Koilamasuchus

Koilamasuchus Temporal range:Middle-Late Triassic
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauria
Clade:	Pseudosuchia
Clade:	Suchia
Genus:	†Koilamasuchus Ezcurraet al.2010
Species
†K. gonzalezdiaziEzcurraet al.2010 (type)

Koilamasuchusis anextinctgenusof indeterminatearchosauriformfrom theTriassicofArgentina.It is based on anexternal moldof a partial postcranial skeleton from theQuebrada de los Fósiles Formation.Due to its incomplete nature, the relationships of this reptile are difficult to establish. Originally described as a non-archosaurarchosauriform,later studies tentatively considered it adoswelliidor asuchianarchosaur.

Discovery

Remains have been found in the Quebrada de los Fósiles Formation of thePuesto Viejo Groupin Argentina.This locality is probablyLadiniantoCarnian(MiddletoLate Triassic) in age, although its exact age is debatable.Koilamasuchusis derived from theLatinword 'koilamas' (which means cavity or pocket) and theGreek'suchus' (which means crocodile). This is in reference to the lateral fossae (pits) on the type specimen's vertebrae. Thetype speciesofKoilamasuchus, K. gonzalezdiazi,refers to Dr. Emilio F. Gonzalez Diaz, thepaleontologistwho discovered the holotype specimen. The fossil has been known since 1981 when it was first described by paleontologistJosé Bonaparte,but it was not described as a new genus and species until 2010.

Theholotypespecimen, MACN-Pv 18119, is anexternal moldof various parts of the postcranial skeleton, including dorsalvertebraeand a dorsalrib,osteoderms,a rightilium,ahumerus,anungual,achevron,a possiblegastralium,twometapodials(eithermetacarpalsormetatarsals), and several indeterminate bones including a possibleradius.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.

Description

In life,Koilamasuchuswould have been a small reptile. Based on the proportions ofEuparkeria,it would have been approximately 50 centimeters (20 inches) in total length.The dorsal vertebrae are tall, particularly the neural spines. Theproximalpart of the shaft of the rib curves outwards slightly, but also possesses a notch which forces the rib to strongly project outwards for a short distance and then gradually curve back inwards. A deep sulcus (groove) is also present on the shaft of the rib. The humerus is hourglass-shaped, with an expanded proximal end more than 3.5 times the width of the shaft. The distal end is also expanded to a slightly lesser degree. A wide, slanting ridge (or tuberosity) runs down the shaft of the humerus. The ilium possesses a completely closedacetabulumwith a triangular lower edge. Thepubicandischialpeduncles are widely separated and the pubic peduncle is rather long while the ischial peduncle is short. One of MACN-Pv 18119's osteoderms is triangular, with a pointed front edge, a slightly rounded rear edge, and a pronounced longitudinal keel. Some other osteoderms are smaller and more oval-shaped.

Classification

Aphylogeneticanalysis was conducted in the 2010 paper which named and described the genus.Koilamasuchuswas found to be closely related toArchosauria,acrown groupthat includes livingcrocodiliansandbirds.It was placed as the sister taxon to acladecontaining Archosauria andErythrosuchidae,afamilyof large basal archosauriforms. According to the resultingphylogenetic tree,Koilamasuchusis morederivedthanOsmolskina,Sarmatosuchus,Fugusuchus(commonly thought to be an erythrosuchid), andproterosuchids(which were found to beparaphyletic). Being more derived than these other taxa,Koilamasuchusis closer to the crown group Archosauria.

Koilamasuchusis considered more derived than proterosuchids and other early archosauriforms because it possesses an iliac preacetabular process, or a bony projection near theacetabulum(part of the hip joint). The pubic peduncle, a projection on the ilium that connects it to thepubisand 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.Koilamasuchushad dorsalosteodermscovering its back, which are also present in erythrosuchids and some archosaurs, but not in more basal archosauriforms such as proterosuchids.

The 2016 description ofTeyujaguaplacedKoilamasuchusin a polytomy withFugusuchusand all other non-proterosuchid archosauriforms.A 2016 study onEuparkeriidaeincludedKoilamasuchusin its phylogenetic analysis, and found that it was a basaldoswelliid.This classification was supported by the presence of a convex illiac blade with rounded anterior and posterior ends. Nevertheless, the authors noted that the large amount of missing data for the genus makes this assignment uncertain.

In another 2016 analysis on archosauriforms,Koilamasuchuswas classified as anarchosaur,specifically a basal member ofSuchia.Some of the features supporting this classification include the presence of a lateral fossa on eachvertebral centra,a humerus with a proximal end that is roughly symmetrical from the front, and a well-developed preacetabular process of the ilium which nevertheless does not extend further forward than the pubic peduncle. However, the osteoderms of MACN-Pv 18119 do not bend along their lateral edge, meaning thatKoilamasuchuswas more basal than advanced suchians such as members ofParacrocodylomorpha.