Phylogeny and taxonomy of crocodilians: summaries for taxon in crocodilian lineage, secondary adaptations of crocodilians, evolutionary relashionships with other amniotes at GeoChemBio.com

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Crocodilian taxonomy and phylogeny

Taxonomic lineage

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Deuterostomia - Chordata - Craniata - Vertebrata - Gnathostomata - Teleostomi - Euteleostomi - Sarcopterygii - Tetrapoda - Amniota - Sauropsida - Sauria - Archosauria - Crocodylidae

Crocodilian taxonomy explained

Coelomata: Coelomate animals have a coelom, a body cavity that develops within the embryonic mesoderm. It has muscles on both sides and is lined with a special structure derived from mesoderm, called peritoneum.
Deuterostomia (vs. Protostomia).: In the deuterostomate lineage, the fertilized egg cleaves radially. Cells divide along the plane either parallel to or at right angles to the long axis of the fertilized egg. The cleavage is indeterminate - cells separated after several divisions can still develop into complete embryos.
Chordata: Chordates have pharyngeal slits and notochord at some stages of their development. The main shared ancestral features of their body plan are (1) dorsal, hollow nervous system; (2) ventral heart; and (3) tail that extends beyond the anus.
Craniata: Craniates include all animals that have cartilaginous or bony skull (or cranium), which protects brain and sensory organs.
Vertebrata: Dorsal vertebrate column replaced the notochord as the primary support in these organisms.
Gnathostomata: The organisms that have jaws. First gnathostomes appeared during Devonian period about 400 million years ago. Jaws evolved from some of the skeletal arches that supported the gill region.
Teleostomi: Key features of this group include an operculum (gill cover) and a pair of respiratory openings. Bony fishes, which gave rise to reptiles, are at the base of this group. At some point, they evolved lung-like sacs that supplemented gills in respiration. The teleostomes include all jawed vertebrates except for cartilaginous fishes (chondrichthyans)) and a group of armored prehistoric fishes, placoderms (Placodermi) that went extinct at the end of Devonian period, 345 million years ago.
Euteleostomi (bony vertebrates): The group includes more than 90% of the living species of vertebrates.
Sarcopterygii (fleshy-finned fishes vs. Actinopterygii, ray-finned fishes): The group of organisms that evolved from ancestor of bony fish with fleshy, lobed-paired fins, which are joined to the body by a single bone. Two extant species belong to genus Latimeria.
Tetrapoda: Animals with two pairs of limbs.
Amniota: Amniotes are first vertebrates that conquer terrestrial habitats due to the development the following morphological changes: (1) egg shell that allowed eggs to be laid in dry places; (2) tough water-proof skin; (3) kidney that can excrete concentrated urine. Early amniotes arose from early tetrapods in Carboniferous period, some 300 million years ago.
Sauropsida ("lizard faced" vs. Theropsida, beast faced, or Mammalia): This group includes reptiles and birds.
Sauria: This group includes all reptiles and birds.
Archosauria: This group includes crocodilians and birds.
Crocodylidae: Fossil data demonstrate that birds' and crocodilians' common ancestor is more recent than that shared between crocodilians and other reptiles such as snakes and turtles. Greater superficial similarities between crocodiles and lizards resulted from divergence in evolutionary rates of crocodilians and birds: the former evolved very slowly (living fossils) and the latter have undergone very rapid evolution.

Secondary characteristics of crocodilians

Some unique crocodilian characteristics suggest that they were developed secondarily by "reversal" of more advanced evolutionary features.

Terrestrial locomotion
When out of the water, alligators prefer to move in sprawling manner (belly is on the ground) along the mud trails that they make between bodies of water. On dry solid surface, alligators use trotting gait called high walk during which they elevate their trunk above the ground and position their limbs between sprawling (characteristic of amphibian and lizards) and erect (characteristic of mammals and dinosaurs) grades. In addition, they have to drag their heavy (up to 28% of total weigh) tail behind them. Speed during non-hurried lumbering is about 1.6 m s^-1 (0.35 mph). Alligators can move as fast as 30 mph in short spurts.

A general paradigm in vertebrate locomotion is that locomotor postures in tetrapods evolved from a sprawling posture, with the limbs held laterally to the body, to an erect posture, with the limbs held directly under the body (sprawling-to-erect paradigm). By exhibiting intermediate locomotion extant crocodilians provided a model of transitional locomotion. However, recent kinematic and phylogenetic analyses suggest that rather than being half-way between sprawlers and erect walkers crocodilians evolved secondarily from erect ancestors.

Photo credit: http://kotomatrix.ru/
Incomplete separation of pulmonary and systemic circulation
Like birds and mammals, all crocodilians have a four-chambered heart, consisting of two autricles and two ventricles. The heart performs two main functions — pumping blood to the lungs (pulmonary circulation), and pumping blood around the body (systemic circulation). For terrestrial organisms the four-chambered heart design allows separation of the pulmonary and systemic circulations and prevents mixing of oxygenated blood from the lungs with deoxygenated blood from the body. The blood returned from the general circulation by the venae cavae and jugular vein enters a large sinus when it passes into the right autricle and then into the right ventricle which discharges into the pulmonary artery and left aorta to be oxygenated in the lungs. The left autricle receives the oxygenated blood from the lungs through the pulmonary veins and transmits it to the left ventricle from which it is sent into the systemic circulation. The right and left aortas communicate by a small opening with a valve that is called the foramen of Panizza. During prolonged periods of time spent underwater in chrocodilians, pulmonary pathway becomes unnessessary and by active constriction of a valve in the heart become checked. As it occurs, the right ventricle and its arteries become overfull and blood passes through the foramen of Panizza entering into the general circulation without first passing through the lungs. It is no exaggeration to say that crocodilians have the most advanced heart we know of. Some researchers argue that the anatomy of crocodilian heart, paleontological evidence as well as developmental studies suggest that unique features of crocodilian heart are secondarily derived, indicating a shift from the complete separation of blood flow of endoderm to the controlled shunting of ectoderms. Thus, the ancestors of extant crocodilians were active and endothermic (warm-blooded), but the lineage reverted to ectodermy (cold-bloodedness) when it invaded the aquatic, ambush predator niche.
Nilsson S. The crocodilian heart and central hemodynamics. Cardioscience. 1994 Sep;5(3):163-6.

Axelsson M. The crocodilian heart; more controlled than we thought? Exp Physiol. 2001 Nov;86(6):785-9. (Free article)

Photo of crocodilian heart exhibit at National Museum of Natural History (Washington, D.C., USA) was used in above illustration.

Evolutionary relashionships among Amniotes

The phylogeny indicates that group Reptilia is paraphyletic because it does not include all the descendants of its common ancestor: birds are not included in the group.