Crocodylia: for our purposes, we can consider Crocodylia as the crown group including all of the living crodiles, alligators, gharials and Tomistoma.
Range: From the Late Jurasic, fl Late Cretaceous.
Characters: Internal nares completely surrounded by pterygoids; centra of almost all vertebrae procoelous. Did not begin decline until climate deteriorated in ~Pc. 4-chambered heart, but effectively 3 compartments.
Image: Two gharials from the Repti-Page. ATW020318.
Gavialidae: Gavialis > Crocodylus niloticus. Eogavialis (Eocene-Oligocene of Africa), Eotomistoma (Late Cretaceous of Mongolia), Gavialis (from Pliocene of India, Asia & South America), Gryposuchus (Pliocene to Pleistocene of Brazil), Ramphosuchus (Pliocene of India -- possibly a tomistomid), Thoracosaurus (Late Cretaceous to Eocene of Europe & North America).
Range: from Late Cretaceous.
Phylogeny: Crocodylia : (Alligatoridae + Crocodylidae) + *.
Introduction: These crocodylians developed a very slender snout, with very small nasal bones, which reduce resistance in the water. There is also a change in the set of jaw muscles being emphasized. All long-snouted crocodyliforms reduce the pterygoideus musculature and enlarge the temporalis assemblage. This is evident even in extinct forms in which the supratemporal fenestra (openings) in the skull are enormous. For these fish eaters, there is little need for the crushing blow the heavy crocodylid skull could potentially produce. A fast, snapping action is far more important. This tendency developed independently in a variety of crocodylian lineages.
This line of long-snouted fisheaters first appeared when dinosaurs still ruled the Earth. The genus Thoracosaurus, only distantly related to modern forms, appeared in the Maastrichtian epoch and continued through to early Eocene, inhabiting Europe and eastern North America. It was formerly included under the Tomistoninae. The Mongolian species Eotomistoma multidentata is even earlier (Turonian?), and did not survive the end of the dinosaurs. There is no guarantee that Eotomistoma really belongs to this group. It had as many as 48 pairs of straight, pointed teeth on each jaw hence the name multidentata).
Modern gavials seem to have evolved in Africa during the Eocene, with the Egyptian species Eogavialis africanus (middle to late Eocene) and E. gavialoides (late Eocene to Early Oligocene).
As with all the crocodile tribe, the Gavaliadae were much more diverse and widespread in the past than they are today. The gigantic Ramphosuchus crassidens from the Pliocene of India may have been as much as 15 to 18 meters in length. The group was common not only in Asia but in Central and South America as well, where they persisted until the Pliocene. Only one species of this lineage survives, the gavial or gharial, Gavialis gangeticus, also known as the 'true' or 'Indian' gavial. It lives in the Indus, Ganges and Brahmaputra river systems, feeding on fish. Despite its large size (upto 6 meters in length) it is harmless to man. The elongate snout has 27-29 teeth on each side, and a long narrow snout that widens at the nostrils. MAK) 020318.
Alligatoridae: Alligator, Caiman, Melanosuchus, Paleosuchus
Range: from Late Cretaceous (Campanian)
Phylogeny: Crocodylia:: Crocodylidae + *.
Introduction: This important and long-lived family of large tropical reptiles (suborder Eusuchia) first appeared during the Campanian epoch of the later Cretaceous period. This group includes the alligators (upto 3 or 4 meters, in the past to 5 or 6 meters) and their smaller and more primitive cousins the caimans of South America, along with a lareg number of extinct (late Cretaceous and Cenozoic) forms. The snout is broad and only moderately elongated, and not demarcated from the rear of the skull. The nasal bones extend forward to meet the premaxillae (snout / upper jaw bones). There are 17-22 teeth on either side of each jaw. The fourth tooth of the lower jaw fits into a pit in the upper jaw and is invisible when the mouth is closed. During the past this group ranged widely through Europe, Asia, North and South America. There are 7 living species, most of which are endangered. (only the American alligator, A. mississippiensis, is relatively secure). 020318.
Phylogeny: Crocodylia:: Alligatoridae + *: Crocodylinae + Tomistominae.
Introduction: There is some dispute over what taxa to include in this family of relatively unspecialised eusuchians. A.S. Romer (Vertebrate Paleontology, 1956) placed here three subfamilies - Alligatorinae, Crocodylinae, and Gavialinae. Rodney Steel (1973) in Encyclopedia of Paleoherpetology vol 16 (Crocodylia) included five subfamilies - Crocodylinae, Alligatorinae, Gavialinae, Thoracosaurinae / Tomistominae (false gavials), and the extinct terrestrial Pristichampsinae. Sometimes the Gavialidae was considered a distinct family, as in the wonderful (and long out of print) natural history book The Living World of Animals, 1970, The Reader's Digest Association). Robert Carrol in his important work Vertebrate Paleontology and Evolution (1988, the last great pre-cladistic book on vertebrate phylogeny) elevates the Alligatorines to family rank as well, while The Fossil Record II (1993, edited by Michale Benton) entry on Reptiles has the Pristichampidae as a seperate family rank, and further distinguishes the extinct Thoracosaurinae from the living false gavials (Tomistominae). Finally, modern cladistic representations undertake further taxonomic inflation by making the Alligators, Crocs, and Gavial groups each superfamilies (-oidea). And what was formerly family Crocodylidae now becomes the clade "Crocodylia"; formerly the ordinal name for the entire group, but now used to define all recent species and all extinct descendents of their most recent common ancestor.
Such a high taxonomic ranking (three superfamilies) is surely unwarranted. There is very little difference among the so-called "crown group crocodylia". All these animals display a relatively conservative form and a pretty basic morphological pattern, although they do differ greatly in skull proportions and shape and distribution of the teeth. (MAK) 020318.
Crocodylinae: Crocodylus, Euthecodon, Osteolaemus, Tomistoma?
Range: from Early Miocene
Phylogeny: Crocodylidae: Tomistominae + *.
Introduction: The Crocodylinae, or true crocodiles, include 12 living species. These crocodiles have 14 or 15 teeth on each side of the mouth. The fourth tooth fits into a pit in the upper jaw, but remains visible when the mouth is closed. Among the largest speceis of thios subfamily (and the whole Crocodylidae group), the Nile crocodile, Crocodylus niloticus (illustrated above) reaches 5 meters. It is found in southern and central Africa and Madagascar. It lives in lairs dug out of river banks and feeds mainly on fish, though it also eats land animals. There are seven surviving subspecies. The stratigraphic range is Early Miocene? - Pliocene to Recent. The Australasian salt-water crocodile Crocodylus porosus is even larger, reaching over 6 meters in length and 2 to 3 tonnes in weight, although nowadays anything approaching 5 meters and 1 tonne would be extraordinary. As it's name indicates, this species is quite capable of swimming out to open sea and migrating between islands of south east asia and far north Australia. Like the Nile crocodile it is an ancient and long-lived species, ranging from the Pliocene to Recent. MAK; ATW040208.
Tomistominae: Gavialosuchus, Tomistoma
Range: From Miocene.
Phylogeny: Crocodylidae: Crocodylinae + *.
Introduction: The Tomistominae, or false gavials are so called because these long-snouted forms resemble Gavials, to which they are only distantly related. The "longirostrine" adaptation to long narrow snouts has appeared a number of times among the Crocodylimorphs. Tomistomines have 20 or 21 teeth on each side side, and, as with the Crocodylinae, the fourth tooth remains visible when the mouth is closed. This is a relatively recent group, appearing suddenly during the Miocene epoch (earlier specimens attributed to this subfamily probably pertain to different groups) and flourished on both sides of the Atlantic. The gigantic Gavialosuchus americanus from the late Miocene and early Pliocene of Florida, attained 14 meters, but other members of this subfamily are smaller. There is only one recent species, the south-east Asian Tomistoma schlegelii (maximum length 5 meters) (now limited to Indonesia and Malysia). 020318.