''Susisuchus'' is an
extinct genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
neosuchia
Neosuchia is a clade within Mesoeucrocodylia that includes all modern extant crocodilians and their closest fossil relatives. It is defined as the most inclusive clade containing all crocodylomorphs more closely related to ''Crocodylus niloticu ...
n
mesoeucrocodylia
Mesoeucrocodylia is the clade that includes Eusuchia and crocodyliforms formerly placed in the paraphyletic group Mesosuchia. The group appeared during the Early Jurassic, and continues to the present day.
Diagnosis
It was long known that M ...
n
crocodyliform
Crocodyliformes is a clade of crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the only pseudo ...
from the
Early Cretaceous of
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
. Fossils have been found from the Nova Olinda Member of the
Aptian-age
Crato Formation
The Crato Formation is a geologic formation of Early Cretaceous (Aptian) age in northeastern Brazil's Araripe Basin. It is an important Lagerstätte (undisturbed fossil accumulation) for palaeontologists. The strata were laid down mostly during ...
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
Eusuchia is a clade of crocodylomorphs that first appeared in the Early Cretaceous with '' Hylaeochampsa''. Along with Dyrosauridae and Sebecosuchia, they were the only crocodyliformes who survived the K-T extinction. Since the other two clades ...
, which includes living
crocodilians. The
type species
In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...
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.
Discovery
![Size of Brazilian fossil crocodylomorphs](https://upload.wikimedia.org/wikipedia/commons/8/80/Size_of_Brazilian_fossil_crocodylomorphs.PNG)
The genus ''Susisuchus'' was first erected with the description of the type species ''S. anatoceps'' in 2003. ''S. anatoceps'' was found in the
Aptian-age
Crato Formation
The Crato Formation is a geologic formation of Early Cretaceous (Aptian) age in northeastern Brazil's Araripe Basin. It is an important Lagerstätte (undisturbed fossil accumulation) for palaeontologists. The strata were laid down mostly during ...
in the Araripe Basin, and was the first crocodyliform to be described from the formation. The
holotype
A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...
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 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'' is a genus of extinct crocodyliform that existed during the Cretaceous period of the late Mesozoic era some 125 to 66 million years ago. Six species of ''Araripesuchus'' are currently known. They are generally considered to be n ...
'' and the
trematochampsid ''
Caririsuchus'', on the basis of characteristics of the
fibula
The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...
and proportions of the
femur
The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...
and
tibia
The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
. 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
holotype
A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...
specimen of ''Susisuchus anatoceps'' includes a skull and lower jaw, forelimbs, parts of the
axial skeleton
The axial skeleton is the part of the skeleton that consists of the bones of the head and trunk of a vertebrate. In the human skeleton, it consists of 80 bones and is composed of six parts; the skull (22 bones), also the ossicles of the middle ...
, and some
osteoderm
Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinc ...
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 The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...
and
parietal bone
The parietal bones () are two bones in the skull which, when joined at a fibrous joint, form the sides and roof of the cranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is nam ...
s that is situated on the posterior border of the
supratemporal fenestra, a hole in the
skull table.
[
]
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 '' Bernissartia'' 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 , the forces encountered during high-walking would have been too great for the paravertebral shield to counteract.[
]
Classification
''Susisuchus'' is closely related to '' Pachycheilosuchus'' from Glen Rose, Texas
Glen Rose is a city in and the county seat of Somervell County, Texas, United States. As of the 2012 census estimate, the city population was 2,502.
History
19th century
The area was first settled in 1849 by Charles Barnard, who opened a tradin ...
, United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
, and '' Isisfordia'' from Winton, Queensland
Winton is a town and locality in the Shire of Winton in Central West Queensland, Australia. It is northwest of Longreach. The main industries of the area are sheep and cattle raising. The town was named in 1876 by postmaster Robert Allen, aft ...
, 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
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and ...
. Below is a cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
from Salisbury ''et al.'' (2006) showing the relationships of ''Susisuchus'' and ''Isisfordia'' within Mesoeucrocodylia:[
''Susisuchus'' has amphicoelus ]thoracic
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
, lumbar and caudal (tail) vertebra
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
e, 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 ]phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
ally redefined as a node-based taxon
Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...
including the last common ancestor of ''Susisuchus anatoceps'' and ''Susisuchus jaguaribensis'' and all of its descendants.[
]
References
{{Taxonbar, from=Q7649091
Neosuchians
Early Cretaceous crocodylomorphs of South America
Cretaceous Brazil
Fossils of Brazil
Crato Formation
Fossil taxa described in 2002
Prehistoric pseudosuchian genera