Tachiraptor

Tachiraptor Temporal range: Early Jurassic
IVIC-P-2687, the holotype right tibia, and IVIC-P-2868, the referred left ischium
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	Neotheropoda
Genus:	†Tachiraptor Langer, Rincón, Ramezani, Solórzano, & Rauhut, 2014
Type species
Tachiraptor admirablis Langer, Rincón, Ramezani, Solórzano, & Rauhut, 2014
Type locality for Tachiraptor admirabilis in Venezuela

Tachiraptor is a genus of carnivorous theropod dinosaur found in an early Jurassic period formation of Venezuela. The type species is Tachiraptor admirabilis.

Discovery

Since the late 1980s, in the Venezuelan state of Táchira, dinosaurian remains were uncovered at a road-cut between La Grita and Seboruco. Most of these belonged to a small herbivore that in 2014 was described as Laquintasaura. However, included in the discoveries were some theropod teeth, indicating a predator must have been present. In 2013, this was affirmed by the discovery of some theropod bones.

In 2014, the type species Tachiraptor admirabilis was named and described by Max Cardoso Langer, Ascanio D. Rincón, Jahandar Ramezani, Andrés Solórzano and Oliver Wilhelm Mischa Rauhut.

The description was based on two fossils, found in a layer of the La Quinta Formation dating from the Early Jurassic Hettangian stage. A maximum age of 200.72 ± 0.32 million years ago has been confidently established, but due to the limits of zircon dating, a precise minimum estimate is not known; the actual age could be considerably younger. Both fossils are from the same location, but assumed to represent two individuals. One of these was the holotype specimen, IVIC-P-2867. It consists of a nearly complete right tibia or shinbone. The second fossil was referred to Tachiraptor admirabilis on the assumption that only one species of neotheropod of such a size was present in the La Quinta Formation. It is specimen IVIC-P-2868, consisting of the damaged upper half of a left ischium, a bone of the pelvis.

Etymology

The genus' name was derived from that of the state of Táchira, were it was discovered, combined with the Latin word for thief, raptor. The specific name (admirabilis) was chosen in reference to the Admirable Campaign of 1813, conducted by Simón Bolívar, for which the type locality of La Grita was of strategic importance.

Description

Tachiraptor was a small bipedal predator. The shinbone has a length of about twenty-five centimetres; from this a total body length of the animal has been deduced of just over 1.5 metres.

The describers established a number of distinguishing traits. One of these is a possible autapomorphy, a unique evolutionary innovation shown just by Tachiraptor. It pertains to the profile, seen from above, of the upper surface of the shinbone. Such surfaces generally have two projections, corners jutting out to behind, left and right. In Tachiraptor the outer projection, at the side of the fibula, has a rear edge making a sharp angle with the outer edge. This way a uniquely sharp point is formed which, uniquely also, extend further to behind than the inner projection at the opposite side.

Apart from the autapomorphy, an unique combination of in themselves not unique traits was shown to exist. The bottom surface of the shinbone was, in its transverse width, about 1.5 times as wide as the longitudinal distance, measured from front to rear. In dinosaurs in general, the lower front of the shinbone is covered by the talus bone. A ridge on the front surface demarcates the upper limit of this area. In Tachiraptor this ridge ran obliquely at an angle of about 35° to the lower edge of the shinbone, covering a vertical distance of about a quarter to a third of the lower shinbone height. At its lower end, this ridge slightly curved upwards, being at this point close to the outer edge of the shinbone, at about a fifth of its transverse width. The shinbone extends to below in two bumps, left and right. When seen from the front, with Tachiraptor a line drawn between the bumps made an angle of 80° with the vertical axis of the bone.

Phylogeny

The study describing Tachiraptor performed a cladistic analysis, establishing its probable evolutionary relationships, or phylogeny, by computing an evolutionary tree assuming the least number of evolutionary changes. This analysis showed that Tachiraptor was a basal member of the Neotheropoda, the subgroup encompassing all but the earliest theropods. It thus was placed low in the evolutionary tree of the neotheropods. Tachiraptor was part of the stem leading to the Averostra, the group all theropods belong to from the Middle Jurassic onwards. Being a sister species of the Averostra, it was described as a "stem-averostran". This made the discovery of Tachiraptor especially important, because before 2014, there were no unequivocal stem-averostrans know at all. Tachiraptor thus reduced their ghost lineage, a to be assumed but as yet unproven line of descent, with twenty-five million years.

Tachiraptor also contributed to a greater knowledge of evolution by confirming that the equatorial zone of the supercontinent Pangea played an important role in the development of early dinosaurs, as already shown by the discovery of Laquintasaura.