Marasuchus
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''Marasuchus'' (meaning "
Mara Mara or MARA may refer to: Arts and entertainment Fictional characters * Mara (''Doctor Who''), an evil being in two ''Doctor Who'' serials * Mara (She-Ra), fictional characters from the ''She-Ra and the Princesses of Power'' and ''The New Advent ...
crocodile") is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of basal dinosauriform
archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...
which is possibly synonymous with '' Lagosuchus''. Both genera lived during the Late
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
in what is now La Rioja Province,
Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
. ''Marasuchus'' contains a single species, ''Marasuchus lilloensis''. ''Marasuchus lilloensis'' was originally designated as ''Lagosuchus lilloensis'' in 1972. It was considered a new species of ''Lagosuchus'', a contemporary archosaur described the previous year. However, a 1994 study argued that the original material of ''Lagosuchus'' was undiagnostic. This would mean that ''Lagosuchus'' and its original species (''Lagosuchus talampayaensis)'' could be considered nomen dubia. Specimens of ''Lagosuchus'' stored at a museum in San Miguel de Tucuman were considered to be more diagnostic than those of ''L. talampayensis'', and thus they were given a new genus: ''Marasuchus''. A 2019 study redescribed the original material of ''Lagosuchus'' and concluded that it was valid and not readily distinguishable from ''Marasuchus lilloensis''. This suggests that ''Marasuchus lilloensis'' is a
junior synonym In taxonomy, the scientific classification of living organisms, a synonym is an alternative scientific name for the accepted scientific name of a taxon. The botanical and zoological codes of nomenclature treat the concept of synonymy differently. ...
of ''Lagosuchus talampayensis''. Specimens referred to the genus ''Marasuchus'' 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 the head of the femur. Nevertheless, it lacked certain dinosaur-like features such as a perforated acetabulum, and it had several
plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, an ...
("primitive") features of the ankle.


Discovery and history

''Marasuchus'' hails from the Chañares Formation of
Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
. This formation has been dated to the early
Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...
(the first stage of the
Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
), about 235 to 234 million years old. Many Chanares fossils, including the first known remains of ''Marasuchus'', were unearthed as a result of a 1964-1965 paleontological expedition. This expedition was undertaken by paleontologists from the MCZ (
Museum of Comparative Zoology The Museum of Comparative Zoology (formally the Agassiz Museum of Comparative Zoology and often abbreviated to MCZ) is a zoology museum located on the grounds of Harvard University in Cambridge, Massachusetts. It is one of three natural-history r ...
at
Harvard Harvard University is a private Ivy League research university in Cambridge, Massachusetts, United States. Founded in 1636 and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher lear ...
) and the MLP ( Museo de La Plata in
La Plata, Argentina La Plata () is the capital city of Buenos Aires province, Argentina. According to the 2022 Argentina census, census, the La Plata Partido, Partido has a population of 772,618 and its metropolitan area, the Greater La Plata, has 938,287 inhabit ...
). Discoveries made during the expedition were later described in a series of papers by Alfred Romer from the MCZ. Around the same time, further expeditions by Jose Bonaparte of the PVL (Paleontología de Vertebrados, Instituto ‘Miguel Lillo’ in San Miguel de Tucuman, Argentina) unearthed additional fossils from the area. ''Marasuchus lilloensis'' is known from several specimens representing most of the animal's skeletal anatomy, although skull material remains limited. 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. Other ''Marasuchus'' 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 associated cervical (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. The referral of PVL 4670 to ''Marasuchus'' (or ''Lagosuchus'') is uncertain due to the absence of diagnostic features clearly shared with other specimens.


Relation to ''Lagosuchus''

When the Tucuman specimen was first described by Romer in 1972, it was placed as the
type specimen In biology, a type is a particular wikt:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to ancho ...
of a new species, ''Lagosuchus lilloensis''. ''Lagosuchus talampayensis'', the
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
of ''Lagosuchus'', 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 of ''L. talampayensis''. In his 1975 review of the genus, Jose Bonaparte regarded the two species as synonymous, with ''L. lilloensis'' as a
junior synonym In taxonomy, the scientific classification of living organisms, a synonym is an alternative scientific name for the accepted scientific name of a taxon. The botanical and zoological codes of nomenclature treat the concept of synonymy differently. ...
of ''L. talampayensis''. Many other sources published between 1972 and 1994 followed this conclusion. However, a later study by
Paul Sereno Paul Callistus Sereno (born October 11, 1957) is a professor of paleontology at the University of Chicago who has discovered several new dinosaur species on several continents, including at sites in Inner Mongolia, Argentina, Morocco and Niger. ...
and Andrea Arcucci (1994) concluded that the original type specimen of ''Lagosuchus'' was poorly preserved and undiagnostic. They argued against assigning other specimens to the genus ''Lagosuchus'', which they regarded as a ''
nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
''. They also noted that many referred PVL specimens had limb proportions and other subtle traits differing from ''Lagosuchus'' specimens stored elsewhere. On this basis, a new genus was erected to contain the PVL specimens. This new genus was called ''Marasuchus'' ("
Mara Mara or MARA may refer to: Arts and entertainment Fictional characters * Mara (''Doctor Who''), an evil being in two ''Doctor Who'' serials * Mara (She-Ra), fictional characters from the ''She-Ra and the Princesses of Power'' and ''The New Advent ...
crocodile"), a nod to the etymology of ''Lagosuchus'' ("
Rabbit Rabbits are small mammals in the family Leporidae (which also includes the hares), which is in the order Lagomorpha (which also includes pikas). They are familiar throughout the world as a small herbivore, a prey animal, a domesticated ...
crocodile"). It was given the
specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...
''Marasuchus lilloensis'', based on Romer's species designation for the Tucuman specimen. Between 1994 and 2019, ''Marasuchus'' was considered a more diagnostic and well-described replacement name for the dinosauriform
taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
known previously as ''Lagosuchus''. In 2019, the type specimen of ''Lagosuchus'' was re-examined by Federico Agnolin and
Martin Ezcurra Martin may refer to: Places Antarctica * Martin Peninsula, Marie Byrd Land * Port Martin, Adelie Land * Point Martin, South Orkney Islands Europe * Martin, Croatia, a village * Martin, Slovakia, a city * Martín del Río, Aragón, Spain * M ...
, who noted that the type specimen actually does share several of the diagnostic traits identified by Sereno and Arucci for ''Marasuchus''. Furthermore, they argued that variations in size and certain features of the skeleton were either
ontogenetic Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
or individually variable. As a result, they referred the PVL specimens back to ''Lagosuchus''. Following their conclusion, ''Marasuchus lilloensis'' could be considered a synonym of ''Lagosuchus talampayensis'' once more.


Description

In terms of proportions, ''Marasuchus'' generally resembled early theropod dinosaurs like ''
Coelophysis ''Coelophysis'' ( Traditional English pronunciation of Latin, traditionally; or , as heard more commonly in recent decades) is a genus of coelophysid Theropoda, theropod dinosaur that lived Approximation, approximately 215 to 201.4 million y ...
''. The limbs were long and slender, with the hindlimbs about twice the length of the forelimbs. These proportions meant that it was probably
bipedal Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' ...
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 of ''Marasuchus'' (PVL 3871) had a femur which was 5.75 cm (2.26) inches in length, about 40-50% larger than the type specimen of ''
Lagosuchus talampayensis ''Lagosuchus'' is an extinct genus of avemetatarsalian archosaur from the Late Triassic of Argentina. The type species of ''Lagosuchus'', ''Lagosuchus talampayensis'', is based on a small partial skeleton recovered from the early Carnian-age Cha ...
.'' Nevertheless, ''Marasuchus'' was still a small and lightly built animal.


Skull

Skull material is very limited for ''Marasuchus'', with the only preserved bones from this region being a
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
(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 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 foramen magnum () is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes thro ...
, the main exit for the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...
) were wide and edged by a pronounced ridge next to the exit holes for the
hypoglossal nerve The hypoglossal nerve, also known as the twelfth cranial nerve, cranial nerve XII, or simply CN XII, is a cranial nerve that innervates all the extrinsic and intrinsic muscles of the tongue except for the palatoglossus, which is innervated b ...
. Bonaparte (1975) additionally described
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...
and quadrate bones similar to those of ''
Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W. K. Parker) is an Extinction, extinct genus of archosauriform reptile from the Triassic of South Africa. ''Euparkeria'' is close to the ancestry of Archosauria, the reptile gro ...
'' attached to PVL 3872's braincase, although these were not mentioned by later studies.


Vertebrae

Almost the entirety of the
spinal column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmen ...
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 sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...
. 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.


Forelimbs

The
scapulocoracoid The scapulocoracoid is the unit of the pectoral girdle that contains the coracoid and scapula. The coracoid itself is a beak-shaped bone that is commonly found in most vertebrates with a few exceptions. The scapula is commonly known as the ''shoulde ...
(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 The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...
,
ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...
, and
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
) 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.


Hip

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 ''Herrerasaurus'' is likely a genus of saurischian dinosaur from the Late Triassic period. Measuring long and weighing around , this genus was one of the earliest dinosaurs from the fossil record. Its name means "Herrera's lizard", after the ...
'' in general shape. The pubis (front lower blade of the hip) was longer than the
ischium The ischium (; : is ...
(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
saurischia Saurischia ( , meaning "reptile-hipped" from the Greek ' () meaning 'lizard' and ' () meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithi ...
n 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 The acetabulum (; : acetabula), also called the cotyloid cavity, is a wikt:concave, concave surface of the pelvis. The femur head, head of the femur meets with the pelvis at the acetabulum, forming the Hip#Articulation, hip joint. Structure The ...
(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.


Hindlimbs

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 ''Dromomeron'' (meaning "running femur") is a genus of Lagerpetidae, lagerpetid Avemetatarsalia, avemetatarsalian which lived around 220 to 211.9 ± 0.7 million years ago. The genus contains species known from Late Triassic-age rocks of the South ...
'', and a similar structure is also present in aphanosaurs, albeit separate from their equivalent of the anterior trochanter. As with other dinosauriforms, the
tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...
(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 Astragalus may refer to: * ''Astragalus'' (plant), a large genus of herbs and small shrubs *Astragalus (bone) The talus (; Latin for ankle or ankle bone; : tali), talus bone, astragalus (), or ankle bone is one of the group of foot bones known ...
and a smaller
calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is t ...
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 Third or 3rd may refer to: Numbers * 3rd, the ordinal form of the cardinal number 3 * , a fraction of one third * 1⁄60 of a ''second'', i.e., the third in a series of fractional parts in a sexagesimal number system Places * 3rd Street (di ...
and fourth metatarsals were the longest, followed by the
second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
, with the first and fifth being only about half the length of the longest. Although not all of the pedal
phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...
(toe bones) were preserved, the phalangeal formula (number of bones per toe) was likely 2-3-4-5-0 as with other dinosauromorphs.


Classification

''Marasuchus'' was part of
Avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid Reptile, reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosau ...
, 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 A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
of basal Dinosauriformes according to Nesbitt (2011), and Dinosauria according to Baron ''et al.'' (2017):


References

{{Taxonbar, from=Q133233 Dinosauromorpha Late Triassic reptiles of South America Triassic Argentina Fossils of Argentina Chañares Formation Fossil taxa described in 1994 Taxa named by Paul Sereno