Sereno & Arcucci 1994
Marasuchus (meaning "Mara crocodile") is a genus of basal dinosauriform archosaur which lived during the late Triassic in what is now La Rioja Province, Argentina. It 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, and it shared certain specific characteristics with that group such, 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 ("primitive") features of the ankle.
Discovery and history
Alfred Romer first discovered fossils of the species in the 1960s in the Chañares Formation of Argentina. This formation was dated to 235 to 234 million years old. Marasuchus is known from several specimens representing most of the animal's skeletal anatomy, although skull material remains limited.
Two species have been named, L. talampayensis and L. lilloensis. The genus (Lagosuchus) and type species (L. talampayensis) are regarded as nomina dubia. The species Marasuchus lilloensis was originally described as a second species of Lagosuchus, L. lilloensis. However, in a restudy of Lagosuchus by Sereno and Arcucci (1994), the authors concluded that the original (type) specimen was too poorly preserved to allow any further specimens to be assigned to the genus. They also noted that the L. lilloensis specimen had limb proportions different from the type species. On this basis, they assigned L. lilloensis to a new genus, Marasuchus.
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. Fossil specimens of Marasuchus were twice as large as the holotype of Lagosuchus talampayensis, its dubious possible senior synonym. Nevertheless, it was still lightly built and small.
Skull material is very limited for Marasuchus, with the only preserved bones from this region being a maxilla (a toothed bone at the side of the snout) preserved in PVL 3870 and braincases preserved in PVL 3870 and 3872. The maxilla was low, with at least 12 teeth which were blade-like and serrated. Some of the teeth near the rear of the bone were less curved and more leaf-shaped, and 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 (a pair of plates at the bottom of the braincase which connect to the roof of the mouth) were short, blade-like, and tilted forwards. In addition, the exoccipitals (a pair of braincase bones adjacent to the foramen magnum, the main exit for the spinal cord) were wide and edged by a pronounced ridge next to the exit holes for the hypoglossal nerve. Bonaparte (1975) 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.
Almost the entirety of the spinal column is present in Marasuchus, barring the tip of the tail. Most of Marasuchus' diagnostic features (i.e. unique or unusual traits which characterize it specifically) occur in its vertebrae. Most of the neck vertebrae were elongated and had offset front and rear ends, creating a long and curved neck like that of other avemetatarsalians (bird-lineage archosaurs). Also like avemetatarsalians, the upward projecting neural spine of the axis vertebra was expanded and trapezoidal rather than peak-like. More uniquely, the neural spines of vertebrae closer to the base of the neck leaned forwards. Vertebrae near the hip were also characteristic to Marasuchus, since their neural spines were also trapezoidal and expanded to such an extent that they contacted those of adjacent vertebrae. Two vertebrae attach to the hip, less than in most dinosaurs which typically acquire three or more in the sacrum. The tail was characteristically elongated, with vertebrae drastically increasing in length towards the tip. The chevrons (spine-like bones projecting under the tail vertebrae) were also elongated in tail vertebrae near the hip, making the tail unusually deep at its base as well.
The scapulocoracoid (shoulder blade) was quite large and broad unlike most other avemetatarsalians. On the other hand, the glenoid (shoulder socket) was directed somewhat backwards (rather than sideways), as is the case with other dinosauriforms. The forelimb bones (consisting of a humerus, ulna, and radius) 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.
The pelvis (hip) shared quite a few similarities with other dinosauriforms not otherwise present in earlier archosauriforms. The ilium (upper blade of the hip) was similar to that of Herrerasaurus in general shape. The pubis (front lower blade of the hip) was longer than the ischium (rear lower blade of the hip), 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 (hip socket). 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.
Modifications to the acetabulum are mirrored in the head of the femur (thigh bone), 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 (shin bone) 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 (known as an ascending process) 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 (foot bones) 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 (toe bones) were preserved, the phalangeal formula (number of bones per toe) was likely 2-3-4-5-0 as with other dinosauromorphs.
Marasuchus was part of Avemetatarsalia, the branch of archosaurs closer to birds and other dinosaurs rather than to crocodilians. More specifically, it was a dinosauriform, meaning that it was closer to dinosaurs than the lagerpetids. Although it was not as close as silesaurids such as Silesaurus, Marasuchus is still one of the most completely known avemetatarsalians, assisting knowledge of the early evolution of dinosaur-like characteristics. The following is a cladogram of basal Dinosauriformes according to Nesbitt (2011), and Dinosauria according to Baron et al. (2017):
- Sereno, Paul C.; Arcucci, Andrea B. (March 1994). "Dinosaurian precursors from the Middle Triassic of Argentina: Marasuchus lilloensis, gen. nov". Journal of Vertebrate Paleontology. 14 (1): 53–73. doi:10.1080/02724634.1994.10011538.
- Claudia A. Marsicano; Randall B. Irmis; Adriana C. Mancuso; Roland Mundil; Farid Chemale (2016). "The precise temporal calibration of dinosaur origins". Proceedings of the National Academy of Sciences of the United States of America. 113 (3): 509–513. doi:10.1073/pnas.1512541112. PMC 4725541. PMID 26644579.
- Nesbitt, SJ (2011). "The early evolution of archosaurs: relationships and the origin of major clades". Bulletin of the American Museum of Natural History. 352: 1–292. doi:10.1206/352.1. ISSN 0003-0090.
- Baron, Matthew G.; Norman, David B.; Barrett, Paul M. "A new hypothesis of dinosaur relationships and early dinosaur evolution". Nature. 543: 501–506. doi:10.1038/nature21700.