Aniksosaurus

Aniksosaurus (meaning "spring lizard", from Modern Greek Άνοιξη, "Spring", referring to the fact it was found on 21 September 1995, the onset of Spring on the Southern Hemisphere) is a genus of dinosaur from what is now present-day Chubut Province, Argentina. It was a theropod, specifically a coelurosaur, which lived in the middle Cretaceous period, between 96-91 million years ago. The type species, Aniksosaurus darwini, was formally described by Rubén Dario Martínez and Fernando Emilio Novas in 2006;[1] the name was first coined in 1995 and reported in the literature in 1997.[2] The specific epithet honors Charles Darwin who visited Patagonia in 1832/1833 during the Voyage of the Beagle.

Description
The type specimen MDT-PV 1/48, discovered in the Bajo Barreal Formation, consists of an articulated right hindlimb, which includes the femur, fibula, tibia and foot. The longest femur discovered for this genus measures 247mm, and the longest tibia, 270mm. On average, the tibia is 13% longer than the femur in Aniksosaurus, an adaptation which has been strongly correlated with the development of cursorial habits in dinosaurs.[3] Martinez and Novas (2006), based on femoral measurements, originally suggested that Aniksosaurus darwini was approximately 2 m (7 ft) long and 70 cm (2 ft) tall at the hip, and weighed up to 65 kilograms (143 pounds).[1] However, recent estimates suggest a total body length between 2 m (7 ft) and 3 m (10 ft) and a body weight between 35 kilograms (77 pounds) and 45 kilograms (99 pounds).[4] In a 2013 study by Ibiricu et al., it was concluded that the holotype and four more individuals referable to Aniksosaurus, were juvenile to sub-adult individuals as shown by a histological analysis. The two individuals analyzed had histological characteristics that suggested an approximate age of three. The morphological evidence supporting ontogenetic immaturity was as follows: (a) the absence of an outer circumferential layer; (b) the absence of secondary (Haversian) osteons; (c) the presence of only a few growth cycles; and (d) the thickness of the zones.

Classification
Martínez and Novas (2006) originally assigned this genus to the clade Coelurosauria based on anatomical features present in the hindlimbs. The paleontologists describe Aniksosaurus as "more derived than some basal coelurosaurians such as compsognathids, Ornitholestes, and coelurids", but less advanced than later coelurosaurs such as Tyrannosaurus and Oviraptor. Aniksosaurus exhibits some of the derived features found in Coelurosauria, such as (a) the ilium has a well developed cuppedicus fossa; (b) the femur possesses an anterior trochanter that is proximally projected, almost reaching the level of the articular head; (c) the greater trochanter is craniocaudally expanded; (d) the femoral head is rectangular-shaped in cranial aspect; (e) and the fibular shaft is craniocaudally narrow. However more recent research by Choiniere et al. (2010) demonstrated that phylogenetically, Aniksosaurus was closer to individuals in the taxon Compsognathidae.

Distinguishing anatomical features
A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism.

According to Martínez and Novas (2006), Aniksosaurus can be distinguished based on the following characteristics:[1]
 * cranial cervical vertebrae have dorsoventrally deep neural arches, and are provided with a pair of cavities at their cranial surfaces
 * the neural canal is wide
 * cranial caudals feature a ventral sagittal keel, and their transverse processes are triangular-shaped in dorsal view
 * manual ungual phalanges are robust
 * the metatarsal and digit IV of the pes are transversely narrow
 * ilium with an extremely expanded brevis shelf (a bone surface on the ilium that functions as an attachment site for tail muscles)
 * the femur possesses a deep notch for the M. iliotrochantericus