Marasuchus

Initially, the first specimens of Marasuchus were assigned as a new species of the genus Lagosuchus as L. lilloensis, together with the type species L. talampayensis. In a review of the genus, paleontologist Jose Bonaparte in 1975 regarded the two species as synonymous, with L. lilloensis as the junior synonym of L. talampayensis. However, a later restudy by Sereno and Arcucci (1994) concluded that the original type specimen of Lagosuchus was too poorly preserved to allow any further specimens to be assigned to the genus, and regarded it as a nomen dubium. They also noted that the L. lilloensis specimen had limb proportions different from the type species. On this basis, they erected the new genus Marasuchus for "L." lilloensis.[1]

In 2019, the type specimen of Lagosuchus was re-examined by Federico Agnolin and Martin Ezcurra, who noted that the type specimen in fact shares several of the diagnostic traits identified by Sereno and Arucci for Marasuchus. Furthermore, they identified variations in size and certain features of the skeleton as being either ontogenetic or individually variable. Based on this, they considered Lagosuchus to be a senior synonym of Marasuchus.[4]

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.[1] 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.[1]

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.[1]

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.[3]

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.[1]

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.[1]