| Named By: | M. D. Ezcurra, A. Lecuona & A. Martinelli in 2010 |
| Time Period: | Early Triassic |
| Location: | Argentina - Quebrada de Los Fosiles Formation |
| Size: | Unavailable |
| Diet: | Unavailable |
| Fossil(s): | Partial remains |
| Classification: | | Chordata | Reptilia | Archosauriformes | |
Koilamasuchus is an extinct genus of Early Triassic archosauriform. Remains have been found from the Quebrada de los Fosiles 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 Jose 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 ribs, osteoderms, an ilium, a humerus, an ungual, a chevron, a possible gastralium, and two metapodials (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 birds. 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 45deg from the bone's vertical axis. This is a derived feature not seen in more basal archosauriforms. Koilamasuchus had dorsal osteoderms covering its back, which are also present in erythrosuchids and some archosaurs, but not in more basal archosauriforms such as proterosuchids.