| Aniksosaurus | |
|---|---|
 | |
| Right femur (MDT-PV 1/3) and right tibia (MDT-PV 1/48) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Genus: | †Aniksosaurus |
| Species: | †A. darwini |
| Binomial name | |
| †Aniksosaurus darwini Martínez & Novas, 2006 | |
Aniksosaurus (meaning "spring lizard", fromModern Greek Άνοιξη, "Spring", referring to the fact it was found on 21 September 1995, the onset of Spring on theSouthern Hemisphere) is agenus ofavetheropoddinosaur from what is nowChubut Province,Argentina. It lived during theCenomanian toTuronian of theCretaceousperiod, between 96-91 million years ago. Thetype species,Aniksosaurus darwini, was formally described from theBajo Barreal Formation of theGolfo San Jorge Basin byRubén Dario Martínez andFernando Emilio Novas in 2006;[1] the name was first coined in 1995 and reported in the literature in 1997.[2] The specific epithet honorsCharles Darwin who visitedPatagonia in 1832/1833 during theVoyage of the Beagle.
The type specimenMDT-PV 1/48, discovered in theBajo Barreal Formation, consists of an articulated right hindlimb, which includes thefemur,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 inAniksosaurus, 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 thatAniksosaurus 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 toAniksosaurus, were juvenile to sub-adult individuals as shown by ahistological 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.
Martínez and Novas (2006) originally assigned this genus to the cladeCoelurosauria based on anatomical features present in the hindlimbs. The paleontologists describeAniksosaurus as "more derived than some basal coelurosaurians such as compsognathids,Ornitholestes, and coelurids", but less advanced than later coelurosaurs such asTyrannosaurus andOviraptor.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. Choiniere et al. (2010) demonstrated thatphylogenetically,Aniksosaurus was closer to individuals in the taxonCompsognathidae.[5] Hartman et al. (2019) instead find it to be amaniraptoromorph outside compsognathids and maniraptorans,[6] while Naish and Cau (2022) find it to be amegaraptoran.[7]
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]
Monospecific bonebeds suggest it was gregarious, and that this behavior gaveAniksosaurus a selective advantage over competitors in itspaleoenvironment.[4] The hypothesis suggesting gregarious (social) behavior is supported by the taphonomic association of individuals from the Bajo Barreal Formation and evidence garnered from analysis of its bone histology. The authors of this study, Ibiricu, Martínez, Casal and Cerda (2013) noted that this particular assemblage is only the second known body fossil association of small, coelurosaurian theropods discovered in South America.[4] The theropod taxa have been interpreted as gregarious areCoelophysis bauri,[8]Syntarsus rhodiensensis,[9]Albertosaurus sarcophagus[10] andSinornithomimus dongi.[11]Aniksosaurus is likely to have exhibited gregariousness only during a specific interval of its life, most likely when it was a sub-adult.
The remains of the type specimenMDT-PV 1/48 were discovered in the Laguna Palacios Ranch locality in the lowerBajo Barreal Formation ofPatagonia in Chubut,Argentina. The specimen was collected by Martínez in 1995 in green sandstone that was deposited between theCenomanianstage and theTuronian stage of theCretaceous period, between 96-91 million years ago, as shown byradiometricAr-Ar dating. This specimen is housed in the collection of the Museo Regional Desiderio Torres - in Sarmiento, Chubut, Argentina. Remains of at least four more individuals, in 2006 assigned as theparatypes, were uncovered at the site.
Dinosaur remains from thisformation include those of theabelisaurid theropodXenotarsosaurus bonapartei, thesauropodsDrusilasaura,Epachthosaurus andCampylodoniscus, thehadrosaurSecernosaurus as well as indeterminate teeth belonging tocarcharodontosaurids anddromaeosaurids.
The nature of the lithology at theAniksosaurus site suggests that the deposits were made by a low-energy, unidirectional, overbankfluvial floodplain deposit, which was produced by a lateral migrating fluvial channel. The observation that sandstone tabular bodies were deposited with coarse sandstones andtuff interclasts also supports thatAniksosaurus was preserved in a fluvial environment. The soft tissue underwent subaerialweathering and then began to decompose, from the contributory action of small vertebrae scavengers as evidenced by a series of small grooves in one of the femora recovered, which was then followed by the partial disarticulation of the skeleton. Disarticulation was followed by reorientation of the remains by fluvial currents, which was then followed by differential burial in what appears to have been at least two stages of sedimentation. No evidence was found to support either cannibalism or intra-specific competition.
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