Welcome to Reptilia!
Background: Today’s lab was an overall
introduction to the basic anatomical features that unite the reptilian clade. The
reproductive, developmental, integumentary, and skeletal adaptations required
to exploit terrestrial environments were a central theme of this lab.
Eggs, Embryos, and Reproduction
Cleidoic Eggs
In
order for some early tetrapods to complete the transition to an exclusively terrestrial lifestyle, a method of reproduction that protected eggs from desiccation and gravity was required. These abiotic
challenges were met by changing from aquatic, soft-shelled, externally
fertilized anamniotic eggs (as seen in most amphibians) to internally fertilized, hard-shelled
cleidoic eggs (as seen in Amniota).
This figure from the Herpetology 4th ed. text book (p. 121) is a good visualization of the reptilian egg structure and highlights the extraembryonic membranes found in members of Amniota. |
A cracked chicken egg displays surprising structural complexity. The nutritive yellow yolk is conspicuous and has two sections of white chalaza attached, which function in centering and providing support to the embryo. The multiple membranes of the albumin supply water and protect the developing embryo from mechanical stress.
With the benefit of a protective hard
shell came the cost of reduced
permeability to gas and waste exchange, which was easily achieved via
diffusion for anamniotic eggs in aquatic environments. In reptilian eggs, gas
exchange occurs through the air cell, which is present at the “rounded” end of
the egg. However, if gas exchange only occurred at one end of the egg, the
gases would need to diffuse the entire length of the egg, making the process
inefficient. Thus, the porous outer layer of the shell allows gas exchange to occur
over the entire surface of the egg.
- Embryos and Development
On the left is a stained Chelydra serpentina embryo showing cartilage (blue) and bone (red). On the right is a series of embryonic stages of Trachemys scripta (Greenbaum 2002). |
In
contrast to the free-living larvae of many amphibians that metamorphose into
adults in the process of indirect development, reptiles exhibit direct development from miniature versions of
adults that grow into full-size adults. The reptilian embryo undergoes various
developmental stages including chondrification, ossification of bones,
elaboration of face and sensory organs, and formation of the intestinal tract. The
above picture shows a turtle embryo that has been cleared of soft tissue
and double-stained, so that blue stain indicates the presence of cartilage,
while bone is stained red.
Hatching
In
order for a young reptile to emerge from the egg, it must break through the
membranes and hard outer shell. A specialized structure called an egg tooth
helps young reptiles penetrate the shell. Some reptiles have an egg tooth that
is merely a protuberance extending from the snout. Alternately, in snakes and
lizards, the egg tooth is a true midline tooth that is lost after hatching. The
conspicuous bulge on the abdomen of the turtle specimen showing an egg tooth is
the remaining yolk of the egg. When turtles hatch from their eggs, they do not
immediately emerge from the nest. There is a period where hatchlings hibernate
in the nest and absorb the last of the yolk.
Integument
Skin
Some examples of shed snake skins. |
1) Stratum corneum – This is the outer most layer that is
comprised of dead cells arranged in alpha and beta keratin layers. A process
known as ecdysis allows the stratum corneum to be shed and replaced by an
identical new layer of cells beneath it. Snake sheds are a great example of
this process.
2) Stratum germinativum – This is the layer of living cells
of the deep epidermis.
3) Dermis – This is the layer of skin which contains nerves,
blood vessels and in some cases osteoderms. Osteoderms present in the dermis
can form a protective layer that is seen in some lizard, turtle and crocodile
species.
It is through the folding of the dermis and epidermis that
scales form on the outer layer of the skin.
Scales
The specimen on the left has rectangular, juxtaposed scales on its ventrum, while the beaded lizard has bead-like scales covering its body. |
Often, reptiles have multiple types
of scales covering their body. There are various terms used to describe the specific
type of scale: cycloid scales – scales with a rounded edge, granular – appear
like tiny beads, keeled – prominent ridge running down the center, mucronate - terminate in a pointed "V" shape, rectangular - scales that appear longer than wide. There
are also terms used to describe the position of the scales: dorsal scales - covering the upper body surface, ventral scales - covering the lower body surface, subcaudal scales - on the underside of the tail, juxtaposed –
placed adjacent to one another, and imbricate – those scales overlapping
like shingles. Snake-specific scale terms include anal scales, which appear as a flap-like cover on or near the cloaca, and apical pits, which are small pits found near the edge or center of the scale. Additionally, there are modified lamellae scales, which are associated with the fingers or toes.
It is important to understand that when describing scales
multiple terms are often applied, for example, a specimen can have “imbricate
keeled mucronate scales” (as exhibited by the specimen pictured above).
Further, reptiles can have different scale types of different parts of the body. Particular attention is given to the scales of the head of snakes and lizards. Scales and scale pits (snakes) are often used as identifying
characteristics, with specific methods of counting employed to differentiate taxa.
The carapace (upper bony shell) and the plastron (ventral bony
plate) of turtles are covered by scutes that are made of keratin.
Glands
(Source) |
Compared to our amphibian friends,
reptiles have fewer glands in the skin that are visible to the naked eye.
Femoral or precloacal glands are typically seen in males, and if these glands
are present on females they are noticeably smaller. Femoral glands are used to emit a waxy secretion used for communication between sexes.
Skeleton
General Regions
(Source) |
1) Atlas – Axis complex: the first two vertebrae of the
cervical region allowing for the movement of the head in all directions.
2) Dorsal/trunk region: some of these vertebrae attach to
ribs.
3) Sacral region: vertebrae that interact with the ilium of
the pelvic girdle.
3) Caudal region: the vertebrae of the tail.
4) Pelvic and pectoral girdles along with their respective
limbs. The development of a robust set of girdles was critical for supporting
weight and allowing for locomotion on land.
Skull
Overall
trends in cranial anatomy as reptiles evolved from fish and amphibian ancestors
included compaction of the skull and a reduction in the number of bones.
Large alligator skulls provide excellent models for identifying specific elements of anatomy.
Large alligator skulls provide excellent models for identifying specific elements of anatomy.
Temporal Fenestration
A fenestra is a “window” or large
opening between bones. Modification of the temporal fenestra within Amniota
shows adaptation to the pressures of moving from an aquatic to a more
terrestrial lifestyle. For example, it is thought that the anapsid skull, which contains no temporal fenestra, is the plesiomorphic
condition. Testudines (an order including turtles) are the only extant reptiles with anapsid skulls.
The dense, anapsid skull of a sea turtle would be supported by water
and thus not have to endure the pressure of gravity.
In addition to reducing the density of the
skull, fenestration allowed for a change in the origin and insertion of
musculature associated with the jaw. Synapsids are represented by Mammalia and
have skulls with one subtemporal fenestra. In herbivorous ungulates, this
singular cranial opening accommodates muscles that function in grinding the jaw
from side to side to chew vegetation.
(Source) |
Euryapsids also only have one fenestra,
but in contrast to synapsids, these extinct reptiles only possessed a
supratemporal fenestra.
The diapsid condition, in which the skull has both
supratemporal and subtemporal fenestrae, is characteristic of most reptiles.
Sources
1. Greenbaum E (2002) A standard series of embryonic stages for the emydid turtle Trachemys scripta. Can. J. Zool. 80: 1350-1370
2. Vitt LJ, Caldwell JP (2014) Herpetology: An introductory biology of amphibians and reptiles, 4th ed. Elsevier inc., Massachusetts pp 121
Sources
1. Greenbaum E (2002) A standard series of embryonic stages for the emydid turtle Trachemys scripta. Can. J. Zool. 80: 1350-1370
2. Vitt LJ, Caldwell JP (2014) Herpetology: An introductory biology of amphibians and reptiles, 4th ed. Elsevier inc., Massachusetts pp 121
No comments:
Post a Comment