Fossil range: Late Cretaceous
Life restoration of Maiasaura.
| Scientific classification
Maiasaura (meaning "good mother lizard") is an extinct genus of large duck-billed dinosaur that lived in the area currently covered by the state of Montana in the Upper Cretaceous Period (Campanian), approximately 74 million years ago. It was one of the last non-avian dinosaurs, and lived alongside Triceratops and Tyrannosaurus shortly before the Cretaceous–Tertiary extinction event.
The first fossils of Maiasaura were discovered by paleontologists Jack Horner and Robert Makela in 1978 in the Two Medicine Formation. The discovery included colonies of fossilized nests containing many still-intact eggs and adult specimens, which led paleontologists to believe that newborn hatchlings stayed with the herd, and that their parents provided for them, at least for a while. This apparent form of parental care led to its discovers to christen the genus with the name Maiasaura.
Maiasaura was large, attaining an adult length of about 9 meters (30 feet) and had the typical hadrosaurid flat beak and a thick nose. It had a small, spiky crest in front of its eyes. The crest may have been used in headbutting contests between males during the breeding season.
This dinosaur was herbivorous. It walked both on two (bipedal) or four (quadrupedal) legs and appeared to have no defense against predators, except perhaps its heavy, muscular tail and its herding behavior. These herds were extremely large and could have comprised as many as 10,000 individuals.
Maiasaura was discovered by dinosaur paleontologist Jack Horner and Robert Makela. The original discovery comprised 15 hatchlings found in a fossilized nest, and subsequent visits to the site yieled fossilized eggs, more hatchlings, sub-adult, and adult Maiasaura. The spherical eggs are classified as belonging to the Spheroolithus variety, and have a diamter of reoughly 5 inches, and a total cubic volume of approximately 55 cubic inches. The dinosaur was named after the discovery of a series of nests with remains of eggshells and hatchlings at "Egg Mountain", in rocks of the Two Medicine Formation near Choteau in western Montana. Careful examination and study of the fossil site led to several discoveries and insights regarding the nurturing habits of Maiasaura, as well as other dinosaurs. Many of the excavated Maiasaura skeletons were particularly large; clearly too large to be hatchlings, which lead to the conclusion that even after hatching, young Maiasaura lived with the herd. Evidence for this form of parental care is also seen in close examination of the leg bones of young Maiasaura, showing that they were not fully formed when the animals hatched, similar to hatchling modern birds, in that the bones of hatchlings lacked well-ossified ends, and and instead ended in calcified cartilage pads. As a result of this, the hatchlings were most likely confined to their nest for upwards of a few weeks. Over 200 specimens, in all age and size ranges, have been found.
Maiasaura varies ontogenetically, in that juvenile and adolescent Maiasaura growth rates indicate highly vascularized woven bone. based on analyses of several specimens, ranging from hatchlings to fully grown adults, it is widelt accepted that Maiasaura reached approximately nine feet in length during its first year. This is consistent with the growth rates of modern-day flightless birds that are warm-blooded, and is faster than the growth rates of cold-blooded reptiles.
Maiasaura lived in herds and it raised its young in nesting colonies. The nests in the colonies were packed closely together, like those of modern seabirds, with the gap between the nests being around 7 meters (23 ft); less than the length of the adult animal. The nests were made of earth or sand, and was repeatedly layered, so as to provide concealment of the eggs from potential predators, and to aid in egg incubation. The nests contained anywhere from 30 to 40 eggs, all laid in a circular or spiral pattern within the nest. The eggs were about the size of ostrich eggs.
The eggs were incubated by the heat resulting from rotting vegetation placed into the nest by the parents, rather than a parent sitting on the nest. Upon hatching, fossils of baby Maiasaura show that their legs were not fully developed and thus they were incapable of walking. Fossils also show that their teeth were partly worn, which means that the adults brought food to the nest.
Evidence for parental care in Maiasaura has also been seen in close examination of the leg bones of young Maiasaura, showing that they were not fully formed when the animals hatched, similar to hatchling modern birds. As a result of this, the hatchlings were most likely confined to their nest for upwards of a few weeks. This is evidenced by the presence of trampled eggshell remains within the nests. If the hatchlings had left the nest as soon as they had hatched, then the eggshells would be left more intact.
Newborn Maiasaura were approximately 12 inches (30 cm) in length, however, by approximately three to five weeks after hatching, the Maiasaura were in lengths of upward of 5 feet (1.5 m) long, and were capable of leaving the nest on their own. Maiasaura grew from a size of 16 to 58 inches (41 to 150 cm) long in the span of their first year. This high rate of growth may be evidence of warm bloodedness. The hatchlings had different facial proportions from the adults, with larger eyes, a shorter snout, and as they grew older, developed a flatter and wider head. These features are associated with cuteness and are common among animals that are dependent on their parents' for survival during the early stages of life.
The individual nests varied in size, but were usually in the range of 10 feet (3 m) in diameter and about 5 feet (1.5 m) in height. The interior of the fossil nest was approximately 6 feet (2 m) in diameter and approximately 2 feet (0.75 m) deep. The average spacing between these nests was about 23 feet (7 m), which indicates that Maiasaura mothers nested close together, as the average size of an adult varied from 26-30 feet (7-8 m). Several Maiasaura specimens have been found in bone-beds that cover miles in some cases. The large number of fossilized specimens in these bone-beds lets paleontologists conclude that Maiasaura nested in colonies.
So far, only the hadrosaurs Maiasaura and Hypacrosaurus, along with an unidentified Argentine sauropod, and unidentified Portugese theropod, a possible therizinosaur, Oviraptor, and Troodon are the only known dinosaurs that have been discovered near clutches of eggs, caring for their young.
Maiasaura lived alongside Orodromeus, Troodon, the ceratopsid Centrosaurus, the tank-like Euoplocephalus and earlier relatives of Tyrannosaurus rex, Daspletosaurus torosus and Albertosaurus. It was among the latest dinosaur species to evolve, prior to the Cretaceous-Tertiary extinction of 65 million years ago.
- ^ a b c d e Brochu, Christopher A.. (2002). A Guide to Dinosaurs. San Francisco, CA: Fog City Press. p. 185. ISBN 1-876778-63-6
- ^ a b c d e f g h i j "Maiasaura." In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. The Age of Dinosaurs. Publications International, LTD. p. 116-117. ISBN 0-7853-0443-6.
- ^ Palmer, D., ed (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 148. ISBN 1-84028-152-9.