Fossil range: Middle Triassic
Life restoration of Gracilisuchus
| Scientific classification
Gracilisuchus (meaning "graceful crocodile") is an extinct genus of tiny (30 cm long) crurotarsan (a group which includes the ancestors of crocodilians) from the Middle Triassic. It was a suchian close to the ancestry of crocodiles, and was at one time thought to be a dinosaur, but this hypothesis has since been rejected. Its fossils were first discovered in the 1970s.
Gracilisuchus was a small member of the Pseudosuchia. The largest skull has a length of slightly over 9 centimetres (3.5 in), and the largest femur has a length of roughly 8 centimetres (3.1 in). Alfred Romer estimated a length of 21 centimetres (8.3 in), but Agustinia Lecuona and Julia Desojo estimated a length of 28 centimetres (11 in) for the body excluding the hip and tail. The weight of Gracilisuchus has been estimated at 1.31 kilograms (2.9 lb).
A number of characteristics of the skull can be used to distinguish Gracilisuchus. Its skull openings are relatively large, with the antorbital fenestra occupying 30-36% of the skull roof's length and the eye socket occupying 35-42% of the skull roof's length. Additionally, the supratemporal fenestra is, uniquely, wider than it is long. Within the eye socket, there is a sclerotic ring, and the ossicles (bony segments) comprising the ring contact but do not overlap each other. Unlike its closest relatives, Turfanosuchus and Yonghesuchus, but convergent upon Tropidosuchus, early theropods, and the Crocodylomorpha, the lacrimal bone is as tall as the eye socket instead of being significantly shorter.
Behind the eye socket, the vertical process of the jugal bone is uniquely straight. The backward process of the jugal is located underneath the forward process of the quadratojugal bone, convergent upon Erpetosuchus, Postosuchus, Polonosuchus, and the Crocodylomorpha, unlike Turfanosuchus and Yonghesuchus where they are the other way around. Furthermore, a postfrontal bone 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, 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 instead of five, like Prestosuchus, Saurosuchus, Fasolasuchus, Batrachotomus, the Rauisuchidae, 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.
There are eight cervical vertebrae. Unlike Turfanosuchus, Euparkeria, Fasolasuchus, Saurosuchus, and sphenosuchians, the suture between the centrum and neural arch of the axis (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, Saurosuchus, the aetosaur Stagonolepis, and phytosaurs. The front border of the neural spine is uniquely high and vertical, while the rear border is concave like Turfanosuchus but unlike Erpetosuchus. Similar to both, the articular process 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, aetosaurs, Saurosuchus, and Riojasuchus. The sides of their centra have long depressions, like Turfanosuchus, aetosaurs, Batrachotomus, and Ticinosuchus.
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. The top margin of the postzygapophyses are convex like Turfanosuchus. There are depressions below the postzygapophyses, which are not seen in any other archosaurs except for Stagonosuchus 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. Like Parringtonia, 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 have somewhat deep cavities underneath, like Turfanosuchus, but they are more poorly developed than Nundasuchus, Batrachotomus, or Stagonosuchus.
Two sacral vertebrae 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.
In the shoulder girdle, the end of the scapular blade is widely and asymmetrically expanded, unlike Turfanosuchus, Batrachotomus, and Ticinosuchus, but similar to the crocodylomorph Dromicosuchus. Unlike Turfanosuchus but like Terrestrisuchus and Dibothrosuchus, the articulation with the radius on the humerus is narrower than the one with the ulna. 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 ilium has a weakly expanded frontal process, like Turfanosuchus, Euparkeria, Postosuchus, and Caiman. The front of the articulation with the pubis reaches further than the process, like Turfanosuchus, Euparkeria, Saurosuchus, Postosuchus, and Lagerpeton. 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. The sacral ribs articulate at this shelf, unlike Turfanosuchus where the shelf is located above. The acetabulum, or hip socket, of Gracilisuchus was larger than other archosauriforms. 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.
Unusually, but like Tropidosuchus, Protosuchus, and Orthosuchus, 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 former is similar to Fasolasuchus and Orthosuchus, and the latter is similar to living crocodilians. An L-shaped lamina 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. There is also a small forward projection like Pseudohesperosuchus. Like Macelognathus and Trialestes, the fourth trochanter, an archosauriform characteristic, is poorly-developed. On the bottom end, the groove separating the articulations with the tibia and fibula is shallow, like Turfanosuchus, Euparkeria, Tropidosuchus, Riojasuchus, Marasuchus, and Lagerpeton. Depressions on the rear and outer surfaces are equally poorly-developed, as in Aetosauroides and Marasuchus. The tibia is 90% of the femur's length, like other basal archosaurs, with a straight shaft, like Euparkeria, Aetosauroides, Neoaetosauroides, 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 have a similarly poorly-developed trochanter.
Like Turfanosuchus and other members of the Crurotarsi, Gracilisuchus has a "crocodile-normal" ankle joint, with the astragalus and calcaneum 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 being preserved on each digit being 2-3-2-2-1 from the first digit to the fifth; the first digit is completely preserved.
Gracilisuchus bore two rows of bony plates known as osteoderms 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, 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.
Romer, A.S. (1972). "The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov." Breviora 389:1-24.