Eosuchus
Encyclopedia
Eosuchus is an extinct genus
of gavialoid
crocodilia
n. It is among the most basal
of all gavialoids, lying crownward
of all other known members of the superfamily
, including earlier members such as Thoracosaurus
and Eothoracosaurus
. Fossil
s have been found from France
as well as eastern North America
in Maryland
, Virginia
, and New Jersey
. The strata from which specimens have been found date back to the late Paleocene
and early Eocene
epochs.
border, assigned to the type species E. lerichei. A second species, E. minor, was named in 2006 from material found from the United States
, having been referred in the past to several different genera of crocodilians. E. minor differs from E. lerichei on the basis of a noticeably wider nasal
and prefrontals
positioned anteriorly further up the skull than the lacrimals
A new species of Gavialis, G. minor, was named by Othniel Charles Marsh
in 1870 on the basis of the holotype
specimen YPM 282, consisting of cranial fragments and isolated vertebrae found from the Manasquan Formation in Monmouth County, New Jersey
. The locality dates back to the Ypresian
stage of the early Eocene
. The species name refers to the relatively small estimated size of the animal, estimated at no more than 2 meters, when compared to other gavialoids such as the modern gharial
, which can grow up to 5 meters in length.
However, this species was later recognized as distinct from Gavialis on the basis of certain aspects of the known cranial material, in particular the large foramen aerum of the quadrate
formed from the epithelial tube
that connects the pneumatic chambers of the quadrate and articular
. Another diagnostic feature thought to distinguish the species from Gavialis was the narrow interfenestral bar of the parietal bone
that is relatively smooth and unsculptured when compared to other gavialoids such as Thoracosaurus. The name Thecachampsoides was proposed for the species G. minor in 1986. A close relationship between T. minor and Eosuchus lerichei was always evident, yet it was not until 2006 that the name Eosuchus was applied to the T. minor specimens, specifically on the basis of a fairly complete specimen called NJSM 15437 from the Vincetown Formation in New Jersey, of which there is a visibly exposed braincase which aids greatly in the taxonomic classification of the genus. The examination of specimens of T. minor with those of E. lerichei yielded many similarities between the two species, including the foramen aerum as well as other features such as a long nasal process
between the premaxilla
e, dentary alvioli arranged in pairs, and a W-shaped basioccipital
tuberosity.
Other material present from the Aquia Formation
of Maryland and Virginia, which dates back to the early Paleocene
, tends to be more complete. Some specimens found from these localities are known from nearly complete skulls that provide a more detailed view of the phylogenetic position of Eosuchus, and further aid in distinguishing E. minor from other gavialoids.
Historically, many other species of extinct gavialoids have been assigned to the genus Gavialis (of which the modern gharial, G. gangeticus, is a member of). These include forms such as Thoracosaurus.
. The genus is distinct from the later Eogavialis
and Neogene
gavialoids in still having a thin descending lamina positioned next to the pterygoid
.
A phylogenetic analysis
conducted in 1996 suggested that Eosuchus may be closely related to tomistomines
such as the extant Tomistoma
. There is further evidence of this relationship shown in the humeri
of these crocodilians; both Eosuchus and tomistomines have a deeply concave deltopectoral crest, unlike the more triangular crests of Gavialis. A nasal-premaxilla contact and similar dentary and maxilla
ry tooth counts also seem to suggest that there may be a close relationship between the genus and later tomistomines, although these features represent more primitive conditions that changed later in gavialoid history. Despite this, it is currently accepted that a close relationship between Eosuchus and tomistomines is not the case, and that the similarities between the two may just be superficial and that these characteristics are plesiomorphic to all gavialoids, being lost in more derived members such as the modern Gavialis.
, and thus probably reflect the actual environments that these animals would have inhabited. It has been proposed that early gavialoids were originally salt-tolerant coastal forms, and the evidence seen in the case of Eosuchus is consistent with this theory. One specimen of E. minor from the Aquia Formation, USNM 299730, has a fossil oyster
attached to the dorsal surface of the rostrum.
The fact that the two species of Eosuchus lived on either side of the Atlantic Ocean
implies that these populations may have been separated geographically from one another while not necessarily having to be separated stratigraphically (that is, if the temporal ranges of the two species coincide with one another). More importantly, the separate biogeographic
ranges of the two species may be evidence for a transoceanic dispersal event from one continent to the other. Since the presumed ages of the localities from which specimens have been found are quite similar yet inexact, it is currently unknown just what continent this dispersal event may have originated. A recent reevaluation of the holotype material of E. lerichei, which in the past has been poorly studied, suggests that it is the more basal species and thus would have been the ancestor of E. minor in Europe.
archosauromorph
was originally named Eosuchus, although it is completely unrelated to the gavialoid of the same name. The name of the archosaur was later changed to Noteosuchus due to this preoccupation.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of gavialoid
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...
crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
n. It is among 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:...
of all gavialoids, lying crownward
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
of all other known members of the superfamily
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
, including earlier members such as Thoracosaurus
Thoracosaurus
Thoracosaurus is an extinct genus of gavialoid crocodilian which existed during the Late Cretaceous and early Paleocene. It contains the species Thoracosaurus neocesariensis in North America and Thoracosaurus macrorhynchus in Europe. A number of species have been referred to this genus, but most...
and Eothoracosaurus
Eothoracosaurus
Eothoracosaurus is an extinct genus of gavialoid crocodilian. It was one of the earliest and most basal gavialoids known. Fossils are present from the Ripley Formation in Mississippi, and date back to the early Maastrichtian stage of the Late Cretaceous...
. Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s have been found 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...
as well as eastern 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...
in Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
, Virginia
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...
, and New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
. The strata from which specimens have been found date back to the late 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...
and early 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...
epochs.
Species
The name Eosuchus was first used in 1907 to describe a single specimen found from northern France near the BelgianBelgium
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...
border, assigned to the type species E. lerichei. A second species, E. minor, was named in 2006 from material found from the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, having been referred in the past to several different genera of crocodilians. E. minor differs from E. lerichei on the basis of a noticeably wider nasal
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
and prefrontals
Prefrontal bone
The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern and extinct lungfish, amphibians and reptiles...
positioned anteriorly further up the skull than the lacrimals
Lacrimal bone
The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.-Lateral or orbital surface:...
A new species of Gavialis, G. minor, was named by Othniel Charles Marsh
Othniel Charles Marsh
Othniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...
in 1870 on the basis of the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
specimen YPM 282, consisting of cranial fragments and isolated vertebrae found from the Manasquan Formation in Monmouth County, New Jersey
Monmouth County, New Jersey
Monmouth County is a county located in the U.S. state of New Jersey, within the New York metropolitan area. As of the 2010 Census, the population was 630,380, up from 615,301 at the 2000 census. Its county seat is Freehold Borough. The most populous municipality is Middletown Township with...
. The locality dates back to the Ypresian
Ypresian
In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between and , is preceded by the Thanetian age and is followed by the Eocene Lutetian age....
stage of the early 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...
. The species name refers to the relatively small estimated size of the animal, estimated at no more than 2 meters, when compared to other gavialoids such as the modern 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...
, which can grow up to 5 meters in length.
However, this species was later recognized as distinct from Gavialis on the basis of certain aspects of the known cranial material, in particular the large foramen aerum of the quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
formed from the epithelial tube
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
that connects the pneumatic chambers of the quadrate and articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
. Another diagnostic feature thought to distinguish the species from Gavialis was the narrow interfenestral bar of the parietal bone
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
that is relatively smooth and unsculptured when compared to other gavialoids such as Thoracosaurus. The name Thecachampsoides was proposed for the species G. minor in 1986. A close relationship between T. minor and Eosuchus lerichei was always evident, yet it was not until 2006 that the name Eosuchus was applied to the T. minor specimens, specifically on the basis of a fairly complete specimen called NJSM 15437 from the Vincetown Formation in New Jersey, of which there is a visibly exposed braincase which aids greatly in the taxonomic classification of the genus. The examination of specimens of T. minor with those of E. lerichei yielded many similarities between the two species, including the foramen aerum as well as other features such as a long nasal process
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
between the premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
e, dentary alvioli arranged in pairs, and a W-shaped basioccipital
Occipital bone
The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...
tuberosity.
Other material present from the Aquia Formation
Aquia Formation
The Aquia Formation is a Paleocene fossil site located near the Potomac River in Maryland and Virginia.-Sharks:*Carcharias hopei*Cretolamna*Otodus obliquus*Paleohypotodus rutoti*Striatolamia striata*Squalus...
of Maryland and Virginia, which dates back to the early 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...
, tends to be more complete. Some specimens found from these localities are known from nearly complete skulls that provide a more detailed view of the phylogenetic position of Eosuchus, and further aid in distinguishing E. minor from other gavialoids.
Historically, many other species of extinct gavialoids have been assigned to the genus Gavialis (of which the modern gharial, G. gangeticus, is a member of). These include forms such as Thoracosaurus.
Phylogenetics
Eogavialis is often referred to as a "thoracosaur", a group that was constructed to include many basal forms of gavialoids that cannot be placed into any specific families of their own, yet do not form a true cladeClade
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...
. The genus is distinct from the later Eogavialis
Eogavialis
Eogavialis is an extinct gavialoid. It superficially resembles Tomistoma schlegelii, the extant false gharial, and consequently material from the genus was originally referred to Tomistoma...
and Neogene
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...
gavialoids in still having a thin descending lamina positioned next to the pterygoid
Lateral pterygoid plate
The lateral pterygoid plate of the sphenoid is broad, thin, and everted; its lateral surface forms part of the medial wall of the infratemporal fossa, and gives attachment to the lateral pterygoid muscle; its medial surface forms part of the pterygoid fossa, and gives attachment to the medial...
.
A phylogenetic analysis
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
conducted in 1996 suggested that Eosuchus may be closely related to tomistomines
Tomistominae
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...
such as the extant Tomistoma
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...
. There is further evidence of this relationship shown in the humeri
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
of these crocodilians; both Eosuchus and tomistomines have a deeply concave deltopectoral crest, unlike the more triangular crests of Gavialis. A nasal-premaxilla contact and similar dentary and maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
ry tooth counts also seem to suggest that there may be a close relationship between the genus and later tomistomines, although these features represent more primitive conditions that changed later in gavialoid history. Despite this, it is currently accepted that a close relationship between Eosuchus and tomistomines is not the case, and that the similarities between the two may just be superficial and that these characteristics are plesiomorphic to all gavialoids, being lost in more derived members such as the modern Gavialis.
Paleobiology
The strata from which both species of Eosuchus have been found were though to have formed in a marginal marine depositional environmentSedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...
, and thus probably reflect the actual environments that these animals would have inhabited. It has been proposed that early gavialoids were originally salt-tolerant coastal forms, and the evidence seen in the case of Eosuchus is consistent with this theory. One specimen of E. minor from the Aquia Formation, USNM 299730, has a fossil oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....
attached to the dorsal surface of the rostrum.
The fact that the two species of Eosuchus lived on either side of the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
implies that these populations may have been separated geographically from one another while not necessarily having to be separated stratigraphically (that is, if the temporal ranges of the two species coincide with one another). More importantly, the separate biogeographic
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
ranges of the two species may be evidence for a transoceanic dispersal event from one continent to the other. Since the presumed ages of the localities from which specimens have been found are quite similar yet inexact, it is currently unknown just what continent this dispersal event may have originated. A recent reevaluation of the holotype material of E. lerichei, which in the past has been poorly studied, suggests that it is the more basal species and thus would have been the ancestor of E. minor in Europe.
Other taxa referred to Eosuchus
A TriassicTriassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
archosauromorph
Archosauromorpha
Archosauromorpha is an infraclass of diapsid reptiles that first appeared during the late Permian and became more common during the Triassic. Included in this infraclass are the groups Rhynchosauria, Trilophosauridae, Prolacertiformes, Archosauriformes, and, tentatively, Choristodera...
was originally named Eosuchus, although it is completely unrelated to the gavialoid of the same name. The name of the archosaur was later changed to Noteosuchus due to this preoccupation.
External links
- Eosuchus in the Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...