Living crocodilians consist of 23 species of crocodiles (Crocodylia), caiman, alligators (Alligatoridae) and gharials (Gavialidae). They are the largest living reptiles and are found throughout warm and tropical climates (Brochu, 2003). They are large, stealthy predators that stalk and ambush prey in freshwater and estuarine habitats (Erickson et. al, 2012). Their diet consists of invertebrates, fish, snakes, turtles, birds and mammals. American alligators (Alligator mississippiensis) are generalists when it comes to what they eat; unlike the more specialized Indian gharial (Gavialis gangeticus) with its long, slender snout and needle-like teeth, it relies on small fish, invertebrates and crustaceans (Erickson et. al, 2012). The gharial and some other species alive today are endangered, however, some are doing quite well and are examples of successful conservation efforts. Diversity of extant species is low compared to extinct ones. Known fossils of extinct crocodilian species are five times greater than their living relatives (Brochu, 2003). This paints the bigger picture of the rich evolutionary history of today’s crocodiles, caiman, alligators and gharials that dates back to the Campanian (72.1–83.6 mya). This wiki will fallow the evolutionary history of crocodilians up to the American Alligator (Alligator mississippiensis) that is still successful today.
“Crocodilians have dominated predatory niches at the water-land interface for over 85 million years” (Erickson et. al, 20012). During the Eocene (34-56 mya), these fossils were deposited on every continent, even Antarctica (Willis & Stilwell, 2000). The crocodilian record has two points of high diversity, first the Early Eocene (47-56 mya), then the Early Miocene (16-23 mya), these peaks correlate with global temperatures maximums. Amazingly they survived the Cretaceous-Tertiary extinction event unharmed (Brochu, 2003). This event occurred 66 mya and marked the end of the Mesazoic Era and ushered in the Cenazoic Era; it caused the extinction of the dinosaurs, as well as 75% of all other plant and animal species. It is believed that a massive asteroid impact in the Gulf of Mexico caused a period of prolonged cold weather throughout the world (Alvarez, 1980). It is estimated that crocodilians underwent mass extinctions fallowing the mid-Miocene global cooling and glacial advancement. The number of genera dropped from 26 to 8 during the Pleistocene, making that time period the highest crocodilian extinction rate over 100 million years (Oaks, 2011). This decline in diversity was survived by extant species because of their ability to withstand cooler temperatures and find suitable habitat during global cooling.
The term Alligatoroidea refers to all alligators, caimans and their extinct ancestors that are more closely related than to Crocodylus and Gavailis (Gray, 1844). Studies show that this group originated in North America during the late Cretaceous (Brochu, 1999). Alligatoroidea had two dispersal events into Europe during the Tertiary but after the Oligocene (22-33 mya) the record is dominated by North America. A. mississippiensis has been present since the Pliestocene (1-2 mya). A. mississippiensis is able to survive cold temperatures by lowering their metabolism, they usually lose their appetite around 80°F and stop eating below 73°F. They can survive the whole winter on food stores.
Many diverse species make up the phylogenetic tree that leads to the American Alligator. The last common ancestor between Alligators and crocodiles is Brevirostres (Brochu, 2003). The giant Deinosuchus is an extinct relative of the alligator who was around 73-80 mya. It is estimated to have been 9-12 meters or 30 to 40 feet in length (Brochu, 2003). This is more than twice the size of the largest American Alligators alive today. Its fossils were discovered in North Carolina. Another relative is the Chinese alligator (A. sinensis). This species is somewhat of a biogeographic mystery. The fact that they do not posses the morphology to allow prolonged salt water crossing, yet its most common ancestor is the American alligator (A. mississippiensis), means they must have migrated using the Beringia. Some believe that this high latitude route was too temperate during the Miocene (5-20mya) when these two species split. However, A. sinensis is the most cold tolerant crocodilian species alive today and they overwinter utilizing a complex set of burrows (Brochu, 2003). Oaks (2011) estimates the divergence of these sister taxa took place ~31-58 mya which is much longer than the fossil record indicates. She believes this means an earlier crossing of Beringia during a warmer period, however she does not exclude the possibility of an overwater route even though they are not a adapted to salt water as some crocodile species (Oaks, 2011)
The generalized condition of the American Alligator (A. mississippiensis) to have a long, broad snout is thought to be a reversal from the earliest ancestor of Alligatoroidea who had bulbous rear dentition and short snouts (Bochu, 2003). Bite force and dentition play an important role in the diversity and success of different crocodilian lineages. There is a nine-fold difference in bite-forces amongst species, from the lowest force in delicate, slender snouts to the highest amongst robust, blunt snouts (Erikson, 2012). This important feature surely had a hand in multiple extinctions and expansions of various lines exploiting various niches. More generalized forms, by definition, have an easier time adapting to environmental changes that cause certain prey items to decline, etc. In the case of crocodilians, its hard to imagine that a stronger bite-force was ever a disadvantage, however the persistence of the gharial today shows that survival of the fittest does not mean survival of the strongest and sometimes finesse out performs brute force depending on the environment and food source.
Another anatomical feature that links extant species to their prehistoric past is the unidirectional airflow through the lungs. This pattern dates back to the basal archosaurs of the Triassic (Farmer, 2010). It is hypothesized that it was present in dinosaurs and non-dinosaurs descendants and can be seen today only in bird and crocodilian lungs. Birds and crocodilians are the only remaining descendants of archosaurs, the group that contained all extinct dinosaurs, they were the predominant terrestrial vertebrates of the Early Triassic around 250 million years ago.
Current and Future Issues
Crocodilian distribution today is closely lined with temperature, so it is hard to say how important that factor has been throughout their evolutionary history and how climate change will impact them in the future. Reptiles are in decline on a global scale today; they are threatened by habitat loss, degradation, invasive species, pollution, disease, unsustainable use and climate change (Gibbon, 2000). Unsustainable use has been reversed by large scale farming, and has reduced the value of illegally harvesting and trading skins. 7-8% of alligators over 3 feet are harvested annually in Florida as well as 50% alligator eggs, these populations are closely monitored and presumably have negligible effects (Gibbon, 2000). The American alligator is one example of effective legislation to protect a native species in Louisiana. Global warming trends may have an effect on crocodiles and alligators because the sex ratio of their hatchlings is determined by nest temperature during incubation (Gibbon, 2003).
Paleontologists have been finding and studying crocodyliformes for nearly 200 years, today advancements in the field along with genetic studies are refining previous hypothesis that were incomplete due to the insular nature of science (Brochu, 2003). Even as technology is developed, studying the fossils and finding new ones is the key to a greater understanding of phylogeny and of the long history of animals like the American alligator.
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