Susisuchus
Encyclopedia
Susisuchus is an extinct genus
of mesoeucrocodylia
n crocodyliform from the Early Cretaceous
of Brazil
. Fossils have been found from the Nova Olinda Member of the Aptian
-age Crato Formation
in the Araripe and Lima Campos Basins of northeastern Brazil. Named in 2003, Susisuchus is the sole member of the family Susisuchidae, and is closely related to the clade
Eusuchia
, which includes living crocodilians. The type species
is S. anatoceps, known from a single partial articulated skeleton that preserves some soft tissue. A second species, S. jaguaribensis, was named in 2009 from fragmentary remains.
-age Crato Formation
in the Araripe Basin, and was the first crocodyliform to be described from the formation. The holotype
skeleton is the most complete known specimen of Susisuchus, including most of the bones except for those of the hindlimbs, and even traces of soft tissue. The specimen is thought to have been a desiccated carcass before it was buried and later fossilized. A second species, S. jaguaribensis, was named in 2009 from the Lima Campos Basin about 115 kilometres (71.5 mi) from where the skeleton of S. anatoceps was uncovered.
A new crocodyliform specimen from the Crato Formation was described in 2009. It can be distinguished from the two other crocodyliforms known from the formation, Araripesuchus
and the trematochampsid Caririsuchus
, on the basis of characteristics of the fibula and proportions of the femur
and tibia
. Since the limb elements differ from all other known crocodyliforms from the Araripe Basin, the specimen was tentatively classified as cf. Susisuchus sp.
specimen of Susisuchus anatoceps includes a skull and lower jaw, forelimbs, parts of the axial skeleton
, and some osteoderm
s. There are traces of soft tissue around both forelimbs and the digits of the right hand. S. jaguaribensis is known from far less material, but the fragmentary remains are enough to diagnose the species. Both species have a contact between the squamosal
and parietal bone
s that is situated on the posterior border of the supratemporal fenestra, a hole in the skull table
.
have biserial paravertebral shields, in which there are only two rows of paravertebral osteoderms. These two rows of large osteoderms became segmented into four smaller rows in Susisuchus. Susisuchus also has two rows of accessory osteoderms to either side of the paravertebral osteoderms which are not part of the shield.
The segmentation of the paravertebral shield allowed for a greater degree of lateral flexibility in the trunk of Susisuchus. This flexibility would have enabled lateral undulation while swimming, resulting in greater efficiency in locomotion. Because the width of the paravertebral shield was not compromised by this segmentation, the back would remain rigid enough to overcome the forces encountered during high-walking, or walking semi-erect. However, the ability to high-walk would have restricted the size of Susisuchus. If Susisuchus grew any larger than 50 kilograms (110.2 lb), the forces encountered during high-walking would have been too great for the paravertebral shield to counteract.
Susisuchus is closely related to Pachycheilosuchus
from Glen Rose, Texas
, United States
, and Isisfordia
from Winton, Queensland
, Australia
. All three genera are highly derived
Early Cretaceous mesoeucrocodylians. In the initial description of Susisuchidae, Isisfordia was considered a possible member of the family, although the genus had not yet been named. However, with its naming in 2006, Isisfordia was placed within Eusuchia as the most basal member of the clade, while Susisuchus was placed outside Eusuchia as its sister taxon. Below is a cladogram
from Salisbury et al. (2006) showing the relationships of Susisuchus and Isisfordia within Mesoeucrocodylia:
Susisuchus has amphicoelus thoracic
, lumbar
and caudal (tail) vertebrae, meaning that the ends of the centra of these vertebrae are concave. The presence of this feature in a crocodyliform with a tetraserial paravertebral shield is unusual, and has been used to erect the family Susisuchidae, of which Susisuchus is the only member. Susisuchus can be considered a transitional form because it has a tetraserial paravertebral shield characteristic of eusuchians, but retains more primitive amphicoelus vertebrae characteristic of more basal crocodylomorphs.
With the description of S. jaguaribensis as a second species of Susisuchus, the genus has been phylogenetically redefined as a node-based taxon including the last common ancestor of Susisuchus anatoceps and Susisuchus jaguaribensis and all of its descendants.
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 mesoeucrocodylia
Mesoeucrocodylia
Mesoeucrocodylia is the name of the clade that includes Eusuchia and the paraphyletic group Mesosuchia. The group appeared during the Early Jurassic, and continues to the present day....
n crocodyliform from the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
of Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. Fossils have been found from the Nova Olinda Member of the Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
-age Crato Formation
Crato Formation
The Crato Formation is a geologic formation of Early Cretaceous age in northeastern Brazil's Araripe Basin. It is an important Lagerstätte for palaeontologists. The strata were laid down mostly during the early Albian age, about 108 million years ago, in a shallow inland sea...
in the Araripe and Lima Campos Basins of northeastern Brazil. Named in 2003, Susisuchus is the sole member of the family Susisuchidae, and is closely related to the 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...
Eusuchia
Eusuchia
Eusuchia is a clade of crocodylomorphs that first appears in the Early Cretaceous with Hylaeochampsa. All living crocodilian species are eusuchians, as are many extinct forms.-Description:...
, which includes living crocodilians. The type species
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...
is S. anatoceps, known from a single partial articulated skeleton that preserves some soft tissue. A second species, S. jaguaribensis, was named in 2009 from fragmentary remains.
Species
The genus Susisuchus was first erected with the description of the type species S. anatoceps in 2003. S. anatoceps was found in the AptianAptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
-age Crato Formation
Crato Formation
The Crato Formation is a geologic formation of Early Cretaceous age in northeastern Brazil's Araripe Basin. It is an important Lagerstätte for palaeontologists. The strata were laid down mostly during the early Albian age, about 108 million years ago, in a shallow inland sea...
in the Araripe Basin, and was the first crocodyliform to be described from the formation. 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...
skeleton is the most complete known specimen of Susisuchus, including most of the bones except for those of the hindlimbs, and even traces of soft tissue. The specimen is thought to have been a desiccated carcass before it was buried and later fossilized. A second species, S. jaguaribensis, was named in 2009 from the Lima Campos Basin about 115 kilometres (71.5 mi) from where the skeleton of S. anatoceps was uncovered.
A new crocodyliform specimen from the Crato Formation was described in 2009. It can be distinguished from the two other crocodyliforms known from the formation, Araripesuchus
Araripesuchus
Araripesuchus is a genus of extinct crocodyliform whose species existed during the Cretaceous period of the late Mesozoic era some 125 to 65 million years ago. They are generally considered to be notosuchians , characterized by their varied teeth types and distinct skull elements...
and the trematochampsid Caririsuchus
Caririsuchus
Caririsuchus is an extinct genus of trematochampsid crocodylomorph. Fossils have been found from the Romualdo Member of the Santana Formation in northeastern Brazil, dating back to the Albian stage of the Early Cretaceous. It was about 1.5m in length....
, on the basis of characteristics of the fibula and proportions of the femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
and tibia
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
. Since the limb elements differ from all other known crocodyliforms from the Araripe Basin, the specimen was tentatively classified as cf. Susisuchus sp.
Description
The holotypeHolotype
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 of Susisuchus anatoceps includes a skull and lower jaw, forelimbs, parts of the axial skeleton
Axial skeleton
The axial skeleton consists of the 80 bones along the central axis of the human body. It is composed of six parts; the human skull, the ossicles of the middle ear, the hyoid bone of the throat, the rib cage, sternum and the vertebral column...
, and some osteoderm
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
s. There are traces of soft tissue around both forelimbs and the digits of the right hand. S. jaguaribensis is known from far less material, but the fragmentary remains are enough to diagnose the species. Both species have a contact between the squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
and 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....
s that is situated on the posterior border of the supratemporal fenestra, a hole in 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...
.
Osteoderms
Susisuchus has a dorsal shield of osteoderms that is similar to those of the more derived clade Eusuchia, which includes living crocodilians. Susisuchus was one of the first mesoeucrocodylians to have a tetraserial paravertebral shield, which is formed from the dorsal osteoderms to make the back more rigid. In a tetraserial paravertebral shield, there are four separate rows of paravertebral osteoderms (osteoderms near the midline of the back). Earlier mesoeucrocodylians such as BernissartiaBernissartia
Bernissartia is an extinct genus of crocodyliform that lived in the Early Cretaceous, around 130 million years ago.At only in length, Bernissartia is one of the smallest crocodyliforms that ever lived. It resembled modern species in many respects, and was probably semi-aquatic...
have biserial paravertebral shields, in which there are only two rows of paravertebral osteoderms. These two rows of large osteoderms became segmented into four smaller rows in Susisuchus. Susisuchus also has two rows of accessory osteoderms to either side of the paravertebral osteoderms which are not part of the shield.
The segmentation of the paravertebral shield allowed for a greater degree of lateral flexibility in the trunk of Susisuchus. This flexibility would have enabled lateral undulation while swimming, resulting in greater efficiency in locomotion. Because the width of the paravertebral shield was not compromised by this segmentation, the back would remain rigid enough to overcome the forces encountered during high-walking, or walking semi-erect. However, the ability to high-walk would have restricted the size of Susisuchus. If Susisuchus grew any larger than 50 kilograms (110.2 lb), the forces encountered during high-walking would have been too great for the paravertebral shield to counteract.
Phylogeny
Pachycheilosuchus
Pachycheilosuchus is an extinct genus of mesoeucrocodylian from the Early Cretaceous of Texas, United States...
from Glen Rose, Texas
Glen Rose, Texas
Glen Rose is a city in Somervell County, Texas, United States. It is the county seat of Somervell County. As of the 2010 census, the city population was 2,444. Glen Rose is part of the Granbury micropolitan area.-19th century:...
, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, and Isisfordia
Isisfordia
Isisfordia is an extinct genus of crocodyliform closely related to crocodilians that lived during the Middle Cretaceous . Its fossils were discovered in the Winton Formation in Isisford, Queensland, Australia in the mid 1990s...
from Winton, Queensland
Winton, Queensland
-Qantas:Winton was one of the founding towns of the Australian airline Qantas. The first board meeting was held at the Winton Club on 10 February 1921.-Waltzing Matilda:...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. All three genera are 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...
Early Cretaceous mesoeucrocodylians. In the initial description of Susisuchidae, Isisfordia was considered a possible member of the family, although the genus had not yet been named. However, with its naming in 2006, Isisfordia was placed within Eusuchia as the most basal member of the clade, while Susisuchus was placed outside Eusuchia as its sister taxon. Below is a cladogram
Cladogram
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...
from Salisbury et al. (2006) showing the relationships of Susisuchus and Isisfordia within Mesoeucrocodylia:
Susisuchus has amphicoelus thoracic
Thoracic vertebrae
In human anatomy, twelve thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. They are intermediate in size between those of the cervical and lumbar regions; they increase in size as one proceeds down the spine, the upper...
, lumbar
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...
and caudal (tail) vertebrae, meaning that the ends of the centra of these vertebrae are concave. The presence of this feature in a crocodyliform with a tetraserial paravertebral shield is unusual, and has been used to erect the family Susisuchidae, of which Susisuchus is the only member. Susisuchus can be considered a transitional form because it has a tetraserial paravertebral shield characteristic of eusuchians, but retains more primitive amphicoelus vertebrae characteristic of more basal crocodylomorphs.
With the description of S. jaguaribensis as a second species of Susisuchus, the genus has been phylogenetically redefined as a node-based taxon including the last common ancestor of Susisuchus anatoceps and Susisuchus jaguaribensis and all of its descendants.