Marasuchus
Marasuchus 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 contains a single species, Marasuchus lilloensis.
Marasuchus lilloensis was originally designated as Lagosuchus lilloensis in 1972. It was considered a new species of Lagosuchus, a contemporary archosaur described the previous year. However, a 1994 study argued that the original material of Lagosuchus was undiagnostic. This would mean that Lagosuchus and its original species could be considered nomen dubia. Specimens of Lagosuchus stored at a museum in San Miguel de Tucuman were considered to be more diagnostic than those of L. talampayensis, and thus they were given a new genus: Marasuchus. A 2019 study redescribed the original material of Lagosuchus and concluded that it was valid and not readily distinguishable from Marasuchus lilloensis. This suggests that Marasuchus lilloensis is a junior synonym of Lagosuchus talampayensis.
Specimens referred to the genus Marasuchus'' possessed some, but not all of the adaptations which traditionally characterized dinosaurs. For example, its proportions indicate that it was likely bipedal as in early dinosaurs. Also, it shared certain specific characteristics with that group, most relating to the hip and the head of the femur. Nevertheless, it lacked certain dinosaur-like features such as a perforated acetabulum, and it had several plesiomorphic features of the ankle.
Discovery and history
Marasuchus hails from the Chañares Formation of Argentina. This formation has been dated to the early Carnian, about 235 to 234 million years old. Many Chanares fossils, including the first known remains of Marasuchus, were unearthed as a result of a 1964-1965 paleontological expedition. This expedition was undertaken by paleontologists from the MCZ and the MLP. Discoveries made during the expedition were later described in a series of papers by Alfred Romer from the MCZ. Around the same time, further expeditions by Jose Bonaparte of the PVL unearthed additional fossils from the area.Marasuchus lilloensis is known from several specimens representing most of the animal's skeletal anatomy, although skull material remains limited. The holotype, PVL 3871, was collected by Bonaparte in 1969 and was initially known as "the Tucuman specimen". This specimen consists of large portions of the tail, left forelimb, hip, and hindlimbs. Other Marasuchus fossils are stored at the PVL as well. PVL 3870 includes skull material, the entire presacral vertebral column, and a nearly complete hip and hindlimbs. PVL 3872 is a braincase and associated cervical vertebrae. PVL 4670 and 4671 each consist of vertebrae from the base of the tail, while PVL 4672 is a sequence of cervical and dorsal vertebrae. The referral of PVL 4670 to Marasuchus is uncertain due to the absence of diagnostic features clearly shared with other specimens.
Relation to ''Lagosuchus''
When the Tucuman specimen was first described by Romer in 1972, it was placed as the type specimen of a new species, Lagosuchus lilloensis. Lagosuchus talampayensis, the type species of Lagosuchus, had been named a few papers earlier in 1971. The two species were differentiated mainly on the basis of the Tucuman specimen being larger than the type specimen of L. talampayensis. In his 1975 review of the genus, Jose Bonaparte regarded the two species as synonymous, with L. lilloensis as a junior synonym of L. talampayensis. Many other sources published between 1972 and 1994 followed this conclusion.However, a later study by Paul Sereno and Andrea Arcucci concluded that the original type specimen of Lagosuchus was poorly preserved and undiagnostic. They argued against assigning other specimens to the genus Lagosuchus, which they regarded as a nomen dubium. They also noted that many referred PVL specimens had limb proportions and other subtle traits differing from Lagosuchus specimens stored elsewhere. On this basis, a new genus was erected to contain the PVL specimens. This new genus was called Marasuchus, a nod to the etymology of Lagosuchus. It was given the specific name Marasuchus lilloensis, based on Romer's species designation for the Tucuman specimen. Between 1994 and 2019, Marasuchus was considered a more diagnostic and well-described replacement name for the dinosauriform taxon known previously as Lagosuchus.
In 2019, the type specimen of Lagosuchus was re-examined by Federico Agnolin and Martin Ezcurra, who noted that the type specimen actually does share several of the diagnostic traits identified by Sereno and Arucci for Marasuchus. Furthermore, they argued that variations in size and certain features of the skeleton were either ontogenetic or individually variable. As a result, they referred the PVL specimens back to Lagosuchus. Following their conclusion, Marasuchus lilloensis could be considered a synonym of Lagosuchus talampayensis once more.
Description
In terms of proportions, Marasuchus generally resembled early theropod dinosaurs like Coelophysis. The limbs were long and slender, with the hindlimbs about twice the length of the forelimbs. These proportions meant that it was probably bipedal and had acquired the upright stance characteristic of dinosaurs. The neck was long, with an S-shaped curve as its default position, while the tail was very long and thin, though deeper at its base. The type specimen of Marasuchus had a femur which was 5.75 cm inches in length, about 40-50% larger than the type specimen of Lagosuchus talampayensis. Nevertheless, Marasuchus was still a small and lightly built animal.Skull
Skull material is very limited for Marasuchus, with the only preserved bones from this region being a maxilla preserved in PVL 3870 and braincases preserved in PVL 3870 and 3872. The maxilla was low, with at least 12 teeth. Most of these teeth were blade-like and serrated, but some of those near the rear of the bone were less curved and more leaf-shaped. The maxilla also possessed interdental plates on its inner surface. The braincase was tall and fairly typical compared to other early archosaurs. However, in a few cases it shared specific similarities with the braincase of early dinosaurs. For example, the basipterygoid processes were short, blade-like, and tilted forwards. In addition, the exoccipitals were wide and edged by a pronounced ridge next to the exit holes for the hypoglossal nerve. Bonaparte additionally described squamosal and quadrate bones similar to those of Euparkeria attached to PVL 3872's braincase, although these were not mentioned by later studies.Vertebrae
Almost the entirety of the spinal column is present in Marasuchus, barring the tip of the tail. Most of MarasuchusForelimbs
The scapulocoracoid was quite large and broad unlike most other avemetatarsalians. On the other hand, the glenoid was directed somewhat backwards, as is the case with other dinosauriforms. The forelimb bones were very slender and shorter than the leg bones, and the forelimb as a whole was about half the size of the hindlimb. No portion of the hand was preserved.Hip
The pelvis shared quite a few similarities with other dinosauriforms not otherwise present in earlier archosauriforms. The ilium was similar to that of Herrerasaurus in general shape. The pubis was longer than the ischium, like dinosauriforms. However, the ischium was also enlarged relative to earlier archosauriforms, as it was longer than the main portion of the ilium. Furthermore, the ischium's contact with the pubis is less extensive than in early archosauriforms and it fails to contact the ilium along the boundary of the pubis, as with silesaurids and saurischian dinosaurs. This "gap" between the ilium and ischium along the edge of the pubis becomes more developed in dinosaurs, where it becomes and open cavity that fills up the entire acetabulum. However, this had not yet evolved in Marasuchus, which retains a bony inner wall of the acetablum. Moreover, the edge of the ischium in Marasuchus retains contact between the ilium and pubis, unlike dinosaurs. Nevertheless, a depression present in that area may be a predecessor to the more advanced condition in dinosaurs.Hindlimbs
Modifications to the acetabulum are mirrored in the head of the femur, which connects to it. A distinct tab of bone known as an anterior trochanter was present on the outer edge of the femoral head, as with other dinosauriforms and to a lesser extent in other avemetatarsalians. In addition, Marasuchus also possessed a ridge of bone known as the trochanteric shelf, which branches down from the anterior trochanter and wraps around the shaft of the femur. A trochanteric shelf is also characteristic of some early dinosaurs, silesaurids, and some specimens of Dromomeron, and a similar structure is also present in aphanosaurs, albeit separate from their equivalent of the anterior trochanter. As with other dinosauriforms, the tibia has a longitudinal groove edged by a sharp flange at its rear outer corner, near the ankle. The tibia was also longer than the femur.The ankle had two main bones: the larger, boxy astragalus and a smaller calcaneum attached to its outer edge. In some aspects, the ankle shared features with other dinosauriforms, such as a vertical triangular branch of the astragalus which rises up in front of the tibia. However, in other aspects the ankle was surprisingly primitive, even compared to earlier avemetatarsalians like pterosaurs and lagerpetids. For example, the rear of the astragalus possesses a vertical groove, and the calcaneum had a knob on its rear edge known as a calcaneal tuber. Unlike lagerpetids or coelophysoids, the astragalus and calcaneum were not fused together. The five metatarsals were thin, elongated, and close together. The third and fourth metatarsals were the longest, followed by the second, with the first and fifth being only about half the length of the longest. Although not all of the pedal phalanges were preserved, the phalangeal formula was likely 2-3-4-5-0 as with other dinosauromorphs.