Chirostenotes

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Caenagnathidae

Genus:Chirostenotes

Species:Chirostenotes pergracilis

Introduction

Chirostenotes was a small omnivorous dinosaur, known from remains found in the Upper Cretaceous in Canada's Dinosaur Park Formation (Alberta, Canada). The species was one of a group of species closely related to birds and is thought to be an offshoot of the dinosaurian lineage that gave rise to birds in the Cretaceous period.

In the Triassic period, the continents of the world were united in a single land mass, known as Pangaea. The first dinosaurs evolved during the Triassic and spread across the world, evolving into a variety of species. In the Jurassic and Cretaceous periods, tectonic movement split Pangaea into two supercontinents, Laurasia and Gondwana, which over time continued to divide into the present continents.

Chirostenotes is a member of a group of bipedal dinosaurs that split from the theropod group in the Cretaceous and evolved bird-like features, including feathers and beaks. Most species related to Chirostenotes were found in Laurasia, suggesting that the common ancestor emerged in North America after the supercontinents split.

Classification

Scientists utilize two primary methods to classify organisms into different categories. Linnaean taxonomy places organisms in a ranked hierarchy based on overall physical similarity, while an alternate system called “cladistics” creates groups called “clades” in an effort to reconstruct the evolutionary relationships between species.

In the Linnaean system, dinosaurs have traditionally been divided into two orders based on the structure of their pelvic bones: Saurischia, or “lizard-hipped,” and Ornithischia, or "bird-hipped." Although this broad division has been challenged by some researchers, it remains in wide use. Chirostenotes is considered a saurischian and placed in the suborder Theropoda, a group of bipedal, largely predatory dinosaurs. Further classification of Chirostenotes and its relatives, known as the oviraptorosaurians, has seen frequent revision and remains subject to much debate. Generally, the genus is placed in the superfamily Caenagnathoidea—which contains “bird-like” theropods generally featuring long skulls, beaks, and feathers—and the family Caenagnathidae. The type species is Chirostenotes pergracilis.

Cladistic analysis places Chirostenotes in a similar overall structure, but has also seen considerable controversy and revision. Most researchers place the genus in the clade Avetheropoda, which shared protofeathers, enlarged brains, and other characteristic features of the skull and chest. From there, it falls into the clade Oviraptorosauria, which were theropods with short, box-shaped skulls, most of which had beaks. Further specification indicates that Chirostenotes falls into the clades Caenagnathoidea (containing theropods with other bird-like features like beaks and true feathers), Caenagnathidae, and possibly Elmisaurinae.

Anatomy

Chirostenotes was a mid-sized dinosaur, reaching lengths of between 2 and 2.9 meters (6.6–9.5 ft) and standing about 1 meter (3.2 ft) at the hip. Chirostenotes was lightly built, with an estimated adult weight of between 30 and 50 kilograms (66–110 lbs).

Chirostenotes was bipedal, with three-toed feet ending in blunt claws. Its arms were well-muscled with three clawed fingers on each hand, with the middle digit extending past the other two, and which might have been used for hooking or for probing. Chirostenotes had a relatively heavy body with a long, flexible neck. Its tail was short compared to many other theropods and was probably used primarily for balance when the animal was in motion.

Impressions of Chirostenotes fossils indicate that the animal may have possessed true feathers like those seen on modern birds. Its head was short with a curved beak and a pronounced bony crest along the top of the skull. The eyes were large, giving the animal strong vision. Chirostenotes may have been cryptically colored, helping the animal to blend into its surroundings.

Intelligence

Scientists have used encephalization quotient (EQ), based on brain and body size, to estimate animal intelligence. Chirostenotes probably had an EQ rating at the upper end of the range for dinosaurs, making it more intelligent than most of the herbivorous dinosaurs in its environment. Chirostenotes was most likely similar in intelligence to a bird or small mammal.

Reproduction and Population

Paleontologists have not yet found eggs or juvenile Chirostenotes specimens, so little is known about the animal's reproductive behavior. Fossilized eggs and nests for other species in the Oviraptorosauria clade indicate that some species maintained nests of up to ten or more eggs and that males may have taken an active role in incubating eggs. Chirostenotes may also have sat on its nest to incubate its eggs like modern birds.

Paleontologists have discovered significant variation among fossils of Chirostenotes and some paleontologists believe that these differences may be the result of sexual dimorphism, with either males or females being larger than the other sex. The size and shape of the animal's crest may also differ between the sexes.

Diet

Chirostenotes possessed a beak similar to a modern ostrich, an herbivorous species, leading some paleontologists to speculate that Chirostenotes was also herbivorous. However, Chirostenotes's beak also had a sharp upper edge, which may have allowed the animal to live an omnivorous lifestyle, feeding on both plants and small animals. Some have speculated that the animal's beak would have been useful for breaking open reptile and dinosaur eggs.

Behavior

Paleontologists have discovered Chirostenotes fossils in isolated locations, indicating that the species may have lived as a solitary forager or in small, family groups. The animal's long, well-muscled legs indicate that it relied on speed and may, therefore, have tended towards open areas where it could build up speed to avoid predators and possibly capture small prey.

Chirostenotes had large eyes and may have been active during the day, relying on its vision for feeding and avoiding danger. Ostriches and other ratites have similarly large eyes, used to scan their environment for danger. The cassowary (Casuarius) is a large flightless bird native to New Guinea that has a skull crest similar to that of Chirostenotes. The cassowary uses its skull to push through dense undergrowth while exploring the forest, and paleontologists have suggested that Chirostenotes may have used its crest for a similar purpose.

Habitat and Other Life Forms

Late Cretaceous Canada was warmer and had more moisture than in the modern period. Gymnosperms like conifers and cycads were still a common form of vegetation, though angiosperms, or flowering plants, had evolved by this period and were beginning to spread across the environment.

There were a variety of reptiles, amphibians, and small mammals living in Cretaceous Canada alongside Chirostenotes. The hadrosaurs were one of the most successful lineages of herbivorous dinosaurs in Cretaceous North America, including species like Saurolophus, a hadrosaur species that ranged through both Canada and Asia. Another successful group of herbivores was the ceratopsians, or horned dinosaurs like Anchiceratops, which may have traveled in herds like modern buffalo.

In addition to small omnivores and predators around the same size as Chirostenotes, there were a variety of larger theropod predators in Cretaceous Canada, including Daspletosaurus, a large predator related to Tyrannosaurus. There were also smaller predators from the dromaeosaur or “raptor” group, like Dromaeosaurus and several varieties of smaller theropod predators, which may have competed with and possibly hunted Chirostenotes.

Research

The history of Chirostenotes pergracilis is a lesson in the science of paleontology. Chirostenotes was first known from a set of hand bones discovered in Dinosaur National Park (Alberta, Canada) in 1924. The specimen was named and described by paleontologist Charles Gilmore. Additional isolated specimens were later found and mistakenly identified as other creatures; some sat in a state of limbo, awaiting identification. In 1988 paleontologists Philip Currie and Dale Russell were able to make sense of various components that had been misidentified and assigned to another genus, and they were able to clearly identify Chirostenotes pergracilis. Subsequently, in 1997, Hans-Dieter Sues identified an unrecognized partial skeleton (skull, pelvic girdle, and vertebrae) as C. pergracilis. Also, a jaw with teeth that had previously been attributed to C. pergracilis was ruled out and given another name.

In the twenty-first century, many cladistic analyses have sought to clarify relationships between the various identified oviraptorosaurs. Several studies specifically examined whether Chirostenotes and Caenagnathus were the same, though most suggested they were indeed distinct. Debate over the exact placement of Chirostenotes within Caenagnathidae continues to be debated.

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