''Lewisuchus'' 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
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 ...
that lived during 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 ...
(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 ...
). As a
silesaurid dinosauriform, it was a member of the group of
reptile
Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s most commonly considered to be the closest relatives of
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 (possibly true dinosaurs themselves). ''Lewisuchus'' was about long. Fossils have been found in 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 ...
.
It exhibited
osteoderms
Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of Extant taxon, extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, Temnospondyli, ...
along its back.
History
The first remains of ''Lewisuchus'' were discovered in a 1964-1965 joint expedition by the
Museo de La Plata and
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 ...
's
Museum of Comparative Zoology. These remains were preserved in a carbonate nodule recovered 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 ...
. It contained a partial skeleton of ''Lewisuchus'', including parts of the skull, braincase, many
vertebra
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 ...
e,
scapulocoracoids,
humeri, and
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.
A
gomphodont and bones from several other species of archosaurs were also preserved in the same nodule.
An isolated lower jaw and foot bones were also initially referred to ''Lewisuchus'', but these were later identified as belong to
proterochampsids. This is also likely true of an
astragalus found alongside the skeleton.
Alfred Romer named ''Lewisuchus'' as a new genus in 1972, based on the partial skeleton which at that point was stored at the Museo de La Plata with catalogue number 64-XI-14-14. The generic name honored Arnold D. Lewis, the chief preparator who discovered the skeleton while working on the recovered nodule. Like many other early archosaurs and crocodilian relatives, it also references ''Soûkhos'', the Greek name for the Egyptian crocodile god
Sobek
Sobek (), also known as Suchus (), was an ancient Egyptian deities, ancient Egyptian deity with a complex and elastic history and nature. He is associated with the Nile crocodile and is often represented as a crocodile-headed humanoid, if not a ...
. The species name "''admixtus''" refers to the mixture of different species preserved along with the holotype.
The holotype is now housed at the Paleontology Museum at the
National University of La Rioja (PULR) with catalogue number PULR 01.
''Pseudolagosuchus''
Another Chañares
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 ...
was described by
Andrea Arcucci in 1987. This archosaur encompassed four specimens stored at the Miguel Lillo Institute (PVL) of the
University of Tucuman, the PULR, and the
Museo Argentino de Ciencias Naturales (MACN). Arcucci named a new archosaur genus based on these specimens: ''Pseudolagosuchus major''. It was originally considered a close relative of its namesake, the small
dinosauriform ''
Lagosuchus''. The holotype of ''Pseudolagosuchus'', PVL 4629, included most of a left hindlimb and pelvis as well as a fragmentary ankle, vertebrae, and ribs. Other specimens included PVL 3454 (fragments of the hip and hindlimb), MACN 18954 (vertebrae and portions of the hindlimb and ankle), and PULR 53 (tail vertebrae and bones from the ankle region). A 1996 study on dinosaur origins by
Fernando Novas also placed a well-preserved femur and tibia (erroneously labelled UPLR 53) within the genus, and this may also be the case with PVL 3455, an indeterminate partial skeleton with some similarities to Arcucci's original material.
Despite not sharing much overlapping material, some paleontologists have proposed that ''Pseudolagosuchus major'' represented the same species of animal as ''Lewisuchus admixtus''. If this is demonstrated to be the case, the name ''Lewisuchus'' would have priority over ''Pseudolagosuchus'' due to having been named earlier. Arcucci argued towards synonymizing the two as early as 1997. A 2010 study in the Journal ''
Nature
Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
'' by
Nesbitt et al. also made a case for their synonymy, as they were approximately the same size, came from the same locality and strata, and both were found to be basal members of the recently named family
Silesauridae.
Many other studies supported these arguments, sometimes even scoring the two as a single taxon in their
phylogenetic analyses
In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as Computational phylogenetics, phylogenetic inference. It infers the relationship among organ ...
.
Several hindlimb, hip, and vertebral fossils (PVL 3456, PULR-V 112 and PULR-V 113) have additionally been referred to ''Lewisuchus'' based on their similarity to ''Pseudolagosuchus'' remains.
However,
Bittencourt ''et al''.'s 2014 redescription of the ''Lewisuchus'' holotype refrained from synonymizing the two due to a lack of data, with the only overlapping material being 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 a few cervical (neck) vertebrae. No unambiguous characteristics were shared between these scraps, and one tibia referred to ''Pseudolagosuchus'' (Novas's 'UPLR 53') seemed to have a curvature different from that of ''Lewisuchus''
's holotype.
A new specimen of ''Lewisuchus'' discovered in 2013 was described by
Martin Ezcurra ''et al''. in 2019, helping to settle the synonymy debate. This new specimen, CRILAR-Pv 552, was a partial skeleton incorporating parts of the skeleton previously only known in either ''Lewisuchus'' (such as the skull) or in ''Pseudolagosuchus'' (such as the hip and hindlimbs). It also provided details of the
premaxilla
The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...
, inner ear, and lower forelimbs, which were not preserved in any previous remains. The individual bones of CRILAR-Pv 552 were basically identical to their counterparts in ''Lewisuchus'' and ''Pseudolagosuchus'' specimens, providing irrefutable evidence that ''Pseudolagosuchus'' referred to the same species of reptile as ''Lewisuchus'', and establishing ''Lewisuchus admixtus'' as the valid name for all fossils previously referenced with either name.
References
{{Taxonbar, from=Q649348
Silesauridae
Late Triassic reptiles of South America
Triassic Argentina
Fossils of Argentina
Chañares Formation
Fossil taxa described in 1972
Taxa named by Alfred Romer