| Marasuchus | |
|---|---|
 | |
| Restored skeleton | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauriformes |
| Genus: | †Marasuchus Sereno & Arcucci1994 |
| Species: | †M. lilloensis |
| Binomial name | |
| †Marasuchus lilloensis | |
| Synonyms | |
| |
Marasuchus (meaning "Mara crocodile") is agenus of basaldinosauriformarchosaur which is possibly synonymous withLagosuchus.[1] Both genera lived during the LateTriassic in what is nowLa Rioja Province,Argentina.Marasuchus contains a single species,Marasuchus lilloensis.[2]
Marasuchus lilloensis was originally designated asLagosuchus lilloensis in 1972.[3] It was considered a new species ofLagosuchus, a contemporary archosaur described the previous year.[4] However, a 1994 study argued that the original material ofLagosuchus was undiagnostic. This would mean thatLagosuchus and its original species (Lagosuchus talampayaensis) could be considerednomen dubia. Specimens ofLagosuchus stored at a museum inSan Miguel de Tucuman were considered to be more diagnostic than those ofL. talampayensis, and thus they were given a new genus:Marasuchus.[2] A 2019 study redescribed the original material ofLagosuchus and concluded that it was valid and not readily distinguishable fromMarasuchus lilloensis. This suggests thatMarasuchus lilloensis is ajunior synonym ofLagosuchus talampayensis.[1]
Specimens referred to the genusMarasuchus 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 thehead of the femur. Nevertheless, it lacked certain dinosaur-like features such as aperforated acetabulum, and it had severalplesiomorphic ("primitive") features of the ankle.[2]
Marasuchus hails from theChañares Formation ofArgentina. This formation has been dated to the earlyCarnian (the first stage of theLate Triassic), about 235 to 234 million years old.[5] Many Chanares fossils, including the first known remains ofMarasuchus, were unearthed as a result of a 1964-1965 paleontological expedition. This expedition was undertaken by paleontologists from the MCZ (Museum of Comparative Zoology atHarvard) and the MLP (Museo de La Plata inLa Plata, Argentina). Discoveries made during the expedition were later described in a series of papers byAlfred Romer from the MCZ. Around the same time, further expeditions byJose Bonaparte of the PVL (Paleontología de Vertebrados, Instituto 'Miguel Lillo' inSan Miguel de Tucuman, Argentina) unearthed additional fossils from the area.[4][3][6]
Marasuchus lilloensis is known from several specimens representing most of the animal's skeletal anatomy, although skull material remains limited.[2][7] 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. OtherMarasuchus 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 associatedcervical (neck) 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 (trunk) vertebrae.[2] The referral of PVL 4670 toMarasuchus (orLagosuchus) is uncertain due to the absence of diagnostic features clearly shared with other specimens.[1]
When the Tucuman specimen was first described by Romer in 1972, it was placed as thetype specimen of a new species,Lagosuchus lilloensis.Lagosuchus talampayensis, thetype species ofLagosuchus, 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 ofL. talampayensis. In his 1975 review of the genus, Jose Bonaparte regarded the two species as synonymous, withL. lilloensis as ajunior synonym ofL. talampayensis.[6] Many other sources published between 1972 and 1994 followed this conclusion.[8][9]
However, a later study byPaul Sereno andAndrea Arcucci (1994) concluded that the original type specimen ofLagosuchus was poorly preserved and undiagnostic. They argued against assigning other specimens to the genusLagosuchus, which they regarded as anomen dubium. They also noted that many referred PVL specimens had limb proportions and other subtle traits differing fromLagosuchus specimens stored elsewhere. On this basis, a new genus was erected to contain the PVL specimens. This new genus was calledMarasuchus ("Mara crocodile"), a nod to the etymology ofLagosuchus ("Rabbit crocodile"). It was given thespecific nameMarasuchus lilloensis, based on Romer's species designation for the Tucuman specimen.[2] Between 1994 and 2019,Marasuchus was considered a more diagnostic and well-described replacement name for the dinosauriformtaxon known previously asLagosuchus.[10][7][11]
In 2019, the type specimen ofLagosuchus was re-examined byFederico Agnolin andMartin Ezcurra, who noted that the type specimen actually does share several of the diagnostic traits identified by Sereno and Arucci forMarasuchus. Furthermore, they argued that variations in size and certain features of the skeleton were eitherontogenetic or individually variable. As a result, they referred the PVL specimens back toLagosuchus. Following their conclusion,Marasuchus lilloensis could be considered a synonym ofLagosuchus talampayensis once more.[1]
In terms of proportions,Marasuchus generally resembled early theropod dinosaurs likeCoelophysis. The limbs were long and slender, with the hindlimbs about twice the length of the forelimbs. These proportions meant that it was probablybipedal 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 ofMarasuchus (PVL 3871) had a femur which was 5.75 cm (2.26) inches in length, about 40-50% larger than the type specimen ofLagosuchus talampayensis. Nevertheless,Marasuchus was still a small and lightly built animal.[3][2][1]
Skull material is very limited forMarasuchus, with the only preserved bones from this region being amaxilla (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. 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 possessedinterdental 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, theexoccipitals (a pair of braincase bones adjacent to theforamen magnum, the main exit for thespinal cord) were wide and edged by a pronounced ridge next to the exit holes for thehypoglossal nerve.[2] Bonaparte (1975) additionally describedsquamosal andquadrate bones similar to those ofEuparkeria attached to PVL 3872's braincase, although these were not mentioned by later studies.[6]
Almost the entirety of thespinal column is present inMarasuchus, barring the tip of the tail. Most ofMarasuchus' 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 theaxis 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 toMarasuchus, 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 thesacrum. 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.[2]
Thescapulocoracoid (shoulder blade) was quite large and broad unlike most other avemetatarsalians. On the other hand, theglenoid (shoulder socket) was directed somewhat backwards (rather than sideways), as is the case with other dinosauriforms. The forelimb bones (consisting of ahumerus,ulna, andradius) 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.[2]
The pelvis (hip) shared quite a few similarities with other dinosauriforms not otherwise present in earlier archosauriforms. Theilium (upper blade of the hip) was similar to that ofHerrerasaurus in general shape. Thepubis (front lower blade of the hip) was longer than theischium (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 withsilesaurids andsaurischian 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 entireacetabulum (hip socket). However, this had not yet evolved inMarasuchus, which retains a bony inner wall of the acetablum. Moreover, the edge of the ischium inMarasuchus 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.[2][7]
Modifications to the acetabulum are mirrored in thehead 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 ofDromomeron, and a similar structure is also present inaphanosaurs, albeit separate from their equivalent of the anterior trochanter. As with other dinosauriforms, thetibia (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.[2]
The ankle had two main bones: the larger, boxyastragalus and a smallercalcaneum 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 orcoelophysoids, the astragalus and calcaneum were not fused together. The fivemetatarsals (foot bones) were thin, elongated, and close together. Thethird andfourth metatarsals were the longest, followed by thesecond, with thefirst andfifth being only about half the length of the longest. Although not all of the pedalphalanges (toe bones) were preserved, the phalangeal formula (number of bones per toe) was likely 2-3-4-5-0 as with other dinosauromorphs.[2]
Marasuchus was part ofAvemetatarsalia, the branch of archosaurs closer to birds and other dinosaurs rather than to crocodilians. More specifically, it was adinosauriform, meaning that it was closer to dinosaurs than the lagerpetids. Although it was not as close assilesaurids such asSilesaurus,Marasuchus is still one of the most completely known avemetatarsalians, assisting knowledge of the early evolution of dinosaur-like characteristics. The following is acladogram of basal Dinosauriformes according to Nesbitt (2011),[7] and Dinosauria according to Baronet al. (2017):[12]
_(Antwerpener_Breiftaube).jpg)
