Lagosuchus
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''Lagosuchus'' is an extinct
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
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 ...
n
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 ...
from 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 ...
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 ...
. 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'', ''Lagosuchus talampayensis'', is based on a small partial skeleton recovered from 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 ...
-age Chañares Formation. The holotype skeleton of ''L. talampayensis'' is fairly fragmentary, but it does possess some traits suggesting that ''Lagosuchus'' was a probable dinosauriform, closely related to
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s. A second potential species of ''Lagosuchus'', ''L. lilloensis'', is based on an assortment of slightly larger and more well-preserved fossils. These larger specimens have been considered much more diagnostic and informative than the original small ''L. talampayensis'' skeleton. As a result, some paleontologists have placed the larger specimens into a new genus, ''
Marasuchus ''Marasuchus'' (meaning "Mara (mammal), Mara crocodile") is a genus of basal Dinosauriformes, dinosauriform archosaur which is possibly synonymous with ''Lagosuchus''. Both genera lived during the Late Triassic in what is now La Rioja Province, A ...
''. ''Marasuchus'' is generally considered one of the more complete early dinosauriforms, useful for estimating ancestral traits for the origin of dinosaurs. This would also render ''Lagosuchus'' 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 ...
, simply a name referring to a fossil which is too fragmentary to have a formal genus. However, other paleontologists support the argument that ''Lagosuchus'' is a valid genus, and that ''Marasuchus'' 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 it.


History

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 ...
''Lagosuchus talampayensis'' was first described by Alfred S. Romer in 1971, who considered it a "
pseudosuchia Pseudosuchia, from Ancient Greek ψεύδος (''pseúdos)'', meaning "false", and σούχος (''soúkhos''), meaning "crocodile" is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely relat ...
n" (then a collection of various non-dinosaurian " thecodonts"). In 1972 he named a second species, '' Lagosuchus lilloensis'', known from larger and more well-preserved specimens. A later review by Jose Bonaparte in 1975 synonymized the two species and considered ''Lagosuchus'' intermediate between "pseudosuchians" 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. Modern authors now consider at least ''L. lilloensis'' to be firmly on the lineage of archosaurs leading to dinosaurs. However, the genus ''Lagosuchus'' is regarded by some to be dubious.
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 considered ''L. talampayensis'' to be undiagnosable in a 1994 study, and reclassified ''L. lilloensis'' as a new genus, ''
Marasuchus ''Marasuchus'' (meaning "Mara (mammal), Mara crocodile") is a genus of basal Dinosauriformes, dinosauriform archosaur which is possibly synonymous with ''Lagosuchus''. Both genera lived during the Late Triassic in what is now La Rioja Province, A ...
.'' In 2019, PULR 09, the holotype skeleton of ''L. talampayensis'', was redescribed 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 ...
. They argued that the skeleton was not only diagnostic, but indistinguishable from specimens of ''Marasuchus lilloensis''. As a result, they supported the synonymy proposed by Bonaparte, referring specimens of ''Marasuchus lilloensis'' back to ''Lagosuchus talampayensis''. The Chañares Formation's age has been through much debate. It has traditionally been considered to belong to the
Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...
stage, the last stage of the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...
.
Radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...
in 2016 has dated the main fossiliferous section of the formation 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 ...
stage, near the start 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 ...
.


Description

''Lagosuchus talampayensis'', in its most restricted form, can only be described based on the incomplete holotype skeleton. It was a lightly built archosaur, with long, slender legs and well-developed feet - features it shares with certain dinosaurs. With its short forelimbs, long shin bones, and narrow stance, it was likely an agile
biped 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'' ' ...
adapted for running.
Gregory S. Paul Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology. He is best known for his work and research on theropoda, theropod dinosaurs and his detailed illustrations, both l ...
inferred that ''Lagosuchus'' was one of the smallest Triassic archosaurs, with a weight of about 167 g, similar in size and ecology to a
weasel Weasels are mammals of the genus ''Mustela'' of the family Mustelidae. The genus ''Mustela'' includes the least weasels, polecats, stoats, ferrets, and European mink. Members of this genus are small, active predators, with long and slend ...
or
ferret The ferret (''Mustela furo'') is a small, domesticated species belonging to the family Mustelidae. The ferret is most likely a domesticated form of the wild European polecat (''Mustela putorius''), as evidenced by the ferret's ability to inter ...
. Thomas Holtz estimated that ''Lagosuchus'' could have obtained a total length of 1.7 ft (51 cm) and a weight similar to that of a
pigeon Columbidae is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with small heads, relatively short necks and slender bills that in some species feature fleshy ceres. ...
(50-500 g).Holtz, Thomas R. Jr. (2012) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'
Winter 2011 Appendix.
/ref> However, these estimates may have been based on specimens referred to ''Marasuchus,'' some of which were significantly larger than the ''Lagosuchus'' holotype.


Vertebrae and forelimbs

The
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...
(trunk)
vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
had centra about three times longer than tall, slightly more elongated than those referred to ''Marasuchus''. Yet the dorsals also had many traits in common with ''Marasuchus'', such as large excavations below the
transverse processes Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
. In addition, they both have trapezoidal
neural spines Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
with thickened upper edges which expand forwards and backwards to contact those of adjacent vertebrae. The contemporary silesaurid '' Lewisuchus'' has a similar neural spine morphology, but it possesses
osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amph ...
s on its neural spines, unlike ''Lagosuchus'' and ''Marasuchus''. The two sacral (hip) vertebrae had large and slightly tapering transverse processes. This is also the case for the first four caudal (tail) vertebrae at the base of the tail. Further back, the caudals have much shorter transverse processes and more elongated centra, like those of ''Marasuchus''. The
scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...
(shoulder blade) was narrow, with a slightly expanded upper tip and a thick longitudinal ridge on its inner surface. The
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 ...
was also quite narrow, with a subtriangular deltopectoral crest in its upper part. The deltopectoral crest extends about 31% down the length of the humerus, making it somewhat less extensive than that of other
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 ...
ns (including ''Marasuchus''). The
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 ...
and
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 ...
are thin, simple, and unusually short, only about 65% the length of the humerus. In contrast, ''Marasuchus'' has a larger ulna with a strong olecranon process. The forelimbs in general were much smaller than the hindlimbs.


Hip and hindlimbs

The
pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...
(hip) is similar to that of ''Marasuchus'', with a thin pubis and a plate-like
ischium The ischium (; : is ...
which has a large ridge on its rear edge. The
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
is elongated and has a slightly inturned femoral head. The femoral head is characteristically 'globose', with a strong projecting convex surface stretching up along its inner and upper portions. This trait is shared with ''Marasuchus'' and lagerpetids. The femur also possessed a knob-like anterior trochanter and a distinct
fourth trochanter The fourth trochanter is a shared characteristic common to archosaurs. It is a protrusion on the posterior-medial side of the middle of the femur shaft that serves as a muscle attachment, mainly for the '' musculus caudofemoralis longus'', the m ...
. 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 ...
and
fibula The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...
were narrow and about 10% longer than the femur. Like other dinosauriforms, the tibia had a strong cnemial crest at the knee and a lateral groove near the ankle. The ankle has a small distal tarsal 3, a larger distal tarsal 4, and a rounded
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 ...
, but the
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 ...
is missing from the fossil. The middle three metatarsals are elongated, with metatarsal III as the longest. Metatarsal V is short and tapers to a sharp tip. Metatarsal I is also short, though still longer than V (unlike ''Marasuchus'').
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. ...
are slender, and an isolated pedal ungual (toe claw) is weakly curved and flattened sideways.


Palaeobiology


Metabolism

It is believed that ''Lagosuchus'' and ''Marasuchus'' were transitional between cold blooded reptiles and warm blooded dinosaurs.


References

{{Taxonbar, from=Q132937 Dinosauromorpha Late Triassic reptiles of South America Triassic Argentina Fossils of Argentina Chañares Formation Taxa described in 1971 Taxa named by Alfred Romer