Avimimus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Avimimidae

Genus:Avimimus

Species:Avimimus portentosus

Introduction

Avimimus was a small omnivorous dinosaur that lived in Late Cretaceous Mongolia. The discovery of Avimimus and related groups of “birdlike” dinosaurs helped to initiate the ongoing debate regarding the relationship between birds and dinosaurs.

In the Triassic and Early Jurassic, the continents were united in a single supercontinent known as Pangaea, which split apart in the Jurassic, giving rise to two supercontinents, Gondwana in the south and Laurasia in the north. By the Late Cretaceous, the continents were continuing to split, approaching their present conformation. Many dinosaur groups migrated between the continents while they were connected by land and then diversified after the continents split, leaving descendants across the world.

Avimimus was a member of the oviraptorosaurids, a diverse group of bipedal dinosaurs that flourished in Mongolia, becoming one of the primary predatory groups. Similar species, descended from a common ancestor, appeared in North America and in Europe.

Classification

Scientists use two major methods to classify organisms. Linnaean taxonomy groups organisms based on overall physical similarity and was used to create the basic hierarchical system that is still used today, which includes divisions such as kingdom, phylum, class, and family.

Avimimus is placed in the Saurischia order, as it is “lizard hipped,” and the Theropoda (“beast feet”) suborder, as it was bi-pedal. Theropoda dinosaurs included many of the Mesozoic's top carnivores. Key characteristics of the suborder include the forward-facing eyes and narrow skull. Within this group, Avimimus is a member of the Oviraptorosauria infraorder, a group noted for their anatomical and evolutionary similarity to modern birds.

An alternate method, called “cladistics,” attempts to create groups of organisms that more accurately represent the evolutionary relationships between species. To accomplish this, scientists focus on the presence or absence of key characteristics and attempt to trace evolution by noting how the characteristic in question was inherited through a common ancestor.

Classification of Avimimus has been historically difficult, as the fossil record of theropods in Asia is limited. In the case of creatures such as Avimimus, there is a debate among scientists regarding the relationship of modern birds to these early theropods. There has been significant disagreement about whether or not oviraptorosaurs should be placed in Aves (birds) clade. A cladistic analysis might put Avimimus close to Caudipteryx, because of a tail feather; another close relative might be Archaeopteryx of the Late Jurassic period, often called the first bird.

As with any dynamic study in the field of science, cladistic analyses serve as hypotheses and can change over time as new specimens are discovered and as the science advances. In 2017 Matthew Baron and his colleagues posited a common ancestor for therapods like Avimimus and the ornithischians, such as Triceratops, and proposed they be reclassified into a new clade, Ornithoscelida.

Anatomy

Avimimus was a small dinosaur, built for speed and agility. The animal had a length of between 1 and 1.2 meters (3 and 4 feet) and stood about 1 meter (3.5 feet) tall. The adult animals probably weighed 12 to 16 kilograms (26 to 35 pounds). The animal had long rear legs terminating in three-toed, clawed feet. Its forelimbs were shorter and thinner, with three long, clawed fingers on each hand. Avimimus had a long neck, compared to its body size, and a long thin tail. The animal's head was also large compared to its body, with large eyes and a large brain. The animal had a relatively short beak, which was lightly serrated, but no teeth.

The body and build of Avimimus is similar to that of modern birds, which are closely related to the group. Examinations of Avimimus's arm bones suggest that the animal may have had feathers in life, though no feather impressions have been found to date. Other members of the oviraptorosaurian group have been discovered with feather imprints.

Intelligence

No direct calculation of EQ has been completed for Avimimus, but other members of its group have been found to have EQ levels higher than 2.0, approximately mid-range among dinosaurs. Avimimus was likely an omnivore, relying on its speed to catch small prey and to avoid predators. Thus, the animal must have had sufficient intelligence to evaluate a wide variety of potential food sources and to effectively avoid danger.

Reproduction and Population

Some closely related dinosaurs, like Oviraptor, have been found in close association to nests of eggs, leading some paleontologists to suspect that many dinosaurs brooded their eggs like modern birds. Avimimus may also have brooded its eggs, but there is no evidence to suggest that the animal provided any type of parental care to its hatchlings.

Fossil beds from the Gobi Desert in Mongolia have revealed large numbers of Avimimus specimens in the same dig site, indicating that the species may have been relatively common within its range. There are still insufficient fossils to estimate population density, but the oviraptorosaurians appear to have been one of the most successful groups in Cretaceous Mongolia.

Diet

Avimimus possessed no teeth and had a short, lightly serrated beak. Most paleontologists believe that Avimimus was an herbivore or omnivore. Some paleontologists have suggested that Avimimus could have been an insectivore, using its speed and agility to capture a variety of insects. The similarity of Avimimus’s beak to modern parrots suggests that plant matter may have formed at least part of the animal's diet.

Behavior

The anatomy of Avimimus suggests that the animal was built for speed, with long legs similar to those of such running animals as ostriches and antelopes. Discoveries of large groups of adult fossils suggest that the animal was social and may have gathered into groups and lived communally. A discovery in Mongolia generated two theories. One theory suggests that Avimimus lived in large non-familial groups that were age segregated. Alternatively, the discovery may suggest flocking or communal roosting behavior among the species.

The large eyes and brain of Avimimus suggest that the animal relied on its vision and relatively quick capacity for evaluation to find food and avoid danger. As a relatively small dinosaur, Avimimus would have faced significant threat of predation by the larger theropods in its environment. The first Avimimus fossils were discovered in an area that had once had a flowing river with some scattered forests and open plains, and paleontologists are uncertain if Avimimus specialized in one part of this varied habitat.

Habitat and Other Life Forms

The climate of Late Cretaceous Mongolia was warmer than the present day, when a dry, arid plain covered most of Mongolia. There were pockets of moist habitat, which became dense forests. Ferns and gymnosperms, including conifers and cycads, were still the dominant forms of vegetation, though angiosperms were quickly spreading throughout many habitats.

Cretaceous Mongolia was host to a wide variety of reptiles, amphibians, and mammals, many closely resembling modern species. There were also several varieties of small birds, though flying reptiles, called pterosaurs, were still the dominant flying vertebrates.

Opportunistic bird mimics like Avimimus lived alongside a variety of hunters that may have preyed on them. These hunters included the small predator Velociraptor and its larger relative Adasaurus. Larger theropod predators were also present including Alioramus and Tarbosaurus. Other members of the oviraptorosaurids in Mongolia included the small predators Ingenia and Chonchoraptor.

One of the most successful groups of herbivores in Mongolia was the ankylosaurs, large armored dinosaurs known for their clubbed or spiked tails. There were many representatives of this group, including Amtosaurus and Pinacosaurus. There were also several varieties of ceratopsians, or “horned dinosaurs,” including Protoceratops and Bagaceratops. In addition, there were several representatives of the hadrosaur, or “duck-billed,” dinosaur group and a few species of sauropods, especially near the forested areas.

Research

The first Avimimus specimen was discovered in 1981 by paleontologist Sergei Kurzanov from a formation in the Gobi Desert of Mongolia. The animal was initially thought to have been tailless, until a more complete specimen was discovered with an intact tail. Examinations of the 1981 specimen revealed unusual bumps on the forearms that were later interpreted as anchoring sites for feathers.

Around 2007–8, paleontologist Phillip Currie and his team discovered a large bed of Avimimus fossils in the Nemegt Formation of the Gobi, leading to speculation that the animal might have lived in groups. By 2016 eighteen Avimimus skeletons, all mixed together, had been unearthed, representing an unprecedented number of Avimimus specimens and the first such assemblage of birdlike dinosaurs found in a bone bed. Currie and his team theorized that the animals could have been gathered either in a protective flock or for temporary purposes such as mating, finding food, or brooding; as no juveniles were found, however, the group might have been age-segregated. In 2018 the researchers proposed a new species of Avimimus, each found in a different area, and concluded Avimimus may have preferred moist environments over dry ones.

Takanobu Tsuihij and his team subsequently examined Avimimus skulls from Nemegt, particularly the nose, cheek, and jaw, and in a 2017 Journal of Vertebrate Paleontology paper suggested Avimimus shared more characteristics with oviraptorosaurs than with modern birds, in contrast to Kurzanov's conclusion.

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