Biology of the Reptilia

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.

Figure 1: Amniota phylogeny, including the Order Crocodylia (indicated by the red star), which is the sister taxon to birds, within Archosauria.

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 2: Of the three basic tooth skeletal orientations, the thecodont tooth is comprised of a deep pocket surrounding the tooth root. The sharp, conical shape of the tooth enables a carnivorous diet.

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.

Figure 4: The crocodylians are the only extant group that have hind limb ankle orientation with an S-like articulation between the astragulus (ast) and the calcaneous (cal) bones. The calcaneous bone possesses a socket that encapsulates the knob of the astragulus, allowing for foot flexibility in holding the limb upright.

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).

Figure 5: Crocodylia vertebral column, showing the transition between thoracic capitulum articulation with the transverse process of the ribs to the cervical capitulum articulation with the centrum of the spine. A=Thoracic Region; B=Capitulum split to centrum; C=Reduced ribs and continued centrum articulation.

The second distinctive characteristic is the presence of a pubic process on the ischium of the pelvic girdle (Fig. 6a &b).

Figure 6a (upper): Pelvic girdle on skeleton with hind limb connected in the acetabulum. (A=Acetabulum; B=Ilium; C= Ischium; D= Pubis). Fig 6b (lower)- View of interior side of pelvic girdle without limb (P= Pubis; Il.= Ilium; Is.= Ischium; a.f.= Acetabulum foramen). Both views are highlighting the pubic process, which is unique to crocodylians.

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).

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).

Figure 9: Top left, Alligatoridae has the broadest snout of the three groups, followed by Crocodylidae (top right), whose snout is slightly narrower with an indentation to support the 4th tooth that pierces and overlaps upward from the mandibular jaw. Gavialidae's narrow snout is the bottom picture.

C) NOSTRILS

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

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 1^st mandibular tooth. The 4^th 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).