Anatotitan

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Hadrosauridae

Genus:Anatotitan

Species:Anatotitan copei

Introduction

Anatotitan is a controversial name for a large herbivorous hadrosaurid, or duck-billed dinosaur, that lived in Late Cretaceous North America, one of the most diverse dinosaur environments in the Mesozoic Era (251–65.5 million years ago). Hadrosaur classification has long been the subject of much debate, and many paleontologists now consider Anatotitan copei and several other proposed species to be synonyms of Edmontosaurus annectens. However, some researchers still make arguments for Anatotitan as a valid separate genus.

Regardless of classification controversy, fossils studied as Anatotitan have helped inform scientific understanding of dinosaur relationships and geologic history. In the Triassic period, the seven continents were united in a single landmass, called Pangaea. The dinosaurs evolved and radiated across the world during this period. When Pangaea began to split apart in the Jurassic Period, it gave rise to two supercontinents: Gondwana and Laurasia. Anatotitan was part of a common herbivore group in Western Laurasia. Paleontologists believe that the species found in the Late Cretaceous evolved from an ancestor that arrived in North America in the Cretaceous, long after Pangaea began to split apart under the influence of tectonic movement. Relatives of Anatotitan were largely absent from Africa and other continents that formed Gondwana.

Classification

Paleontologists use two major systems for classifying organisms. Linnaean taxonomy uses overall physical similarity to place organisms into different groups. By contrast, cladistics seeks to create groups of organisms based on evolutionary relationships; these groups are called “clades.” Cladistic analysis focuses on important key traits and attempts to trace the inheritance of a trait among descendants of a common ancestor.

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. According to Linnaean taxonomy, Anatotitan is an ornithischian and further placed in the infraorder Ornithopoda, a group united by their bird-like feet. Anatotitan has long been included in the family Hadrosauridae, popularly known as the “duck-billed” dinosaurs, and, according to some classification, the sub-family Hadrosaurinae, which comprised the larger hadrosaurs that lacked cranial crests.

Cladistic analysis also traditionally placed Anatotian in the clades Ornithopoda and Hadrosaurinae, due to the dinosaur's skull and the hip characteristics, as well as its lack of a cranial crest. While taxonomic analysis often grouped Anatotitan with the Early Cretaceous African dinosaur Ouranosaurus, cladistic analysis suggests that these two species evolved to have similar characteristics but from separate ancestors.

In the twenty-first century a growing body of research sought to reduce the number of valid hadrosaurid species, regardless of which classification system was used. Some scientists suggested that many fossils proposed as distinct species in fact represented various stages of growth of just a few different species. Anatotitan was a prominent example, as it was argued that this large animal was simply the full adult form of Edmontosaurus annectens (several other Edmontosaurus, species were also identified as younger forms). Support for this idea slowly increased among paleontologists, though some continued to consider Anatotitan valid.

Anatomy

Anatotitan (or Edmontosaurus) was a large dinosaur, with adults ranging from 9 to 12 meters (29–40 ft) long and standing as high as 4 or 5 meters (13–16 ft). The species could weigh between 3,000 and 4,000 kilograms (6,600–8,800 lbs). Anatotitan's rear legs were longer than its forelegs and terminated in three-toed feet with blunt, hoof-like claws.

The rear half of the animal was much heavier than the front, indicating that it may have favored a bipedal stance, with the upper body held low to the ground. The upper arms were well-muscled and featured claws that could be used for locomotion and defense.

Anatotitan's skull was over a meter (3.2 ft) long with a wide, flat bill and rows of leaf-shaped teeth. Anatotitan had unusually deep nasal cavities that may have been surrounded by pockets of soft tissue. These sacks may have inflated to allow the animals to make deep noises or even to serve as a visual signal.

The original Anatotitan specimen discovered revealed impressions of skin with pebbled, mosaic scales. The animal had a long, thick tail covered in thick muscle. The tail was probably used to balance the animal as it reared to reach vegetation.

Intelligence

Scientists have used encephalization quotient (EQ), based on brain and body size, to estimate animal intelligence. Hadrosaurs are generally thought to have had EQ ratings in the range of 1.5 to 2.0, which is lower than many modern grazing and herding animals, the group that most closely resembles the behavioral patterns ascribed to the hadrosaurs. They may have had intelligence similar to a small bird or large reptile.

Though Anatotitan faced a wide variety of potential predators, the species’ size would have protected it from all but the mid-size and larger predators. Some paleontologists believe that hadrosaurs also used herding behavior to avoid predators.

Reproduction and Population

Paleontologists assume that Anatotitan was oviparous, or laid eggs, but no specific Anatotitan eggs or nests have been found—perhaps in part due to confusion over hadrosaur diversity. Hadrosaurs in general laid groups of eggs in nests made from plant matter and debris. Paleontologists believe that hadrosaurs did not incubate their eggs by sitting on their nests, as many modern birds do, but instead covered the nests in vegetation, which gave off heat as it decayed.

Examinations of hatchling and young hadrosaurs have led some paleontologists to speculate that the young required parental care for the first few weeks after hatching. Young hadrosaurs may have stayed with their parents until they were subadults.

Diet

Paleontologists believe that Anatotitan was an herbivore and may have fed on a variety of plants in its environment, including conifers, ferns, and semi-aquatic vegetation. The long, flat bill of Anatotitan—which modern studies suggest looked less like a duck's bill and more like a beak, in contrast to popular conceptions—was probably used to crop leaves and stems from surrounding vegetation, which was then passed to the rows of masticating teeth in the animal's cheeks.

Dinosaur dung associated with some hadrosaur species has revealed the remains of fibrous plant matter, which appear to be masticated leaves and other plant parts. Hadrosaurs, like many modern herbivores, had large stomachs that may have served as fermentation chambers, allowing vegetation to decompose before being passed through the intestines.

Behavior

Scientists have found fossil beds containing multiple hadrosaurs, suggesting that they congregated in groups. However, paleontologists still remain somewhat uncertain whether hadrosaurs lived in groups or whether solitary animals died while gathered around a common food source. Groups of fossils containing both adult and young animals provide potential evidence that the species gathered in family groups. Herd behavior increases group defense and is common in large herbivores living alongside many predators.

Along with speculation about social behavior, scientists have debated whether hadrosaurs such as Anatotitan (or Edmontosaurus) were migratory. The diversity in hadrosaur fossils has also been interpreted as evidence of dimorphism, possibly including sexual dimorphism. However, further research is required before these theories can be considered conclusive.

Habitat and Other Life Forms

In the Late Cretaceous, much of central North America was covered by a shallow inland sea surrounded by a variety of swamps and floodplains. Elsewhere there were tropical and subtropical forests, dry forests, arid scrub, and deserts.

Gymnosperms were the dominant form of vegetation, and there were numerous species of conifers and cycads throughout North America and dozens of species of ferns and their spore-bearing relatives. The first angiosperms appeared during this period and rapidly spread. There were numerous reptiles, amphibians, and small mammals, many of which resembled extant species. In addition, the first birds had evolved, though the skies were still dominated by pterosaurs, flying reptiles that evolved alongside the dinosaurs in the Triassic.

North America supported a wide variety of dinosaur life, including other species of hadrosaurs such as Maiasaura. There were also several species of horned dinosaurs, or ceratopsians, like Triceratops and Torosaurus. Another major herbivore group was the ankylosaurs, like Ankylosaurus, known for their heavily-armored bodies and tails equipped with defensive spikes and clubs. As far as predatory dinosaurs were concerned, Anatotitan shared its environment with the tyrannosaurid Albertosaurus, one of the largest known predatory dinosaurs, and with a number of smaller predators like the dromaeosaur Troodon.

Research

The first Anatotitan specimen was collected in 1882 from the Hell's Creek Formation in what is now the Black Hills area of South Dakota. The specimen was discovered by comparative anatomist Jacob L. Wortman and his associate RS Hill, and was studied by pioneering American paleontologist Edward Drinker Cope. Cope placed the specimen within the hadrosaur group, but incorrectly discerned its relationships to other known species.

Later fossils studied by paleontologist Othniel Marsh were eventually determined to be of the same species. The genus was originally called Trachodon, but the species was renamed as Anatosaurus in the early 1940s when the hadrosaurs were reorganized. Further revisions changed the name again until the genus was given the unofficial name Anatotitan in literature from the late 1980s and early 1990s. As additional specimens were found, these too acquired the Anatotitan name. However, the validity and proper use of the name Anatotitan remained controversial.

In the twenty-first century evidence began to mount that Anatotitan might be the same species as Edmontosaurus annectens. Studies by paleontologists Nicolás Campione and David Evans published in 2009 and 2011 helped shift most researchers in favor of this reconsideration. Under their hypothesis the many proposed species of Edmontosaurus, along with Anatotitan and others previously thought the be relatives, are consolidated into just two species: Edmontosaurus annectens and Edmontosaurus regalis. Debate over whether too many or too few hadrosaurs, and dinosaurs in general, are formally recognized remains a significant issue.

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