Lab 3 Crocodylians:
Aquatic Armored Ambush Predators
By: Meghan Kelley and Katie Sagarin
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Introduction:
Modern reptiles have origins in the Carboniferous period, and today can be divided into two major groups: Lepidosauria includes lizards, snakes, and tuataras, and Archosauria includes crocodiles, birds, and possibly turtles (Fig. 1). Order Crocodylia, within Archosauria, contains at least 25 extant species from three different families: Gavialidae, Alligatoridae, and Crocodylidae.
Our lab was based on the diversity within the clade Crocodylia, focusing on general morphology, post-cranial anatomy, and cranial anatomy as a diagnostic framework for identifying the three main families within the Order Crocodylia, in addition to general diet and reproductive glands.
Our lab was based on the diversity within the clade Crocodylia, focusing on general morphology, post-cranial anatomy, and cranial anatomy as a diagnostic framework for identifying the three main families within the Order Crocodylia, in addition to general diet and reproductive glands.
Figure 1: Amniota phylogeny, including the Order Crocodylia (indicated by the red star), which is the sister taxon to birds, within Archosauria.
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I. General Morphology:
First of all, some general characteristics of Crocodylians include: elongated snouts, thecodont teeth that are rooted to jaws (Fig. 2), laterally compressed tails, unfused osteoderms (Fig. 3), and a crocodile normal ankle (Fig. 4).
Figure 3: The crocodile skin is made up of sharp looking ridges that are supported by bony osteoderms beneath the surface, creating a sort of armor of protection. |
II. Post-Cranial Anatomy
Crocodiles have a few distinctive
characteristics in their post-cranial skeleton, one of which is ribs with two points of articulation to the vertebral column. "Two-headed" ribs is an ancestral condition of
tetrapods but is not found in other reptiles. The more anterior head within the rib is called the
capitulum, and the more posterior head is called the tuberculum. The position of the capitulum changes between the thoracic (upper trunk) and cervical (neck) vertebrae. There is a transitional point on the vertebrae in which the capitulum position changes from articulation with the transverse process (thoracic) to articulation with the centrum (cervical) (Fig. 5).
The second
distinctive characteristic is the presence of a pubic process on the ischium of
the pelvic girdle (Fig. 6a &b).
III. Diagnostic Cranial Anatomy
Crocodylians are the only extant amniotes
aside from mammals to have a secondary palate. The secondary palate is formed
by processes of the palatine bones, which fuse to separate the mouth cavity from
the nasal passage. This allows passage of air directly from the nose to the lungs even when the mouth
is full of water (see internal nares on Fig. 7).
To differentiate between the families of Crocodylia, external snout and skeletal cranial morphology are the most useful for identifying individual families. We used the following characteristics:
Figure 7: Internal nares and secondary palate of the crocodylians. |
To differentiate between the families of Crocodylia, external snout and skeletal cranial morphology are the most useful for identifying individual families. We used the following characteristics:
A) SYMPHYSIS
OF THE JAW
The symphysis of
the jaw is the area of the suture between the left and right mandibles. In
Alligatoroidea, the symphysis is short, whereas in Crocodylidae, the symphysis
is long (Fig. 8).
Figure 8: Alligator (left) and crocodile (right) mandibular comparisons of symphyses. |
B) SNOUT BREADTH
Generally,
Alligatoroidea has a broader snout than Crocodylidae, and Gavialidae has an
especially narrow snout (Fig. 9).
Within
Alligatoroidea, Alligatorinae has an extension of the nasals that divides the
nares, whereas Caimaninae (caimans) have no such extension of the nasals.
Neither Crocodylidae nor Gavialidae has split nostrils.
D) OTHER DISTINCTIVE CHARACTERS
D) OTHER DISTINCTIVE CHARACTERS
Male members of
Gavialidae have a boss (knob) on the end of their snout. Members of
Crocodylidae have deep pockets or even holes in their upper jaw to allow for a
large 1st mandibular tooth. The 4th mandibular tooth
grows outside of the upper jaw, and as a consequence the upper jaw may bear an
indentation (Fig. 9).
IV. Reproduction
Crocodylians are similar to other reptiles
in that they have internal fertilization and lay cleidoic eggs, but differ in the antero-posterior position of their cloaca and the possession of cloacal glands (Fig. 10); males also have a unique
“penis.” Some crocodylians invest energy in child care, helping their
young to hatch with gentle bites to the eggshell, and guarding and protecting
them during their first year of life (Fig. 11).
Figure 10: Position of the cloacal glands on a caimen specimen. |
Figure 11: Example of prenatal care in crocodylians. |
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