Ceratosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Sauropsida

Order: Saurischia

Family: Ceratosauridae

Genus:Ceratosaurus

Species:Ceratosaurus nasicornis

Introduction

Ceratosaurus was a large predatory dinosaur living in Late Jurassic North America. Part of a large group of predatory dinosaurs that were most common in South America and Africa, Ceratosaurus lived alongside Allosaurus and members of its immediate group, which were the dominant predatory dinosaurs in the northern hemisphere.

In the Triassic period, the continents of the world were united in a single land mass known as Pangaea. In the Jurassic and Cretaceous, tectonic shifts caused the supercontinent to split into two continents—Laurasia in the north and Gondwana in the south. After the breakup of Pangaea, dinosaurs evolved into unique northern and southern varieties.

Ceratosaurus was descended from an ancestral species that lived in both the southern and northern hemisphere before the breakup of Pangaea. Members of the group were found throughout South America, with a few species, like Ceratosaurus, in the northern hemisphere.

Classification

Paleontologists use two primary methods to organize animals into different groups. Linnaean taxonomy, or “systematics,” groups organisms according to overall physical similarity, while another method, called “cladistics,” places organisms into groups called “clades” intended to reflect the evolutionary relationships between species.

Systematic analysis places Ceratosaurus in the order Saurischia, because of the “lizard-like” orientation of the animal's pelvis. From there, the genus is placed in the suborder Theropoda, which comprises bipedal dinosaurs with shorter arms and similar skulls, and in the family Ceratosauridae, the members of which share similarities in skull and teeth structure.

A cladistic analysis classifies Ceratosaurus as a member of the Theropoda clade, which is defined largely by the presence of a furcula or “wishbone” and a reduction in the number of fingers. The dinosaur then falls under the Averostra clade, which is defined by asymmetrical teeth and a narrow, deep skull shape. Ceratosaurus also falls under the Ceratosauria clade, which includes dinosaurs with reduced finger and hand size and an increased number of vertebrae.

In 2000, paleontologists James Madsen and Samuel Welles named two additional species of Ceratosauruss: Ceratosaurus magnicornis and Ceratosaurus dentisulcatus.

Anatomy

Ceratosaurus was a large dinosaur, measuring between 5.2 and 7 meters (17-22.9 ft) long and standing approximately 4 meters (13.1 ft) at the hip. Paleontologists estimate that the dinosaur may have weighed between 500 and 900 kilograms (1,102–1,984 lbs). The species had reduced hands but long, powerful legs terminating in three large, clawed toes.

The animal had a large skull relative to its body size, with rows of sharp teeth in both jaws. The teeth in the upper jaw were longer, relative to the size of the skull, than in most theropod predators. Ceratosaurus was named for its nasal horn, which consisted of two elongated bones fused above the animal's snout. The species had large nasal cavities and forward-facing eyes.

Ceratosaurus was sturdily built, with a muscular tail that was partially flexible and able to swing from side to side. The tail is a key defining feature, as most theropods had stiff tails. The species was also unique among predatory dinosaurs in that it had rows of bony osteoderms along its back, forming a sort of rudimentary armor. While osteoderms were common among herbivorous dinosaurs in the Thyreophora group, only the ceratosaurs developed similar armor among the theropods.

Intelligence

Theropods (of which Ceratosaurus was a member) were the one group of dinosaurs that exceeded the 2.0 upper limit for dinosaur EQ ratings. A few theropods measured had EQ ratings as high as 5.8, making them similar in intelligence to some predatory mammals. Most dinosaurs were less intelligent, on average, than mammals and birds living similar lifestyles.

Reproduction and Population

Paleontologists believe that most dinosaurs were oviparous (egg-laying), and research has shown that some theropods laid nests of ten or more eggs. Paleontologists also believe that large theropods covered their nests with vegetation to insulate the eggs from the sun and aid in incubation, similar to behavior observed in modern crocodiles and alligators. Ceratosaurus might also have used vegetation for incubation, as it was too large to sit on its nest and incubate its eggs in the manner of modern birds.

Paleontologists are uncertain whether both sexes of Ceratosaurus were armed with a skull horn or whether the feature was specific to either males or females. Males and females of many bird and reptile species differ in size or other aspects of appearance, a characteristic called sexual dimorphism.

Diet

The long, cutting teeth of Ceratosaurus establish firmly that the animal was a carnivore. Most paleontologists believe that it was probably a predatory dinosaur that used speed and strength to hunt a variety of prey. There were many herbivorous dinosaur species in Jurassic North America, including the iguanodonts, bipedal herbivores that may have traveled in large herds, and the sauropods, massive herbivores that emerged from the same dinosaur order as the theropod predators.

Paleontologists have found Ceratosaurus teeth and other fragmentary remains in areas that were once coastal habitat or near floodplain environments. Some paleontologists have suggested that the long, flexible tail of Ceratosaurus might have allowed the species to swim, perhaps hunting fish and aquatic reptiles. In contrast, many other theropods had stiff tails, strengthened by bony struts used for balance when running.

Behavior

Some paleontologists have suggested that Ceratosaurus might have lived and hunted in groups, as similar behavior has been theorized for one of the species’ competitors, the larger theropod Allosaurus. Paleontologists have not discovered Ceratosaurus fossils in groups, and many paleontologists therefore believe that the species may have been solitary.

Paleontologists are uncertain about the function of the nasal horn in Ceratosaurus. Rather than a defensive weapon, paleontologists have suggested that it may have been used to attract mates or perhaps in contests between animals seeking to impress potential mates. Alternatively, the dorsal osteoderms and nasal horn might have been used to protect the body as the animal charged through tough vegetation in the forests.

Habitat and Other Life Forms

Late Jurassic North America had a diverse collection of habitats ranging from dry scrub to lush conifer forests. Tall conifer trees and cycads, another common variety of seed-bearing plant, dominated the landscape while wetter areas with more moisture supported ferns and horsetails. There were a wide variety of reptiles, amphibians, and small mammals living alongside the large dinosaur fauna. Ceratosaurus might have competed for prey with the other large theropods, such as Allosaurus and the large predator Torvosaurus, which could reach lengths of more than 9 meters (29.5 ft). There were also smaller predatory dinosaurs present, like the agile Ornitholestes, a small bird-like theropod that hunted a variety of small animals.

The Late Jurassic was the time of the giant sauropods, such as Diplodocus, which could grow to more than 50 meters (164 ft), and the smaller Camarasaurus, which was smaller but still dwarfed its predators at lengths of more than 20 meters (65.6 ft). Predators also had the option of pursuing smaller herbivores, like the bipedal ornithopod Dryosaurus or the small ornithischian Othnielia. Herbivores like Stegosaurus, an armored plant eater with impressive dermal armor, also lived in the area and may have fallen victim to predators like Ceratosaurus.

Research

Ceratosaurus was first described by pioneering paleontologist Othniel Marsh from a specimen discovered in Utah in 1884. The original specimen was later combined with remnants discovered in Colorado and redefined as a species by paleontologist Charles Gilmore. Several additional species in the Ceratosaurus genus have been discovered since 2000, most notably a specimen from Portugal, indicating that the genus may have been widespread. Further research from Robert Bakker suggested that Ceratosaurus was one of a small group of dinosaurs living semi-aquatic lifestyles, though further evidence is needed to confirm this hypothesis.

Between 2014 and 2019, the National Museum of Natural History, which owns one of the largest specimens of Ceratosaurus, replaced its mounted exibit with a free-standing skeleton cast in order to store the original bones to allow full access for scientists.

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