Marasuchus(meaning "Maracrocodile ") is agenusof basaldinosauriformarchosaurwhich is possibly synonymous withLagosuchus.[1]Both genera lived during the LateTriassicin what is nowLa Rioja Province,Argentina.Marasuchuscontains a single species,Marasuchus lilloensis.[2]
| Marasuchus Temporal range:Late Triassic,
| |
|---|---|
| |
| Restored skeleton | |
Scientific classification
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauriformes |
| Genus: | †Marasuchus Sereno& Arcucci1994 |
| Species: | †M. lilloensis
|
| Binomial name | |
| †Marasuchus lilloensis | |
| Synonyms | |
| |
Marasuchus lilloensiswas originally designated asLagosuchus lilloensisin 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 ofLagosuchuswas undiagnostic. This would mean thatLagosuchusand its original species (Lagosuchus talampayaensis)could be considerednomen dubia.Specimens ofLagosuchusstored at a museum inSan Miguel de Tucumanwere 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 ofLagosuchusand concluded that it was valid and not readily distinguishable fromMarasuchus lilloensis.This suggests thatMarasuchus lilloensisis ajunior synonymofLagosuchus talampayensis.[1]
Specimens referred to the genusMarasuchuspossessed 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]
Discovery and history
editMarasuchushails from theChañares FormationofArgentina.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 ZoologyatHarvard) and the MLP (Museo de La PlatainLa Plata, Argentina). Discoveries made during the expedition were later described in a series of papers byAlfred Romerfrom the MCZ. Around the same time, further expeditions byJose Bonaparteof the PVL (Paleontología de Vertebrados, Instituto ‘Miguel Lillo’ inSan Miguel de Tucuman, Argentina) unearthed additional fossils from the area.[4][3][6]
Marasuchus lilloensisis 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. OtherMarasuchusfossils 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]
Relation toLagosuchus
edit
When the Tucuman specimen was first described by Romer in 1972, it was placed as thetype specimenof a new species,Lagosuchus lilloensis.Lagosuchus talampayensis,thetype speciesofLagosuchus,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. lilloensisas ajunior synonymofL. talampayensis.[6]Many other sources published between 1972 and 1994 followed this conclusion.[8][9]
However, a later study byPaul SerenoandAndrea Arcucci(1994) concluded that the original type specimen ofLagosuchuswas 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 fromLagosuchusspecimens stored elsewhere. On this basis, a new genus was erected to contain the PVL specimens. This new genus was calledMarasuchus( "Maracrocodile "), a nod to the etymology ofLagosuchus( "Rabbitcrocodile "). It was given thespecific nameMarasuchus lilloensis,based on Romer's species designation for the Tucuman specimen.[2]Between 1994 and 2019,Marasuchuswas considered a more diagnostic and well-described replacement name for the dinosauriformtaxonknown previously asLagosuchus.[10][7][11]
In 2019, the type specimen ofLagosuchuswas re-examined byFederico AgnolinandMartin 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 eitherontogeneticor individually variable. As a result, they referred the PVL specimens back toLagosuchus.Following their conclusion,Marasuchus lilloensiscould be considered a synonym ofLagosuchus talampayensisonce more.[1]
Description
edit
In terms of proportions,Marasuchusgenerally 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 probablybipedaland 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,Marasuchuswas still a small and lightly built animal.[3][2][1]
Skull
editSkull 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 plateson 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 describedsquamosalandquadratebones similar to those ofEuparkeriaattached to PVL 3872's braincase, although these were not mentioned by later studies.[6]
Vertebrae
editAlmost the entirety of thespinal columnis 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 vertebrawas 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]
Forelimbs
editThescapulocoracoid(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]
Hip
editThe 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 ofHerrerasaurusin 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 withsilesauridsandsaurischiandinosaurs. 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 inMarasuchusretains 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]
Hindlimbs
editModifications 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,Marasuchusalso 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, boxyastragalusand a smallercalcaneumattached 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. Thethirdandfourthmetatarsals were the longest, followed by thesecond,with thefirstandfifthbeing 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]
Classification
edit
Marasuchuswas 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 assilesauridssuch asSilesaurus,Marasuchusis still one of the most completely known avemetatarsalians, assisting knowledge of the early evolution of dinosaur-like characteristics. The following is acladogramof basal Dinosauriformes according to Nesbitt (2011),[7]and Dinosauria according to Baronet al.(2017):[12]
References
edit- ^abcdeAgnolin, Federico L.; Ezcurra, Martin D. (2019)."The Validity ofLagosuchus talampayensisRomer, 1971 (Archosauria, Dinosauriformes), from the Late Triassic of Argentina "(PDF).Breviora.565(1): 1–21.doi:10.3099/0006-9698-565.1.1.ISSN0006-9698.S2CID201949710.
- ^abcdefghijklmSereno, 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.Bibcode:1994JVPal..14...53S.doi:10.1080/02724634.1994.10011538.
- ^abcRomer, Alfred Sherwood (11 August 1972)."The Chañares (Argentina) Triassic reptile fauna. XV. Further remains of the thecodontsLagerpetonandLagosuchus".Breviora.394:1–7.
- ^abRomer, A. S. (1971)."The Chanares (Argentina) Triassic reptile fauna. X. Two new but incompletely known long-limbed pseudosuchians".Breviora.378:1–10.
- ^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.Bibcode:2016PNAS..113..509M.doi:10.1073/pnas.1512541112.PMC4725541.PMID26644579.
- ^abcJose, Bonaparte (1975)."Nuevos materiales deLagosuchus talampayensisRomer (Thecodontia-Pseudosuchia) y su significado en el origen de los Saurischia: Chañarense inferior, Triásico medio de Argentina "(PDF).Acta Geológica Lilloana.13(1): 5–90.
- ^abcdNesbitt, SJ (2011)."The early evolution of archosaurs: relationships and the origin of major clades"(PDF).Bulletin of the American Museum of Natural History.352:1–292.doi:10.1206/352.1.hdl:2246/6112.ISSN0003-0090.S2CID83493714.
- ^Arcucci, Andrea (1987)."Un nuevo Lagosuchidae (Thecodontia-Pseudosuchia) de la fauna de Los Chanares (edad reptil Chanarense, Triasico Medio), La Rioja, Argentina"(PDF).Ameghiniana.24(1–2): 89–94.
- ^Arcucci, Andrea B. (1989-11-01). "Locomotor structures in the Middle Triassic archosaurs from Los Chañares (La Rioja, Argentina)".Historical Biology.3(1–2): 85–95.Bibcode:1989HBio....3...85A.doi:10.1080/08912968909386515.ISSN0891-2963.
- ^Novas, Fernando E. (1996-12-31). "Dinosaur monophyly".Journal of Vertebrate Paleontology.16(4): 723–741.Bibcode:1996JVPal..16..723N.doi:10.1080/02724634.1996.10011361.ISSN0272-4634.JSTOR4523770.
- ^Ezcurra, Martín D. (2016-04-28)."The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms".PeerJ.4:e1778.doi:10.7717/peerj.1778.ISSN2167-8359.PMC4860341.PMID27162705.
- ^Baron, Matthew G.; Norman, David B.; Barrett, Paul M. (2017). "A new hypothesis of dinosaur relationships and early dinosaur evolution".Nature.543(7646): 501–506.Bibcode:2017Natur.543..501B.doi:10.1038/nature21700.PMID28332513.S2CID205254710.
