Koilamasuchus
Encyclopedia
Koilamasuchus is an extinct genus
of Early Triassic
archosauriform. Remains have been found from the Quebrada de los Fósiles Formation of the Puesto Viejo Group in Argentina
. The holotype
specimen of Koilamasuchus gonzalezdiazi, the type species
, is the most well preserved and best known specimen of a basal archosauriform in South America. The fossil has been known since 1981 when it was first described by paleontologist José Bonaparte
, but it was not described as a new genus and species until 2010.
The holotype specimen is an external mold of various parts of the postcranial skeleton, including dorsal vertebrae and rib
s, osteoderm
s, an ilium
, a humerus
, an ungual
, a chevron
, a possible gastralium
, and two metapodial
s (either metacarpals or metatarsals). The mold has several diagnostic features, such as an elongated illiac blade that is convex along its upper margin, that indicate that it is an archosauriform.
A phylogenetic analysis was conducted along with the naming of the genus. Koilamasuchus was found to be closely related to Archosauria, a crown group
that includes living crocodilians and bird
s. It was placed as the sister taxon to a clade
containing Archosauria and Erythrosuchidae
, a family
of large basal archosauriforms. According to the resulting phylogenetic tree
, Koilamasuchus is more derived
than Osmolskina
, Sarmatosuchus
, Fugusuchus
(commonly thought to be an erythrosuchid), and proterosuchids (which were found to be paraphyletic). Being more derived than these other taxa, Koilamasuchus is closer to the crown group Archosauria.
Koilamasuchus is considered more derived than proterosuchids and other early archosauriforms because it possesses an iliac preacetabular process, or a bony projection near the acetabulum
(part of the hip joint). The pubic peduncle, a projection on the ilium that connects it to the pubis
and forms part of the front edge of the acetabulum, is angled less than 45° from the bone's vertical axis. This is a derived feature not seen in more basal archosauriforms. Koilamasuchus had dorsal osteoderm
s covering its back, which are also present in erythrosuchids and some archosaurs, but not in more basal archosauriforms such as proterosuchids.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of Early Triassic
Early Triassic
The Early Triassic is the first of three epochs of the Triassic period of the geologic timescale. It spans the time between 251 ± 0.4 Ma and 245 ± 1.5 Ma . Rocks from this epoch are collectively known as the Lower Triassic, which is a unit in chronostratigraphy...
archosauriform. Remains have been found from the Quebrada de los Fósiles Formation of the Puesto Viejo Group in Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. The holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
specimen of Koilamasuchus gonzalezdiazi, the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
, is the most well preserved and best known specimen of a basal archosauriform in South America. The fossil has been known since 1981 when it was first described by paleontologist José Bonaparte
José Bonaparte
José Fernando Bonaparte, Ph.D. , is an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists like Rodolfo Coria...
, but it was not described as a new genus and species until 2010.
The holotype specimen is an external mold of various parts of the postcranial skeleton, including dorsal vertebrae and rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...
s, osteoderm
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
s, an ilium
Ilium
-Places:* Ilion or, Latinized, Ilium, another name for the legendary city of Troy, hence the title of Homer's Iliad*Ilium , an ancient city in Epirus...
, a humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
, an ungual
Ungual
An ungual is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and other ungulates have ungual phalanges, as did the sauropods and horned dinosaurs. A claw is a highly modified ungual phalange.As an adjective, ungual means related to nail, as in periungual .-External...
, a chevron
Chevron (anatomy)
A chevron is one of a series of bones on the ventral side of the tail in many reptiles, dinosaurs , and some mammals such as kangaroos and manatees....
, a possible gastralium
Gastralium
Gastralia are dermal bones found in the ventral body wall of crocodilian and Sphenodon species. They are found between the sternum and pelvis, and do not articulate with the vertebrae...
, and two metapodial
Metapodial
Metapodials are long bones of the hand and feet which connect the digits to the centers. In humans, five are present in each hand and foot....
s (either metacarpals or metatarsals). The mold has several diagnostic features, such as an elongated illiac blade that is convex along its upper margin, that indicate that it is an archosauriform.
A phylogenetic analysis was conducted along with the naming of the genus. Koilamasuchus was found to be closely related to Archosauria, a crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
that includes living crocodilians and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s. It was placed as the sister taxon to a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
containing Archosauria and Erythrosuchidae
Erythrosuchidae
Erythrosuchidae are a family of large basal archosauromorph carnivores that lived from the later Early Triassic to the early Middle Triassic . Their fossil remains are known so far from South Africa , the Perm region of Russia, and China...
, a family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of large basal archosauriforms. According to the resulting phylogenetic tree
Phylogenetic tree
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
, Koilamasuchus is more derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
than Osmolskina
Osmolskina
Osmolskina is a genus of archosauriform reptile which lived during the Early Triassic in what is now Poland. The type species, Osmolskina czatkowicensis, was described by Magdalena Borsuk−Białynicka and Susan Evans in 2003. The generic name honors the late female Polish paleontologist Halszka...
, Sarmatosuchus
Sarmatosuchus
Sarmatosuchus is an extinct genus of archosaurin reptile found in sediments of early Triassic age and know from a single species Sarmatosuchus otschevi. The holotype and only specimen was found in litified river deposits of the Donguz Formation exposed near the Berdyanka River, Orenburg region,...
, Fugusuchus
Fugusuchus
Fugusuchus is an extinct genus of erythrosuchid archosauriform. It was one of the earliest and most basal erythrosuchids. The genus is known from a single fossil from the middle Early Triassic Heshankou Formation in Shanxi, China. The partial skeleton consists of an incomplete skull, parts of the...
(commonly thought to be an erythrosuchid), and proterosuchids (which were found to be paraphyletic). Being more derived than these other taxa, Koilamasuchus is closer to the crown group Archosauria.
Koilamasuchus is considered more derived than proterosuchids and other early archosauriforms because it possesses an iliac preacetabular process, or a bony projection near the acetabulum
Acetabulum
The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.-Structure:...
(part of the hip joint). The pubic peduncle, a projection on the ilium that connects it to the pubis
Pubis
Pubis may refer to:* Pubis * Mons pubis, a padding of fat that protects the pubis bone...
and forms part of the front edge of the acetabulum, is angled less than 45° from the bone's vertical axis. This is a derived feature not seen in more basal archosauriforms. Koilamasuchus had dorsal osteoderm
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
s covering its back, which are also present in erythrosuchids and some archosaurs, but not in more basal archosauriforms such as proterosuchids.