''Gracilisuchus'' (meaning "slender crocodile") is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of tiny

pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...

n (a group which includes the ancestors of

crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...

ns) from the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

. It contains a single species, ''G. stipanicicorum'', which is placed in the clade

Suchia Suchia is a clade of archosaurs containing the majority of pseudosuchians (crocodilians and their extinct relatives). It was defined as the least inclusive clade containing '' Aetosaurus ferratus'', '' Rauisuchus tiradentes'', '' Prestosuchus ch ...

, close to the ancestry of

crocodylomorphs Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cr ...

. Both the genus and the species were first described by

Alfred Romer Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution. Biography Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer an ...

in 1972.

Discovery

Discovery of the holotype

A four-month expedition spanning 1964 and 1965 in the

Ischigualasto-Villa Unión Basin The Ischigualasto-Villa Unión Basin ( es, Cuenca de Ischigualasto-Villa Unión) is a small sedimentary basin located in the Argentine Northwest, Argentina. It is located in the southwestern part of La Rioja Province and the northeastern part of ...

of La Rioja Province,

was conducted by Alfred Romer and his colleagues, who consisted of researchers from the

Museum of Comparative Zoology A museum ( ; plural museums or, rarely, musea) is a building or institution that cares for and displays a collection of artifacts and other objects of artistic, cultural, historical, or scientific importance. Many public museums make these ...

(MCZ) at

Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...

. While the first two months of the expedition were unfruitful, excavations near the Chañares River and the Gualo River, soon uncovered specimens belonging to a wide variety of

tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...

groups. Among these was the skeleton of a small

suchia Suchia is a clade of archosaurs containing the majority of pseudosuchians (crocodilians and their extinct relatives). It was defined as the least inclusive clade containing '' Aetosaurus ferratus'', '' Rauisuchus tiradentes'', '' Prestosuchus ch ...

n archosaur, discovered about north of the Chañares River. The skeleton was stored at the

La Plata Museum The La Plata Museum ( es, Museo de la Plata) is a natural history museum in La Plata, Argentina. It is part of the (Natural Sciences School) of the UNLP (National University of La Plata). The building, long, today houses 3 million fossils and ...

(MLP), which had supported the expedition, under the specimen number MLP 64-XI-14-11. The specimen has since been transferred to the Museum of Paleontology at the

National University of La Rioja The National University of La Rioja ( es, Universidad Nacional de La Rioja, UNLAR) is an Argentine national university, situated in the city of La Rioja, capital of La Rioja Province. Its precursor, the Provincial University of La Rioja, was ...

(PULR), where it bears the specimen number PULR 08. This specimen, which would become 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 several ...

of ''Gracilisuchus'', consists of a partial skull, an incomplete vertebral column, parts of the scapula and humerus,

gastralia Gastralia (singular gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In these ...

, and several associated osteoderms. Several other specimens are mixed in with these remains on the same slab. A vertebral series ("Series A"), originally identified as the tail of ''Gracilisuchus'', has been reassigned to ''Tropidosuchus''. Another series of cervical vertebrae ("Series B") underwent a similar reassignment. A scapula and

coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...

overlapping the holotype's limb bones likely belongs to a second ''Gracilisuchus''. There are three ilia, none of which belong to the holotype; one belongs to a specimen of ''

Lagosuchus ''Lagosuchus'' is an extinct genus of avemetatarsalian archosaur from the Late Triassic of Argentina. The type species of ''Lagosuchus'', ''Lagosuchus talampayensis,'' is based on a small partial skeleton recovered from the early Carnian-age Cha� ...

'', another to ''Tropidosuchus''. A right hindlimb and a left femur and tibia have been assigned to ''Tropidosuchus'', and another one has been assigned to ''Lagosuchus''. The right hindlimb's foot was originally assigned to ''Gracilisuchus''.

Additional specimens and description

Three other specimens, stored at the MCZ, were found by Romer and his team in the same locality as the holotype. They are MCZ 1147, a nearly complete skull with jaws; MCZ 4118, parts of the skull, a well-preserved neck, and other elements; and MCZ 4116, a crushed skull and jaw with material from the rest of the body, some of it pertaining to a smaller specimen.

José Bonaparte José Fernando Bonaparte (14 June 1928 – 18 February 2020) was an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists . One of the best-known Argentine paleo ...

from the Miguel Lillo Institute (PVL) later collected additional material from the site between 1970 and 1972, including two new specimens: PVL 4597, the "Tucuman specimen", which includes a nearly complete skull and jaws, nearly complete cervical and dorsal vertebrae, partial sacral and caudal vertebrae, the hip girdle, most of the left hindlimb, and part of the hindlimb, which is 20% larger than the holotype; and PVL 4612, a nearly complete skull and jaws. In 1972, Romer described the specimens his team had discovered in the journal ''Breviora''. He named a new

for the specimens, ''Gracilisuchus'', with the prefix ''Gracili-'' referring to "the obviously graceful build of the little reptile". Furthermore, he placed them in the type and only species ''G. stipanicicorum'', which honors the work of Pedro and Maria Stipanicic in the

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...

and

paleobotany Paleobotany, which is also spelled as palaeobotany, is the branch of botany dealing with the recovery and identification of plant remains from geological contexts, and their use for the biological reconstruction of past environments (paleogeogr ...

of the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

period. In this paper, Romer also provided a skeletal reconstruction of ''Gracilisuchus'', due to his impression that nearly the entire skeleton was available. However, several elements had been erroneously referred by him. The lack of complete knowledge regarding its anatomy hampered subsequent studies of ''Gracilisuchus'', until papers from Lecuona, Desojo, and

Diego Pol {{Short pages monitor Gracilisuchus BW

A number of characteristics of the skull can be used to distinguish ''Gracilisuchus''. Its skull openings are relatively large, with the

antorbital fenestra An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with archosauriforms, first appearing during the Triassic Period. Among extant archosaurs, birds ...

occupying 30-36% of the

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 and are part of the dermatocranium. In comparati ...

's length and the eye socket occupying 35-42% of the skull roof's length. Additionally, the

supratemporal fenestra The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

is, uniquely, wider than it is long. Within the eye socket, there is a

sclerotic ring Sclerotic rings are rings of bone found in the eyes of many animals in several groups of vertebrates, except for mammals and crocodilians. They can be made up of single bones or multiple segments and take their name from the sclera. They are bel ...

, and the ossicles (bony segments) comprising the ring contact but do not overlap each other. Unlike its closest relatives, ''

Turfanosuchus ''Turfanosuchus'' is a genus of archosauriform reptile, likely a gracilisuchid archosaur, which lived during the Middle Triassic (Anisian) of northwestern China. The type species, ''T. dabanensis'', was described by C.C. Young in 1973, based on ...

'' and ''

Yonghesuchus ''Yonghesuchus'' is an extinct genus of Late Triassic archosaur reptile. Remains have been found from the early Late Triassic Tongchuan Formation in Shanxi, China. It is named after Yonghe County, the county where fossils were found. Currently o ...

'', but convergent upon '' Tropidosuchus'', early

theropods Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

, and the

Crocodylomorpha Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...

, the

lacrimal bone The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...

is as tall as the eye socket instead of being significantly shorter. Behind the eye socket, the vertical process of the

jugal bone The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anatomy ...

is uniquely straight. The backward process of the jugal is located underneath the forward process of the

quadratojugal bone The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...

, convergent upon ''

Erpetosuchus ''Erpetosuchus'' is an extinct genus of pseudosuchian from the Late Triassic. The type species of ''Erpetosuchus'' is ''E. granti''. It was first described by E. T. Newton in 1894 for remains found in northeastern Scotland, including four specim ...

'', ''

Postosuchus ''Postosuchus'', meaning "Crocodile from Post", is an extinct genus of rauisuchid reptiles comprising two species, ''P. kirkpatricki'' and ''P. alisonae'', that lived in what is now North America during the Late Triassic. ''Postosuchus'' is a me ...

'', ''

Polonosuchus ''Polonosuchus'' is a genus of rauisuchid known from the late Triassic (Carnian age) of Poland. It was a huge predator about 5–6 metres in length and, like all rauisuchians, was equipped with a large head of long sharp teeth. The legs were plac ...

'', and the Crocodylomorpha, unlike ''Turfanosuchus'' and ''Yonghesuchus'' where they are the other way around. Furthermore, a

postfrontal bone The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

is present, with an outer process that is uniquely long and extends over the back of the eye socket. There is also a

postparietal bone Postparietals are cranial bones present in fish and many Tetrapod, tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were Dermal bone, dermal bones situated alo ...

, which is small and triangular. At the back of the skull, the posttemporal foramen is large relative to the skull's width. Also unlike ''Turfanosuchus'' and ''Yonghesuchus'', ''Gracilisuchus'' has four teeth in the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

instead of five, like ''

Prestosuchus ''Prestosuchus'' (meaning "Prestes crocodile") is an extinct genus of pseudosuchian in the group Loricata, which also includes ''Saurosuchus'' and ''Postosuchus''. It has historically been referred to as a "rauisuchian", and was the defining mem ...

'', ''

Saurosuchus ''Saurosuchus'' (meaning "lizard crocodile") is an extinct genus of large loricatan pseudosuchian archosaur that lived in South America during the Late Triassic period. It was a heavy, ground-dwelling, quadrupedal carnivore, likely being the apex ...

'', ''

Fasolasuchus ''Fasolasuchus'' is an extinct genus of loricatan. Fossils have been found in the Los Colorados Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina that date back to the Norian stage of the Late Triassic, making it one of ...

'', ''

Batrachotomus ''Batrachotomus'' is a genus of prehistoric archosaur. Fossils of this animal have been found in southern Germany and dated from the Ladinian stage of the Middle Triassic period, around 242 to 237 million years ago. ''Batrachotomus'' was descri ...

'', the

Rauisuchidae Rauisuchidae is a group of large (up to or more) predatory Triassic archosaurs. There is some disagreement over which genera should be included in Rauisuchidae and which should be in the related Prestosuchidae and Poposauridae, and indeed whethe ...

, and the Crocodylomorpha. There is no cutting edge, or carina, at the front of the premaxillary teeth, and they lack serrated denticles on either the front or rear edges.

Vertebrae

There are eight

cervical vertebrae In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...

. Unlike ''Turfanosuchus'', ''

Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W.K. Parker) is an extinct genus of archosauriform from the Middle Triassic of South Africa. It was a small reptile that lived between 245-230 million years ago, and was close ...

'', ''Fasolasuchus'', ''Saurosuchus'', and

sphenosuchia Sphenosuchia is a suborder of basal crocodylomorphs that first appeared in the Triassic and occurred into the Middle Jurassic. Most were small, gracile animals with an erect limb posture. They are now thought to be ancestral to crocodyliforms, a ...

ns, the suture between the

centrum (Latin for ''center'') may refer to: Places In Greenland * Nuuk Centrum, a district of Nuuk, Greenland * Centrum Lake, Greenland In the Netherlands * Amsterdam-Centrum, the inner-most borough of Amsterdam, Netherlands * Rotterdam Centrum, a borou ...

and

neural arch 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 characteristic i ...

of the

axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis *Axis (mathematics), a designator for a Cartesian-coordinate ...

(second cervical) bears an unrounded, triangular upward projection. There is a long, narrow longitudinal keel on the bottom of the axis, which is also seen in ''

Riojasuchus ''Riojasuchus'' is an extinct genus of Late Triassic ( Norian) quadrupedal archosaur. ''Riojasuchus'' is a member of Ornithosuchidae, a family of facultatively bipedal carnivores that were geographically widespread during the Late Triassic. Tw ...

'', ''Saurosuchus'', the

aetosaur Aetosaurs () are heavily armored reptiles belonging to the extinct order Aetosauria (; from Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs ...

Stagonolepis ''Stagonolepis'' is an extinct genus of stagonolepidid aetosaur known from the Late Triassic (Carnian stage) Hassberge Formation of Germany, the Drawno Beds of Poland, and the Lossiemouth Sandstone of Scotland. Supposed fossils from North and ...

'', and

phytosaur Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent ...

s. The front border of the

neural spine 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 characteristic i ...

is uniquely high and vertical, while the rear border is concave like ''Turfanosuchus'' but unlike ''Erpetosuchus''. Similar to both, the

articular process The articular processes or zygapophyses (Greek ζυγον = "yoke" (because it links two vertebrae) + απο = "away" + φυσις = "process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vertebr ...

known as the postzygapophysis projects over the back of the centrum, but it is uniquely in the horizontal plane. The remaining cervicals have poorly-developed keels on their bottom surfaces, in contrast to ''Erpetosuchus'', ''

Nundasuchus ''Nundasuchus'' is an extinct genus of crurotarsan, possibly a suchian archosaur related to Paracrocodylomorpha. Remains of this genus are known from the Middle Triassic Manda beds of southwestern Tanzania. It contains a single species, ''Nundas ...

'', aetosaurs, ''Saurosuchus'', and ''Riojasuchus''. The sides of their centra have long depressions, like ''Turfanosuchus'', aetosaurs, ''Batrachotomus'', and ''

Ticinosuchus ''Ticinosuchus'' is an extinct genus of suchian archosaur from the Middle Triassic (Anisian - Ladinian) of Switzerland and Italy. Description One of only a handful of fossil reptiles that have been found in Switzerland, ''Ticinosuchus'' (mea ...

''. In at least the fourth, sixth, and seventh cervicals, there are narrow, rounded "tables" at the base of the neural spines, like ''Turfanosuchus'' and ''Euparkeria''. There are circular depressions at the front of the neural spines, above the neural arches, a trait shared with ''Turfanosuchus''. The postzygapophyses are located at the same level as another set of processes, the prezygapophyses, like ''Turfanosuchus'', ''Erpetosuchus'', and ''

Ornithosuchus ''Ornithosuchus'' (meaning "bird crocodile") is an extinct genus of pseudosuchians from the Late Triassic (Carnian) Lossiemouth Sandstone of Scotland. It was originally thought to be the ancestor to the carnosaurian dinosaurs (such as ''Allosauru ...

''. The top margin of the postzygapophyses are convex like ''Turfanosuchus''. There are depressions below the postzygapophyses, which are not seen in any other

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...

s except for ''

Stagonosuchus ''Stagonosuchus'' is an extinct genus of loricatan, or possibly a species of ''Prestosuchus''. Fossils have been found from the Late Triassic Manda Formation in Tanzania that are Anisian in age. Unlike other rauisuchians, which have a prominent ...

'' and ''Batrachotomus'', where they are deeper. Another set of processes known as the parapophyses extend backwards onto longitudinal ridges, like ''Nundasuchus'', ''Batrachotomus'', and ''Postosuchus''. There are sixteen

dorsal vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical ...

. Like ''

Parringtonia ''Parringtonia'' is an extinct genus of Triassic archosaur within the family Erpetosuchidae, known from the type species ''Parringtonia gracilis''. It is known from a single specimen, NHMUK R8646, found from the Anisian-age Manda Formation of Ta ...

'', ''Nundasuchus'', and other archosaurs, the articulating surfaces of their centra are flat. The keels on their bottom surfaces are again very weak, which is unlike ''Riojasuchus'', ''Erpetosuchus'', ''Parringtonia'', and ''Nundasuchus'', but like aetosaurs. There are no "tables" on the neural spines either, unlike ''Turfanosuchus'', ''Erpetosuchus'', ''Parringtonia'', aetosaurs, ''Nundasuchus'', and ''Ticinosuchus''. Like ''Turfanosuchus'', ''Parringtonia'', and ''Nundasuchus'', the prezygapophyses are located at the same level as the top of the front of some of the centra. Some of the sideways-projecting

transverse processes 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 characteristic i ...

have somewhat deep cavities underneath, like ''Turfanosuchus'', but they are more poorly developed than ''Nundasuchus'', ''Batrachotomus'', or ''Stagonosuchus''. Two

sacral vertebrae The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part ...

are known. The transverse processes of the first sacral, which are nearly fused to the ribs, are separated from the neural spines by two pairs of grooves which form an acute angle. The second sacral's neural arch also bears a long depression on its top surface. The appearance of these characteristics are more similar to ''Turfanosuchus'' than ''Nundasuchus''. Uniquely, the outer edges of the first pair of sacral ribs are longer than their inner edges. The second pair has a more prominent expansion, also seen in ''Turfanosuchus'', ''Euparkeria'', and ''Saurosuchus''. The two pairs did not contact each other, like ''Euparkeria''. There were at least 16 caudal (tail) vertebrae. Their neural spines do not have accessory processes at their front margins, like ''Turfanosuchus'' and ''Euparkeria'' but unlike other basal archosaurs. They also lack "tables", unlike ''Turfanosuchus'' and ''Parringtonia''.

Limbs

In the

shoulder girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of t ...

, the end of the

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...

r blade is widely and asymmetrically expanded, unlike ''Turfanosuchus'', ''Batrachotomus'', and ''Ticinosuchus'', but similar to the crocodylomorph '' Dromicosuchus''. Unlike ''Turfanosuchus'' but like ''

Terrestrisuchus ''Terrestrisuchus'' is an extinct genus of very small early crocodylomorph that was about long. Fossils have been found in Wales and Southern England and date from near the very end of the Late Triassic during the Rhaetian, and it is known by t ...

'' and ''

Dibothrosuchus ''Dibothrosuchus'' is a genus of sphenosuchian, a type of basal crocodylomorph, the clade that comprises the crocodilians and their closest kin. It is known from several partial skeletons and skulls. These fossils were found in Lower Jurassic ...

'', the articulation with the

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

on the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

is narrower than the one with the

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

. The width of the bottom end of the humerus is 2.5 times that of the shaft, like ''Batrachotomus'' (2.5), ''Ticinosuchus'' (2.7), ''Postosuchus'' (2.4), and ''Terrestrisuchus'' (2.4), but smaller than ''Turfanosuchus'' (3.75). In the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

, the ilium has a weakly expanded frontal process, like ''Turfanosuchus'', ''Euparkeria'', ''Postosuchus'', and ''

Caiman A caiman (also cayman as a variant spelling) is an alligatorid belonging to the subfamily Caimaninae, one of two primary lineages within the Alligatoridae family, the other being alligators. Caimans inhabit Mexico, Central and South America fro ...

''. The front of the articulation with the pubis reaches further than the process, like ''Turfanosuchus'', ''Euparkeria'', ''Saurosuchus'', ''Postosuchus'', and ''

Lagerpeton ''Lagerpeton'' is a genus of lagerpetid avemetatarsalian, comprising a single species, ''L. chanarensis''. First described from the Chañares Formation of Argentina by A. S. Romer in 1971, ''Lagerpeton'' anatomy is somewhat incompletely kn ...

''. Meanwhile, the rear process is long, like ''Turfanosuchus'' and ''Postosuchus''. There is a shelf along the bottom of this process, most similar to those of ''Terrestrisuchus'', ''Dromicosuchus'', and ''

Marasuchus ''Marasuchus'' (meaning " Mara crocodile") is a genus of basal dinosauriform archosaur which is possibly synonymous with ''Lagosuchus''. Both genera lived during the Late Triassic in what is now La Rioja Province, Argentina. ''Marasuchus'' cont ...

''. The sacral ribs articulate at this shelf, unlike ''Turfanosuchus'' where the shelf is located above. The

acetabulum The acetabulum (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure There are three bones of the ''os coxae'' (hip bone) that c ...

, or hip socket, of ''Gracilisuchus'' was larger than other

archosauriforms Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthi ...

. Like ''Turfanosuchus'', ''Marasuchus'', ''Lagerpeton'', and other archosaurs, the acetabulum does not bear a perforation, and there is a buttress above the acetabulum for 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 with ...

. Unusually, but like ''Tropidosuchus'', ''

Protosuchus ''Protosuchus'' is an extinct genus of carnivorous crocodylomorph from the Early Jurassic. The name ''Protosuchus'' means "first crocodile", and is among the earliest animals that resemble crocodilians. ''Protosuchus'' was about in length and ...

'', and ''

Orthosuchus ''Orthosuchus'' (meaning "straight crocodile") is an extinct genus of protosuchian crocodyliform that lived during the Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series ( ...

'', the articulation with the ilium on the pubis is short. There is a small bony tongue projecting downwards from this articulation, which has only been recognized in ''Postosuchus''. There is no prominent surface of the acetabulum on the pubis, nor is there a visible articulation with the

ischium The ischium () form ...

; the former is similar to ''Fasolasuchus'' and ''Orthosuchus'', and the latter is similar to living

ns. An L-shaped

lamina Lamina may refer to: Science and technology * Planar lamina, a two-dimensional planar closed surface with mass and density, in mathematics * Laminar flow, (or streamline flow) occurs when a fluid flows in parallel layers, with no disruption betwee ...

is present on the rear surface of the "apron" of the pubis. The articulation between the two halves of the ischium is characteristically close to the top of the bone, with the separated portion being only 22% the length of the bone; ''Ornithosuchus'' may have a similar condition. Roughly the bottom 55% of the femur is bowed, resulting in a sigmoidal shape. The top of the femoral head is expanded towards the midline, with the expansion closely resembling those of ''Fasolasuchus'', ''Postosuchus'', and the phytosaur ''

Parasuchus ''Parasuchus'' is an extinct genus of basal phytosaur known from the Late Triassic (late Carnian to early Norian stage) of Andhra Pradesh and Madhya Pradesh, India. At its most restricted definition, ''Parasuchus'' contains a single species, ' ...

''. There is also a small forward projection like '' Pseudohesperosuchus''. Like ''

Macelognathus ''Macelognathus'' is an extinct genus of sphenosuchian crocodylomorph from the Late Jurassic. Originally it was believed be a turtle and later a dinosaur. It lived in what is now Wyoming, in North America.Moodie, R.L. 1908. The relationship of ...

'' and ''

Trialestes ''Trialestes'' is an extinct genus of Late Triassic (Carnian) crocodylomorphs that lived in South America. It has been classified as a dinosaur in the past due it being adapted as a terrestrial, running carnivore. It is classified in Sphenosuchi ...

'', the

fourth trochanter The fourth trochanter is a shared characteristic common to archosaurs. It is a knob-like feature on the posterior-medial side of the middle of the femur shaft that serves as a muscle attachment, mainly for the ''musculus caudofemoralis longus'', ...

, an archosauriform characteristic, is poorly-developed. On the bottom end, the groove separating the articulations with the

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 ...

and

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 is ...

is shallow, like ''Turfanosuchus'', ''Euparkeria'', ''Tropidosuchus'', ''Riojasuchus'', ''Marasuchus'', and ''Lagerpeton''. Depressions on the rear and outer surfaces are equally poorly-developed, as in ''

Aetosauroides ''Aetosauroides'' (meaning "''Aetosaurus''-like") is an extinct genus of aetosaur from the Late Triassic of South America. It is one of four aetosaurs known from South America, the others being ''Neoaetosauroides'', '' Chilenosuchus'' and ''Aetob ...

'' and ''Marasuchus''. The tibia is 90% of the femur's length, like other basal archosaurs, with a straight shaft, like ''Euparkeria'', ''Aetosauroides'', ''

Neoaetosauroides ''Neoaetosauroides'' is an extinct genus of primitive aetosaur. Its type and only species is ''N. engaeus''. Fossils have been found in Los Colorados Formation outcropping along the Sierra Morada River in the Ischigualasto-Villa Unión Basin in ...

'', ''Fasolasuchus'', ''Postosuchus'', and ''Lagerpeton''. The bottom end of the bone is wider than it is long, like ''Dromicosuchus''. There is a bend about a quarter of the way down from the top of the fibula, where the elongate, weakly-developed iliofibular trochanter is located. ''Euparkeria'', ''Marasuchus'', ''Terrestrisuchus'', ''Dromicosuchus'', and ''

Effigia ''Effigia'' was an extinct genus of shuvosaurid known from the Late Triassic of New Mexico, south-western USA. With a bipedal stance, long neck, and a toothless beaked skull, ''Effigia'' and other shuvosaurids bore a resemblance to the ornithomim ...

'' have a similarly poorly-developed trochanter. Like ''Turfanosuchus'' and other members of the

Crurotarsi Crurotarsi is a clade of archosauriform reptiles that includes crocodilians and stem-crocodilians and possibly bird-line archosaurs too if the extinct, crocodile-like phytosaurs are more distantly related to crocodiles than traditionally thought ...

, ''Gracilisuchus'' has a "crocodile-normal" ankle joint, with the

astragalus ''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...

and

calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. St ...

being joined with a "peg-and-socket" joint. Unlike ''Turfanosuchus'', ''Euparkeria'', and ''Marasuchus'', the astragalus has a "screw-joint" articulation with the tibia, with slightly divergent articulating surfaces. Like ''Turfanosuchus'' and ''Euparkeria'', the hollow on the front of the astraglus covers more than half of the surface. Unlike those two, the inner face of the astragalus has one flat surface instead of two. The calcaneum has a "sliding" articulation with the fibula like ''Turfanosuchus'' and other pseudosuchians. There is a notch on the back of the bone, like in ''Turfanosuchus'', ''Aetosauroides'', ''Fasolasuchus'', ''Dromicosuchus'', ''Protosuchus'', and ''Caiman''. The tuber beside the notch is directed backwards and is wider than high, like ''Turfanosuchus'' and aetosaurs. There are five digits in the foot, with the number of

phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...

being preserved on each digit being 2-3-2-2-1 from the first digit to the fifth; the first digit is completely preserved.

Osteoderms

''Gracilisuchus'' bore two rows of bony plates known as

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 (extinct amp ...

s above its neck and torso, with the first pair beginning immediately behind the skull. They do not appear to continue over the hip; this may be associated with the lack of "tables" in the neural spines of the rear vertebrae, or it may be an artifact of preservation. Like ''Turfanosuchus'', ''Ticinosuchus'', ''Saurosuchus'', and ''

Qianosuchus ''Qianosuchus'' is an extinct genus of aquatic poposauroid archosaur from the middle Triassic (Anisian) Guanling Formation of Pan County, China. It is represented by two nearly complete skeletons and a crushed skull preserved in the limestone. '' ...

'', there were two pairs of osteoderms over each vertebra. Each osteoderm slightly overlaps the one immediately behind it, and the left osteoderm of each row is slightly further forward than the right, creating an asymmetrical appearance. This staggered arrangement is also seen in ''Euparkeria'', ''Ticinosuchus'', ''Nundasuchus'', ''Qianosuchus'', ''Prestosuchus'', and ''Saurosuchus''. While osteoderms from the frontmost row are triangular, osteoderms further behind are leaf-shaped. These leaf-shaped osteoderms have small forward projections where they meet each other at the midline, like ''Turfanosuchus'' and ''Euparkeria'' but unlike ''Postosuchus'', ''Batrachotomus'', and ''Saurosuchus'', which possess osteoderms with forward projections situated further to the sides. The surface of each osteoderm bears a longitudinal midline crest, with a depression on either side. This is similar to ''Saurosuchus'' and ''Batrachotomus'', but unlike ''Turfanosuchus'', ''Euparkeria'', ''Erpetosuchus'', ''Parringtonia'', and ''Postosuchus'', which all have crests not on the midline. Different specimens of ''Gracilisuchus'' have different osteoderm surface textures; some are smooth like ''Turfanosuchus'', while others bear radial pits and grooves like ''Erpetosuchus''.

Classification

Interpretation as an ornithosuchid

Romer considered ''Gracilisuchus'' to be, "quite clear y, a relative of the Scottish ''Ornithosuchus'', owing to similarities in skull structure and other skeletal features. When

Alick Walker Alick Donald Walker (26 October 1925 – 4 December 1999) was a British palaeontologist, after whom the ''Alwalkeria'' genus of dinosaur is named. He was born in Skirpenbeck, near York and attended Pocklington School from 1936 to 1943. He began a ...

described ''Ornithosuchus'' in 1964, he suggested that ''

Teratosaurus ''Teratosaurus'' is a genus of rauisuchians known from the Triassic Stubensandstein (Löwenstein Formation - Norian stage) of Germany. It is estimated to be 6 meters (19.5 ft) long. Discovery In 1860, Sixt Friedrich Jakob von Kapff at th ...

'' and ''

Sinosaurus ''Sinosaurus'' (meaning "Chinese lizard") is an extinct genus of theropod dinosaur which lived during the Early Jurassic Period. It was a bipedal carnivore approximately in length and in body mass. Fossils of the animal were found at the Lufen ...

'' were its closest relatives, collectively forming the family

Ornithosuchidae Ornithosuchidae is an extinct family of pseudosuchian archosaurs (distant relatives of modern crocodilians) from the Triassic period. Ornithosuchids were quadrupedal and facultatively bipedal (e.g. like chimpanzees), meaning that they had the abi ...

. He further suggested that they were theropod

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s. At this time, theropods were divided into two groups following Romer's 1956 classification: the

Coelurosauria Coelurosauria (; from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyrann ...

(long-necked, small-headed theropods) and

Carnosauria Carnosauria is an extinct large group of predatory dinosaurs that lived during the Jurassic and Cretaceous periods. Starting from the 1990s, scientists have discovered some very large carnosaurs in the carcharodontosaurid family, such as ''Gig ...

(short-necked, large-headed theropods). Walker considered ornithosuchids to belong to the latter group, due to strong morphological similarities between the limb girdles of ''Ornithosuchus'', ''

Albertosaurus ''Albertosaurus'' (; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 71 million years ago. The type species, ''A. sarcophagus'', was app ...

'', ''

Gorgosaurus ''Gorgosaurus'' ( ; ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (Campanian), between about 76.6 and 75.1 million years ago. Fossil remains have been found in the Can ...

'', and ''

Antrodemus ''Antrodemus'' ("chamber bodied") is a dubious genus of theropod dinosaur from the Kimmeridgian-Tithonian age Upper Jurassic Morrison Formation of Middle Park, Colorado. It contains one species, ''Antrodemus valens'', first described and named a ...

'' (=''

Allosaurus ''Allosaurus'' () is a genus of large carnosaurian theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic epoch (Kimmeridgian to late Tithonian). The name "''Allosaurus''" means "different lizard" alluding to ...

''). He identified ornithosuchids as the ancestral Triassic stock from which Jurassic and Cretaceous carnosaurs originated. This assessment remained fairly popular in ensuing years, and was adopted by Romer for the 1966 edition of his textbook ''

Vertebrate Paleontology Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord. It also tries to connect, by us ...

''. By the time of Romer's description of ''Gracilisuchus'' in 1972, the geographic range of ornithosuchids had expanded to include the Argentinian ''

Venaticosuchus ''Venaticosuchus'' is a genus of pseudosuchian archosaurs from the family Ornithosuchidae. Known from a single species, ''Venaticosuchus rusconii,'' this genus is described based on an incomplete skull and jaw (as well as a lost partial forelimb ...

'' and ''Riojasuchus'', which had been referred to the family by Bonaparte in 1969. Romer noted that ''Gracilisuchus'' was the smallest and oldest known member of the group to date, and accordingly had a fairly basal morphology (notwithstanding supposedly aberrant traits such as the partial closure of the

infratemporal fenestra An infratemporal fenestra, also called the lateral temporal fenestra or simply temporal fenestra, is an opening in the skull behind the orbit in some animals. It is ventrally bordered by a zygomatic arch. An opening in front of the eye sockets, ...

). However, he had reservations regarding Walker's identification of ornithosuchids as dinosaurs, noting basal archosaur traits such as the closed acetabulum, osteoderms, and crocodile-normal ankle. Thus, he considered the supposedly carnosaurian features to be products of convergence.

Reidentification as a non-ornithosuchid

Doubts arose in following years regarding whether ''Gracilisuchus'' was actually referable to the Ornithosuchidae. In 1979, Arthur Cruickshank separated pseudosuchians ("crocodile-line" archosaurs) into two groups based on whether they bore "crocodile-normal" or "crocodile-reversed" (where the peg and socket are located on the opposite bones) ankles. He observed that, while ''Gracilisuchus'' had a "crocodile-normal" joint, other ornithosuchids had a "crocodile-reversed" joint; he thus removed ''Gracilisuchus'' from the Ornithosuchidae. Donald Brinkman noted in 1981 that, without further information regarding the origin of "crocodile-reversed" joints, it would be possible that "crocodile-normal" joints represent the basal condition, which was retained in ''Gracilisuchus'', with "crocodile-reversed" joints representing a specialization of later ornithosuchids. However, at the same time, Brinkman noted a number of other traits in ''Gracilisuchus'' that differ from "advanced" ornithosuchids. These include the rectangular antorbital fenestra; the rounded bottom of the eye socket due to the absence of a prominent projection of the jugal bone in front of the eye; the tall, slender quadratojugal bone; the lack of a gap between the teeth of the premaxilla and

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

; the rectangular infratemporal fenestra; a concave flange on the rear of the

squamosal bone 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 ...

; the rear of the

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

of the lower jaw being unforked, and extending backwards only above the mandibular fenestra; the splenial bone forming the bottom of the jaw as opposed to being restricted to its inner surface; the lack of keels in the cervical vertebrae; and the presence of two osteoderm rows over each vertebra instead of one. The first, fourth, and fifth of these are shared with ''Euparkeria'', leading Brinkman to consider them as basal traits; however, ''Euparkeria'' is more similar to "advanced" ornithosuchids with respect to the other traits. According to Brinkman, this left two possibilities: either ''Gracilisuchus'' was

derived Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...

from the basal ornithosuchid condition in a manner different from other ornithosuchids, or it represents a different

evolutionary radiation An evolutionary radiation is an increase in taxonomic diversity that is caused by elevated rates of speciation, that may or may not be associated with an increase in morphological disparity. Radiations may affect one clade or many, and be rapid ...

altogether, separate from the ornithosuchids. He noted intriguing similarities between ''Gracilisuchus'' and members of the Sphenosuchia; the third and sixth of the above traits are shared with ''

Sphenosuchus ''Sphenosuchus'' ("wedge crocodile") is an extinct genus of crocodylomorph from the Early Jurassic Elliot Formation of South Africa, discovered and described early in the 20th century. The skull is preserved very well but other than elements of ...

'' and ''Pseudohesperosuchus'', while the ninth is shared with ''

Lewisuchus ''Lewisuchus'' is a genus of archosaur that lived during the Late Triassic (early Carnian). As a silesaurid dinosauriform, it was a member of the group of reptiles most commonly considered to be the closest relatives of dinosaurs (possibly tr ...

''. Thus, in a

phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

, Brinkman tentatively depicted ''Gracilisuchus'' as being closer to the "crocodile-line"

Erythrosuchidae Erythrosuchidae (meaning "red crocodiles" in Greek) are a family of large basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian). Naming The family Erythrosuchidae was named b ...

, the Rauisuchidae, and the

Stagonolepididae Aetosaurs () are heavily armored reptiles belonging to the extinct order Aetosauria (; from Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs m ...

(=Aetosauria) than the Ornithosuchidae and the

Euparkeriidae Euparkeriidae is an extinct family of small carnivorous archosauriforms which lived from the Early Triassic to the Middle Triassic (Anisian). While most other early archosauriforms walked on four limbs, euparkeriids were probably facultative ...

Inclusion in the Suchia and Crurotarsi

Phylogenetic analysis 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 ...

soon supported Brinkman's hypothesis that ''Gracilisuchus'' was closer to "crocodile-line" archosaurs. In 1988,

Michael Benton Michael James Benton One or more of the preceding sentences incorporates text from the royalsociety.org website where: (born 8 April 1956) is a British palaeontologist, and professor of vertebrate palaeontology in the School of Earth Sciences ...

and James Clark published a phylogenetic analysis incorporating ''Gracilisuchus'' and 16 other

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

. It was recovered as a member of the Suchia, a group defined by Bernard Krebs as being characterized by a "crocodile-normal" ankle, among other characteristics. Within the Suchia, they found that ''Gracilisuchus'' was the

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 t ...

of crocodylomorphs and "pseudosuchians" (which they restrictively defined to include rauisuchids and stagonolepidids). It was differentiated from the latter two by the presence of postparietals and the absence of: a pit between the

basioccipital The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull this area is rough and uneven, and is joined t ...

and basisphenoid bones; fusion between the

atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographic ...

(first cervical) and the intercentrum, an element below the axis; accessory processes on the caudal neural spines; and osteoderms on the bottom of the tail. Meanwhile, ornithosuchids were closer to dinosaurs in the Ornithosuchia, and ''Euparkeria'' was the sister taxon of the group containing suchians and ornithosuchians. In 1990,

Paul Sereno Paul Callistus Sereno (born October 11, 1957) is a professor of paleontology at the University of Chicago and a National Geographic "explorer-in-residence" who has discovered several new dinosaur species on several continents, including at sites ...

and Andrea Arcucci suggested that ornithosuchids - in which they included ''Gracilisuchus'' - were actually closer to the conventional "crocodile-line" archosaurs than dinosaurs. They named the collective group of Suchia, Ornithosuchidae, and Parasuchia (=Phytosauria) as the

, and noted some uniting characteristics: the strong inward arch of the top of the humerus; the bottom end of the fibula being wider than the top; the specialized "sliding" articulation between the fibula and calcaneum; the "screw-joint" articulation between the tibia and astragalus; the robust tuber of the calcaneum, with a flared bottom end; and a "recess" at the back of the top end of the pubis. ''Gracilisuchus'' was identified as an exception for additional uniting traits, such as the robust trochanter on the fibula and the single row of osteoderms per vertebra. This classification was further revised by Sereno in 1991, when he removed ''Gracilisuchus'' from the ornithosuchids, and used it to define Suchia's contents.

Phylogenetic "limbo"

Subsequent analyses did not reach a consensus on the relationships between ''Gracilisuchus'' and other crurotarsans. Despite this ambiguity, ''Gracilisuchus'' has been widely used as the staple

outgroup Outgroup may refer to: * Outgroup (cladistics), an evolutionary-history concept * Outgroup (sociology), a social group {{disambig ...

, or taxon representing a basal condition, in analyses of more derived pseudosuchians such as crocodylomorphs, including both

crocodyliforms 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 ...

and sphenosuchians (now known to be a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

assemblage of non-crocodyliform crocodylomorphs). J. Michael Parrish's 1993 analysis noted that a "splint-like" fifth

metatarsal The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the med ...

in the foot, likely a consequence of the compaction of the foot, united ''Gracilisuchus'' with its sister group, the newly-defined

Paracrocodylomorpha Paracrocodylomorpha is a clade of pseudosuchian archosaurs. The clade includes the diverse and unusual group Poposauroidea as well as the generally carnivorous and quadrupedal members of Loricata, including modern crocodylians. Paracrocodylomorpha ...

(Poposauridae and Crocodylomorpha). They formed the Rauisuchia along with the Rauisuchidae. However, Parrish also noted that ''Gracilisuchus'' differed from other rauisuchians in the absence of an ossification at the back of the top of the skull, and the absence of a fenestra between the premaxilla and maxilla. In a 1994 analysis, Lars Juul moved ''Gracilisuchus'' inside the Paracrocodylomorpha, placing it as the sister taxon of ''Postosuchus'' (then a poposaurid). Paracrocodylomorpha, in turn, was united with the Ornithosuchidae to form the Dromaeosuchia. Both analyses suggested that the squamosal flange of ''Gracilisuchus'' was homologous with that of ''Postosuchus'' and crocodylomorphs. Descriptions of material from ''Erpetosuchus'' in the early 2000s were accompanied by further analyses incorporating ''Gracilisuchus''. In a 2000 description of North American material, Paul Olsen,

Hans-Dieter Sues Hans-Dieter Sues (born January 13, 1956) is a German-born American paleontologist who is Senior Scientist and Curator of Vertebrate Paleontology at the National Museum of Natural History of the Smithsonian Institution in Washington, DC. He receiv ...

, and

Mark Norell Mark Allen Norell (born July 26, 1957) is an American paleontologist, acknowledged as one of the most important living vertebrate paleontologists. He is currently the chairman of paleontology and a research associate at the American Museum of Na ...

recovered ''Gracilisuchus'' as more derived than ''Stagonolepis'' but more basal than ''Postosuchus'', ''Erpetosuchus'', and crocodylomorphs. Later, in a 2002 revision of the genus, Benton and Walker found contrasting hypotheses for the position of ''Gracilisuchus'': as being more derived than a group containing ''Ornithosuchus'' and rauisuchians (''Saurosuchus'', ''Batrachotomus'', ''Prestosuchus''); or being in a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...

with ''Ornithosuchus'' and rauisuchians. In both cases, it was more basal than the same group in Olsen and colleagues' analysis, being united by a ridge on the squamosal bone above the supratemporal fenestra and the absence of a foramen on the quadrate. In an analysis for the 2004 second edition of ''

The Dinosauria ''The Dinosauria'' is an extensive book on dinosaurs, compiled by David B. Weishampel, Peter Dodson, and Halszka Osmólska. It has been published in 2 editions, with the first edition published in 1990, consisting of material from 23 scientists.B ...

'', Benton performed another phylogenetic analysis, finding ''Gracilisuchus'' to be the sister taxon of the Phytosauria (by then renamed from the Parasuchia). They formed a polytomy with Ornithosuchidae, which was in a basal position relative to the Suchia (defined to include Stagonolepididae, ''Postosuchus'', and Crocodylomorpha), ''Fasolasuchus'', and the

Prestosuchidae Prestosuchidae (in its widest usage) is a polyphyletic grouping of carnivorous archosaurs that lived during the Triassic. They were large active terrestrial apex predators, ranging from around in length. They succeeded the Erythrosuchidae as the ...

. Later, in 2006, the same relationship with the Phytosauria was recovered by Chun Li and colleagues in a phylogenetic analysis conducted for the supplementary material of the description of ''Qianosuchus''. They found that group to be in a polytomy with Ornithosuchidae; ''Qianosuchus''; the group of ''Postosuchus'' and Crocodylomorpha; and the group of Stagonolepididae, ''Fasolasuchus'', and Prestosuchidae.

Stephen Brusatte Stephen Louis Brusatte (born April 24, 1984) is an American paleontologist and evolutionary biologist, who specializes in the anatomy and evolution of dinosaurs. He was educated at the University of Chicago for his BS degree, at the University of ...

, Benton, Desojo, and Max Langer conducted the most comprehensive phylogenetic analysis at the time in 2010. They noted ''Gracilisuchus'' had been a "singleton" taxon in prior analyses, one that couldn't be definitely placed in any particular group. In their own analysis, ''Gracilisuchus'' was the sister taxon of a group containing ''Erpetosuchus'' and the Crocodylomorpha, which along with the Aetosauria (by then renamed from the Stagonolepididae) formed one branch of the Suchia. Although they found strong support for this grouping in the form of eight

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

(shared traits), with two of them (involving the ossification and position of the

perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ca ...

atic foramen of the braincase) being unambiguous, later assessment noted that this may have resulted from poor non-rauisuchian sampling. Meanwhile, ''

Revueltosaurus ''Revueltosaurus'' ("Revuelto lizard") is an extinct genus of suchian pseudosuchian from Late Triassic (late Carnian to middle Norian stage) deposits of New Mexico, Arizona and North Carolina, United States. Many specimens, mostly teeth, have be ...

'' and ornithosuchids formed a group on the opposite branch of the Suchia, being closer to rauisuchians. In 2011,

Sterling Nesbitt Sterling Nesbitt (born March 25, 1982, in Mesa, Arizona) is an American paleontologist best known for his work on the origin and early evolutionary patterns of archosaurs. He is currently an associate professor at Virginia Tech in the Department of ...

conducted another, better-sampled phylogenetic analysis. Contrary to Parrish and Juul, he found that ''Gracilisuchus'' acquired a squamosal flange independently of ''Postosuchus'' and crocodylomorphs. Although he recovered a variety of positions for ''Gracilisuchus'' in different trees, all of them agreed that ''Gracilisuchus'' was a basal suchian, and was closer to crocodylomorphs than the non-archosaurian phytosaurs. The consensus found ''Gracilisuchus'' to be in a polytomy with a group of ''Revueltosaurus'' and Aetosauria; ''Turfanosuchus''; and a group of ''Ticinosaurus'' and Paracrocodylomorpha. He noted that the removal of ''Turfanosuchus'' allied ''Gracilisuchus'' with ornithosuchids in one possible tree, with this relationship being based upon unambiguous synapomorphies such as the presence of three premaxillary teeth; the length of the pubis being more than 70% that of the femur; the pubis being longer than the ischium; the presence of a midline gap at the bottom of the ischium; and the lack of a "hook" at the top end of the fifth metatarsal.

Formation of the Gracilisuchidae

The redescription of the hindlimbs of ''Gracilisuchus'' by Lecuona and Desojo in 2011 allowed additional data to be incorporated into subsequent analyses. Lecuona and Desojo also noted that the poor development of the fourth trochanter and femoral head was shared with members of the Sphenosuchia, which allowed for the possibility that they formed a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

group. However, they also noted that Nesbitt's analysis provides support for a position outside the Crocodylomorpha, due to the absence of a perforated acetabulum. In 2014, an analysis led by Richard Butler, modified from that of Nesbitt, suggested for the first time that ''Gracilisuchus'' formed a group with ''Turfanosuchus'' and ''Yonghesuchus'', two basal suchians with similarly convoluted taxonomic histories. The group was named the Gracilisuchidae. They found that Gracilisuchidae was the sister taxon of a group containing ''Ticinosuchus'' and the Paracrocodylomorpha, with all of these collectively forming the sister taxon to a group consisting of ''Revueltosaurus'' and the Aetosauria; both sub-groups had previously been recovered by Nesbitt. Strong support was obtained for the Gracilisuchidae in the form of six unambiguous synapomorphies: a process on the back of the premaxilla that fits into a slot on outer surface of the

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 by their junction, form the bridge of the upper one third of the nose. Eac ...

; the nasal bordering the top of the antorbital fenestra; the tapering

frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.''Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, par ...

; the presence of a depression on the bottom of the calcaneal tuber; the osteoderms bending downwards at their outer edges; and the presence of a triangular process bearing a clear apex on the maxilla. Within the Gracilisuchidae, Butler and colleagues noted that ''Gracilisuchus'' was likely closer to ''Yonghesuchus'' than ''Turfanosuchus'', on account of three synapomorphies: the contact between the squamosal and

postorbital bone The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...

s continuing backwards along much of the former's bottom surface; the jugal stopping short at its rear end of the infratemporal fenestra; and the conjunction of the basisphenoid and

parasphenoid The parasphenoid is a bone which can be found in the cranium of many vertebrates. It is an unpaired dermal bone which lies at the midline of the roof of the mouth. In many reptiles (including birds), it fuses to the endochondral (cartilage-derived) ...

being located between plate-like, triangular projections of the basioccipital known as tubera, with the basipterygoid processes at the base of the basisphenoid being at least 1.5 times longer than it is wide. However, poor support for this relationship was obtained, partially due to the incompleteness of material referred to ''Yonghesuchus''. Lecuona, Desojo, and Pol conducted another analysis, building upon the work of Butler and colleagues as well as Lecuona's 2013 thesis, in 2017 to accompany their redescription of ''Gracilisuchus''. They uncovered the same phylogenetic arrangements within the Gracilisuchidae and in relation to other pseudosuchians. However, their analysis was able to provide a well-resolved tree even with the inclusion of the erpetosuchids (''Erpetosuchus'' and ''Parringtonia''); the inclusion of erpetosuchids had collapsed Gracilisuchidae into a polytomy in Butler and colleagues' analysis. Lecuona and colleagues added two synapomorphies of Gracilisuchidae to those listed by Butler: the absence of the jugal's contribution to the postorbital bar behind the eye socket, and the articulations with the fibula and astragalus forming a continuous structure on the calcaneum. They also removed the original character involving the calcaneal tuber. Finally, they added one synapomorphy uniting ''Gracilisuchus'' and ''Yonghesuchus'': the absence of the postorbital's contribution to the border of the infratemporal fenestra.

Palaeobiology

Posture

Owing to its supposed affinities with ''Ornithosuchus'', Romer reconstructed ''Gracilisuchus'' as a

facultative biped A facultative biped is an animal that is capable of walking or running on two legs (bipedal), as a response to exceptional circumstances ( facultative), while normally walking or running on four limbs or more. In contrast, obligate bipedalism is ...

in his initial description. Both Walker and Bonaparte noted that the forelimbs of ornithosuchids were considerably reduced relative to the hindlimbs, with likewise reduced fingers that were more suitable for grasping than locomotion. Although the hand is not preserved in ''Gracilisuchus'', Romer noted that its forelimbs were three-fifths the lengths of the hindlimbs, like ornithosuchids; however, this interpretation was based on material which has since been reassigned.

Palaeoecology

Stratigraphic context

The locality where ''Gracilisuchus'' was discovered is known as the Los Chañares locality, and is located at . It consists of

badlands Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded."Badlands" in ''Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 2, p. 47. They are characterized by steep slopes, m ...

at the base of a latitudinal

escarpment An escarpment is a steep slope or long cliff that forms as a result of faulting or erosion and separates two relatively level areas having different elevations. The terms ''scarp'' and ''scarp face'' are often used interchangeably with ''escar ...

, with the exposed rocks being composed of

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

grains, along with glass shards, embedded in a

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

. These rocks belong to the lower portion of the

Chañares Formation The Chañares Formation is a Carnian-age geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Province, Argentina. It is characterized by drab-colored fine-grained volcaniclastic claystones, siltstones, and sandstones ...

, which locally overlies the

Tarjados Formation The Tarjados Formation is an Early to Middle Triassic geologic formation in the Ischigualasto-Villa Unión Basin of La Rioja Province in northwestern Argentina. The red to whitish sandstones and conglomerates of the formation were deposited in a ...

and underlies the Los Rastros Formation. Starting about above the boundary between the Tarjados and Chañares Formations, brown

concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...

s of

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...

are present throughout the rock. It is in these concretions that the remains of ''Gracilisuchus'' and other tetrapods have been found. Unlike other

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...

(

) formations, there are no dinosaur remains in the Chañares Formation. This has led researchers to assign a

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...

age to the Chañares. Owing to shared

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

l components, correlations with the ''

Dinodontosaurus ''Dinodontosaurus'' (meaning "terrible-toothed lizard") is a genus of dicynodont therapsid. It was medium to large dicynodont of the Triassic (with skull up to long) and had a beak corneum. It lived in the Middle Triassic but disappeared in the ...

'' Assemblage Zone of the

Santa Maria Formation The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age (Late Triassic), and is notable for its fossils of cynodonts, "rauisuchian" pseudosuchians, and early dinosaurs and oth ...

in Brazil have been used to assign a

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic). ...

age to the Chañares.

Argon–argon dating Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer ...

of the

Ischigualasto Formation The Ischigualasto Formation is a Late Triassic fossiliferous formation and Lagerstätte in the Ischigualasto-Villa Unión Basin of the southwestern La Rioja Province and northeastern San Juan Province in northwestern Argentina. The formation da ...

(which locally overlies the Los Rastros) in 1993 also produced an age near the Ladinian–Carnian boundary, thus constraining the Chañares to the Ladinian. However, more accurate

uranium–lead dating Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routi ...

subsequently necessitated revisions of the

geologic time scale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochrono ...

. The Ischigualasto was reassigned to the late Carnian, making a Carnian age possible for the Chañares. This was reaffirmed by uranium-lead dating of Chañares deposits in 2016, indicating an early Carnian age of 236 to 234 million years.

Notes

References

External links

Crocodylomorpha fossil record
{{Taxonbar, from=Q943527 Gracilisuchids Late Triassic pseudosuchians Late Triassic reptiles of South America Triassic Argentina Fossils of Argentina Fossil taxa described in 1972 Prehistoric pseudosuchian genera