Amargasaurus (encyclopedia entry)

Weighing 3–5 tons (6,600–11,000 lbs), Amargasaurus is considerably smaller than some of its better-known sauropod cousins, such as Apatosaurus and Brachiosaurus. Amargasaurus, however, has an important distinguishing feature that has baffled scientists since its discovery.  Amargasaurus has peculiar elongated spines that protrude from the animal’s vertebrae, which are up to 50 cm (21 inches) long in the neck region. The purpose of these high, pointed spines continues to fuel debate among paleontologists.  Some argue that the spines may have formed a sail-like structure used for temperature regulation, defense or mating displays. Others believe that the spines were covered in flesh, creating a hump-like structure similar to that seen in bison and buffalo today.

Classification

Scientists classify dinosaurs in different ways. The Linnaean system is a hierarchical classification system that narrows species into order, genera, and species. In the late twentieth and early twenty-first century, paleontologists moved towards cladistic or phylogenetic classification systems to classify dinosaurs according to their common ancestry and shared physical characteristics. In a cladistic analysis, Amargasaurus is a member of the larger sauropodomorpha suborder of dinosaurs, and the infraorder sauropoda.  Apatosaurus and Brachiosaurus are among some of the best known sauropods. They reached over 20 m (60 ft) in length and weighed up to 50 tons (110,000 lbs). Sauropods share a number of common physical characteristics, including small heads suspended on long necks, long tails, large round bodies, large pillar-like legs, peg-like teeth and nares (nostril channels) positioned high on the skull. Sauropods were herbivorous (plant-eating) animals that walked on four legs.

Cladistic analysis is based on more specific physical characteristics; for example the shape of the head, shape of the nostrils and features of bone and skeletal arrangement.  Amargasaurus is a member of the dicraeosauridae clade or “group” that consists of only two other genera to date, Brachytrachelopan and Dicraeosaurus. The dicraeosauridae clade is characterized by being smaller than other sauropods and having a comparatively shorter neck. This group of dinosaurs also has elongated neural spines that form pointed spikes that protrude from the animals’ vertebrae along the neck, back and tail. Based on fossil records to date, this group appears to be isolated to Gondwana, the land mass in the southern hemisphere. It is important to note that cladistic analyses can change over time as new specimens are discovered and as the science advances.

Anatomy

While Amargasaurus was small for a sauropod, it still weighed up to 5 tons (11,000 lbs) and stood up to 4 m (12 ft) tall. Fossilized sauropod tracks demonstrate that these dinosaurs walked on all four feet with their tails held off the ground. They were lumbering creatures that moved at around 5 km (3 mi) an hour.  The position of sauropod nares high on the skull originally led paleontologists to believe that Amargasaurus and its cousins may have been semi-aquatic animals, using their long necks to breathe at the water’s surface while their bodies were submerged. This theory was refuted, however, when scientists demonstrated that sauropod lungs would not be strong enough to function under the weight of water pressure. It is agreed today that sauropods were terrestrial animals.

The most striking feature of Amargasaurus‘s anatomy is a row of spikes, called neural spines, which protrude from the animal’s vertebrae. They are up to 50 cm (21 in) long in the cervical (neck) region.  Paleontologists debate the purpose of these elongated spines. Some theorize that the spines were webbed with skin, forming a sail-like structure that may have been useful for temperature regulation, defense or mating displays. Critics argue, however, that the spines would have offered little defense, except to intimidate potential predators by exaggerating the animal’s size. They assert that Amargasaurus was too heavy and inflexible to wield the spines defensively. Some paleontologists believe that the spines were covered in flesh and skin, creating a hump similar to that seen in bison and buffalo today. They propose that the hump was an adaptation for energy storage and heat shielding in habitats where there was little shade. While the debate continues, artistic renditions tend to interpret Amargasaurus with webbing between the spines.

Intelligence

Scientists measure animals’ intelligence using the encephalization quotient (EQ), which is determined through the ratio of the animal’s brain weight to body weight. Endothermic (warm-blooded) animals demonstrate larger brains than ectothermic (cold-blooded) animals. Assuming that a higher EQ equals higher intelligence, scientists can predict the intelligence of animals, living and extinct. Paleontologist James A. Hopson estimated the brain to body weight ratio of dinosaurs by rating them in comparison to the brain to body weight ratio of a crocodile. Hopson developed a scale that showed if a dinosaur’s EQ was smaller or larger than expected.

With extremely small heads compared with their massive bodies, creatures in the sauropodomorpha suborder have the lowest EQ of all dinosaurs. Their EQ score is less than 0.5 (Hopson), compared with 1.0 for a crocodile, up to 2.0 for theropods and 5.8 for dromaeosaurids. Amargasaurus‘s low EQ further implies that these animals were docile, lumbering giants.

Reproduction & Population

In 1997, an expedition in Patagonia discovered a nesting ground that contained thousands of fossilized dinosaur eggs and embryos. Scientists identified that the eggs came from sauropod dinosaurs. The finding confirmed that these dinosaurs were oviparous (egg-laying) and revealed a great deal about their reproduction habits.

Each egg was approximately 12 to 15 cm in diameter. They were arranged in clusters consisting of six to twelve eggs. Scientists inferred from the high concentration of clusters that sauropod dinosaurs laid their eggs in communal nesting grounds. While scientists cannot definitively say if sauropod dinosaurs cared for their eggs, the close proximity of egg clusters combined with the large size of these dinosaurs suggest that parental care of the eggs, or brooding, would have been difficult. Alternatively, it is possible that adults protected the nesting ground from a distance. It is not known if dinosaurs cared for their young after hatching, but herding behaviour demonstrated in fossilized track sites imply that the young were adopted into adult groups. Scientific investigation of sauropod bones indicates that these animals grew quickly after hatching, probably reaching sexual maturity in less than twenty years.

Diet

The shape of Amargasaurus‘s teeth indicates that it was an herbivore (it ate only plants). Its peg-like teeth were designed for eating a diet that consisted of mainly low-lying plants, such as ferns, cycads and small conifers. While researchers agree that Amargasaurus was mainly a terrestrial animal (it lived on land), its long neck may also have been used for feeding along river banks and under water.

Amargasaurus ingested its food by stripping leaves from branches and swallowing them whole without chewing. Since the Amargasaurus weighed over 3 tons (6,600 lbs), its energy requirements meant that it spent the majority of each day grazing. Amargasaurus‘s high food demand also suggests that these dinosaurs roamed over a wide geography.

Researchers have proposed that Amargasaurus and other sauropods may have reared on their hind legs to reach higher foliage. Proponents of this idea suggest that the animal used its rear legs and tail as a tripod. Critics, however, argue that the large size of these animals, coupled with their long necks, would have made balancing difficult.

Behavior

Fossilized trackways (footprints preserved in mud) suggest that Amargasaurus, like other sauropods, was probably a docile animal that moved slowly in herds. The diversity of prints found in trackways implies that these large animals sometimes moved in mixed groups. It is not known if Amargasaurus had a simple or complex social structure or whether the species preferred a social or solitary existence. Scientists do know that their large size prevented them from walking quickly. Traveling in numbers, therefore, may have provided protection from predators, especially for the young.

Being an herbivorous animal with a large calorie requirement, Amargasaurus needed to move from place to place in search of food. These big animals would have been capable of destroying large areas of vegetation quickly as they foraged. It is estimated, therefore, that they ranged 20 to 40 km (12–25 mi) a day in search of food. Amargasaurus‘s population and distribution is unknown, but scientists know that the Early Cretaceous period was dominated by sauropod species.

Habitat & Other Life Forms

The climate of the Early Cretaceous period was warm and humid, contributing to lush forests and fertile plains. In addition to gymnosperms (non-flowering, seed-bearing plants), angiosperms (flowering plants) appeared for the first time and began to quickly evolve into many different species. This evolutionary step supported an increase in the number and diversity of insects, as well as the first appearance of small mammals.

Amargasaurus lived in the geographic area we know today as Patagonia, at the southernmost tip of South America. The area looked much different in the Cretaceous period than it does today.  South America was still part of Gondwana and the Andes Mountain Range had not yet been formed. The geography consisted of low-lying marsh areas, fertile plains and forested hills vegetated by ferns, cycads and conifers.

Amargasaurus shared its environment with other giant, herbivorous dinosaur species, such as Rayosaurus, Agustinia and other sauropods. It’s likely that these docile animals grazed and moved together in herds.  Carnivorous dinosaurs had not yet evolved into giant species, but Amargasaurus may have been vulnerable to predators such as Tyrannotitan, a 12-meter-long, meat-eating theropod.

Research

Argentinean paleontologist Leonardo Salgado unearthed the Amargasaurus fossil in 1984 in the La Armaga Canyon in the Neuquén province in southern Argentina. Neuquén is located in Patagonia, a geographic area that comprises the southernmost part of South America, straddling the national borders of Argentina and Chile. Salgado collaborated with his mentor, José F. Bonaparte, in naming the new genus Amargasaurus, meaning “Lizard from La Amarga.” Bonaparte is one of the most prolific dinosaur discoverers and researchers of all time. Excavations at the La Amarga Formation have unearthed a variety of dinosaur fossils from the Early Cretaceous period, including sauropods, theropods, stegosaurs and crocodilians. Argentina is proving to be an extremely valuable source for dinosaur fossils, now accounting for around 10 percent of dinosaur species discovered worldwide.

Scientists continue to debate about whether dinosaurs were endothermic (warm-blooded) or ectothermic (cold-blooded). The majority of scholars tend to agree, however, that the large size of sauropods coupled with the warm environment of the Mesozoic era, probably meant that sauropods behaved like warm-blooded animals, even though they lacked an endothermic metabolism. Given its large size and warm environment, Amargasaurus‘s greatest thermoregulation risk would have been from over-heating. Thermoregulation is a key factor contributing to the debate about the purpose of Amargasaurus‘s peculiar neural spines.

Vocabulary

Dicraeosauridae: A family of sauropod dinosaurs, of which there are only three genera: Brachytrachelopan, Dicraeosaurus, and Amargasaurus. Dicraeosauridae are characterized by shorter necks than other sauropods, and tall neural spines.

Diplodocoidea: A super family of sauropods characterized by extremely long necks and tails. Members of the dicraeosauridae group are an exception to the diplodocoidea rule because of their shorter necks and small stature.

Encephalization quotient (EQ): Ratio of an organism’s brain weight to the expected brain weight of a similarly sized organism. Used by scientists to estimate intelligence.

Ectothermic: A trait of cold-blooded animals whereby the regulation of their body temperature is dependent upon external environmental factors.

Endothermic: A trait of warm-blooded animals whereby they generate their body heat at a fairly constant temperature, despite changes in the surrounding climate.

Nares: Nostrils or nasal passages.

Neural spine: A projection that points backward from the vertebrae. In the case of Amargasaurus, the neural spines are so exaggerated that they form bizarre spikes.

Oviparous: An egg-laying trait whereby young develop outside of the mother’s body.

Bibliography

Books:

Fastovsky, David E. and David B. Weishampel. “Sauropodomorpha: the Big, the Bizarre and the Majestic.” The Evolution and Extinction of Dinosaurs. 2nd ed. New York: Cambridge University Press, 2005.

McIntosh, John S., M.K. Brett-Surman and James O. Farlow. “Sauropods.” The Complete Dinosaur. Eds. James O. Farlow and M.K. Brett-Surman. Bloomington, IN: Indiana University Press, 1999.

Periodicals:

Bailey, Jack Bowman. “Neural Spine Elongation in Dinosaurs: Sailbacks or Buffalo Backs?” Journal of Paleontology 71.6 (1997): 1124-1146.

Chiappe, Luis et al. “Sauropod Eggs and Embryos from the Late Cretaceous of Patagonia”. Extended Abstracts of the First International Symposium on Dinosaur Eggs and Babies (2000): 23-29.

O’Connor, Michael P. and Peter Dodson. “Biophysical Constraints on the Thermal Ecology of Dinosaurs.” In Paleobiology 25.3 (1999): 341-368.

Websites:

American Museum of Natural History. http://www.amnh.org/exhibitions/expeditions/dinosaur/patagonia/discovery.html “The Discovery.” Accessed June 2010.

University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/diapsids/saurischia/sauropoda.html “The Sauropod Dinosaurs.” Accessed June 2010.

About the Author

Kylie Grimshaw Hughes, BA Hons, MA. Kylie has a Bachelor of Arts degree with Honors in Labor Studies and Politics from the University of Adelaide, Australia, and a Master of Arts degree in Communication Studies from the University of South Australia.

Written by Kylie Hughes for EBSCO Publishing

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