Chindesaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Herrerasauridae

Genus:Chindesaurus

Species:Chindesaurus bryansmalli

Introduction

Chindesaurus was a small predatory dinosaur that lived in Late Triassic North America and was first discovered in Arizona. It is, in fact, one of the geologically oldest known American dinosaurs. A primitive dinosaur, Chindesaurus shared its environment with a variety of prehistoric reptiles, representing a lineage that dominated the terrestrial environment for millions of years before the Triassic period (approx. 251 to 201 million years ago).

In the Triassic period, the continents of the earth were united in a single landmass, known as Pangaea. Dinosaurs first evolved during this period and began to spread across the globe. In the Jurassic period, Pangaea began to split as a result of tectonic forces, giving rise to two supercontinents, Gondwana and Laurasia. When the supercontinents divided, species became isolated and developed into a variety of species.

Chindesaurus represents one of the earliest offshoots of the dinosaurs that evolved before tectonic shifting began to drive the continents apart. The species lived alongside members of its ancestor group, the dinosauromorphs, which were an offshoot of prehistoric reptiles and the ancestor group of all dinosaur species.

Classification

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

Within the Linnaean system, dinosaurs have long been split into two orders, Ornithischia and Saurischia, based on their skeletal structure. (Although some researchers have challenged this basic division, it remains in wide use.) Chindesaurus falls into the saurischian order, which comprises dinosaurs known for more lizard-like pelvic bones, as opposed to the more bird-like pelvises of the ornithischians. From there it is generally placed in the suborder Theropoda, which contains bipedal and typically predatory dinosaurs.

Further taxonomic analysis of Chindesaurus is still under debate as it possessed many primitive characteristics, making it difficult to determine precise relationships. For this reason, its family assignation is sometimes considered incertae sedis, meaning “of uncertain placement.” This phrase is used when a dinosaur cannot be appropriately placed within a particular taxonomic group or system. However, Chindesaurus is most often placed in the family Herrerasauridae, a family made up of relatively small, primitive saurischians called herrerasaurids. Among the oldest dinosaurs known, herrerasaurids were small-to-medium-sized, sharp-toothed, and lizard-hipped carnivores that were similar to later theropods but were extinct by the close of the Triassic.

Cladistic analysis also places Chindesaurus in the clade Saurischia, based on the orientation of its pelvic bones and special articulation of the vertebrae. Paleontologists are unsure as to whether the genus should further should be placed in a clade with the herrerasaurids because it lacks certain features found in this group. Some researchers classify Chindesaurus as a basal saurischian, leaving its higher classification open to further discovery or interpretation. However, many other scientists do consider it a herrerasaurid. More broadly, there is much disagreement as to the placement of herrerasaurids as a whole, especially whether they should be included as theropods.

Anatomy

Chindesaurus was a small dinosaur, reaching a length of 2 to 2.3 meters (6.6-7.5 feet) and weighing about 30 to 38 kilograms (66-84 lbs), about the size of a large dog. Chindesaurus had long thin legs and was bipedal, using its rear limbs to walk, and presumably using its smaller forelimbs to aid in feeding. Both its hands and feet ended in claws, though the claws of the feet were adapted to running. The animal had a long, flexible tail comprising nearly half of its overall length.

Paleontologists are uncertain about the animal's integument, but it may have been covered in mosaic scales, similar to those seen on later dinosaurs. Chindesaurus lived in forested floodplain environments and may have been cryptically colored to blend into its environment. It had large eyes and jaws lined with small, sharp teeth.

Intelligence

Scientists have used encephalization quotient (EQ), based on brain and body size, to estimate animal intelligence. A specific EQ rating for Chindesaurus has not been established, but it may have had similar intelligence to a modern small reptile. Later theropod dinosaurs would develop EQ levels higher than most other dinosaurs, evolving increased intelligence to further predatory efficiency. As Chindesaurus was a primitive member of this group, it is likely that the species possessed relatively low intelligence.

Reproduction and Population

Chindesaurus was most likely oviparous, or egg-laying, but no eggs or fossilized juveniles have been found to confirm that theory. Nests found for other theropod species indicate that some dinosaurs laid clutches of two eggs at a time and brooded nests filled with ten or more eggs. Chindesaurus may have displayed similar reproductive behavior.

As Chindesaurus is known from only a few specimens, paleontologists are uncertain whether males and females were similar in appearance and/or size. In some dinosaur species, males appear to have taken a role in brooding nests of eggs, while in other species females appear to have been the primary caretakers of eggs.

Diet

Judging from the animal's dentition (development and arrangement of the teeth), paleontologists have determined that Chindesaurus was most likely predatory. It presumably preyed upon a variety of small reptiles, amphibians, and other small animals. The animal's long legs and flexible spine indicate that it may have relied upon speed and agility when hunting.

In addition to small prey, there were several larger herbivorous reptiles and a few species of herbivorous dinosaur living in the same environments as Chindesaurus. It is possible that the species preyed on larger animals, perhaps inflicting wounds with its teeth and allowing the prey to expire before feeding.

Behavior

Paleontologists know little about the behavior and lifestyle of Chindesaurus. While later theropods distantly related to Chindesaurus are believed to have displayed some social behavior, this evolutionary adaptation may have emerged only in later species. Chindesaurus fossils have been found in environments that were partially forested floodplains in the Late Triassic, leading paleontologists to speculate that Chindesaurus was a solitary predator, stalking the wetlands for small prey items and living in nearby forested areas.

Habitat and Other Life Forms

Late Triassic North America was warmer than in the modern period and the landscape was dotted with lakes, rivers, and floodplain environments. Gymnosperms, or plants such as conifers and cycads that reproduce by producing uncovered seeds, and pteridophytes, which include ferns and tree ferns, were the dominant types of vegetation. There were a variety of small reptiles and amphibians in the environment.

In addition to Chindesaurus, other predators in the environment included the bipedal dinosaur Coelophysis, a swift predator that may have hunted in groups. The rauisuchian Postosuchus, a bipedal predator related to modern crocodiles and alligators, also inhabited the same areas as Chindesaurus.

Relatively recently, paleontologists have discovered that in some environments, like Late Triassic Arizona, dinosaurs coexisted with “dinosauromorphs,” the ancestral reptiles that represent a transitional evolutionary stage between reptiles and dinosaurs. Several species of dinosauromorphs, like Eucoelophysis, have been discovered from sediments in the Late Triassic. Dinosauromorphs included both predatory and herbivorous species and may have competed with the true dinosaurs in these habitats.

Research

Chindesaurus was discovered in 1984 by paleontologist Bryan Small. The find was made at the Late Triassic Chinle Group strata in the Petrified Forest National Park of Arizona, and the specimen was fully described by paleontologists Robert Long and Philip A. Murry in 1995. The species was named Chindesaurus bryansmalli for Chinde Point, a topographic feature that took its name from the Navajo word for “ghost” or “evil spirit,” and for Small.

At the time of its discovery and description, Chindesaurus drew much media attention as it was considered one of the oldest known dinosaurs. With only a few fragmentary specimens discovered over the years, paleontologists relied on reconstructions to determine the dinosaur's characteristics. Much subsequent research focused on classification. In particular, scientists continue to debated whether Chindesaurus should be considered a basal saurischian or a herrerasaurid.

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