Tomistominae
Encyclopedia
Tomistominae is a subfamily of crocodylians that includes one living species, the False gharial
. Many more extinct species are known, extending the range of the subfamily back to the Eocene
epoch. In contrast to the False gharial, which is a freshwater species that lives only in southeast Asia, extinct tomistomines had a global distribution and lived in estuaries and along coastlines.
The classification of tomistomines among Crocodylia is uncertain; while traditionally thought to be within Crocodyloidea
, molecular evidence suggests that they may be more closely related to true gharial
s as members of Gavialoidea
.
bone of the lower jaw is long and slender, forming a distinctive "V" shape not seen in gharials.
and North Africa
. The oldest known tomistomine is Kentisuchus spenceri
from England
, although a possible tomistomine fossil from the Paleocene
of Spain
is even older. Other early tomistomines include Maroccosuchus zennaroi
from Morocco
and Dollosuchus dixoni
from Belgium
. These early tomistomines inhabited the Tethys Ocean
, which covered much of Europe and North Africa during the Paleogene. Several early tomistomines are found in coastal marine deposits, suggesting that they lived along the shoreline or in estuaries. Extinct gavialoids are also thought to have been coastal animals. The marine lifestyles of these early forms likely allowed tomistomines to spread around the Tethys, forming a northern population in Europe and a southern in North Africa.
Later in the Eocene and Oligocene
, tomistomines spread across Asia. The middle Eocene species Ferganosuchus planus
and Dollosuchus zajsanicus are known from Kazakhstan
and
Kyrgyzstan
. Tomistomines reached China and Taiwan with the late Eocene species "Tomistoma" petrolicum and the Miocene
species Penghusuchus pani
. One species, "Tomistoma" tandoni, lived in India
during the middle Eocene. During this time, the Indian subcontinent was separated from mainland Asia, creating a barrier to species that could not tolerate salt water. The Obik Sea, which separated Europe from Asia, also impeded travel. Tomistomines were able to cross these areas, indicating that they had tolerance to salt water.
Tomistomines crossed the Atlantic Ocean and spread into the Americas in the Oligocene, Miocene, and Pliocene
. The earliest known neotropical tomistomine is Charactosuchus kuleri
from Jamaica
. A close relationship has been proposed between C. kuleri and D. zajsanicus from Belgium, suggesting that tomistomines migrated from Europe to the Americas through the De Geer land bridge
connecting Norway
to Greenland
and the North American mainland or the Thule land bridge connecting Scotland, Iceland, Greenland, and the North American mainland. The genus Thecachampsa was present along the eastern coast of North America during the Oligocene, Miocene, and Pliocene.
Tomistomines disappeared from Europe during the Oligocene but returned by the end of the epoch. They diversified and became common in the middle Miocene. One tomistomine, Tomistoma coppensi, is known from the late Miocene of Uganda
. The appearance of tomistomines in central Africa is unusual because there is little evidence of late Miocene species in North Africa, an area where they must have traveled through from Europe.
Tomistomines may have traveled from Africa into Asia when Arabia collided with the Eurasian continent in the Early Miocene. However, Asian Miocene tomistomines may also have descended from the Eocene tomistomines that were already present in eastern Asia. Tomistomines spread throughout the Indian subcontinent during this time. One species, Rhamphosuchus crassidens
, was one of the largest crocodilians that ever lived, growing to an estimated 8 to 11 m (26.2 to 36.1 ft). New species such as Toyotamaphimeia machikanensis
were present in Japan in the Pleistocene
. In southeast Asia however, there is little fossil evidence of the tomistomines that preceded the False gharial. Therefore, its relation with extinct species is unclear.
Morphological
Molecular
The False gharial Tomistoma is similar to the true gharial Gavialis in that it is longirostrine, meaning that it has a long, narrow snout. While crocodiles are also longirostrine, their snouts are not as narrow as those of Tomistoma and Gavialis. Details of the morphology of Tomistoma and Gavialis suggest that the two are only distantly related and that long snouts evolved independently in each lineage as a result of convergent evolution
. Morphologically, tomistomines seem to be more closely related to crocodylids, having shared a more recent common ancestor with crocodiles than with the gharial. Under this phylogeny, gharials split with the common ancestors of tomistomines, crocodiles, and alligators over 65 million years ago in the Late Cretaceous
. Below is a cladogram from Piras et al. (2007) placing tomistomines within Crocodyloidea:
Below is a cladogram from Shan et al. (2009):
Recent molecular analyses of crocodylians find a closer relationship between Tomistoma and Gavialis. According to this phylogeny, the two forms are sister taxa, more closely related to each other than either is to any other living crocodylian. If they are sister taxa, the divergence of the two forms is thought to have occurred in the Eocene
or mid-Miocene
. If Tomistoma and Gavialis are considered to be closely related, the morphological similarities between Tomistoma and crocodiles must be convergent
. In this case, convergence leads to a problem known as long branch attraction
, where similar but independently evolved features in Tomistoma and crocodiles misleadingly imply that these two highly derived
groups are closely related. In effect, these shared characters mask the closer relationship between Tomistoma and Gavialis.
Long branch attraction can be resolved if differences are observed between ancestral members of each group. If the two groups were closely related, early members should have primitive traits from a common ancestor and should lack the derived traits that cause long branch attraction. Tomistomine fossils do not resolve long branch attraction when their characters are added to phylogenetic analyses, so conflict still exists between morphological and molecular phylogenies.
Some studies have combined morphological and molecular characters in their analyses. Analyses that include morphological characters but still rely heavily on the sequences of DNA find a close relation between Tomistoma and Gavialis. Analyses that use more characters from bones and fossils uphold the close relationship of Tomistoma with crocodiles.
False gharial
The false gharial , also known as the Malayan gharial, false gavial, or Tomistoma is a freshwater crocodile of the Crocodylidae family with a very thin and elongated snout...
. Many more extinct species are known, extending the range of the subfamily back to the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
epoch. In contrast to the False gharial, which is a freshwater species that lives only in southeast Asia, extinct tomistomines had a global distribution and lived in estuaries and along coastlines.
The classification of tomistomines among Crocodylia is uncertain; while traditionally thought to be within 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...
, molecular evidence suggests that they may be more closely related to true gharial
Gharial
The gharial , , also called Indian gavial or gavial, is the only surviving member of the once well-represented family Gavialidae, a long-established group of crocodilians with long, slender snouts...
s as members of Gavialoidea
Gavialoidea
Gavialoidea is one of three superfamilies of crocodylians, the other two being Alligatoroidea and Crocodyloidea. Although many extinct species are known, only the gharial Gavialis gangeticus and possibly the False gharial Tomistoma schlegelii are alive today.-Classification:Gavialoidea contains the...
.
Description
Tomistomines have narrow or longirostine snouts like gharials. The living False gharial lives in fresh water and uses its long snout and sharp teeth to catch fish, although true gharials are more adapted toward piscivory, or fish-eating. Despite the similarity with gharials, the shapes of bones in tomistomine skulls link them with crocodiles. For example, both tomistomines and crocodiles have thin postorbital bars behind the eye sockets and a large socket for the fifth maxillary tooth. The splenialSplenial
The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side between the angular and suprangular....
bone of the lower jaw is long and slender, forming a distinctive "V" shape not seen in gharials.
Evolutionary history
Tomistomines first appeared in the Eocene in EuropeEurope
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and North Africa
North Africa
North Africa or Northern Africa is the northernmost region of the African continent, linked by the Sahara to Sub-Saharan Africa. Geopolitically, the United Nations definition of Northern Africa includes eight countries or territories; Algeria, Egypt, Libya, Morocco, South Sudan, Sudan, Tunisia, and...
. The oldest known tomistomine is Kentisuchus spenceri
Kentisuchus
Kentisuchus is an extinct genus of tomistomine . It is considered one of the most basal members of the subfamily. Fossils have been found from England that date back to the early Eocene...
from England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, although a possible tomistomine fossil from the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
of Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
is even older. Other early tomistomines include Maroccosuchus zennaroi
Maroccosuchus
Maroccosuchus is an extinct genus of tomistomine crocodilian from the early Eocene of Morocco....
from Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
and Dollosuchus dixoni
Dollosuchus
Dollosuchoides is an extinct tomistomine. . It is a basal form possibly related to Kentisuchus, according to several phylogenetic analyses that have been conducted in recent years, and is the oldest known tomistomine to date. Fossils have been found from Belgium and the United Kingdom...
from Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
. These early tomistomines inhabited the Tethys Ocean
Tethys Ocean
The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...
, which covered much of Europe and North Africa during the Paleogene. Several early tomistomines are found in coastal marine deposits, suggesting that they lived along the shoreline or in estuaries. Extinct gavialoids are also thought to have been coastal animals. The marine lifestyles of these early forms likely allowed tomistomines to spread around the Tethys, forming a northern population in Europe and a southern in North Africa.
Later in the Eocene and Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, tomistomines spread across Asia. The middle Eocene species Ferganosuchus planus
Ferganosuchus
Ferganosuchus is an extinct genus of gavialid crocodilian. The genus is thought to be a tomistomine, although it has been suggested that Ferganosuchus may be a more basal gavialoid. Fossils have been found in the region of Kazakhstan and Kyrgyzstan that date back the middle Eocene...
and Dollosuchus zajsanicus are known from Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...
and
Kyrgyzstan
Kyrgyzstan
Kyrgyzstan , officially the Kyrgyz Republic is one of the world's six independent Turkic states . Located in Central Asia, landlocked and mountainous, Kyrgyzstan is bordered by Kazakhstan to the north, Uzbekistan to the west, Tajikistan to the southwest and China to the east...
. Tomistomines reached China and Taiwan with the late Eocene species "Tomistoma" petrolicum and the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
species Penghusuchus pani
Penghusuchus
Penghusuchus is an extinct genus of tomistomine gavialid crocodilian. It is known from a skeleton found in Upper Miocene rocks of Penghu Island, off of Taiwan. It appears to be related to two other fossil Asian tomistomines: Tomistoma petrolica of southeastern China and Toyotamaphimeia...
. One species, "Tomistoma" tandoni, lived in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
during the middle Eocene. During this time, the Indian subcontinent was separated from mainland Asia, creating a barrier to species that could not tolerate salt water. The Obik Sea, which separated Europe from Asia, also impeded travel. Tomistomines were able to cross these areas, indicating that they had tolerance to salt water.
Tomistomines crossed the Atlantic Ocean and spread into the Americas in the Oligocene, Miocene, and Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
. The earliest known neotropical tomistomine is Charactosuchus kuleri
Charactosuchus
Charactosuchus is an extinct genus of crocodilian. It was assigned to the family Crocodylidae in 1988. Specimens have been found from Colombia, Brazil, Jamaica, and possibly Florida and South Carolina...
from Jamaica
Jamaica
Jamaica is an island nation of the Greater Antilles, in length, up to in width and 10,990 square kilometres in area. It is situated in the Caribbean Sea, about south of Cuba, and west of Hispaniola, the island harbouring the nation-states Haiti and the Dominican Republic...
. A close relationship has been proposed between C. kuleri and D. zajsanicus from Belgium, suggesting that tomistomines migrated from Europe to the Americas through the De Geer land bridge
Land bridge
A land bridge, in biogeography, is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonise new lands...
connecting Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
to Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
and the North American mainland or the Thule land bridge connecting Scotland, Iceland, Greenland, and the North American mainland. The genus Thecachampsa was present along the eastern coast of North America during the Oligocene, Miocene, and Pliocene.
Tomistomines disappeared from Europe during the Oligocene but returned by the end of the epoch. They diversified and became common in the middle Miocene. One tomistomine, Tomistoma coppensi, is known from the late Miocene of Uganda
Uganda
Uganda , officially the Republic of Uganda, is a landlocked country in East Africa. Uganda is also known as the "Pearl of Africa". It is bordered on the east by Kenya, on the north by South Sudan, on the west by the Democratic Republic of the Congo, on the southwest by Rwanda, and on the south by...
. The appearance of tomistomines in central Africa is unusual because there is little evidence of late Miocene species in North Africa, an area where they must have traveled through from Europe.
Tomistomines may have traveled from Africa into Asia when Arabia collided with the Eurasian continent in the Early Miocene. However, Asian Miocene tomistomines may also have descended from the Eocene tomistomines that were already present in eastern Asia. Tomistomines spread throughout the Indian subcontinent during this time. One species, Rhamphosuchus crassidens
Rhamphosuchus
Rhamphosuchus is an extinct relative of the modern false gharial. It inhabited what is now the Indian sub-continent in the Miocene...
, was one of the largest crocodilians that ever lived, growing to an estimated 8 to 11 m (26.2 to 36.1 ft). New species such as Toyotamaphimeia machikanensis
Toyotamaphimeia
Toyotamaphimeia is an extinct genus of tomistomine from the Pleistocene of Japan closely related to the false gharial, which lived 400.000 years ago. This relationship is reflected in the fact that it was originally described as a member of the same genus, Tomistoma....
were present in Japan in the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
. In southeast Asia however, there is little fossil evidence of the tomistomines that preceded the False gharial. Therefore, its relation with extinct species is unclear.
Phylogeny
The False gharial Tomistoma is similar to the true gharial Gavialis in that it is longirostrine, meaning that it has a long, narrow snout. While crocodiles are also longirostrine, their snouts are not as narrow as those of Tomistoma and Gavialis. Details of the morphology of Tomistoma and Gavialis suggest that the two are only distantly related and that long snouts evolved independently in each lineage as a result of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
. Morphologically, tomistomines seem to be more closely related to crocodylids, having shared a more recent common ancestor with crocodiles than with the gharial. Under this phylogeny, gharials split with the common ancestors of tomistomines, crocodiles, and alligators over 65 million years ago in 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...
. Below is a cladogram from Piras et al. (2007) placing tomistomines within Crocodyloidea:
Below is a cladogram from Shan et al. (2009):
Recent molecular analyses of crocodylians find a closer relationship between Tomistoma and Gavialis. According to this phylogeny, the two forms are sister taxa, more closely related to each other than either is to any other living crocodylian. If they are sister taxa, the divergence of the two forms is thought to have occurred in the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
or mid-Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
. If Tomistoma and Gavialis are considered to be closely related, the morphological similarities between Tomistoma and crocodiles must be convergent
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
. In this case, convergence leads to a problem known as long branch attraction
Long branch attraction
Long branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...
, where similar but independently evolved features in Tomistoma and crocodiles misleadingly imply that these two highly derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
groups are closely related. In effect, these shared characters mask the closer relationship between Tomistoma and Gavialis.
Long branch attraction can be resolved if differences are observed between ancestral members of each group. If the two groups were closely related, early members should have primitive traits from a common ancestor and should lack the derived traits that cause long branch attraction. Tomistomine fossils do not resolve long branch attraction when their characters are added to phylogenetic analyses, so conflict still exists between morphological and molecular phylogenies.
Some studies have combined morphological and molecular characters in their analyses. Analyses that include morphological characters but still rely heavily on the sequences of DNA find a close relation between Tomistoma and Gavialis. Analyses that use more characters from bones and fossils uphold the close relationship of Tomistoma with crocodiles.