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.
Sereno & Arcucci 1994
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):
Aphanosauria ("hidden lizards") is group of reptiles distantly related to dinosaurs (including birds). They were at the base of a group known as Avemetatarsalia, one of two main branches of archosaurs. The other main branch, Pseudosuchia, includes modern crocodilians. Aphanosaurs possessed features from both groups, indicating that they are the oldest and most primitive known clade of avemetatarsalians, at least in terms of their position on the archosaur family tree. Other avemetatarsalians include the flying pterosaurs, small bipedal lagerpetids, herbivorous silesaurids, and the incredibly diverse dinosaurs, which survive to the present day in the form of birds. Aphanosauria is formally defined as the most inclusive clade containing Teleocrater rhadinus and Yarasuchus deccanensis but not Passer domesticus (House sparrow) or Crocodylus niloticus (Nile crocodile). This group was first recognized during the description of Teleocrater. Although only known by a few genera, Aphanosaurs had a widespread distribution across Pangaea in the Middle Triassic.They were fairly slow quadrupedal long-necked carnivores, a biology more similar to basal archosaurs than to advanced avemetatarsalians such as pterosaurs, lagerpetids, and early dinosaurs. In addition, they seemingly possess 'crocodile-normal' ankles (with a crurotarsal joint), showing that 'advanced mesotarsal' ankles (the form acquired by many dinosaurs, pterosaurs, lagerpetids, and advanced silesaurids) were not basal to the whole clade of Avemetatarsalia. Nevertheless, they possessed elevated growth rates compared to their contemporaries, indicating that they grew quickly, more like birds than modern reptiles. Despite superficially resembling lizards, the closest modern relatives of aphanosaurs are birds.Avemetatarsalia
Avemetatarsalia (meaning "bird metatarsals") is a clade name established by British palaeontologist Michael Benton in 1999 for all crown group archosaurs that are closer to birds than to crocodilians. An alternate name is Pan-Aves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more closely related to birds than to crocodilians. Almost all avemetatarsalians are members of a similarly defined subgroup, Ornithodira. Ornithodira is defined as the last common ancestor of dinosaurs and pterosaurs, and all of its descendants.Members of this group include the Dinosauromorpha, Pterosauromorpha, the genus Scleromochlus, and Aphanosauria. Dinosauromorpha contains more basal forms, including Lagerpeton and Marasuchus, as well as more derived forms, including dinosaurs. Birds belong to the dinosaurs as members of the theropods. Pterosauromorpha contains Pterosauria, which were the first vertebrates capable of true flight. Aphanosauria is a Triassic group of gracile carnivorous quadrupeds which was recognized in 2017.Averostra
Averostra, or "bird snouts", is a clade that includes most theropod dinosaurs that have a promaxillary fenestra (fenestra promaxillaris), an extra opening in the front outer side of the maxilla, the bone that makes up the upper jaw. Two groups of averostrans, the Ceratosauria and the Orionides, survived into the Cretaceous period. When the Cretaceous–Paleogene extinction event occurred, ceratosaurians and two groups of orionideans within the clade Coelurosauria, the Tyrannosauroidea and Maniraptoriformes, were still extant. Only one subgroup of maniraptoriformes, Aves, survived the extinction event and persisted to the present day.Avetheropoda
Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs.Chañares Formation
The Chañares Formation is a geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Provence, Argentina. The claystones and tuffs of the formation date to the Carnian stage of the Late Triassic and were deposited in a fluvial to lacustrine environment.
The formation represents the onset of the first syn-rift phase in the Triassic rift basin and is the lowermost stratigraphic unit of the Agua de la Peña Group, unconformably overlying the Tarjados Formation of the Paganzo Group. The Chañares Formation is overlain by the Ischichuca Formation and both formations have a combined maximum thickness of 750 metres (2,460 ft).
The Chañares Formation has provided a rich faunal assemblage, including many of the earliest crocodylomorph fossils, as Tropidosuchus, Chanaresuchus, and Gualosuchus, as well as other archosaurs; Lewisuchus admixtus, Lagerpeton, Marasuchus lilloensis, Gracilisuchus, Luperosuchus and Pseudolagosuchus major. Cynodonts are represented by Probainognathus and Massetognathus and other therapsids include Dinodontosaurus.Dinosauriformes
Dinosauriformes is a clade of archosaurian reptiles that include the dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is Asilisaurus, dating to about 245 million years ago in the Anisian age of the middle Triassic period.Dinosauromorpha
Dinosauromorpha is a clade of archosaurs that includes the clade Dinosauria (dinosaurs), and all animals more closely related to dinosaurs than to pterosaurs. Birds are the only surviving dinosauromorphs.Eodromaeus
Eodromaeus (meaning "dawn runner") was a genus of basal theropod dinosaur known from the Late Triassic (Carnian) Valle de la Luna Member of the Ischigualasto Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina. It has been cited by Sereno as resembling a supposed common ancestor to all dinosaurs, the "Eve" of the dinosaurs.Epipophyses
Epipophyses are bony projections of the cervical vertebrae found in archosauromorphs, particularly dinosaurs (including some basal birds). These paired processes sit above the postzygapophyses on the rear of the vertebral neural arch. Their morphology is variable and ranges from small, simple, hill-like elevations to large, complex, winglike projections. Epipophyses provided large attachment areas for several neck muscles; large epipophyses are therefore indicative of a strong neck musculature.The presence of epipophyses is a synapomorphy (distinguishing feature) of the group Dinosauria. Epipophyses were present in the basal-most dinosaurs, but absent in the closest relatives of the group, such as Marasuchus and Silesaurus. They were typical for most dinosaur lineages; however, they became lost in several derived theropod lineages in the wake of an increasingly S-shaped curvature of the neck.Several scientific papers have observed that epipophyses were present in various non-dinosaur archosauromorphs. These include several pseudosuchians (Batrachotomus, Revueltosaurus, Xilousuchus, Effigia, Hesperosuchus), basal avemetatarsalians (aphanosaurs) non-archosaur archosauriforms (Vancleavea, Halazhaisuchus), rhynchosaurs, several tanystropheids, and allokotosaurs. Sauropod-oriented paleontologist Mike Taylor has informally suggested that epipophyses were also present in the vertebrae of certain pterosaurs.Hyposphene-hypantrum articulation
The hyposphene-hypantrum articulation is an accessory joint found in the vertebrae of several fossil reptiles of the group Archosauromorpha. It consists of a process on the backside of the vertebrae, the hyposphene, that fits in a depression in the front side of the next vertebrae, the hypantrum. Hyposphene-hypantrum articulations occur in the dorsal vertebrae and sometimes also in the posteriormost cervical and anteriormost caudal vertebrae.In most tetrapods including the human, the vertebrae are connected with each other only via the centrum and the zygapophysis joints. Additional joints like the hyposphene-hypantrum articulations, which add rigidity to the vertebral column, are found in several different reptile lineages; a known example are the zygosphene-zygantrum articulations found in snakes.Hyposphene-hypantrum articulations are found in several unrelated groups within the Archosauromorpha. They occur especially in large forms, for example in rauisuchids and in silesaurids and – within the Dinosauria – in saurischians. They evolved to make the vertebral column more rigid and stable and probably had supported the gigantism in sauropod dinosaurs.Early Dinosauromorphs (early ancestors of dinosaurs) like Marasuchus, Lagosuchus and Euparkeria as well as ornithischian dinosaurs lack hyposphene-hypantrum articulations. Because these articulations are absent in both saurischian ancestors and all non-saurischian dinosaurs, they are considered a synapomorphy (a distinctive feature) of the Saurischia, as proposed by Gauthier (1986). Hyposphene-hypantrum articulations are found in all the basal members of the Saurischia. However, they became lost in several saurischian lineages. They were present in the derived and birdlike dromaeosaurid Rahonavis, but are lost in modern day's birds, probably due to their highly modified vertebrae. Within the Sauropodomorpha, they were present in prosauropods and most sauropods, but became independently lost in two cretaceous sauropod lineages, the Titanosauria and the Rebbachisauridae.The hyposphene usually consists of a vertical ridge and is situated below the postzygapophysis, whereas the hypantrum is situated between the prezygapophysis. In sauropods, this joint is extremely variable.Ixalerpeton
Ixalerpeton (meaning "leaping reptile") is a genus of small, bipedal dinosauromorphs in the lagerpetid family, containing one species, I. polesinensis. It lived in the Late Triassic of Brazil alongside the sauropodomorph dinosaur Buriolestes.Lagerpetidae
The Lagerpetidae (; originally Lagerpetonidae) is a family of basal dinosauromorphs. Members of the family are known from Late Triassic of Argentina, Arizona, Brazil, New Mexico, and Texas. Lagerpetids were typically small, although some, like Dromomeron gigas, were fairly large. Lagerpetid fossils are very rare, the most common finds are of the hindlimbs, which possessed a number of unique features.Lagosuchus
Lagosuchus is a genus of small avemetatarsalian archosaurs from the middle Triassic period. It is generally thought to be closely related to dinosaurs, as a member of the Dinosauromorpha. Its fossils were found in the Chañares Formation of Argentina, the dating of which is uncertain; some sources date it to the Middle Triassic whilst others date it to the earliest Carnian.Lutungutali
Lutungutali (meaning "high hip" in the Bemba language) is an extinct genus of silesaurid dinosauriform from the Middle Triassic of Zambia. The single type species of the genus is Lutungutali sitwensis. Lutungutali was named in 2013 and described from a fossil specimen, holotype NHCC LB32, including hip bones and tail vertebrae. The specimen was collected in 2009 from the upper Ntawere Formation, which dates to the Anisian stage of the Middle Triassic. Lutungutali is the first known silesaurid from Zambia and, along with the Tanzanian silesaurid Asilisaurus and dinosauriform Nyasasaurus, the oldest bird-line archosaur known from body fossils (i.e. parts of the skeleton).Orionides
Orionides is a clade of tetanuran theropod dinosaurs from the Middle Jurassic to the Present. The clade includes most theropod dinosaurs, including birds.Pseudolagosuchus
Pseudolagosuchus (meaning "false Lagosuchus") is a genus of dinosauromorph from the Middle Triassic (Ladinian) Chañares Formation of Argentina. It may be a junior synonym of Lewisuchus, but there is very little overlapping material. It was a small reptile which was probably about 1 meter (3.3 ft) long, 30 centimeters (1 ft) tall, and weighed approximately 2 kilograms (4.4 lb). It is known only from a pubis, a femur, a tibia, and vertebrae. Both Sterling Nesbitt, Christian Sidor et al. (2010) and Matthew Baron, David Norman and Paul Barrett (2017) treated this taxon as being synonymous with Lewisuchus.Silesauridae
Silesauridae is an extinct clade of Triassic dinosauriformes consisting of the closest known relatives of dinosaurs. As indicated by coprolite contents, some silesaurids such as Silesaurus may have been insectivorous, feeding selectively on small beetles and other arthropods.Silesaurus
Silesaurus is a genus of silesaurid dinosauriform from the Late Triassic, approximately 230 million years ago in the Carnian faunal stage of what is now Poland.
Fossilized remains of Silesaurus have been found in the Keuper Claystone in Krasiejów near Opole, Silesia, Poland, which is also the origin of its name. The type species, Silesaurus opolensis, was described by Jerzy Dzik in 2003. It is known from some 20 skeletons, making it one of the best-represented of the early dinosauriformes.