Lagerpeton
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''Lagerpeton'' is a genus of lagerpetid
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, comprising a single species, ''L. chanarensis''. First described from the Chañares Formation of Argentina by A. S. Romer in 1971, ''Lagerpeton'' anatomy is somewhat incompletely known, with fossil specimens accounting for the
pelvic girdle The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the Ilium (bone) ...
,
hindlimb A hindlimb or back limb is one of the paired articulated appendages ( limbs) attached on the caudal ( posterior) end of a terrestrial tetrapod vertebrate's torso.http://www.merriam-webster.com/medical/hind%20limb, Merriam Webster Dictionary-H ...
s, posterior presacral, sacral and anterior caudal vertebrae. Skull and shoulder material has also been described. The name comes from the
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
(, "hare") plus (, "reptile").


Discovery

''Lagerpeton'' fossils have only been collected from the Chañares Formation in
La Rioja Province, Argentina La Rioja (), officially Province of La Rioja is a province of Argentina located in the west of the country. The landscape of the province consists of a series of arid to semi-arid mountain ranges and agricultural valleys in between. It is in one ...
. The first of these fossils were discovered in a 1964-1965 expedition by the
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 ...
(MCZ) and Museo de la Plata (MLP), although some were also discovered in 1966 by paleontologists from the Miguel Lillo Institute (PVL) of the University of Tucuman. Alfred Romer named ''Lagerpeton chanarensis'' in 1971, based on a complete right hindlimb discovered during the MCZ-MLP expedition. The specimen was initially stored at the Museo de la Plata with catalogue number MLP 64-XI-14-10, but by 1986 it had been transferred to the Paleontology Museum at the National University of La Rioja (PULR) and given the designation PULR 06, though some studies alternatively call it UPLR 06 or UNLR 06. Some of the foot bones from this specimen have gone missing. Romer also mentioned PVL material collected by Jose Bonaparte. In 1972, Romer described MCZ 4121, which was a specimen smaller than the holotype. It was preserved in a nodule alongside the holotype of '' Lewisuchus admixtus'' and a few '' Lagosuchus'' bones. MCZ 4121 represents a few vertebrae, a pair of
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 ...
s (mislabeled as belonging to ''Lagosuchus'') and portions of the hip and hindlimbs, including two complete
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 ...
s. He also suggested that ''Lagerpeton'' was the probable identity of several incomplete
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 ...
e 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. ...
e preserved along with several gomphodont skeletons in slab MCZ 3691. However, later authors have doubted the referral of most MCZ material to ''Lagerpeton'', with only the MCZ 4121 femurs being confidently referred to the genus. Andrea Arcucci described two PVL specimens, PVL 4619 and 4625, in 1986. PVL 4619, the PVL specimen mentioned by Romer, was a partial skeleton including a complete pelvis and left hindlimb, as well as a partial right hindlimb. PVL 4625 was another skeleton discovered later and originally described as including portions of the left hip, left hindlimb, and vertebral column.
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 Arcucci redescribed the known material in 1994 and mentioned that an isolated partial femur of this species was also present at the PVL, although
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 ...
(2016) noted that the provided catalogue number, PVL 5000, actually referred to a notoungulate
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
. Further preparation of PVL 4625 has revealed the presence of a
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 ...
,
dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...
, and cranial fragments.


Description

''Lagerpeton'' is estimated to have been 70 cm (28 in) in length based on the length of the hindlimb; the most complete hindlimb specimen, from PVL 4619, measures 257.9mm from proximal femur to distal
ungual An ungual (from Latin ''unguis'', i.e. ''nail'') is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and ungulates have ungual phalanges, as did the sauropod Sauropoda (), whose members are known as sauropods (; ...
. Body mass has been estimated as no more than , based on the slender cross section of limb bones and estimates between more derived dinosauromorphs, such as '' Silesaurus'', and basal
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 ...
ns like ''
Eoraptor ''Eoraptor'' () is a genus of small, lightly built, Basal (phylogenetics), basal sauropodomorpha, sauropodomorph dinosaur. One of the earliest-known dinosaurs and one of the earliest sauropodomorphs, it lived approximately 231 to 228 million yea ...
''. Twenty one autapomorphic characters have been identified in ''L. chanarensis'', these include: the anterior inclination of the posterior dorsal
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 ...
, the hook-shaped
femoral head The femoral head (femur head or head of the femur) is the highest part of the thigh bone (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 ...
and the length of digit IV and
metatarsal The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...
IV being greater than digit III and metatarsal III. ''L. chanarensis'' lacks many
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 ...
ian characters, such as the anterior trochanter, placing it basal within
Dinosauromorpha Dinosauromorpha is a clade of avemetatarsalians ( archosaurs closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lagerpetids ...
or even outside the group altogether.


Classification

Early to late
Olenekian In the geologic timescale, the Olenekian is an age (geology), age in the Early Triassic epoch (geology), epoch; in chronostratigraphy, it is a stage (stratigraphy), stage in the Lower Triassic series (stratigraphy), series. It spans the time betw ...
trackways from Poland have yielded footprints of a ''Lagerpeton''-like quadrupedal dinosauromorph. This
ichnogenus An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxon'' comes from the Ancient Greek (''íchnos'') meaning "track" and English , itself derived from ...
, named '' Prorotodactylus'' shares multiple synapomorphic characters with ''Lagerpeton'' including approximately parallel digits II, III and IV, fused metatarsus, digitigrade posture and reduced digits I and V. ''Prorotodactylus'' also shares the, previously autapomorphic, Pes (anatomy) morphology of ''Lagerpeton''. If this ichnogenus represents a close relative of ''Lagerpeton'', it would push back the origin of this taxon to the Early Triassic; as a quadrupedal basal dinosauromorph, it also raises questions debating the theory that bipedalism is ancestral to dinosaurs. ''Lagerpeton'' is the namesake of the family Lagerpetidae, a group of small avemetatarsalians which coexisted alongside dinosaurs for much 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 ...
. Lagerpetids are traditionally considered the most basal clade within Dinosauromorpha and the sister taxon to Dinosauriformes.Baron, M.G., Norman, D.B., and Barrett, P.M. (2017)
A new hypothesis of dinosaur relationships and early dinosaur evolution
''Nature'', 543: 501–506.
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 ...
simplified after Kammerer, Nesbitt & Shubin (2012): More recently described fossil material for the group instead suggests that lagerpetids are early pterosauromorphs, more closely related to pterosaurs than to dinosaurs.


Palaeogeography

The oldest fossils of ''L. chanarensis'' were found in the Chañares Formation and originate from the Upper Middle Triassic (
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 ...
) of
Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...
, southern
Pangaea Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous period approximately 335 mi ...
. All ''Lagerpeton'' specimens share this geographic location, including other fossils from the Lower Late Triassic (
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 ...
).
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 ...
of volcanic material in the formation has narrowed the formation and entire fossil assemblage found there to between 236 and 234 million years old.


Locomotion

It has been suggested that the extant analogues most similar to ''L. chanarensis'' are small bipedal mammals, which are often saltators. Three morphological characteristics in ''L. chanarensis'' fossils have been putatively cited as evidence of saltation in this taxon: inclined neural spines, a small pelvic girdle, and didactyly.


Neural spines

The neural spines of the posterior dorsal vertebrae are inclined anteriorly, a character not observed in any other
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 ...
, but common in saltatory mammals. This feature is suggested to allow for greater vertebral flexibility, correlated with leaping and bounding locomotor styles.


Pelvic girdle

Relative to the hindlimb length, the pelvic girdle is remarkably small. The distance from the pelvic girdle to the femur is therefore also small, more so than most other archosaurs apart from closely related taxa. This reduction in distance may increase the force production during hip extension in extant small mammals.


Didactyl foot

The narrow and functionally didactyl pes are a further similarity to modern saltators. By condensing into a single unit, the metatarsus gains strength without the drawback of increased weight. It also appears likely that, consequently to the reduction of digit II, digit IV may have elongated to balance the pes. The hypothesis of saltatorial locomotion is debated, however. Vertebral adaptations of extant organisms exceed those seen in ''Lagerpeton''; the sacral vertebrae of modern saltators are fused and the neural spines reduced. Furthermore, the size of the pelvic girdle and lateral digital reduction may be equally used as evidence for cursorial locomotion.


References

{{Taxonbar, from=Q514042 Lagerpetidae Triassic archosaurs Middle Triassic reptiles of South America Triassic Argentina Fossils of Argentina Chañares Formation Fossil taxa described in 1971 Taxa named by Alfred Romer