Sebecus
Encyclopedia
Sebecus is an extinct genus
of sebecid
crocodylomorph from the Eocene
of South America
. Fossils have been found in Patagonia
. Like other sebecosuchia
ns, it was entirely terrestrial and carnivorous. The genus is currently represented by a single species, the type
S. icaeorhinus. Several other species have been referred to Sebecus, but were later reclassified as their own genera.
The name Sebecus is a Latinisation
of Sebek (also called Sobek), the crocodile
god of ancient Egypt
. Sebek was considered an alternative to the Greek
χάμψαι, or "champsa" in crocodilian nomenclature (the Greek historian Herodotus
claimed that champsa was the Egyptian word for crocodile).
The specific name icaeorhinus of the type species is derived from the Greek words εικαίοs and ρύγχος. Εικαίοs means "random" or "not according to plan" and ρύγχος means "snout", in reference to the animal's unusually deep snout.
paleontologist George Gaylord Simpson
in 1937, Sebecus was one of the first known sebecosuchians. Simpson described the type species, S. icaeorhinus, from a fragmented skull and lower jaw. The specimen was discovered by the American Museum of Natural History
's First Scarritt Expedition to Patagonia
. Teeth had been known since 1906 when Argentine paleontologist Florentino Ameghino
associated them with carnivorous dinosaurs. The more complete material found by Simpson firmly established that the new animal was a crocodyliform. Although Simpson's fossil was considered one of the best finds of the expedition, Simpson described the genus only briefly in 1937. He noted its unusual ziphodont dentition in which the teeth were laterally compressed and serrated. Simpson was preparing a more detailed monograph on the genus, but entered the United States Army
before its completion. Another American paleontologist, Edwin Harris Colbert
, completed Simpson's work, thoroughly describing the genus and placing it in a new family, Sebecidae
. Colbert placed Sebecus and the Cretaceous
baurusuchid
Baurusuchus
(also from South America) in the suborder Sebecosuchia (erected by Simpson for all ziphodont crocodylomorphs), as both had deep snouts and ziphodont teeth.
In 1965, American paleontologist Wann Langston, Jr.
named a second species, S. huilensis, from the Miocene
La Venta Formation in Columbia. S. huilensis was named on the basis of skull fragments. The deposits are Friasian
in age (about 15 million years old), extending the range of the genus into the Neogene
by around 40 million years. In 1977, remains were described from the Miocene of Peru
.
A third species of Sebecus, S. querejazus, was named in 1991 from the early Paleocene
Santa Lucia Formation
in Bolivia
. This extended the range of Sebecus back to the beginning of the Paleogene
, soon after the Cretaceous-Tertiary extinction event
. In 1993, Gasparini et al. described Sebecus carajazus. This was not a fourth species but a misspelling, or lapsus calami, of Sebecus querejazus.
A 2007 study of sebecids reclassified several species. The two species S. huilensis and S. querejazus were given their own genera, Zulmasuchus
and Langstonia
, respectively. Langstonia huilensis, named after Langston, was distinguished from Sebecus by its narrower snout and widely spaced teeth. Zulmasuchus querejazus, named after Zulma Gasparini, one of the authors of the study, differs from Sebecus in its wider snout.
, or eye sockets. The supratemporal fenestrae, two holes on the skull table
, are relatively small.
Laterally compressed, or ziphodont teeth, are characteristic of Sebecus and other sebecosuchians. Although the teeth vary in size, they are homodont, having a similar shape throughout the jaw. At the tips of the upper and lower jaws, the teeth are rounder in cross-section. The fourth dentary tooth is raised in the lower jaw to form an effective canine
. The foremost teeth of the lower jaw are much smaller and lower than the fourth tooth. At the tip of the jaw the first dentary tooth is procumbent, or directed forward. The teeth of the upper and lower jaws form an alternate pattern to allow the jaw to close tightly. A notch is present between the maxilla
and premaxilla
bones of the upper jaw, accommodating the fourth dentary tooth when the jaw is closed. The procumbent first dentary teeth fit between the first and second premaxillary teeth. This close fit allows the serrated edges of the teeth shear with one another.
The articulation between the articular
and quadrate bone
s at the jaw joint is well developed. Along with the broad downturned "wings" formed by the pterygoid and ectopterygoid bones at the bottom of the skull, this articulation restricts the jaw to up-and-down movement. The jaw movement and close shearing of the teeth suggest that Sebecus was carnivorous. Its compressed, blade-like teeth would have been well-suited for cutting meat. In contrast, living crocodilians have circular, widely spaced teeth and usually consume their food in large pieces. Sebecus likely consumed food in a manner more similar to theropod dinosaur
s than living crocodilians. In particular, the teeth of tyrannosaurids bear the closest resemblance to those of Sebecus. Both animals have serrated teeth with rounded projections called denticles, and sharp clefts between the denticles called diaphyses. These diaphyses compress meat fibers between the serrations and rip them apart. Ultrastructural analyses using electron microscope
s have revealed microwear scratches on the teeth that are suggestive of this form of cutting.
Colbert's monograph on Sebecus included a description of the brain, Eustachian tube
s, and jaw musculature. Details of these soft tissues were inferred from characteristics of the skull and endocast
s, or molds of its interior. The deep snout of Sebecus makes the shape of its brain somewhat different from those of living crocodiles, although its structure is the same. The olfactory bulb
is elongate and makes up a significant portion of the brain. The cerebrum is narrow and long in comparison to crocodilians, and tapers toward the olfactory bulb. The temporal lobe
s are somewhat smaller than those of living crocodilians. Colbert interpreted the smaller cerebrum of Sebecus as a sign of primitiveness, with an evolutionary trend toward larger brain size in crocodilians. Hans C. E. Larsson performed a 2001 study of the endocranial anatomy of the dinosaur Carcharodontosaurus saharicus, comparing the ratio of its cerebrum to its total brain volume to the ratios of other prehistoric reptiles. Larsson found that Sebecus and Allosaurus fragilis had similar ratios to C. saharicus, falling within the 95% confidence range characterizing living reptile species.
The Eustachian tubes, passages that connect the middle ear
with the pharynx
, are very complex in crocodilians. Unlike those of other vertebrates, the tubes are made of several interconnected branches. This branching is fully developed in Sebecus, and probably appeared much earlier in crocodylomorphs.
The jaw muscles of Sebecus were likely similar to those of living crocodilians, but the distinctively deep skull of Sebecus indicates that the muscles were longer. In Sebecus, the shape of the skull and jaws provides more room for adductor muscles, or muscles that close the jaws. The supratemporal fenestrae at the top of the skull are relatively wide, allowing for the passage of large muscles. In crocodilians, the depressor muscle that opens the jaws originates near the top of the skull and inserts into a projection at the back of the jaw called the retroarticular process. Living crocodilians have a straight retroarticular process at the back of the jaw and a low point of origin for the depressor muscle. Sebecus, with its deeper skull, has a higher point of origin for the depressor, but the retroarticular process curves upward to make the length of the depressor about the same as it is in crocodilians. Like living crocodilians, the depressor muscle of Sebecus was relatively underdeveloped. Therefore, while the closure of the jaws would have been very strong, the ability to open the jaws would be much weaker.
Sebecus has been placed in various positions among metasuchia
n crocodyliforms. It has often been placed in a larger sebecosuchian clade
. Carvalho et al. (2004) used Sebecus in their definition of Sebecidae, considering the clade to include the most recent common ancestor
of Sebecus and Libycosuchus
and all of its descendants. Carvalho et al. also established a sebecisuchian clade that was defined using Sebecus and referred to as Baurusuchoidea. Sebecus and other sebecosuchians are often contrasted with the smaller-bodied Cretaceous
notosuchia
ns. Turner and Calvo (2005) considered Sebecosuchia to be the sister taxon of Notosuchia and placed Sebecus as a basal
member the clade. Several recent phylogenetic analyses have resulted in a polyphyletic Sebecosuchia, with some members such as Baurusuchus being more closely related to notosuchians and other such as Sebecus forming a clade of metasuchians more distantly related to Notosuchia. Larsson and Sues (2007) named this clade Sebecia
.
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 sebecid
Sebecidae
Sebecidae is an extinct family of prehistoric crocodylomorphs. They lived mostly in Argentina.This group included many medium and large - sized genera, from Sebecus to a giant indeterminate unnamed species from the Miocene.-Phylogeny:...
crocodylomorph from 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...
of 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...
. Fossils have been found in Patagonia
Patagonia
Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...
. Like other sebecosuchia
Sebecosuchia
Sebecosuchia is an extinct group of mesoeucrocodylian crocodyliforms that includes the families Sebecidae and Baurusuchidae. The group first appeared in the Late Cretaceous with the baurusuchids and went extinct in the Miocene with the last sebecids. Fossils have been found primarily from South...
ns, it was entirely terrestrial and carnivorous. The genus is currently represented by a single species, the type
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
S. icaeorhinus. Several other species have been referred to Sebecus, but were later reclassified as their own genera.
Etymology
Latinisation (literature)
Latinisation is the practice of rendering a non-Latin name in a Latin style. It is commonly met with for historical personal names, with toponyms, or for the standard binomial nomenclature of the life sciences. It goes further than Romanisation, which is the writing of a word in the Latin alphabet...
of Sebek (also called Sobek), the crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
god of ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...
. Sebek was considered an alternative to the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
χάμψαι, or "champsa" in crocodilian nomenclature (the Greek historian Herodotus
Herodotus
Herodotus was an ancient Greek historian who was born in Halicarnassus, Caria and lived in the 5th century BC . He has been called the "Father of History", and was the first historian known to collect his materials systematically, test their accuracy to a certain extent and arrange them in a...
claimed that champsa was the Egyptian word for crocodile).
The specific name icaeorhinus of the type species is derived from the Greek words εικαίοs and ρύγχος. Εικαίοs means "random" or "not according to plan" and ρύγχος means "snout", in reference to the animal's unusually deep snout.
History and species
Named by AmericanUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
paleontologist George Gaylord Simpson
George Gaylord Simpson
George Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...
in 1937, Sebecus was one of the first known sebecosuchians. Simpson described the type species, S. icaeorhinus, from a fragmented skull and lower jaw. The specimen was discovered by the American Museum of Natural History
American Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...
's First Scarritt Expedition to Patagonia
Patagonia
Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...
. Teeth had been known since 1906 when Argentine paleontologist Florentino Ameghino
Florentino Ameghino
Florentino Ameghino was an Argentine naturalist, paleontologist, anthropologist and zoologist.Born in Luján, son of Italian immigrants, Ameghino was a self-taught naturalist, and focused his study on the lands of the southern Pampas...
associated them with carnivorous dinosaurs. The more complete material found by Simpson firmly established that the new animal was a crocodyliform. Although Simpson's fossil was considered one of the best finds of the expedition, Simpson described the genus only briefly in 1937. He noted its unusual ziphodont dentition in which the teeth were laterally compressed and serrated. Simpson was preparing a more detailed monograph on the genus, but entered the United States Army
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...
before its completion. Another American paleontologist, Edwin Harris Colbert
Edwin Harris Colbert
Edwin Harris Colbert was a distinguished American vertebrate paleontologist and prolific researcher and author. He received his A.B. from the University of Nebraska, then his Masters and Ph.D. from Columbia University, finishing in 1935.Born in Clarinda, Iowa, he grew up in Maryville, Missouri...
, completed Simpson's work, thoroughly describing the genus and placing it in a new family, Sebecidae
Sebecidae
Sebecidae is an extinct family of prehistoric crocodylomorphs. They lived mostly in Argentina.This group included many medium and large - sized genera, from Sebecus to a giant indeterminate unnamed species from the Miocene.-Phylogeny:...
. Colbert placed Sebecus and the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
baurusuchid
Baurusuchidae
Baurusuchidae is a Gondwanan family of mesoeucrocodylians that lived during the Late Cretaceous. It is a group of terrestrial hypercarnivorous crocodilians from South America and possibly Pakistan. Baurusuchidae has been defined as a clade containing the most recent common ancestor of Baurusuchus...
Baurusuchus
Baurusuchus
Baurusuchus is an extinct genus of baurusuchid mesoeucrocodylian from the Late Cretaceous of Brazil. It was a terrestrial predator and scavenger, about 3.5 to 4 meters long. Baurusuchus lived during the Turonian to Santonian stages of the Late Cretaceous Period, in Adamantina Formation, Brazil...
(also from South America) in the suborder Sebecosuchia (erected by Simpson for all ziphodont crocodylomorphs), as both had deep snouts and ziphodont teeth.
In 1965, American paleontologist Wann Langston, Jr.
Wann Langston, Jr.
Wann Langston, Jr. is an American paleontologist and professor at the University of Texas at Austin. He has worked on a number of different reptiles and amphibians in his long career, beginning with the 1950 description of the theropod dinosaur Acrocanthosaurus...
named a second species, S. huilensis, from 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...
La Venta Formation in Columbia. S. huilensis was named on the basis of skull fragments. The deposits are Friasian
Friasian
The Friasian age is a period of geologic time within the Miocene epoch of the Neogene used more specifically with South American Land Mammal Ages. It follows the Santacrucian and precedes the Colloncuran age....
in age (about 15 million years old), extending the range of the genus into the 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...
by around 40 million years. In 1977, remains were described from the Miocene of Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
.
A third species of Sebecus, S. querejazus, was named in 1991 from 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...
Santa Lucia Formation
Santa Lucia Formation
The Santa Lucia Formation is a Mesozoic geologic formation. Fossil ornithopod tracks have been reported from the formation.-See also:* List of dinosaur-bearing rock formations** List of stratigraphic units with ornithischian tracks*** Ornithopod tracks...
in Bolivia
Bolivia
Bolivia officially known as Plurinational State of Bolivia , is a landlocked country in central South America. It is the poorest country in South America...
. This extended the range of Sebecus back to the beginning of the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
, soon after the Cretaceous-Tertiary extinction event
Cretaceous-Tertiary extinction event
The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...
. In 1993, Gasparini et al. described Sebecus carajazus. This was not a fourth species but a misspelling, or lapsus calami, of Sebecus querejazus.
A 2007 study of sebecids reclassified several species. The two species S. huilensis and S. querejazus were given their own genera, Zulmasuchus
Zulmasuchus
Zulmasuchus is an extinct genus of bretesuchid sebecosuchian mesoeucrocodylian. Its fossils have been found in Paleocene-age rocks of the Santa Lucia Formation in Bolivia. Zulmasuchus was named in 2007 by Alfredo Paolillo and Omar Linares for fossils originally described by Buffetaut and...
and Langstonia
Langstonia
Langstonia is an extinct genus of sebecid mesoeucrocodylian. Its fossils have been found in middle Miocene-age rocks from Colombia, in the "Monkey Beds" of the Villavieja Formation...
, respectively. Langstonia huilensis, named after Langston, was distinguished from Sebecus by its narrower snout and widely spaced teeth. Zulmasuchus querejazus, named after Zulma Gasparini, one of the authors of the study, differs from Sebecus in its wider snout.
Description
Unusual among crocodyliforms, Sebecus has a deep, narrow snout. The nares, or nostrils, open anteriorly at the tip of the snout. While most crocodilians have flat skulls that are raised near the eyes and postorbital region behind the eyes, the skull of Sebecus is essentially level. The great depth of the snout makes most of the length of its upper margin level with the margin of the orbitsOrbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
, or eye sockets. The supratemporal fenestrae, two holes on the skull table
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...
, are relatively small.
Laterally compressed, or ziphodont teeth, are characteristic of Sebecus and other sebecosuchians. Although the teeth vary in size, they are homodont, having a similar shape throughout the jaw. At the tips of the upper and lower jaws, the teeth are rounder in cross-section. The fourth dentary tooth is raised in the lower jaw to form an effective canine
Canine
Canine may refer to:* Domestic dog* Animals belonging to the family Canidae, or the sub-family Caninae* Canine tooth* Ralph Canine, American soldier and government administrator* A character in Glenn Martin, DDS...
. The foremost teeth of the lower jaw are much smaller and lower than the fourth tooth. At the tip of the jaw the first dentary tooth is procumbent, or directed forward. The teeth of the upper and lower jaws form an alternate pattern to allow the jaw to close tightly. A notch is present between the 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...
and 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....
bones of the upper jaw, accommodating the fourth dentary tooth when the jaw is closed. The procumbent first dentary teeth fit between the first and second premaxillary teeth. This close fit allows the serrated edges of the teeth shear with one another.
The articulation between the 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...
and quadrate bone
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...
s at the jaw joint is well developed. Along with the broad downturned "wings" formed by the pterygoid and ectopterygoid bones at the bottom of the skull, this articulation restricts the jaw to up-and-down movement. The jaw movement and close shearing of the teeth suggest that Sebecus was carnivorous. Its compressed, blade-like teeth would have been well-suited for cutting meat. In contrast, living crocodilians have circular, widely spaced teeth and usually consume their food in large pieces. Sebecus likely consumed food in a manner more similar to theropod dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s than living crocodilians. In particular, the teeth of tyrannosaurids bear the closest resemblance to those of Sebecus. Both animals have serrated teeth with rounded projections called denticles, and sharp clefts between the denticles called diaphyses. These diaphyses compress meat fibers between the serrations and rip them apart. Ultrastructural analyses using electron microscope
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
s have revealed microwear scratches on the teeth that are suggestive of this form of cutting.
Colbert's monograph on Sebecus included a description of the brain, Eustachian tube
Eustachian tube
The Eustachian tube is a tube that links the nasopharynx to the middle ear. It is a part of the middle ear. In adult humans the Eustachian tube is approximately 35 mm long. It is named after the sixteenth-century anatomist Bartolomeo Eustachi...
s, and jaw musculature. Details of these soft tissues were inferred from characteristics of the skull and endocast
Endocast
An endocast is the internal cast of a hollow object, often specifically used for an endocasts of the cranial vault. Endocasts can be man-made for examining the properties of a hollow, inaccessible space, or occur naturally through fossilisation....
s, or molds of its interior. The deep snout of Sebecus makes the shape of its brain somewhat different from those of living crocodiles, although its structure is the same. The olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...
is elongate and makes up a significant portion of the brain. The cerebrum is narrow and long in comparison to crocodilians, and tapers toward the olfactory bulb. The temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
s are somewhat smaller than those of living crocodilians. Colbert interpreted the smaller cerebrum of Sebecus as a sign of primitiveness, with an evolutionary trend toward larger brain size in crocodilians. Hans C. E. Larsson performed a 2001 study of the endocranial anatomy of the dinosaur Carcharodontosaurus saharicus, comparing the ratio of its cerebrum to its total brain volume to the ratios of other prehistoric reptiles. Larsson found that Sebecus and Allosaurus fragilis had similar ratios to C. saharicus, falling within the 95% confidence range characterizing living reptile species.
The Eustachian tubes, passages that connect the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
with the pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, are very complex in crocodilians. Unlike those of other vertebrates, the tubes are made of several interconnected branches. This branching is fully developed in Sebecus, and probably appeared much earlier in crocodylomorphs.
The jaw muscles of Sebecus were likely similar to those of living crocodilians, but the distinctively deep skull of Sebecus indicates that the muscles were longer. In Sebecus, the shape of the skull and jaws provides more room for adductor muscles, or muscles that close the jaws. The supratemporal fenestrae at the top of the skull are relatively wide, allowing for the passage of large muscles. In crocodilians, the depressor muscle that opens the jaws originates near the top of the skull and inserts into a projection at the back of the jaw called the retroarticular process. Living crocodilians have a straight retroarticular process at the back of the jaw and a low point of origin for the depressor muscle. Sebecus, with its deeper skull, has a higher point of origin for the depressor, but the retroarticular process curves upward to make the length of the depressor about the same as it is in crocodilians. Like living crocodilians, the depressor muscle of Sebecus was relatively underdeveloped. Therefore, while the closure of the jaws would have been very strong, the ability to open the jaws would be much weaker.
Classification
Because it is represented by relatively complete fossil material, Sebecus has been used to define larger groups of crocodyliforms such as Sebecidae and Sebecosuchia. The suborder Sebecosuchia was established to group Sebecus with Baurusuchus and has grown to include many other sebecid and baurusuchid members. While Sebecus and Baurusuchus are well known, other forms are known from only a few fragmentary specimens.Sebecus has been placed in various positions among metasuchia
Metasuchia
Metasuchia is a major clade within the superorder Crocodylomorpha. It is split into two main groups, Notosuchia and Neosuchia. Notosuchia is an extinct group that contains primarily small-bodied Cretaceous taxa with heterodont dentitions. Neosuchia includes extant crocodylians and basal taxa such...
n crocodyliforms. It has often been placed in a larger sebecosuchian clade
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...
. Carvalho et al. (2004) used Sebecus in their definition of Sebecidae, considering the clade to include the most recent common ancestor
Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
of Sebecus and Libycosuchus
Libycosuchus
Libycosuchus is an extinct genus of North African crocodylomorph possibly related to Notosuchus. It was terrestrial, living approximately 95 million years ago in the Cenomanian stage of the Late Cretaceous...
and all of its descendants. Carvalho et al. also established a sebecisuchian clade that was defined using Sebecus and referred to as Baurusuchoidea. Sebecus and other sebecosuchians are often contrasted with the smaller-bodied Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
notosuchia
Notosuchia
Notosuchia is a suborder of primarily Gondwanan mesoeucrocodylian crocodylomorphs that lived during the Cretaceous. Fossils have been found from South America, Africa, and Asia...
ns. Turner and Calvo (2005) considered Sebecosuchia to be the sister taxon of Notosuchia and placed Sebecus as a 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:...
member the clade. Several recent phylogenetic analyses have resulted in a polyphyletic Sebecosuchia, with some members such as Baurusuchus being more closely related to notosuchians and other such as Sebecus forming a clade of metasuchians more distantly related to Notosuchia. Larsson and Sues (2007) named this clade Sebecia
Sebecia
Sebecia is an extinct clade of mesoeucrocodylian crocodyliforms that includes peirosaurids and sebecids. It was first constructed in 2007 to include Hamadasuchus, Peirosauridae, and Sebecus. It was initially considered to be the sister taxon of the clade Neosuchia, which includes living...
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External links
- Sebecus 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...