Alligatoridae
Temporal range: Cretaceous - Holocene,[1]83–0 Ma
American alligator (Alligator mississippiensis)
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Crocodilia
Clade: Globidonta
Family: Alligatoridae
Gray, 1844
Subfamilies

The family Alligatoridae of crocodylians includes alligators and caimans.

Phylogeny

A. olseni fore limb
A. olseni fore limb
Alligator prenasalis fossil
Alligator prenasalis fossil

The superfamily Alligatoroidea includes all crocodilians (fossil and extant) that are more closely related to the American alligator than to either the Nile crocodile or the gharial.[2] This is a stem-based definition for alligators, and is more inclusive than the crown group Alligatoridae.[3] As a crown group, Alligatoridae only includes the last common ancestor of all extant (living) alligators, caimans, and their descendants (living or extinct), whereas Alligatoroidea, as a stem group, also includes more basal extinct alligator ancestors that are more closely related to living alligators than to crocodiles or gavialids. When considering only living taxons (neontology), this makes Alligatoroidea and Alligatoridae synonymous, and only Alligatoridae is used. Thus, Alligatoroidea is only used in the context of paleontology.

The below cladogram shows Alligatoridae's relationships to other crocodilians, based on the results of a 2018 tip dating study by Lee & Yates that simultaneously used morphological, molecular (DNA sequencing), and stratigraphic (fossil age) data.[3]

Crocodylia
Alligatoroidea

Leidyosuchus

Diplocynodon

Globidonta

Stangerochampsa

Brachychampsa

Navajosuchus

Alligatoridae
Caimaninae

Caiman

Melanosuchus

Paleosuchus

(stem group)
Alligatorinae

Alligator

(stem group)
(crown group)
(stem group)
(stem group)

extinct basal Crocodilians (including Mekosuchinae)

Longirostres
Crocodyloidea

extinct basal crocodiles

Crocodylidae (crown group)

(stem group)
Gavialoidea

extinct basal Gavialoids

Gavialidae

Gavialis

Tomistoma

(crown group)
(stem group)
(crown group)
(crown group)

The below cladogram shows the internal relationships within Alligatoridae:[4][5]

Alligatoridae
Caimaninae

Culebrasuchus mesoamericanus

Eocaiman cavernensis

Tsoabichi greenriverensis

crown group caimans

Paleosuchus palpebrosus Cuvier's dwarf caiman

Paleosuchus trigonatus Smooth-fronted caiman

Centenariosuchus gilmorei

Purussaurus neivensis

Mourasuchus

Orthogenysuchus olseni

Caiman crocodilus Spectacled caiman

Caiman yacare Yacare caiman

Caiman latirostris Broad-snouted caiman

Caiman lutescens

Melanosuchus fisheri

Melanosuchus niger Black caiman

Alligatorinae

Ceratosuchus burdoshi

Hassiacosuchus haupti

Navajosuchus mooki

Wannaganosuchus brachymanus

Arambourgia gaudryi

Allognathosuchus polyodon

Allognathosuchus wartheni

Procaimanoidea kayi

Alligator

Alligator prenasalis

Alligator mcgrewi

Alligator olseni

Alligator sinensis Chinese alligator

Alligator thomsoni

Alligator mefferdi

Alligator mississippiensis American alligator

Evolution

The superfamily Alligatoroidea is thought to have split from the crocodile-gharial lineage in the late Cretaceous, about 87 million years ago.[6][7] Leidyosuchus of Alberta is the earliest known genus. Fossil alligatoroids have been found throughout Eurasia as land bridges across both the North Atlantic and the Bering Strait have connected North America to Eurasia during the Cretaceous, Paleogene, and Neogene periods. Alligators and caimans split in North America during the early Tertiary or late Cretaceous (about 53 million[7] to about 65 million years ago[6]) and the latter reached South America by the Paleogene, before the closure of the Isthmus of Panama during the Neogene period. The Chinese alligator split from the American alligator about 33 million years ago[7] and likely descended from a lineage that crossed the Bering land bridge during the Neogene. The modern American alligator is well represented in the fossil record of the Pleistocene.[8] The alligator's full mitochondrial genome was sequenced in the 1990s.[9] The full genome, published in 2014, suggests that the alligator evolved much more slowly than mammals and birds.[10]

True alligators

The lineage including alligators proper (Alligatorinae) occurs in the fluvial deposits of the age of the Upper Chalk in Europe, where they did not die out until the Pliocene age. The true alligators are today represented by two species, A. mississippiensis in the southeastern United States, which can grow to 15 ft (4.6 m) and weigh 1000 lbs (453 kg)[11] and the small A. sinensis in the Yangtze River, China, which grows to an average of 5 ft (1.5 m). Their name derives from the Spanish el lagarto, which means "the lizard".

Caimans

C. crocodilus at the Helsinki Tropicario Zoo aquarium in Helsinki, Finland in 2010

In Central and South America, the alligator family is represented by six species of the subfamily Caimaninae, which differ from the alligator by the absence of a bony septum between the nostrils, and having ventral armour composed of overlapping bony scutes, each of which is formed of two parts united by a suture. Besides the three species in Caiman, the smooth-fronted caimans in genus Paleosuchus and the black caiman in Melanosuchus are described. Caimans tend to be more agile and crocodile-like in their movements, and have longer, sharper teeth than alligators.[12]

C. crocodilus, the spectacled caiman, has the widest distribution, from southern Mexico to the northern half of Argentina, and grows to a modest size of about 2.2 m (7.2 ft). The largest is the near-threatened Melanosuchus niger, the jacaré-açu or large or black caiman of the Amazon River basin. Black caimans grow to 5.0 m (16.5 ft), with the largest recorded size 5.79 m (19.0 ft). The black caiman and American alligator are the only members of the alligator family that pose the same danger to humans as the larger species of the crocodile family.

Although caimans have not been studied in depth, scientists have learned their mating cycles (previously thought to be spontaneous or year-round) are linked to the rainfall cycles and the river levels, which increases chances of survival for their offspring.

Taxonomy

See also: List of crocodilians

An alligator nest at Everglades National Park, Florida, United States
An alligator nest at Everglades National Park, Florida, United States
Spectacled caiman head
Black caiman, Jauaperi River, Amazonia
Head of smooth-fronted caiman

References

  1. ^ Family Alligatoridae (Alligators and Caiman) Archived 2016-03-04 at the Wayback Machine University of Bristol. Quote:"The Alligatoridae appears in the Upper Cretaceous while the genus Alligator first occurs in the Oligocene."
  2. ^ Brochu, Christopher A. (2003). "Phylogenetic approaches toward crocodylian history" (PDF). Annual Review of Earth and Planetary Sciences. 31 (31): 357–97. Bibcode:2003AREPS..31..357B. doi:10.1146/annurev.earth.31.100901.141308.[permanent dead link]
  3. ^ a b Michael S. Y. Lee; Adam M. Yates (27 June 2018). "Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil". Proceedings of the Royal Society B. 285 (1881). doi:10.1098/rspb.2018.1071.
  4. ^ Hastings, A. K.; Bloch, J. I.; Jaramillo, C. A.; Rincon, A. F.; MacFadden, B. J. (2013). "Systematics and biogeography of crocodylians from the Miocene of Panama". Journal of Vertebrate Paleontology. 33 (2): 239. doi:10.1080/02724634.2012.713814. S2CID 83972694.
  5. ^ Brochu, C. A. (2011). "Phylogenetic relationships of Necrosuchus ionensis Simpson, 1937 and the early history of caimanines". Zoological Journal of the Linnean Society. 163: S228–S256. doi:10.1111/j.1096-3642.2011.00716.x.
  6. ^ a b Oaks, J.R. (2011). "A time-calibrated species tree of Crocodylia reveals a recent radiation of the true crocodiles". Evolution. 65 (11): 3285–3297. doi:10.1111/j.1558-5646.2011.01373.x. PMID 22023592. S2CID 7254442.
  7. ^ a b c Pan, T.; Miao, J.-S.; Zhang, H.-B.; Yan, P.; Lee, P.-S.; Jiang, X.-Y.; Ouyang, J.-H.; Deng, Y.-P.; Zhang, B.-W.; Wu, X.-B. (2020). "Near-complete phylogeny of extant Crocodylia (Reptilia) using mitogenome-based data". Zoological Journal of the Linnean Society. 191 (4): 1075–1089. doi:10.1093/zoolinnean/zlaa074.
  8. ^ Brochu, Christopher A. (1999). "Phylogenetics, Taxonomy, and Historical Biogeography of Alligatoroidea". Society of Vertebrate Paleontology Memoir. 6: 9–100. doi:10.2307/3889340. JSTOR 3889340.
  9. ^ Janke, A.; Arnason, U. (1997). "The complete mitochondrial genome of Alligator mississippiensis and the separation between recent archosauria (birds and crocodiles)". Molecular Biology and Evolution. 14 (12): 1266–72. doi:10.1093/oxfordjournals.molbev.a025736. PMID 9402737.
  10. ^ Green RE, Braun EL, Armstrong J, Earl D, Nguyen N, Hickey G, Vandewege MW, St John JA, Capella-Gutiérrez S, Castoe TA, Kern C, Fujita MK, Opazo JC, Jurka J, Kojima KK, Caballero J, Hubley RM, Smit AF, Platt RN, Lavoie CA, Ramakodi MP, Finger JW, Suh A, Isberg SR, Miles L, Chong AY, Jaratlerdsiri W, Gongora J, Moran C, Iriarte A, McCormack J, Burgess SC, Edwards SV, Lyons E, Williams C, Breen M, Howard JT, Gresham CR, Peterson DG, Schmitz J, Pollock DD, Haussler D, Triplett EW, Zhang G, Irie N, Jarvis ED, Brochu CA, Schmidt CJ, McCarthy FM, Faircloth BC, Hoffmann FG, Glenn TC, Gabaldón T, Paten B, Ray DA (2014). "Three crocodilian genomes reveal ancestral patterns of evolution among archosaurs". Science. 346 (6215): 1254449. doi:10.1126/science.1254449. PMC 4386873. PMID 25504731.
  11. ^ "American alligator". animals.nationalgeographic.com. National Geographic Society.
  12. ^ Guggisberg, C.A.W. (1972). Crocodiles: Their Natural History, Folklore, and Conservation. p. 195. ISBN 978-0-7153-5272-4.
  13. ^ Adam P. Cossette (2020). "A new species of Bottosaurus (Alligatoroidea: Caimaninae) from the Black Peaks Formation (Palaeocene) of Texas indicates an early radiation of North American caimanines". Zoological Journal of the Linnean Society. Online edition. doi:10.1093/zoolinnean/zlz178.