Dilophosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Dilophosauridae

Genus: Dilophosaurus

Species: Dilophosaurus wetherilli

Introduction

The fossilized remains of Dilophosaurus wetherilli, a 6-meter (19.6-ft) long bipedal carnivore from the Early Jurassic period, were discovered in Arizona in 1942 by paleontologist Sam Welles. Skeletons of individual dinosaurs in various states of fragmentation have been retrieved from the Kayenta Formation in northern Arizona where Welles made his initial discovery. Some were missing parts of the skull and body, but a few individuals were complete enough to provide an incredible amount of information about Dilophosaurus's anatomy.

Classification

The Linnaean system is a hierarchical classification system that puts organisms with shared physical traits together in groups. The groups are ranked from the least specific to the most specific. In the Linnaean system, Dilophosaurus belongs to the suborder Theropoda. Theropods were bipedal dinosaurs often distinguished by three main fingers and one or two vestigial fingers on each hand, three main toes and two vestigial toes on each foot, and hollow bones. Dilophosaurus was a basal theropod, one of the earliest theropods to appear on the planet. In the twenty-first century, the placement of Dilophosaurus within a specific family was contested, with some placing it within its own family Dilophosauridae, others placing it within Coelophysidae, and still others making different proposals.

As of early 2020, the Dilophosaurus wetherilli was the only widely accepted species of Dilophosaurus. A theropod specimen that was discovered in the Lufeng Formation in China in 1993 was placed in the Dilophosaurus genus and given the name Dilophosaurus sinensis, but controversy surrounded the classification because of numerous anatomical differences between D. sinensis and D. wetherilli. These differences include the shape of the head crest, hands, and jaws, as well as the number of teeth, which suggest that the specimen belongs in an entirely different genus. Some scientists argued that D. sinensis should be assigned to the Sinosaurus genus, but others, even if they saw merit in this case, suggested that the specimen had still not been studied well enough to make a definitive decision and that there were differences between D. sinensis and Sinosaurus triassicus.

The phylogenetic, or cladistic, system of classification is an alternate means of classification that takes into account the evolutionary relationships among organisms. It places organisms into clades, or individual groupings defined by shared physical traits that indicate evolutionary history. Each clade contains all of the descendents of an original common ancestor (which is also the most recent common ancestor) plus the ancestor itself. Though also debated in the early twenty-first century, Dilophosaurus was traditionally believed to belong to the Coelophysoidea clade, which consists of theropods characterized by long skulls with long, tapering jaws.

Anatomy

Dilophosaurus was a slender, bipedal theropod with a sinewy, curved neck, a powerful body, and an elongated tail. A carnivore, its jaws were filled with sharp, pointed teeth, but the tip of its upper jaw, the pre-maxilla, was only weakly connected to the rest of the upper jaw. Its most easily identifiable feature was a rounded double head crest, which may have served as a sexual display. The crest, which began near the nostrils and extended along the top of the skull past the eyes, flared upward in a U-shape.

Because Dilophosaurus was bipedal, its arms were shorter and weaker than its legs. It had three main toes and two vestigial toes on each foot, and three main fingers and one vestigial finger on each hand. One of the fingers on each hand was opposable, allowing it to grasp objects. All of the digits on its hands and feet were tipped with claws.

Intelligence

Theropods, with the exception of some ornithopods, had the highest EQ of all dinosaurs, most likely because as bipedal predators, they required cunning and manual dexterity to hunt prey. The EQ of Dilophosaurus has not yet been calculated, but based on the EQs of other theropods, it could easily have been as high as 1.0. Herbivorous dinosaurs that did not hunt their prey (again with the exception of some ornithopods) were on the lower end of the EQ spectrum.

Reproduction and Population

Most paleontologists agree that dinosaurs laid eggs. Several hundred dinosaur nests have been discovered, complete with fossilized eggs attributable to different dinosaur genera. The nests, located all over the world, vary in structure from simple pits to more elaborate vegetation-lined holes, and each contain anywhere from a few to more than 12 eggs.

Some of the nests were situated close together, indicating that the owners bred in social colonies. Paleontologists debate about the amount of care dinosaurs provided their young. The discovery of fossilized juvenile dinosaurs in a number of nests suggests that at least some parental involvement occurred. No evidence of nesting behavior in Dilophosaurus exists, however.

While many perplexing questions about dinosaur reproduction have been answered, dinosaur mating behavior remains a mystery. One of the few theories about it concerns the function of head crests. Initially thought to have a more physiological function, some theories have proposed that head crests like the one Dilophosaurus had served as sexual displays to attract mates.

Diet

Dilophosaurus was a carnivore with sharp teeth well suited for tearing and chewing meat. But because its pre-maxilla was only weakly connected to the rest of its upper jaw, the power of its bite was compromised. That it killed prey with its teeth was doubtful as its claws better served that purpose.

No one knows exactly what Dilophosaurus's diet consisted of, but by comparing Dilophosaurus to modern predators, some assumptions can be made. To conserve valuable energy, modern predators typically go after prey that is easiest to catch. Dilophosaurus probably did the same, targeting the youngest, oldest, or sickest of animals. It may also have hunted in groups with others of its species, similar to the way lions hunt.

Comparing Dilophosaurus to other theropods sheds light on its feeding habits as well. Dilophosaurus possibly fed on fish because it possessed physical features similar to those of the fish-eating theropod Spinosaurus. These include high placement of the nostrils on the muzzle (presumably to prevent inhalation of splashing water), interlocking front teeth, and arms that were long enough to catch fish underwater.

Behavior

The remains of three Dilophosaurus wetherilli were discovered within 20 feet of each other in the Kayenta Formation in northern Arizona. Some paleontologists point to bone beds containing fossils of only one species of dinosaur to substantiate the theory that dinosaurs lived with members of their own kind. To them, the close proximity of the remains is evidence that Dilophosaurus lived with others of its species in groups.

Whether dinosaurs lived in groups or herds is of great interest because it opens up a wide range of possible dinosaur behaviors. For instance, if carnivorous dinosaurs like Dilophosaurus did live in single-species groups, group members perhaps acted cooperatively, hunting in packs and uniting against larger predators. They may even have had a means of basic communication. Dilophosaurus fossils do not reveal such information, however, so these behaviors are purely speculative.

Habitat and Other Life Forms

The supercontinent Pangaea that had formed between 330 and 300 million years ago was in the process of breaking apart during the Early Jurassic, which was the time period in which Dilophosaurus inhabited the area of the modern-day Kayenta Formation in the southwestern United States. The Kayenta region was, at the time, closer to the equator than it is today, so the climate was undoubtedly hot. The Kayenta region was also a fluvial plain throughout which streams deposited sediment and added humidity to the air.

As of yet, few traces of plant life from the Early Jurassic period have been found in the Kayenta Formation. Fossil records from there do indicate, however, an abundance of Early Jurassic animal life such as marine invertebrates (bivalves and crustaceans), frogs, turtles, crocodilians, tritylodontids (mammal-like reptiles), thyreophoran (armor-plated) dinosaurs, prosauropods, and a variety of other theropods. Trackways, which are dinosaur footprints preserved in rock, provide additional evidence that theropods travelled through the Kayenta region during the Early Jurassic period.

Research

Not much research has been done on Dilophosaurus other than for identification purposes. After discovering the first Dilophosaurus specimens in the Kayenta Formation in 1942, Welles took them to the University of California, Berkley, to be studied. Unaware of what he was dealing with, he waited until 1954 to write a paper, incorrectly assigning them to the Megalosaurus genus. Debate ensued about the age of the specimens. Welles returned to the Kayenta Formation in 1964 to sample the rocks, from which he determined Dilophosaurus's age to be Early Jurassic. He also found a much more complete individual with an obvious double head crest, a crest that was smashed or eroded on earlier specimens. Obviously not a Megalosaurus, Welles changed its name to Dilophosaurus wetherilli.

In 2019, a study was published in which researchers analyzed the range of motion of the forelimbs of the Dilophosaurus in an effort to glean more information about the dinosaur's behavior. As a result of this study, the authors inferred that because the shoulders of the Dilophosaurus likely had a limited range of motion and the dinosaur's forelimbs were shorter, it probably could only take hold of prey from within the area beneath its chest. Therefore, they argued, the Dilophosaurus likely made first contact with prey using its mouth rather than its hands.

Bibliography

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