Friday, February 10, 2012

Lab #3. Crocodylia. Alex Valigosky and Jeff Walker

A selection of preserved caimans

Crocodyliformes, including a distant early ancestor of modern members of Crocodylia, arose some 220 mya. Crocodylia arose in the Late Cretaceous and include 23 extant members of the superfamilies: Crocodyloidea (Crocodiles), Alligatoroidea (Caimans and Alligators),  and Gavialoidea. Debate still exists on their exact phylogenetic relationships due to apparent differences in molecular and morphological trees. Members of the previously mentioned superfamilies can generally be distinguished based on skull anatomy. Regardless, all members of Crocodylia are derived Archosauromorphs and therefore exhibit the diapsid skull condition - as demonstrated below, snout morphology can also vary a great deal as well between members of Crocodylia.

The superfamilies of Crocodylia can be separated based on cranial anatomy, a feature that we are continuing to learn about. The bones or structures that are important in this process are highlighted below: the palatine process of the palatine (blue), the anterior processes of the surangular, (yellow), the mandibular symphysis where the rami meet (red), and the ectopterygoid (green). Conditions and variations of these bones assists in placement of crocodilians in superfamilies.

All crocodilians share the same similar post-cranial skeletal anatomy.  Some important aspects of the skeleton include the pelvic and pectoral girdles, and the anatomy of the vertebral column (above: caudal vertebrae, sacral vertebrae, lumbar vertebrae, thoracic vertebrae, and cervical vertebrae)

 Pictured to the right are the bones of the pectoral girdle: the interclavicle, the scapula, the procoracoid, and the sternum (not seen - connects the procoracoids)

Pictured below are the bones of the pelvic girdle:  the ilium, the ischium, and the pubis, as well as: the sacral vertebrae (where the ilium attaches) and the ungual phalanx (the the claw bearing phalanges)

Crocodilians have cloacal placement that differs from what we observed in lizards and other reptiles. As opposed to being oriented transversely, their cloacas run along the length of the body. Males have a similar cloaca to females, but within is a "penis." Quotation marks are used because this serves the same function as a mammalian penis, but is not homologous. This "penis" was observed by using forceps to tease open the cloaca. All Crocodilians are oviparous and lay multiple eggs per clutch, some up to 48. These eggs are laid on land, as the young hatchlings will need to breathe air. The nests are dug into the ground near the water. The mother includes vegetation in the nest as a means of insulation. A striking uniqueness in these groups is the extensive parental care that young Crocodilians receive. After hatching, the young spend a lot of time with their mother who will look after them. This extreme parental care is very unique among reptiles.

Caiman cloaca and "penis"

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