Wednesday, February 8, 2012

Reptiles Lab 3 - Crocodylia (Serna & Thompson)

Where: JCU lab
When: Friday 3 Feb 2012 (1:30-4:30 pm)

What We Did: This week’s lab focused on the clade of Crocodylia. Crocodilians are large, robust, and elongate reptiles. The extant, or current, crocodilians arose during the Late Cretaceous (~70 million years ago). There are currently 23 described species within three super families: the Alligatoroidea, Crocodyloidea, and Gavialoidea. All members are oviparous (egg-layers) and nest on land near vegetation. One unique characteristic of crocodilians is that they care for their young, including helping the babies hatch, protection, and transportation. Additionally, the male copulatory organ in the cloacal wall is anatomically different from that of turtles, snakes, and lizards.

Overall, there exist three families of crocodilians: Gavialidae, Alligatoridae, and Crocodylidae. Gavialidae contains two genera (Gavialis and Tomistoma), Crocodylidae contains three genera (Crocodylus, Mecistops, and Osteolaemus), and there are two Alligatoridae subfamilies (Alligatorinae and Caimaninae) with one (Alligator) and three (Caiman, Melanosuchus, and Paleosuchus) genera, respectively.

Parts of the Skeleton

Here is a photo of a typical member of Crocodylia. Note the short, stocky limbs. Directly posterior to the skull is the atlas/axis complex. The neck contains cervical vertebrae while the back contains thoracic (with ribs) and lumbar (without ribs) vertebrae. Sacral vertebrae articulate with the ilia (hip) and the remainder of the vertebrae are caudal (tail).

The limbs, pelvic, and pectoral girdles were further analyzed. From the vertebral articulation of the pectoral girdle to the extremities is: suprascapular cartilage, scapula, coracoid, sternum (parallel to spine), humerus, radius (on the same side as digit I), ulna, carpals, metacarpals, phalanges. The "hand" of a crocodylian has 5 digits total. From the vertebral (sacral) articulation of the pelvic girdle to the extremities is: ilium, pubis (with gastralia), ischium, femur, tibia (on the same side as digit I), fibula, tarsals, metatarsals, phalanges. The "foot" of a crocodylian has 4 digits total.

The Crocodilian Skull

Members of Crocodylia have very distinct diapsid skulls. These reptiles display thecodont dentition, with teeth in deep sockets. There are variations within the bones and processes of the lower jaw that help place crocodilians into their respective superfamilies. The distinguishing features are: Gavialidae genera have lower rami joined anteriorly by a long symphysis; Alligatoridae genera have anterior surangular processes (in lower jaw) of the same length and the ectopterygoid abuts separately from the maxilla tooth row; Crocodylidae genera have short anterior processes of the palatine (seen in ventral view of skull) that do not extend beyond the suborbital fenestrae.

Here is an Alligator sp. skull with cranial bones labeled, both dorsally (left) and ventrally (right). The dorsal view clearly shows the diapsid condition with both supra- and subtemporal fenestrae.

Below is the same Alligator sp. skull with lateral cranial bones labeled.

Snout Morphology

The snout variation is a reflection of the diet variety that these animals have. The long, slender snout of a species of Gavialidae (not pictured, but similar to top skull) reflects its adaptation of being piscivorous; the morphology allows for quick and accurate movements to grab fish out of the water. The broad, U-shaped snout of the American alligator (Alligator mississippiensis; middle) is adaptive for grabbing and holding on to large, strong prey trying to flee. The even broader snout of this (accurately-named) broad-snout caiman (Caiman latirostris; bottom) is shorter for crushing the shells of turtles, crustaceans, and other hard-shelled prey.


As seen from the above cladogram, molecular data and morphological data both give different relationships among Crocodylia. For instance, molecular data show Tomistoma and Gavialis as sister taxa, but morphologically they are not. However, both data sets agree on the sister relationship between the Crocodylidae genera Crocodylus and Osteolaemus.

Here is the collection of specimens that were on display in lab. These hatchlings were all identified to be caimans, members of subfamily Caimaninae, of family Alligatoridae. The distinguishing feature of these individuals is the prominent bony ridge posterior to (or behind) the eyes (seen in the picture below). Other members of Alligatoridae are easily identified by yellow markings along the body.

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