this to the morphological features of the skull considered under the description of the lizard above.

The majority of snakes possess large head scales.  Familiarize yourself with the nomenclature of these scales.  Identify the supraocular, postoculars, temporals, preoculars, prefrontal, internasal, nasals, loreal, upper labials, lower labials, frontal, parietal, mental, anterior chin shields, and posterior chin shields.

The scales on the remainder of the body are divisible into dorsal, ventral, and caudal scales.  The dorsal scales are the smaller ones covering the upper part of the body.  They may be smooth, slightly keeled, or keeled.  What is their condition in the garter snake?  The ventrals (or gastroteges) are the large, transversely elongated scales covering the neck and trunk region, and the caudals are located on the underside of the tail.  The latter are sometimes called subcaudals or urosteges and may be in either one or two series.  That is the arrangement of these scales in Thamnophis?  The anal plate is the large scale covering the anus.  It may consist of one large scale or be divided into two scales by a diagonal or longitudinal slit.  What is the condition in Thamnophis?

Counts of dorsal, ventral, or caudal scales are often of importance in identification of snakes.  Study the methods of making these and record the various counts for your specimen.

Sexual dimorphism
Sexual dimorphism is not as pronounced in snakes as in lizards.  The presence of the retracted hemipenes in the male tends to make the region behind the cloaca stouter that in the female, but this is sometimes not too readily apparent.  Males often have longer tails that females of the same species.  In some snakes, secondary sex characteristics are present in males on the form of tubercles on the chin shields or on the scales above the anus on each side.

Read pages 184-198 in Seigel et. al.

Order Crocodylia – Crocodiles, etc.

Read pages 213 – 215 of Ballinger and Lynch.

Adapted from J. N. Layne

Reptile Index