| Nhandumirim | |
|---|---|
 | |
| Skeletal diagram ofNhandumirim, as an early sauropodomorph, following all the subsequent studies after its original description | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Family: | †Saturnaliidae |
| Genus: | †Nhandumirim Marsolaet al.2019 |
| Species: | †N. waldsangae |
| Binomial name | |
| †Nhandumirim waldsangae Marsolaet al. 2019 | |
Nhandumirim (meaning "smallrhea" in theTupi language) is agenus of basalsauropodomorphdinosaur from theCarnian age ofLate TriassicBrazil. It is currently considered asaturnaliid sauropodomorph. Thetype and only species,Nhandumirim waldsangae, is known from a single immature specimen includingvertebrae, achevron, pelvic material, and a hindlimb found in theSanta Maria Formation inRio Grande do Sul.
Nhandumirim is differentiated from other Santa Maria dinosaurs such asStaurikosaurus andSaturnalia on the basis of its more gracile, long-legged proportions and several more specific skeletal features. However, it is noteworthy that the holotype is an immature individual. Several features of thetibia led the describers to consider the genus to be possibly the earliesttheropod, but some analyses in their study offer alternative positions withinSaurischia.[1] All subsequent studies consideredNhandumirim an earlysauropodomorph, closely related toSaturnalia andChromogisaurus.[2][3][4][5][6]
Nhandumirim is known from a single partial skeleton, LPRP/USP 0651, which includes several vertebrae, a right ilium, and most of a right hindlimb. This skeleton was found at the site of Cerro da Alemoa (also known as Waldsanga or Sanga do Mato)[7] inSanta Maria, Rio Grande do Sul. Waldsanga is a historically important Triassic site which also preserved thetype specimens ofSaturnalia,Rauisuchus,Gomphodontosuchus, andAlemoatherium.[1][7] The site preserves Carnian-age sediments of theSanta Maria Formation, and LPRP/USP 0651 specifically comes from the top of the Alemoa member on that site. Thegeneric nameNhandumirim roughly translates to "smallrhea" in theTupi-Guarani language. Thespecific name refers to Waldsanga.[1]
Thedorsal vertebrae have spool-shapedcentra which are about 1.4 times longer than high, making them more elongated than those ofherrerasaurids. Several areas on the dorsals are incised, such as the sides of the centrum (which each have a shallow depression) and the rear of the neural arch (which has postzygapophyseal centrodiapophyseal and centrodiapophyseal fossae). There were likely threesacral vertebrae, with at least the first one articulating with the hip akin to the "primordial" first sacral of other reptiles. This contrasts withSaturnalia in which the first sacral vertebra is an incorporated dorsal vertebra without the hallmarks of a primordial sacral. The sacral vertebrae were thick and wide, but not fused to each other or their respective sacral ribs. The most complete sacral rib was fan shaped when seen from above and has a cross section which gradually curves upwards towards the front, also unlikeSaturnalia which has a more L-shaped cross section. Thecaudal vertebrae increase in length and gradually reduce the size of theirtransverse processes from the base of the tail to the tip. Caudals at the base of the tail have pronounced midline keels along their entire lower edge. This characteristic is seemingly unique toNhandumirim, as the only otherdinosauromorphs with ventral keels in proximal caudals areDracoraptor (which had paired, not midline, keels) andEfraasia (in which they were restricted to the front third of the centrum). Thezygapophyses of the caudals are short, unlike the longer joints of herrerasaurids andneotheropods.[1]
Theilium is generally typical for early dinosaurs, with a perforated acetabulum, conspicuous antitrochanter, and a postacetabular process which was much longer than the preacetabular process. The ischiadic peduncle is expanded in a front-to-back direction like that of neotheropods. There is an L-shaped scar on the inner edge of the iliac blade which received the first sacral vertebra, and there was enough room for two more sacrals behind it. The outer surface of postacetabular process has two holes and a pronounced incision along its lower edge, known as a brevis fossa. In most dinosaurs which have a brevis fossa, it clearly starts at the base of the postacetabular process, according to a ridge which delineates its upper edge. However,Nhandumirim's brevis fossa only occupies the rear 3/4ths of the postacetabular process, with no clear interaction with the main portion of the ilium.[1]
Thefemur is slender and thin-walled. The proximal portion possessed early dinosaurian hallmarks such as an anterior trochanter, a facies articularis antitrochanterica, well-developed medial tubera separated by a deep ligament sulcus, and an asymmetrical and pronounced fourth trochanter. The femoral head also possessed a deep and curved groove, but the appearance of this groove is known to be variable within some dinosaur species, so it is not particularly informative forNhandumirim's classification. The trochanteric shelf was not present, but a homologous muscle attachment scar did exist in the same area, along with a more proximally-located anterolateral scar similar to that reported in somesilesaurids. Unlike other early dinosaurs, the dorsolateral trochanter was characteristically short, ending quite a distance away from the femoral head like the anterior trochanter. The distal portion had a widemedial condyle and muscle scars similar to those ofHerrerasaurus.[1]
Thetibia was poorly preserved but did possess several features similar to those of neotheropods. These include a tall facet for the ascending process of theastragalus and a distal tip which was wide and flattened when seen from below and boxy when seen from the front. There was also a diagonal mound-like tuberosity on the anterior surface of the tibia.Nhandumirim is the only known dinosaur to possess both these neotheropod-like traits and a diagonal tuberosity. The more completefibula was elongated (~10% longer than the femur) and had scars for the tibial ligament and iliofibularis muscle near the knee. There was a semicircular facet for the ascending process of the astragalus on the portion of the fibula contacting the heel, a characteristic unique toNhandumirim.Metatarsal II is straight, with a flattened proximal portion and boxy distal portion.Metatarsal IV has a more irregularly-shaped distal portion but is also straight, an unusual feature more akin toLagerpeton andpterosaurs rather than the more curved bone of other dinosaurs. Isolatedphalanges are variable in proportions, with one having a wide proximal articulation and the rest having tall, triangular proximal articulations and more well-developed joint surfaces.Unguals are triangular in cross section and curved, though not to the extent seen in more advanced theropods.[1]
The describers ofNhandumirim tested its relations using twophylogenetic analyses focused around the origin of dinosaurs. The first one, created previously byCabreiraet al. (2016),[8] considered it the basalmost theropod, as thesister taxon toneotheropods. This is supported by a wideischiadic peduncle and several traits of thetibia. The Cabreiraet al. (2016) analysis is notable for excluding several other traditional basal theropods from the group. These includeDaemonosaurus,Tawa,Chindesaurus, andEodromaeus, which are considered basalsaurischians according to their results.[1]
AddingNhandumirim to the second analysis, created byNesbitt &Ezcurra (2015),[9] led to inconclusive results. In the strict consensus tree (average result ofmost parsimonious trees),Nhandumirim is part of a broadpolytomy at the base ofSaurischia, along with several traditional basal theropods,herrerasaurids, andEoraptor (which, on the other hand, is frequently considered an early sauropodomorph in most recent analyses). This is because it is a "wildcard" taxon with several equally likely positions within Saurischia. These positions may includeNhandumirim as the sister taxon to all other saurischians, as the sister taxon toEoraptor, or as the sister taxon to all other theropods (which also includes herrerasaurids in this option). Regardless, the authors considered unlikely thatNhandumirim is particularly close toStaurikosaurus orSaturnalia.[1]
Nonetheless, the phylogenetic position ofNhandumirim was also tested in the article describingGnathovorax cabreirai, which recovered it as asaturnaliinesauropodomorph, close-associated withSaturnalia andChromogisaurus.[2] Using a modified version of this dataset, an article depicting a novel phylogenetic hypothesis for the position ofsilesaurids, two follow studies,[3][4][5] as well as a study on early dinosaurs from South America based on another dataset also corroborated this position.[6]
The following is a cladogram showingNhandumirim's placement using Cabreiraet al. (2016)'s analysis:[1]
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