Amargasaurus
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''Amargasaurus'' (; "La Amarga lizard") is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of sauropod
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
from the Early Cretaceous
epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...
(129.4–122.46 mya) of what is now
Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...
. The only known skeleton was discovered in 1984 and is virtually complete, including a fragmentary skull, making ''Amargasaurus'' one of the best-known sauropods of its epoch. ''Amargasaurus'' was first described in 1991 and contains a single known
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, ''Amargasaurus cazaui''. It was a large animal, but small for a sauropod, reaching in length. Most distinctively, it sported two parallel rows of tall spines down its neck and back, taller than in any other known sauropod. In life, these spines could have stuck out of the body as solitary structures that supported a
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
ous sheath. An alternate hypothesis, now more favored, postulates that they could have formed a scaffold supporting a skin sail. They might have been used for display, combat, or defense. ''Amargasaurus'' was discovered in
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s of the La Amarga Formation, which dates back to the
Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is precede ...
and late Aptian
stage Stage or stages may refer to: Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly British theatre newspaper * Sta ...
s of the Early Cretaceous. A herbivore, it shared its environment with at least three other sauropod genera, which might have exploited different food sources in order to reduce competition. ''Amargasaurus'' probably fed at mid-height, as shown by the orientation of its inner ear and the articulation of its neck vertebrae, which suggest a habitual position of the snout above the ground and a maximum height of . Within the Sauropoda, ''Amargasaurus'' is classified as a member of the family
Dicraeosauridae Dicraeosauridae is a family of diplodocoid sauropods who are the sister group to Diplodocidae. Dicraeosaurids are a part of the Flagellicaudata, along with Diplodocidae. Dicraeosauridae includes genera such as '' Amargasaurus'', '' Suuwassea' ...
, which differs from other sauropods in showing shorter necks and smaller body sizes.


Description

''Amargasaurus'' was small for a sauropod, measuring in length and weighing approximately . It followed the typical sauropod
body plan A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually applied to animals, envisages a "blueprin ...
, with a long tail and neck, a small head, and a barrel-shaped trunk supported by four column-like legs. The neck of ''Amargasaurus'' was shorter than in most other sauropods, a common trait within the
Dicraeosauridae Dicraeosauridae is a family of diplodocoid sauropods who are the sister group to Diplodocidae. Dicraeosaurids are a part of the Flagellicaudata, along with Diplodocidae. Dicraeosauridae includes genera such as '' Amargasaurus'', '' Suuwassea' ...
. Measuring in length, the neck corresponded to 136% of the length of the dorsal
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...
. This is comparable to ''
Dicraeosaurus ''Dicraeosaurus'' (Gr. , ' "bifurcated, double-headed" + Gr. , ' "lizard") is a genus of diplodocoid sauropod dinosaur that lived in what is now Lindi Region, Tanzania during the late Jurassic period. The genus was named for the neural spines on ...
'' (123%) but greater than in the extremely short-necked form ''
Brachytrachelopan ''Brachytrachelopan'' is a short-necked sauropod dinosaur from the Late Jurassic ( Oxfordian to Tithonian) of Argentina. The holotype and only known specimen (Museo Paleontológico Egidio Feruglio MPEF-PV 1716) was collected from an erosion ...
'' (75%). The neck consisted of thirteen cervical vertebrae, which were opisthocoelous (convex at the front and hollow at the back), forming
ball-and-socket joint The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
s with neighboring vertebrae. The trunk was made out of nine
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...
and probably five fused sacral vertebrae. The foremost dorsals were opisthocoelous, while the remaining dorsals were amphiplatyan (flat on both ends). Robust transverse processes (lateral projections connecting to the ribs) indicate a strongly developed rib cage. The dorsal vertebrae of ''Amargasaurus'' and other dicraeosaurids lacked
pleurocoels Skeletal pneumaticity is the presence of air spaces within bones. It is generally produced during development by excavation of bone by pneumatic diverticula (air sacs) from an air-filled space, such as the lungs or nasal cavity. Pneumatization is h ...
, the deep lateral excavations that were characteristic for other sauropods. The most striking features of the skeleton were the extremely tall, upwardly projecting
neural spine The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
s on the neck and anterior dorsal vertebrae. From the third cervical onward, these neural spines were bifurcated along their entire length, forming a double row. They were circular in cross section and tapered towards their tips. The tallest spines could be found on the middle part of the neck, where they reached on the 8th cervical. On the neck, they were bowed backward, projecting above the adjacent vertebra. A similar elongated neural spine has been described from the neck region of the closely related ''
Bajadasaurus ''Bajadasaurus'' is a genus of sauropod dinosaur from the Early Cretaceous epoch ( late Berriasian to Valanginian stages, between 145 and 132.9 million years ago) of northern Patagonia, Argentina. It was first described in 2019 based on a single ...
'' in 2019. Unlike in ''Amargasaurus'', this spine was bowed frontward and broadened toward the tips. The last two dorsal vertebrae, the hip, and the foremost tail in ''Amargasaurus'' also had elongated spines; these were not bifurcated but flared into a paddle-shaped upper end. The pelvic region was relatively wide, judging from the long, laterally projecting transverse processes of the sacral vertebrae. The forelimbs were somewhat shorter than the hind limbs, as in related sauropods. Most of the hand and foot bones were not preserved, but ''Amargasaurus'' probably possessed five digits each as in all sauropods. Only the rear part of the skull is preserved. It likely showed a horselike, broad snout equipped with pencil-like teeth, as seen in related sauropods for which more complete skulls are known. As in other dicraeosaurids, the (nostril opening) was situated in the posterior half of the skull, diagonally above the (eye opening), which was proportionally large. As in most other dinosaurs, the skull featured three additional openings ( fenestrae). The , located below the orbit, was long and narrow. Behind the orbit was the , which in dicraeosaurids was uniquely small and can be seen when the skull is viewed from the side. This contrasts with other diapsid reptiles, where these openings were directed upward, thus being visible only in top view. The
antorbital fenestra An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with archosauriforms, first appearing during the Triassic Period. Among extant archosaurs, bird ...
would have been located in front of the eye opening, although this region is not preserved. An unusual feature were small openings seen on the backside of the skull, the so-called parietal openings or fontanelles. In other
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s, these openings are usually seen only in juveniles and would close as the individual grows. Skull features shared with ''Dicraeosaurus'' but absent in most other sauropods included the fused and the notably long , bony extensions connecting the with the .


Discovery

The only known skeleton (specimen number MACN-N 15) was discovered in February 1984 by Guillermo Rougier during an expedition led by Argentine paleontologist
José Bonaparte José Fernando Bonaparte (14 June 1928 – 18 February 2020) was an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists . One of the best-known Argentine paleo ...
. This was the eighth expedition of the project "Jurassic and Cretaceous Terrestrial Vertebrates of South America", which was supported by the
National Geographic Society The National Geographic Society (NGS), headquartered in Washington, D.C., United States, is one of the largest non-profit scientific and educational organizations in the world. Founded in 1888, its interests include geography, archaeology, an ...
and initiated in 1975 to improve on the sparse knowledge of the Jurassic and Cretaceous tetrapod life of South America. The same excursion uncovered the nearly complete skeleton of the horned
theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...
''
Carnotaurus ''Carnotaurus'' (; ) is a genus of theropod dinosaur that lived in South America during the Late Cretaceous period, probably sometime between 71 and 69 million years ago. The only species is ''Carnotaurus sastrei''. Known from a single well-p ...
''. The discovery site is located in the La Amarga arroyo in the Picún Leufú Department of Neuquén Province in northern
Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and g ...
, south of Zapala. The skeleton stems from
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s of the La Amarga Formation, which dates to the
Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is precede ...
through early Aptian stages of the Early Cretaceous, or around 130 to 120 million years ago. The skeleton is reasonably complete and includes a partial skull. Sauropod skull bones are rarely found, and the ''Amargasaurus'' skull is only the second skull known from a member of the Dicraeosauridae. Major parts of the skeleton were found in their original anatomical position: the vertebral column of the neck and back, which consisted of 22 articulated vertebrae, was found connected to both the skull and the sacrum. Of the skull, only the temporal region and the braincase are preserved. The sacrum, despite being partly eroded prior to burial, is fairly complete. Most of the tail is missing, with three anterior, three middle, and one posterior vertebrae being preserved, along with fragments of several others. The shoulder girdle is known from the
scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...
(shoulder blade) and
coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
(which sits on the lower end of the scapula), while the pelvis is merely known from the ilium (the uppermost of the three pelvic bones). The limbs are equally fragmentary, with the manus (hand) and most of the pes (foot) missing. The skeleton is currently stored in the collection of the
Bernardino Rivadavia Natural Sciences Museum The Bernardino Rivadavia Natural Sciences Argentine Museum ( es, Museo Argentino de Ciencias Naturales Bernardino Rivadavia) is a public museum located in the Caballito section of Buenos Aires, Argentina. History and overview The museum owes it ...
in
Buenos Aires Buenos Aires ( or ; ), officially the Autonomous City of Buenos Aires ( es, link=no, Ciudad Autónoma de Buenos Aires), is the capital and primate city of Argentina. The city is located on the western shore of the Río de la Plata, on South ...
. The first, unofficial, mention of ''Amargasaurus'' as a new genus of dinosaur was published by Bonaparte in the 1984 Italian book ''Sulle Orme dei Dinosauri''. Here, the species was designated as ''Amargasaurus groeberi'', honoring
Pablo Groeber Pablo Groeber (born Paul Friedrich Karl Gröber; 1885–1964) was a German geologist known for his contributions to the understanding of the geology of Tien Shan in Central Asia and the Andes of Nequén and Mendoza Province in Argentina. See als ...
, which was changed into ''Amargasaurus cazaui'' in the official description published several years later. The official description, written in Spanish, was published in 1991 by Leonardo Salgado and Bonaparte in the Argentinian scientific journal ''
Ameghiniana ''Ameghiniana'' is a peer-reviewed scientific journal covering palaeontology published by the Asociación Paleontológica Argentina. It is named after the 19th century Italian Argentine palaeontologist Florentino Ameghino. The discovery of many ...
''. The name ''Amargasaurus'' alludes to the site of discovery, the La Amarga Arroyo. La Amarga is also the name of a nearby town, as well as the
geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...
the remains were recovered from. The word ''amarga'' is
Spanish Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many Latin American countries **Spanish cuisine Other places * Spanish, Ontario, Can ...
for "bitter", while ''sauros'' is
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
for "lizard". The one
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
(''A. cazaui'') is named in honor of Luis Cazau, a
geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althoug ...
with the YPF
oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
company, which at the time was state-owned. In 1983, Cazau informed Bonaparte's team about the paleontological significance of the La Amarga Formation, leading to the discovery of the skeleton. One year later, Salgado and Jorge O. Calvo published a second paper focusing on the description of the skull.


Classification

''Amargasaurus'' is classified as a member of the Dicraeosauridae, a
family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
ranked A ranking is a relationship between a set of items such that, for any two items, the first is either "ranked higher than", "ranked lower than" or "ranked equal to" the second. In mathematics, this is known as a weak order or total preorder of ...
clade within the
Diplodocoidea Diplodocoidea is a superfamily of sauropod dinosaurs, which included some of the longest animals of all time, including slender giants like '' Supersaurus'', ''Diplodocus'', ''Apatosaurus'', and '' Amphicoelias''. Most had very long necks and lo ...
. Currently, this clade consists of nine species belonging to eight genera. These include '' Lingwulong shenqi'' from the Early or Middle Jurassic of China and four species from the Late Jurassic: ''Brachytrachelopan mesai'' from Argentina; '' Suuwassea emilieae'' from the Morrison Formation of the United States; and ''Dicraeosaurus hansemanni'' and ''Dicraeosaurus sattleri'' from the Tendaguru beds of
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands ...
. ''Amargasaurus'' was the first dicraeosaurid known from the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
, although additional dicraeosaurids from the Lower Cretaceous have been described more recently, including '' Pilmatueia faundezi'', '' Amargatitanis macni'', and '' Bajadasaurus pronuspinax'', which are all from Argentina. An unnamed specimen from Brazil indicates that this group persisted at least until the end of the Early Cretaceous. Most analyses find ''Dicraeosaurus'' and ''
Brachytrachelopan ''Brachytrachelopan'' is a short-necked sauropod dinosaur from the Late Jurassic ( Oxfordian to Tithonian) of Argentina. The holotype and only known specimen (Museo Paleontológico Egidio Feruglio MPEF-PV 1716) was collected from an erosion ...
'' to be more closely related to each other than to ''Amargasaurus''. ''Suuwassea'' is generally recovered as the most basal member of the family. A 2015 analysis by Tschopp and colleagues came to the preliminary result that two poorly known genera from the Morrison Formation, '' Dyslocosaurus polyonychius'' and '' Dystrophaeus viaemalae'', might be additional members of the Dicraeosauridae. Together with the
Diplodocidae Diplodocids, or members of the family Diplodocidae ("double beams"), are a group of sauropod dinosaurs. The family includes some of the longest creatures ever to walk the Earth, including ''Diplodocus'' and ''Supersaurus'', some of which may have ...
and the
Rebbachisauridae Rebbachisauridae is a family of sauropod dinosaurs known from fragmentary fossil remains from the Cretaceous of South America, Africa, North America, Europe and possibly Central Asia. Taxonomy In 1990 sauropod specialist Jack McIntosh included t ...
, the Dicraeosauridae is nested inside the
Diplodocoidea Diplodocoidea is a superfamily of sauropod dinosaurs, which included some of the longest animals of all time, including slender giants like '' Supersaurus'', ''Diplodocus'', ''Apatosaurus'', and '' Amphicoelias''. Most had very long necks and lo ...
. All members of the Diplodocoidea are characterized by their box-shaped snout and narrow teeth restricted to the foremost portion of the jaws. Both the Dicraeosauridae and the Diplodocidae are characterized by bifurcated neural spines of the cervical and dorsal vertebra. In the Dicraeosauridae, the bifurcated neural spines were strongly elongated, a trend reaching its extreme in ''Amargasaurus''. The following
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
by Gallina and colleagues (2019) shows the presumed relationships between members of the Dicraeosauridae:


Paleobiology


Vertebral spines

Both the function and the appearance in life of the extremely elongated and bifurcated vertebral spines remain elusive. Salgado and Bonaparte, in 1991, suggested the spines represented defense weapons against predators, arguing that they tapered towards their tips. They also could have served for display, perhaps for courtship or to intimidate rivals. Some subsequently published life restorations showed the double row of spines supporting two parallel skin sails.
Gregory Paul Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dino ...
, in 1994, considered this possibility unlikely, noting that neck sails would have reduced neck flexion, and that the spines were circular in cross-section rather than flattened as is the case in sail-bearing animals. Instead, he found that this shape indicates that the spines supported a keratinous sheath that would have extended the length of the spines in life. The spines could have been used for display or as weapons both against predators and members of the same species, as the animal might have been able to point its most anterior spines forward by bending its neck. He also hypothesized that the spines could have been clattered together to generate sound. Keratinous sheaths covering the spines were also shown in a 1999 skeletal restoration published by Salgado. Jack Bailey, in 1997, argued that the spines resembled those of sail-bearing
pelycosaur Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is ...
s like '' Dimetrodon''. According to this author, ''Amargasaurus'' might also have possessed such a sail, which might have been used for display. Unlike those of pelycosaurs, the neural spines of ''Amargasaurus'' were bifurcated, forming a double row along the neck and back. As the space between both rows was merely , the existence of two parallel sails seems unlikely. Instead, Bailey suggested the spines represented a scaffold which was completely enveloped by a single skin. Neural spines from the
penultimate Penult is a linguistics term for the second to last syllable of a word. It is an abbreviation of ''penultimate'', which describes the next-to-last item in a series. The penult follows the antepenult and precedes the ultima. For example, the main ...
dorsal vertebra to the foremost tail vertebrae also were strongly elongated, but different in structure, forming a single row of paddle-shaped projections. According to Bailey, these projections resembled those of modern humped
ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, ...
s such as the bison, indicating the presence of a fleshy hump above the hips. Bailey suggested similar humps for other dinosaurs with strongly elongated neural spines, such as ''
Spinosaurus ''Spinosaurus'' (; ) is a genus of spinosaurid dinosaur that lived in what now is North Africa during the Cenomanian to upper Turonian stages of the Late Cretaceous period, about 99 to 93.5 million years ago. The genus was known first f ...
'' and '' Ouranosaurus''. Daniela Schwarz and colleagues, in 2007, concluded that the bifurcated neural spines of diplodocids and dicraeosaurids enclosed an
air sac Air sacs are spaces within an organism where there is the constant presence of air. Among modern animals, birds possess the most air sacs (9–11), with their extinct dinosaurian relatives showing a great increase in the pneumatization (presence ...
, which would have been connected to the lungs as part of the
respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies ...
. In ''Dicraeosaurus'', this air sac (the so-called supravertebral diverticulum) would have rested on top of the neural arch and filled the entire space between the spines. In ''Amargasaurus'' the upper two-thirds of the spines would have been covered by a sheath of keratin, restricting the air sac to the space between the lower one-third of the spines. A cover of either keratin or skin is indicated striations on the surface of the spines similar to those of bony horn cores of today's
bovid The Bovidae comprise the biological family of cloven-hoofed, ruminant mammals that includes cattle, bison, buffalo, antelopes, and caprines. A member of this family is called a bovid. With 143 extant species and 300 known extinct species, ...
s. In 2016, Mark Hallett and Mathew Wedel suggested that the backwards-directed spines might have been able to skewer predators when the neck was abruptly drawn backwards during an attack. A similar defense strategy is found in today's
giant sable antelope The giant sable antelope or royal sable antelope (''Hippotragus niger variani''), also known in Portuguese as the ''palanca-negra-gigante'', is a large, rare subspecies of the sable antelope native and endemic to the region between the Cuango an ...
and
Arabian oryx The Arabian oryx (''Oryx leucoryx'') or white oryx is a medium-sized antelope with a distinct shoulder bump, long, straight horns, and a tufted tail. It is a bovid, and the smallest member of the genus ''Oryx'', native to desert and steppe areas o ...
, which can use their long, backwards directed horns to stab attacking lions. Apart from the possible function in defense, the spines may have been used for display, either for the intimidation of rivaling individuals or for courtship. Hallett and Wedel also hypothesized that rival males might have interlocked their spines for neck wrestling. Pablo Gallina and colleagues (2019) described the closely related ''Bajadasaurus'', which had neural spines similar to those of ''Amargasaurus'', and suggested that both genera employed them for defense. A defense function would have been especially effective in ''Bajadasaurus'' as the spines were directed forwards and would have reached past the tip of the snout, deterring predators. The keratinous sheath that likely covered the spines might have extended their length by 50%, as seen in some modern
even-toed ungulate The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...
s. Such extended sheaths would have made the delicate spines more resistant to damage—likely a critical threat, as the bases of the spines form the roof of the spinal chord. In 2022, a detailed study by Ignacio A. Cerda and colleagues analyzing the structure, morphology, and microanatomy of the vertebral spines of ''Amargasaurus'' and an indeterminate dicraeosaurid (also from the La Amarga Formation) suggested that the spines were not covered in a keratinous sheath as previously believed. Osteohistology of the spines suggests that they were likely, if not exclusively, covered in a sail of skin. The spines are also highly vascularized and bear cyclical growth marks, adding credence to this theory.


Senses and posture

Paulina Carabajal and colleagues, in 2014, CT-scanned the skull, allowing for the generation of three-dimensional models of both the cranial
endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...
(the cast of the brain cavity) and the inner ear. Using these models, the cranial endocast was shown to encompass in volume. The inner ear was tall and wide. The lagena, the part containing the
hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s for hearing, was rather short, indicating that the sense of hearing would have been poorer in ''Amargasaurus'' than in other sauropods for which inner ears have been studied. The first skeletal reconstructions show the skull in a near-horizontal posture. Salgado, in 1999, argued that such a posture would have been anatomically impossible due to the elongated neural spines of the neck vertebrae. Instead, he envisaged the head in a nearly vertical orientation. The habitual orientation of the head is usually reflected by the orientation of the
semicircular canals The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The ...
of the inner ear, which housed the sense of balance ( vestibular system). Using their three-dimensional model of the inner ear, Carabajal and colleagues suggested that the snout faced downwards at an angle of roughly 65° relative to the horizontal. A similar value has recently been proposed for the related ''
Diplodocus ''Diplodocus'' (, , or ) was a genus of diplodocid sauropod dinosaurs, whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a neo-Latin term derived from Greek δι ...
''. The neutral posture of the neck can be approximated based on how the cervical vertebrae attached to each other. According to Carabajal and colleagues, the neck was gently sloping downwards, so that the snout would have rested above the ground in neutral posture. In reality, neck posture would have varied according to the respective activities of the animals. Raising of the neck, e.g. for reaching an alert position, would have been constricted by the elongated neural spines, not permitting heights greater than .


Locomotion

''Amargasaurus'' was
quadrupedal Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor ...
(moved on four legs), and probably was unable to rear on its hind legs. Salgado and Bonaparte, in 1991, suggested that ''Amargasaurus'' was a slow walker, as both the forearms and lower legs were proportionally short, as a feature common to slow-moving animals. This was contradicted by Gerardo Mazzetta and Richard Fariña in 1999, who argued that ''Amargasaurus'' was capable of rapid locomotion. During locomotion, leg bones are strongly affected by
bending moment In solid mechanics, a bending moment is the reaction induced in a structural element when an external force or moment is applied to the element, causing the element to bend. The most common or simplest structural element subjected to bending mo ...
s, representing a limiting factor for the maximum speed of an animal. The leg bones of ''Amargasaurus'' were even more sturdy than those of today's
white rhinoceros The white rhinoceros, white rhino or square-lipped rhinoceros (''Ceratotherium simum'') is the largest extant species of rhinoceros. It has a wide mouth used for grazing and is the most social of all rhino species. The white rhinoceros consists ...
, which is adapted to galloping.


Paleoecology

''Amargasaurus'' stems from
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s of the La Amarga Formation, which is part of the
Neuquén Basin Neuquén Basin ( es, Cuenca Neuquina) is a sedimentary basin covering most of Neuquén Province in Argentina. The basin originated in the Jurassic and developed through alternating continental and marine conditions well into the Tertiary. The bas ...
and dates to the
Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is precede ...
and late Aptian of the Early Cretaceous. Most vertebrate fossils, including ''Amargasaurus'', have been found in the lowermost (oldest) part of the formation, the Puesto Antigual Member. This member is approximately in thickness and mainly composed of sandstones deposited by braided rivers. The ''Amargasaurus'' skeleton itself was recovered from a layer composed of sandy conglomerates. The sauropod fauna of the La Amarga Formation was diverse and included the basal rebbachisaurid '' Zapalasaurus'', the
dicraeosaurid Dicraeosauridae is a family of diplodocoid sauropods who are the sister group to Diplodocidae. Dicraeosaurids are a part of the Flagellicaudata, along with Diplodocidae. Dicraeosauridae includes genera such as ''Amargasaurus'', ''Suuwassea'', ' ...
''
Amargatitanis ''Amargatitanis'' (meaning " Amarga giant") is a genus of dicraeosaurid sauropod dinosaur (a type of large, long-necked quadrupedal herbivorous dinosaur) from the Barremian-age (Lower Cretaceous) La Amarga Formation of Neuquén, Argentina. Cl ...
'', and unnamed remains of basal titanosauriforms. 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. ''Amargasaurus'' probably fed above ground level at heights of up to , as evidenced by the anatomy of its neck and inner ear. Rebbachisaurids like ''Zapalasaurus'' presumably fed at ground-level, while basal Titanosauriforms exploited food sources at higher levels. Other dinosaurs of the La Amarga Formation include the stegosaur '' Amargastegos''; predatory dinosaurs include the small
ceratosaur Ceratosaurs are members of the clade Ceratosauria, a group of dinosaurs defined as all theropods sharing a more recent common ancestor with ''Ceratosaurus'' than with birds. The oldest known ceratosaur, ''Saltriovenator'', dates to the earlies ...
''
Ligabueino ''Ligabueino'' (meaning "Ligabue's little one") is a genus of noasaurid dinosaur named after its discoverer, Italian doctor Giancarlo Ligabue. It is known only from an extremely fragmentary specimen, measuring 79 cm (2.6 ft) long, foun ...
'', and the presence of a large
tetanuran Tetanurae (/ˌtɛtəˈnjuːriː/ or "stiff tails") is a clade that includes most theropod dinosaurs, including megalosauroids, allosauroids, tyrannosauroids, ornithomimosaurs, compsognathids and maniraptorans (including birds). Tetanurans ar ...
is indicated by teeth. Other than dinosaurs, the formation is notable for the cladotherian mammal '' Vincelestes'', the only mammal known from the Early Cretaceous of South America.
Crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...
s are represented by the trematochampsid '' Amargasuchus'' – the
holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...
of this genus was found in association with the ''Amargasaurus'' bones.


References


External links


English translation of original description (with figures)
PDF a
The Polyglot Paleontologist
{{Taxonbar, from=Q311282 Dicraeosaurids Aptian life Barremian life Early Cretaceous dinosaurs of South America Cretaceous Argentina Fossils of Argentina Fossil taxa described in 1991 Taxa named by José Bonaparte