| Amargatitanis | |
|---|---|
 | |
| Skeletal reconstruction, known material in teal | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Superfamily: | †Diplodocoidea |
| Family: | †Dicraeosauridae |
| Genus: | †Amargatitanis Apesteguía 2007 |
| Species: | †A. macni |
| Binomial name | |
| †Amargatitanis macni Apesteguía 2007 | |
Amargatitanis (meaning "Amarga giant") is agenus ofdicraeosauridsauropoddinosaur (a type of large, long-neckedquadrupedalherbivorous dinosaur) from theBarremian-aged (Lower Cretaceous)La Amarga Formation ofNeuquén,Argentina. It is known from a single, incomplete postcranial skeleton consisting of a partialhindlimb,ischium, and twovertebrae. These remains were unearthed by Argentine paleontologist José Bonaparte in 1983 during an expedition by theMuseo Argentino de Ciencias Naturales and later described as a new genus and species,Amargatitanis macni bySebastián Apesteguía. The genus name comes from the words Amarga, where the fossils were collected, andtitanis meaning "titan". Its species name is in reference to the MACN (Museo Argentino de Ciencias Naturales), where the remains are stored.
Though initially thought to be atitanosaur, recent research has found it to be adicraeosaurid, a group of short-necked, smaller sauropods. It reached around 12 meters (39.7 feet) long and 3.5 metric tons (3.9 short tons) according to one estimate. Like other dicraeosaurids, it has robust limbs, a long tail, and tallneural spines. However, much of its anatomy can only be inferred due to the lack of preserved bones. It lived in a diverse ecosystem that included higherbrowsing titanosauriforms and smaller, lower browsing dicraeosaurids likeAmargatitanis itself.
During an expedition by theMuseo Argentino de Ciencias Naturales from March 11th to 16th of 1983,ArgentinepaleontologistJosé Bonaparte collected several fossils from four sites along the La Amarga stream inNeuquén,Argentina. At site number "1", an associated skeleton of a largesauropod was unearthed by Bonaparte who noted the discovery in his personal journal. Bonaparte initially recorded the specimen as being from thePichi Picún Leufú Formation, although they were subsequently determined to be from theLa Amarga Formation. These remains were taken to the Museo Argentino de Ciencias Naturales where they were cataloged under the specimen number MACN PV N53.[1][2]
The skeleton remained undescribed until 2007, when Argentine researcherSebastián Apesteguía named itAmargatitanis macni. The genus name comes from Amarga, where theholotype was found, andtitanis meaning "titan". Its species name,macni, is to honor the Museo Argentino de Ciencias Naturales (MACN).[2] Apesteguía erroneously assigned ascapula (MACN PV N34) and six caudal vertebrae (MACN PV N51) to the specimen, despite coming from other rock layers and being found in 1986.[1][3] It was originally assigned to the groupTitanosauria due to the anatomy of the scapula and caudal vertebra, but a 2016 reanalysis of the holotype (MACN PV N53) fossils demonstrated thatAmargatitanis was instead adicraeosauriddipolodocoid. The study, authored by researcherPablo Gallina, also removed MACN PV N34 and N51 from the holotype material, which were referred to an indeterminatetitanosauriform, and considered only the dicraeosaurid bones to be from the genus. Since Gallina's paper, several phylogenetic studies have recoveredAmargatitanis as a dicraeosaurid.[4][5][6]
Amargatitanis macni is known from a single specimen, theholotype MACN PV N53, which was found in theBarremian-aged Puesto Antigual Member of theLa Amarga Formation, approximately 129 to 123 million years old. This specimen represents a mature individual[7] and consists of an incomplete anteriorcaudal vertebra, an incomplete middle caudal vertebra, an incomplete rightischium, and most of the right hind limb, including thefemur, partialtibia andfibula,astragalus, and the distal half of the firstmetatarsal. Field notes indicate that two phalanges and another metatarsal of this specimen were also collected, but as of 2016 they could not be located.[1] A pair of anteriordorsal vertebrae from a dicraeosaurid that were also found in the La Amarga Formation, MOZ-Pv 6126-1 and MOZ-Pv 6126-2, could be additional remains ofAmargatitanis, or belong to the other La Amarga Formation dicraeosaurid,Amargasaurus.[3]
As a dicraeosaurid,Amargatitanis would have resembled better known members of the family such asDicraeosaurus andAmargasaurus, which had shorter necks than most other sauropods and longneural spines extending from the vertebrae of the neck and trunk.[8] It was a small sauropod,[9] with one study estimating its mass at 11.406 metric tons (12.573 short tons), slightly larger than the same study's estimate forAmargasaurus of 10.195 metric tons (11.238 short tons).[10] Mass estimates forAmargasaurus have been noted to vary widely depending on the method used,[11] and studies using other methods, not yet applied toAmargatitanis, have foundAmargasaurus to weigh 3.304 metric tons (3.642 short tons)[12] to 4.376 metric tons (4.824 short tons).[13]
From what is known of the caudal (tail) vertebrae ofAmargatitanis, they generally resemble those of other dicraeosaurids. Thecentra (or "bodies") of these vertebrae are typical offlagellicaudatans, with gently concave anterior (front) faces and flat posterior (backside) faces. The prezygapophyseal centrodiapophysial fossae, which are deep depressions on the anterior surface of thetransverse processes of the caudal vertebrae found in most diplodocoids and some macronarians, differ from other sauropods in that they contain a thin vertical lamina (sheet of bone).[1] The caudal vertebrae ofAmargatitanis resemble those ofPilmatueia, but not most other dicraeosaurids, in lacking ventrolateral ridges.[14] The ischium generally resembles that of other flagellicaudatans, although it resembles thebrachiosauridGiraffatitan in that much of its posterior margin is straight.[1]
The hind limb bones generally resemble those of other dicraeosaurids. The femur is distinctive in that it is much wider at its proximal (upper) end than at its distal (lower) end, whereas in other sauropods, includingDicraeosaurus, both ends are approximately equal in width.[1] A similar condition may have been present in another dicraeosaurid,Pilmatueia.[15] As in other dicraeosaurids, the lateral (outer) border of the femur is gently curved and lacks the distinct lateral bulge characteristic of titanosauriforms. The tibia has a lowcnemial crest, similar to that ofDicraeosaurus andAmargasaurus and unlike the higher cnemial crest found in most macronarians. The posterior fossa of the astragalus differs from other sauropods in being deep and crescent-shaped, and in the arrangement of theforamina (openings) within it. The distal end of the first metatarsal has an oval shape, rather than being constricted in the middle to form distinct medial and lateral condyles as inDicraeosaurus. The metatarsal has a distinct posterolateral (pointing both to the side and backwards) projection, as in other flagellicaudatans.[1]
Apesteguía originally hypothesized in 2007 thatAmargatitanis was a member of Titanosauria, a group of large,macronarian sauropods, based on features of the femur and scapula. However, two later papers argued that these titanosaurian traits were instead a misinterpretation of broken elements.[16][1] In 2012, D'Emic in turn regardedAmargatitanis as anomen dubium and that the fossils could not be from the same individual, making it achimera.[17] A 2016 study by Pablo Gallina found that characteristics such as the lack of a lateral bulge on the femur and presence ofprocoelous vertebrae (centra being concave at the front and convex at the back) meantAmargatitanis belonged to the family Dicraeosauridae. This claim was supported byphylogenetic analyses conducted by Gallina and later studies, which typically foundAmargatitanis at the base of the family.[15][5][1] In 2022, researcherGuillermo Windholz and colleagues argued that South American dicraeosaurids formed a clade based on their biogeography and results of phylogenetic analyses, withAmargatitanis being especially closely related toPilmatueia andBajadasaurus. They also hypothesized that Dicraeosauridae originated in North America, with the earliest known genera beingSuuwassea andSmitanosaurus.[18][14]
Bajpai and colleagues, in 2023, recoveredAmargatitanis as an advanced dicraeosaurid inpolytomy withPilmatueia and a group containingBrachytrachelopan,Dicraeosaurus, and the contemporaryAmargasaurus. The results of their phylogenetic analyses are shown in the cladogram below:[5]
In a 2021 paper, two Argentine researchers, Guillermo J. Windholz and Ignacio A. Cerda, examined the bonehistology ofAmargatitanis using thin sections from a femur. The study sought toevaluate the age of the specimen by counting the number and spacing of cyclical growth marks (concentric lines similar to thegrowth rings of a tree), which formed in periods of slow growth. Sauropods are thought to be characterised by high rates of continuous growth, and, as a consequence, cyclical growth marks usually form only after sexual maturity is reached. Growth continued after sexual maturity, with adult size being reached much later.[7]
The femur ofAmargatitanis bears 19 cyclical growth marks, indicating that the individual was at least 19 years old at the time of death. The six outermost growth marks are closely spaced, a structure known as an external fundamental system, indicating that the individual had not only reached sexual maturity but also its adult size. This assessment is supported by the extensiveremodeling of the cortical bone, a trait associated with anatomical and sexual maturity. The high number of cyclical growth marks seen inAmargatitanis is unusual for sauropods, and possibly indicates that some sauropods grew discontinuously rather than continuously.[7]
Amargatitanis stems fromsedimentary rocks of the La Amarga Formation, which is part of theNeuquén Basin and dates to the Barremian and lateAptian of the Early Cretaceous. Most vertebrate fossils, includingAmargatitanis, have been found in the lowermost (oldest) part of the formation, the Puesto Antigual Member. This member is approximately 29 meters (95 ft) in thickness and mainly composed of sandstones deposited bybraided rivers.[19] The sauropod fauna of the La Amarga Formation was diverse and included the basalrebbachisauridZapalasaurus, the dicraeosauridAmargasaurus, and unnamed remains of basaltitanosauriforms.[2][20] The high diversity suggests that different sauropod species exploited different food sources in order to reduce competition. Basal titanosauriforms showed proportionally longer necks, longer forelimbs, and broader tooth crowns than dicraeosaurids and rebbachisaurids, suggesting greater feeding heights.[21]Amargatitanis andAmargasaurus probably fed above ground level at heights of up to 2.7 meters (8.9 ft), as evidenced by the anatomy of their neck and inner ear. Rebbachisaurids likeZapalasaurus presumably fed at ground-level, while basal Titanosauriforms exploited food sources at higher levels.[21]
Other dinosaurs of the La Amarga Formation include an indeterminatestegosaur; predatory dinosaurs include the smallceratosaurLigabueino, and the presence of a largetetanuran is indicated by teeth.[22] Other than dinosaurs, the formation is notable for thecladotherian mammalVincelestes, the only mammal known from the Early Cretaceous of South America.[23]Crocodylomorphs are represented by thetrematochampsidAmargasuchus.[24]
{{citation}}: CS1 maint: work parameter with ISBN (link){{citation}}: CS1 maint: work parameter with ISBN (link)
