Globidonta
Encyclopedia
Globidonta is a clade
of alligatoroids that includes alligator
s, caiman
s, and closely related extinct forms. It is defined as a stem-based clade including Alligator mississippiensis (the American Alligator) and all forms more closely related to it than to Diplocynodon
. The group's fossil range extends back into the Late Cretaceous
with early alligatoroids such as Albertochampsa
and Brachychampsa
. Extinct globidontans were particularly common in North America
and Eurasia
, and their modern range also includes South America
. The oldest known globidontan is Acynodon
from France
, which is also the most basal
.
Basal globidontans are characterized by their blunt snouts and bulbous teeth. Modern globidontans have flattened snouts and more conical teeth, and are seen as more generalized than earlier globidontans. Generalized forms are usually expected to be ancestral to more specialized forms rather than descendants of them, so it is unusual for basal members of the group to appear specialized. This seems to conflict with the "Law of the Unspecialized" first proposed by Edward Drinker Cope
in 1894. Under the Law of the Unspecialized, morphological change is always directed toward specialization, and specialized forms can never become "unspecialized" again. This pattern of change, while not seen in globidontans, can be observed in basal members of Alligatoroidea
and Crocodyloidea
.
Flat-snouted globidontans occurred two times in the evolution of the clade: once in caimans and once in alligators. Alligator sinensis, the Chinese Alligator
, has a snout that is somewhat blunt and could be considered specialized. However, its snout is not nearly as blunt as those of more basal globidontans such as Albertachampsa.
If the last common ancestor of Diplocynodon and globidontans was more like Diplocynodon, it would have had a generalized snout shape. It is also possible that the generalized form of Diplocynodon may also have arisen from a specialized blunt-snouted ancestor. Baryphracta
, a diplocynodontine closely related to Diplocynondon, has a blunt snout and may have been similar in appearance to such an ancestor.
modified from Martin (2007):
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of alligatoroids that includes alligator
Alligator
An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....
s, caiman
Caiman
Caimans are alligatorid crocodylians within the subfamily Caimaninae. The group is one of two subfamilies of the family Alligatoridae, the other being alligators. Caimans inhabit Central and South America. They are relatively small crocodilians, with most species reaching lengths of only a few...
s, and closely related extinct forms. It is defined as a stem-based clade including Alligator mississippiensis (the American Alligator) and all forms more closely related to it than to Diplocynodon
Diplocynodon
Diplocynodon is an extinct genus of alligatoroid that lived during the Paleocene to middle Miocene 49 million years ago in Europe. It looked very similar to the modern caiman in that it was small and had bony armour scutes covering its neck, back, belly, and tail...
. The group's fossil range extends back into the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
with early alligatoroids such as Albertochampsa
Albertochampsa
Albertochampsa is an extinct genus of globidontan alligatoroid from the Late Cretaceous of Alberta. It was named in 1972 by Bruce Erickson, and the type species is A. langstoni. It is known from a skull from the Campanian-age Dinosaur Park Formation, where it was rare; Leidyosuchus is the most...
and Brachychampsa
Brachychampsa
Brachychampsa is an extinct genus of alligatoroid. Specimens have been found from New Mexico, Colorado, Wyoming, Montana, North and South Dakota, New Jersey, and Saskatchewan. One specimen has been found from the Darbasa Formation of Kazakhstan, although the species status is indeterminant for the...
. Extinct globidontans were particularly common in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Eurasia
Eurasia
Eurasia is a continent or supercontinent comprising the traditional continents of Europe and Asia ; covering about 52,990,000 km2 or about 10.6% of the Earth's surface located primarily in the eastern and northern hemispheres...
, and their modern range also includes South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. The oldest known globidontan is Acynodon
Acynodon
Acynodon is an extinct genus of alligatoroid crocodilian. When first described it was placed within the family Alligatoridae, but has since been reclassified as a globidontan. It is the oldest and most primitive globidontan known to date, with fossils being found from France, Spain, Italy, and...
from France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, which is also the most basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
.
Basal globidontans are characterized by their blunt snouts and bulbous teeth. Modern globidontans have flattened snouts and more conical teeth, and are seen as more generalized than earlier globidontans. Generalized forms are usually expected to be ancestral to more specialized forms rather than descendants of them, so it is unusual for basal members of the group to appear specialized. This seems to conflict with the "Law of the Unspecialized" first proposed by Edward Drinker Cope
Edward Drinker Cope
Edward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...
in 1894. Under the Law of the Unspecialized, morphological change is always directed toward specialization, and specialized forms can never become "unspecialized" again. This pattern of change, while not seen in globidontans, can be observed in basal members of Alligatoroidea
Alligatoroidea
Alligatoroidea is a superfamily of crocodilians that evolved in the Late Cretaceous period. Cladistically, it is defined as Alligator mississippiensis and all crocodylians more closely related to A. mississippiensis than to either Crocodylus niloticus or Gavialis gangeticus ....
and Crocodyloidea
Crocodyloidea
Crocodyloidea is a superfamily of crocodilians that evolved in the Late Cretaceous period. Cladistically, it is defined as Crocodylus niloticus and all crocodylians more closely related to C. niloticus than to either Alligator mississippiensis or Gavialis gangeticus .- Phylogeny :Cladogram after...
.
Flat-snouted globidontans occurred two times in the evolution of the clade: once in caimans and once in alligators. Alligator sinensis, the Chinese Alligator
Chinese Alligator
The Chinese alligator or Alligator Alligator sinensis) is one of two known living species of Alligator, a genus in the family Alligatoridae. The Chinese alligator is native only to China...
, has a snout that is somewhat blunt and could be considered specialized. However, its snout is not nearly as blunt as those of more basal globidontans such as Albertachampsa.
If the last common ancestor of Diplocynodon and globidontans was more like Diplocynodon, it would have had a generalized snout shape. It is also possible that the generalized form of Diplocynodon may also have arisen from a specialized blunt-snouted ancestor. Baryphracta
Baryphracta
Baryphracta is an extinct genus of diplocynodontid crocodilian. It is closely related to the genera Diplocynodon and Tadzhikosuchus. Fossils have been found from the Messel Pit near Messel, Germany, which was deposited during the Middle Eocene epoch of the Paleogene period...
, a diplocynodontine closely related to Diplocynondon, has a blunt snout and may have been similar in appearance to such an ancestor.
Phylogenetics
Below is a cladogramCladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
modified from Martin (2007):