ZOOL 403 Chapter Notes - Chapter 4: Cranial Kinesis, Squamata, Anapsid

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Published on 9 May 2020
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ZOOL 403 Fall 2017
Week 9
Form and function in reptiles
Objectives
1. To examine the external morphology of the diapsids, particularly that pertaining to the
integument.
2. To gain an understanding of the functional consequences of fenestration and margination of
the skull in turtles and squamates.
3. To appreciate the structure and function of the turtle shell and the constraints of this
arrangement on turtle locomotion and feeding.
4. To understand the importance of cranial kinesis and tooth morphology in the feeding
adaptations of squamates.
5. To appreciate the diversity of locomotor adaptations in squamates.
Skin
Members of the Lepidosauromorpha and Archosauromorpha have in common several
characters that are evident externally. The most obvious trait is the possession of epidermal
keratinized structures known as scales. This thickened layer of keratin acts as a barrier that
prevents moisture loss to the atmosphere, thus allowing individuals to occupy dry, terrestrial
environments for prolonged periods of time without excessive water loss. In turtles, crocodilians
and some lizards, the scales are associated with bony plates (osteoderms) in the dermal layer
that provide protection from predators.
What are scales? How would you distinguish the scales of a lizard from those of the
perch?
If scales serve the purpose of protection and prevention of moisture loss, how is it
advantageous for there to be thinner areas of less keratinized epidermis between the
scales?
What would be the disadvantage of a continuous thick layer of keratin in the epidermis?
Another integumentary structure, claws, make their first appearance in these
vertebrates. These hard, keratinous cone-like sheaths encase the terminal
phalanges of the digits.
What functions do claws serve and what features of terrestrial life are associated with
their possession?
The Head Skeleton
Diapsid head skeletons exhibit a number of shapes and structural differences that are useful for
classifying major groups. The most obvious variation is apparent in the pattern of fenestration in
the temporal region of the skull. The primitive condition in amniotes is a temporal region that is
not pierced by openings. This condition is known as anapsid. All modifications seen in the
temporal region of sauropsid head skeletons are possibly derived from this anapsid condition. In
some groups, the temporal region has remained anapsid, but emarginations that develop from
the caudal region forward often result in the opening of large regions within outer bones of the
9-1
Scales are the keratinized structures
Anapsids-no openings (fenestrae) in the temporal region
in some groups-temporal region has remained anapsid (testudinidae)
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ZOOL 403 Fall 2017
Week 9
dermatocranium. The large muscles of the jaw are able to occupy this space. Other groups
possess fenestrae. Fenestrae are openings in the outer dermatocranium that serve the same
function as the emarginations of the anapsid condition, but are of independent phylogenetic
origin. All other “reptiles” are derived from the diapsid condition, meaning that they possess two
temporal fenestrae. Numerous modifications have occurred.
1. Members of the Testudines all exhibit the anapsid condition, where there are no fenestrae in
the temporal region. To accommodate for muscle expansion and enlargement, emargination of
this region has occurred. (Please note that much new evidence has called into question the
placement of turtles in the category of anapsid reptiles. Some researchers believe that turtles
are, instead, highly modified diapsids.)
Compare the specimens exhibiting varying degrees of emargination – in which areas of
the skull do you see bone loss by emargination?
2. All other living "reptiles" possess skulls that display various forms of the diapsid condition,
where there are two fenestrae in the temporal region of the head. These openings lie between
the post- orbital and squamosal bones. The reptile that exhibits the skull that most closely
resembles what is thought to be the original diapsid condition is the tuatara. In the tuatara, both
temporal arches remain fully intact. In lizards, the lower temporal arch no longer remains.
Modifications to the diapsid condition in modern forms may be associated with the phenomenon
of cranial kinesis, where there is a loosening of joints in the skull that allow for greater flexibility
during prey capture and feeding.
How is the diapsid pattern modified in snakes?
Examine and compare the arch patterns in the skulls of the tuatara, a lizard and a snake.
Can you identify the articulation points at which there will be movement between two
bones?
How would you measure the movement between bones?!
How do you think cranial kinesis might affect the way that prey is caught and ingested?!
Squamates
The Integument of Squamates
As described in the first “reptile” lab, the skin of squamates is composed of many keratinous
thickenings of the epidermis known as scales. Examine a cross section of snake skin to see the
arrangement of the epidermis and dermis.
The size, shape, numbers, and arrangement of the scales vary between groups and are species
typical traits. As such, they may be used as diagnostic characters in the identification of
unknown species. As well as varying between different groups, scale size also varies between
positions on the same individual. For example, scales on the head are usually larger than those
found on the rest of the body. Also, in snakes, scales on the venter (underside) are larger and
wider than on anywhere else on the body. These ventral scales called gastrosteges likely
aid in the locomotor ability of snakes.
Most snakes and lizards possess elongated, overlapping, or imbricated, scales that have knobs
or keels on their outer surfaces. The knobs and keels increase the stiffness of these scales.
Variations on this pattern include flattened, granular scales, raised, spiny scales, and horny
scutes. Scales may be crescent-shaped, round, oblong, diamond-like, or polygonal.
Examine the amphisbaenid and note the presence of annuli on the surface.
9-2
Tuatara-most closest diapsid
Cranial kinesis-loosening of joints in skull that allow for greater flexibility
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ZOOL 403 Fall 2017
Week 9
Colour and Pattern:
As in the amphibians, the colours and patterns on the skin of squamates are very significant
from a number of perspectives. Probably the most common function of colour in squamates is
for camouflage or crypsis. Whether the animal is predator or prey, the ability to blend in with its
surroundings is a very effective means of avoiding detection. The patterns may be banded,
striped, spotted, or mottled (or combinations of these). Cryptic colouring often depends on the
terrain and amount of vegetation found in the habitat. Colouring can also be important for
thermoregulation, intersexual and intrasexual interaction, and as threats, or as a warning to
predators of toxicity. One group of lizards even has the ability to change colour very quickly in
response to environmental stimuli such as light, heat, or the presence of adversaries.
Can these annuli be used to age the amphisbaenid in the same manner as they were
used in the fish? Why or why not?
Explain the mechanism of rapid colour change that occurs in chameleons.
There are also a number of associated structures derived from the integument. Modified scales
may form spines or spikes that can form crests along the head, dorsum, or tail. Another type of
modified scale, the tubercle, may be found anywhere on the body. Examine the iguana (Iguana
iguana) to see each of these modified scales.
Large flaps of skin with varying functions may also be present in a number of locations on the
body. They may form dorsal crests, dewlaps or throat fans, frills or parachuting membranes.
The size and presence of these structures may be sexually dimorphic characters that serve a
display function.
Can you think of any other possible functions of these skin flaps?
Feeding
Lizards, depending on their size, eat many different types of food. Many feed on insects and
other invertebrates, as these are quite abundant in the environments many lizards are found in.
Plant matter (leaves and fruits), eggs and other vertebrates also make up the diets of some
lizards. Snakes, on the other hand, are exclusively carnivorous, and feed on vertebrates such
as frogs, fishes, lizards, birds and rodents. Some snakes are capable of taking prey as large as
a small antelope, while others specialize on snails or even eggs. Snakes possess a highly
modified jaw and suspensorium structure that allows them to ingest large prey items relative to
their own body size (i.e. some snakes can ingest prey that are as much as 1.5 times their own
weight!).
Although they appear to have rather crude feeding methods, squamates exhibit a number of
interesting cranial and dental modifications associated with more effective prey capture and
ingestion. As outlined in the first “reptile” laboratory, snakes and lizards possess a modified
diapsid skull, in which the lower temporal arch is greatly reduced or absent. In snakes, the
upper temporal arch (post orbital squamosal) is also absent. The loss of these arches
contributes significantly to the flexibility of the jaws. Snakes also exhibit a high degree of cranial
kinesis in other areas of the skull. The jaw musculature associated with a kinetic skull tends to
be more finely subdivided to permit a wider range of movement and control. Examine the snake
head skeleton on display and compare it to a head skeleton of a lizard. Note where the
skeletons appear to be kinetic.
9-3
memorize
Large flaps of skin=may be sexually dimorphic
Lizards skull-Lower temporal fenestra absent
Snakes skull- both upper and lower fenestrae absent, snakes exhibit a high degree of cranial kinesis in
other areas of skull-wider range of movement and control
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