Maiasaura
Maiasaura, meaning "Good Mother Lizard," was a large herbivorous dinosaur from the Late Cretaceous period, first discovered in Montana. Recognized as a prominent member of the hadrosaur family, often referred to as "duck-billed" dinosaurs, Maiasaura lived in herds and exhibited social behaviors that have led to debates about parental care. Fossil evidence, including nesting sites with young and hatchlings, suggests that these dinosaurs may have engaged in communal breeding, although some researchers argue that such behaviors may not necessarily indicate parental care. Maiasaura could reach lengths of up to 9 meters (30 feet) and stood more than 2.5 meters (8.2 feet) tall at the hips, displaying a robust body adapted for herbivory with a broad beak for grazing on various plant materials.
The species thrived in a warm climate with lush forests, inhabited alongside other dinosaurs, small mammals, and reptiles, while being preyed upon by theropods and dromeosaurs. Maiasaura's social structure may have included seasonal migrations in search of food, similar to modern herd animals. The knowledge about Maiasaura has been enriched by extensive fossil findings, making it a key species for understanding the behaviors and ecology of hadrosaurs during the Cretaceous period.
Maiasaura
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Ornithischia
Family: Hadrosauridae
Genus:Maiasuara
Species:Maiasuara peeblesorum
Introduction
Maiasaura was a large herbivorous dinosaur first discovered in sediments from the Late Cretaceous period in Montana. A common representative of the hadrosaur group in North America, Maiasaura was part of one of the more successful lineages of herbivores during the Cretaceous period. Fossilized nests of Maiasaura juveniles living together prompted a debate about whether dinosaurs engaged in parental care of their offspring.
During the Triassic period, the continents were united in a single supercontinent known as Pangaea. This supercontinent split during the Jurassic period and gave rise to two supercontinents, Gondwana in the south and Laurasia in the north. In the Cretaceous period, these supercontinents continued to split into smaller landmasses and began to resemble the modern distribution of continents.
Ornithopod dinosaurs like Maiasaura are one of the most successful groups in North America. They spread across all the major continents during the Jurassic period and diversified into many species. Similar species have been found on continents that were formerly part of the Pangea supercontinent, suggesting the population spread before the breakup of the supercontinents.
Classification
Biologists use two major systems to classify organisms. The traditional Linnaean system classifies organisms by overall physical similarity and was used to create most of the currently used categories. In this system, Maiasaura is considered a member of the family Hadrosauridae, sometimes called the “duck-billed” dinosaurs. It was one of the most widespread herbivore groups in North America, Europe, and parts of Asia. Hadrosauridae were further subdivided into two subfamilies, lambeosaurines and the typically larger hadrosaurines, with Maiasaura belonging to the latter. The hadrosaurines are characterized by the lack of a bony crest attached to the skull.
A second classification system, called cladistics, seeks to create groups that better reflect the evolutionary relationships between species. Cladistics generates “clades,” or groups with a common ancestor, by focusing on a few key physical characteristics and attempting to pinpoint the origin of “new” features. Maiasaura belonged to the clade Ornithopoda, or the “bird-footed” dinosaurs. The other defining characteristics of this group included a horn-like beak and armorless, scaled skin.
Anatomy
Maiasaura, like other hadrosaurs, was partially bipedal and could stand on its hind legs, using its heavy tail for balance. A study published in 2015 showed that juvenile Maiasaura were primarily bipedal, and transitioned to quadrupedal as they aged. Fossils of other hadrosaurs reveal skin with mosaic scales, and Maiasaura likely shared this characteristic.
Adult Maiasaura could reach up to 9 meters (30 ft) from head to tail and stood more than 2.5 meters (8.2 ft) tall at the hips. Estimates place the weight of adults at up to 3 tons (6,614 lbs). Maiasaura had longer rear legs than front, though its front legs were well-muscled, and the animal probably walked in a quadrupedal manner part of the time. The rear claws were hoof-like, while the front arms had four-fingered hands with smaller claws.
Maiasaura's head had a flattened bill, presumably to chew vegetation. The animal had small bony crests in front of its eyes, though paleontologists are unsure of their function. Skeletons also reveal a pronounced bony spine, which may have been covered in a crest of skin.
Intelligence
Paleontologist James A. Hopson estimated dinosaur intelligence by calculating the ratio of a dinosaur's brain to body mass, called the encephalization quotient (EQ). Most dinosaurs fell within a range of 0.1 to 5.8, on a scale that extends to 8.0 (in comparison, humans have an EQ of 7.44 and dolphins measure 5.31). Although some scientists believe that EQ provides a poor measure of intelligence, living animals with higher EQ values often rank highly in other measures of intelligence.
The ornithopods fall within the 1.0 to 1.5 range, placing them at an average level compared to other herbivore groups. The Maiasaura would have needed sufficient intelligence to negotiate the dangers of the Late Cretaceous of North America, which included a variety of predatory dinosaurs that most likely hunted them. Paleontologists estimate that Maiasaura and closely related hadrosaurs were probably less intelligent than mammals that occupy similar niches but may have had similar intelligence to modern crocodilians or some birds, like chickens or ducks.
Reproduction and Population
Paleontologists have discovered “nesting sites” with nests of hatchling and young Maiasaura still present. These findings may indicate parental care, or it may indicate that Maiasaura juveniles gathered together for protection, a behavior seen in some modern lizards. The nests were closely grouped, about 6 to 12 meters (20–40 ft) apart, suggesting that the animals used a central egg-laying location, like socially-breeding birds.
Some skeletal features of Maiasaura hatchlings indicate that the young may not have been able to care for themselves for months after hatching, which would have meant parental care was required. Other paleontologists disagree, noting that Maiasaura hatchlings show similar development to other species that do not provide parental care.
While still under debate, Maiasaura provides thought-provoking evidence that dinosaurs may have nested in groups and displayed some parental care. Nests have been found containing young and hatchlings from other hadrosaurs, suggesting that the behavior may have been common to the group.
Diet
Maiasaura was an herbivore with a wide bill suitable for cropping leaves and stems. The animal's large, broad beak indicates that it may have been a generalist, eating a wide variety of plant matter. The presence of flat cheek teeth in the hadrosaurs was an adaptation unique to the group and suggests that hadrosaurs mashed and chewed their food before swallowing.
Maiasaura fossils have been found near sediments that were part of ancient creeks and rivers, indicating that Maiasaura may have browsed near water sources. A wide variety of plant matter has been found near Maiasaura fossils, including ferns, conifers, and cycads.
Behavior
Fossil sites have been found containing thousands of Maiasaura fossils in close proximity, suggesting that Maiasaura travelled in herds. Based on knowledge of the plants growing in the area, paleontologists have suggested that Maiasaura may have migrated seasonally to obtain food.
If Maiasaura gathered in herds, its behavior might have been similar to modern herd animals like wildebeest, oxen, and other hoofed mammals. In these herd animals, adults utilize communal defense to intimidate and drive off predators. Maiasaura might likewise have used group defense to drive off raptors and large carnivores that occupied its habitat.
Habitat and Other Life Forms
During the Late Cretaceous period, central North America was covered by a shallow sea. The climate was warm in the surrounding areas, with lush forests of both gymnosperms, like cycads and conifers, and the angiosperms, or flowering plants. A variety of small reptiles, amphibians, and crocodilians lived alongside Maiasaura, as well as several varieties of small mammals.
Birds had begun to appear throughout North America, but the skies were still dominated by pterosaurs, a large group of flying reptiles that existed from the late Triassic to the end of the Cretaceous periods, and which included Quetzalcoatlus and Pteranodon.
There were more than ten other species of hadrosaurs living in North America, including Prosaurolophus and Brachylophosaurus. There were also several varieties of ceratopsians, or “horned dinosaurs” such as Styracosaurus and Brachyceratops, that have been discovered near Maiasaura fossils.
A number of predatory dinosaurs lived in Late Cretaceous North America, including members of the dromeosaur or “raptor” group, bird-like predators that may have hunted in packs. Other large bipedal carnivores such as Deinodon and Daspletosaurus—theropod dinosaurs related to Tyrannosaurus—have been found in the same region and may have hunted hadrosaurs like Maiasaura.
Research
Maiasaura was named by paleontologist John Horner and research partner Bob Makela in 1979. The species shows a typical example of hadrosaur anatomy but is considered one of the most important hadrosaur discoveries because it indicates Maiasaura utilized social breeding and other herd behavior. Because there are a variety of Maiasaura fossils available, detailed examinations of bones have allowed for estimates of bone histology and more accurate estimates of growth rates. In 2015, a detailed study of Maiasaura was published in Paleobiology. The study examined all aspects of the dinosaur's life, including its growth, social behavior, and environment. The study was the most detailed life history of any known dinosaur and resulted in a model to with other dinosaurs can be compared.
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