Dinosaurs Live highlight 08: Dilophosaurus

Dilophosaurus (Greek: di for "two", lophos "crest", and sauros "lizard") was a theropod dinosaur from the Sinemurian stage of the Early Jurassic Period, about 193 million years ago. The first specimens were described in 1954, but it was not until over a decade later that the genus received its current name. It is one of the earliest known Jurassic theropods and one of the least understood.

Description
Dilophosaurus measured around six meters (20 ft) long and may have weighed half a ton. The most distinctive characteristic of Dilophosaurus is the pair of rounded crests on its skull, possibly used for display. Studies by Robert Gay show no indication that sexual dimorphism was present in the skeleton of Dilophosaurus, but says nothing about crest variation. The teeth of Dilophosaurus are long, but have a fairly small base and expand basally. Another skull feature was a notch behind the first row of teeth, giving Dilophosaurus an almost crocodile-like appearance, similar to the putatively piscivorous spinosaurid dinosaurs. This "notch" existed by virtue of a weak connection between the premaxillary and maxillary bones of the skull. This conformation led to the early hypothesis that Dilophosaurus scavenged off dead carcasses, with the front teeth being too weak to bring down and hold large prey.

Classification
Dilophosaurus may be a primitive member of the clade containing both ceratosaurian and tetanuran theropods. Alternatively, some paleontologists classify this genus as a large coelophysoid.

Dinosaurs Live highlight 07: Sinosauropteryx

Sinosauropteryx (meaning "Chinese reptilian wing", in Chinese 中华龙鸟: zhonghua longniao) was the first genus of dinosaur outside of Avialae (birds and their immediate relatives) to be found with evidence of feathers. They were covered with "furry" coats of very simple filament-like feathers. Structures that indicate colouration have been preserved for some of the feathers, which also makes Sinosauropteryx the first non-avialian dinosaurs where colouration has been determined. Colouration includes a banded tail with reddish and light bands. Some contention has arisen with an alternative interpretation of the filamentous impression as remains of collagen fibres.

Sinosauropteryx were small theropods with unusually long tails and short arms. They were close relatives of the similar but older genus Compsognathus, both genera belonging to the family Compsognathidae. Only one species of Sinosauropteryx has been named: S. prima, meaning "first" in reference to its status as the first feathered non-avialian dinosaur species discovered. A handful of specimens have been described. One specimen previously assigned to this genus represents either a second, as-yet unnamed species or a related genus.

Sinosauropteryx prima lived in what is now northeastern China during the early Cretaceous period. They were among the first dinosaurs discovered from the Yixian Formation in Liaoning Province, and were members of the remarkable Jehol Biota. Well-preserved fossils of this species illustrate many aspects of their biology, such as their diet and reproduction.

Sinosauropteryx prima were small bipedal theropods, noted for their short arms, large first fingers (thumbs), and long tails. The species includes some of the smallest known adult non-avian theropod specimens, with the holotype specimen measuring only 68 cm (27 in) in length, including the tail. This individual was relatively young. The longest known specimen reaches up to 1.07 m (3.5 ft) in length, with an estimated weight of 0.55 kg (1.2 lb).

Sinosauropteryx were anatomically similar to Compsognathus, differing from their European relatives in their proportions. The skulls of Sinosauropteryx were 15% longer than their thigh bones, unlike Compsognathus, in which the skulls and thigh bones are approximately equivalent in length. The arms of Sinosauropteryx (humerus and radius) were only 30% the length of their legs (thigh bone and shin), compared to 40% in Compsognathus. Additionally, Sinosauropteryx had several features unique among all other theropods. S. prima had 64 vertebrae in their tails. This high number helps give them the longest tails relative to body length of any theropod species. Their hands were long compared to its arms, about 84% to 91% of the length of the rest of the arm (humerus and radius), and half the length of the foot. The first and second digits were about the same length, with a large claw on the first digit. The first fingers were large, being both longer and thicker than either of the bones of the forearm. The teeth differed slightly based on position: those near the tips of the upper jaws (on the premaxillae) were slender and lacked serrations, while those behind them (on the maxillae) were serrated and laterally compressed. The teeth of the lower jaws were similarly differentiated.

A pigmented area in the abdomen of the holotype has been suggested as possible traces of organs,[2] and was interpreted as the liver by John Ruben and colleagues, which they described as part of a crocodilian-like "hepatic piston" respiratory system. A later study, while agreeing that the pigmented area represented something originally inside the body, found no defined structure and noted that any organs would have been distorted by the processes that flattened the skeleton into an essentially two-dimensional form. Dark pigment is also present in the eye region of the holotype and another specimen.

Dinosaurs Live highlight 06: Maiasaura

Maiasaura (from the Greek "μαία + σαύρα", meaning "caring mother lizard") is a large duck-billed dinosaur genus that lived in the area currently covered by the state of Montana in the Upper Cretaceous Period (Campanian), about 74 million years ago.

Description
Maiasaura was large, attaining an adult length of about 9 metres (30 ft) and had the typical hadrosaurid flat beak and a thick nose. It had a small, spiky crest in front of its eyes. The crest may have been used in headbutting contests between males during the breeding season.

This dinosaur was herbivorous. It walked both on two (bipedal) or four (quadrupedal) legs and appeared to have no defense against predators, except, perhaps, its heavy muscular tail and its herd behaviour. These herds were extremely large and could have comprised as many as 10,000 individuals. Maisaura lived in an inland habitat.

Reproducation
Maiasaura lived in herds and it raised its young in nesting colonies. The nests in the colonies were packed closely together, like those of modern seabirds, with the gap between the nests being around 7 metres (23 ft); less than the length of the adult animal. The nests were made of earth and contained 30 to 40 eggs laid in a circular or spiral pattern. The eggs were about the size of ostrich eggs.

The eggs were incubated by the heat resulting from rotting vegetation placed into the nest by the parents, rather than a parent sitting on the nest. Upon hatching, fossils of baby Maiasaura show that their legs were not fully developed and thus they were incapable of walking. Fossils also show that their teeth were partly worn, which means that the adults brought food to the nest.

The hatchlings grew from a size of 16 to 58 inches (41 to 150 cm) long in the span of their first year. At this point, or perhaps after another year, the animal left the nest. This high rate of growth may be evidence of warm bloodedness. The hatchlings had different facial proportions from the adults, with larger eyes and a shorter snout. These features are associated with cuteness and are common among animals that are dependant on their parents for survival during the early stages of life.

Dinosaurs Live highlight 05: Spinosaurus

Spinosaurus (meaning "spine lizard") is a genus of theropod dinosaur which lived in what is now North Africa, from the lower Albian to lower Cenomanian stages of the Cretaceous period, about 112 to 97 million years ago. This genus was first known from Egyptian remains discovered in 1912 and described by German paleontologist Ernst Stromer in 1915. The original remains were destroyed in World War II, but additional material has come to light in recent years. It is unclear whether one or two species are represented in the fossils reported in the scientific literature. The best known species is S. aegyptiacus from Egypt, although a potential second species S. maroccanus has been recovered from Morocco.

Spinosaurus may be the largest of all known carnivorous dinosaurs, even larger than Tyrannosaurus and Giganotosaurus. Estimates published in 2005 and 2007 suggest that it was 12.6 to 18 metres (41 to 59 ft) in length and 7 to 20.9 tonnes (7.7 to 23.0 short tons) in weight. The skull was long and narrow like that of a modern crocodilian. Spinosaurus is thought to have eaten fish; evidence suggests that it lived both on land and in water like a modern crocodilian. The distinctive spines of Spinosaurus, which were long extensions of the vertebrae, grew to at least 1.65 meters (5.4 ft) long and were likely to have had skin connecting them, forming a sail-like structure, although some authors have suggested that the spines were covered in fat and formed a hump. Multiple functions have been put forward for this structure, including thermoregulation and display.

Size
Spinosaurus is known for its neural spines, its large size, and its elongated skull. Since its discovery, Spinosaurus has been a contender for the longest and largest theropod dinosaur. Both Friedrich von Huene in 1926 and Donald F. Glut in 1982 listed it as among the most massive theropods in their surveys, at 15 meters (49 ft) in length and upwards of 6 tons in weight. In 1988, Gregory Paul also listed it as the longest theropod at 15 meters (49 ft), but gave a lower mass estimate of 4 tonnes (4.4 short tons).

The lower estimates for Spinosaurus would imply that the animal was shorter and lighter than Carcharodontosaurus and Giganotosaurus. The Therrien and Henderson study has been criticized for the choice of theropods used for comparison (e.g., most of the theropods used to set the initial equations were tyrannosaurids and carnosaurs, which have a different build than spinosaurids), and for the assumption that the Spinosaurus skull could be as little as 1.5 meters (4.9 ft) in length. Improvement of the precision of size estimates for Spinosaurus requires the discovery of more complete remains as available for some other dinosaurs, especially the limb bones of Spinosaurus which are "hitherto unknown."

Neural Spines
Very tall neural spines growing on the back vertebrae of Spinosaurus formed the basis of what is usually called the animal's "sail." The lengths of the neural spines reached over 10 times the diameters of the vertebral bodies from which they extended. The neural spines were slightly longer front to back at the base than higher up, and were unlike the thin rods seen in the pelycosaur finbacks Edaphosaurus and Dimetrodon, contrasting also with the thicker spines in the contemporary iguanodont Ouranosaurus.

Spinosaurus sails were unusual, although other dinosaurs, namely the ornithopod Ouranosaurus, which lived a few million years earlier in the same general region as Spinosaurus, and the South American sauropod Amargasaurus, might have developed similar structural adaptations of their vertebrae. The sail is possibly analogous (not homologous) to that of the Permian synapsid Dimetrodon, which lived before the dinosaurs even appeared; these similarities are due to parallel evolution.

The structure may also have been more hump-like than sail-like, as noted by Stromer in 1915 ("one might rather think of the existence of a large hump of fat, to which the neural spines gave internal support") and by Jack Bowman Bailey in 1997. In support of his "buffalo-back" hypothesis, Bailey argued that in Spinosaurus, Ouranosaurus, and other dinosaurs with long neural spines, the spines were relatively shorter and thicker than the spines of pelycosaurs (which were known to have sails); instead, the dinosaurs' neural spines were similar to the neural spines of extinct hump-backed mammals such as Megacerops and Bison latifrons.

Skull
The skull had a narrow snout filled with straight conical teeth that lacked serrations. There were six or seven teeth on each side of the very front of the upper jaw, in the premaxillae, and another twelve in both maxillae behind them. The second and third teeth on each side were noticeably larger than the rest of the teeth in the premaxilla, creating a space between them and the large teeth in the anterior maxilla; large teeth in the lower jaw faced this space. The very tip of the snout holding those few large anterior teeth was expanded, and a small crest was present in front of the eyes. Using the dimensions of three specimens known as MSNM V4047, UCPC-2, and BSP 1912 VIII 19, and assuming that the postorbital part of the skull of MSNM V4047 had a shape similar to the postorbital part of the skull of Irritator, Dal Sasso et al. (2005) estimated that the skull of Spinosaurus was 1.75 meters (5.7 ft) long.[5] The Dal Sasso et al. skull length estimate was questioned because skull shapes can vary across spinosaurid species.

Dinosaurs Live highlight 04: Dimetrodon

Dimetrodon (meaning "two measures of teeth") was a predatory synapsid genus that flourished during the Permian period, living between 280–265 million years ago (during the Artinskian to Capitanian stages).

As a synapsid it was more closely related to mammals than to true reptiles such as lizards and snakes. It is classified as a pelycosaur. Fossils of Dimetrodon have been found in North America and Europe. Dimetrodon had a sail on its back, which is thought to have been used for regulating body temperature, or for display.

Description
Dimetrodon was one of the largest land animals and the apex predator of its time. Dimetrodon’s diet could have included freshwater sharks, amphibians, reptiles and other amniotes. The amphibian Eryops and freshwater shark Xenacanthus were prey. Humeruses of Eryops and skulls of Xenacanthus were found to have teeth marks matching the shape of the teeth of Dimetrodon.[1] Dimetrodon probably hunted based on sight and smell. Different types ranged in length from 90 to 400 centimetres (35 to 160 in) and weighing 14 to 300 kilograms (31 to 660 lb).

The structure of the bones indicate that it was cold-blooded and had low metabolism. There are few channels in the bones, which indicates limited circulation. For proper metabolism, Dimetrodon needed external heat. The growth pattern of Dimetrodon is unclear. Analysis based on the length and ossification of the thigh, the ulna and the humerus, shows a poor correlation between the size and relative age of individuals. There was a difference in habitat between juveniles and adults in Dimetrodon. Younger animals lived mainly in habitats with plenty of shelter, such as swamps and reed-lined banks. The adults preferred the open areas of flood plains.[4] This is the finding of fossils from the Wichita Group of Texas. The length of the upper arm and thigh, the main measure that was used to distinguish between young and adult specimens, were examined and it was determined what type of sediment the fossils occurred. A similar difference in distribution between young and adult specimens, was also found for Ophiacodon and Eryops and is also known from modern crocodiles and water turtles.

Teeth
Dimetrodon has two types of teeth, shearing teeth and sharp canine teeth. Its name, in fact, means "two-measures of teeth". Dimetrodon was one of the first animals with differentiated teeth and the teeth were suitable for killing animals then tearing them to pieces. Dimetrodon has a large skull with a temporal fenestra behind each eye orbit on the lateral surface, a distinguishing feature of synapsid skull. This made possible new attachment sites for jaw muscles, which could run faster and create mastication. Based on the osteology of the temporal region, the posterior part of the palate and mandible, powerful jaw muscles of Dimetrodon was found to have differentiated. Two groups of muscles have been reconstructed: the adductors and the pterygoideus. The adductors were from temporal to the inside of the lower jaw and caused the closing of the jaws. The pterygoideus ran from the Pterygoid processes of the sphenoid and the sphenoid bone to the articular, one of the bones in the lower jaw, and caused a backward movement of the mandible. The development of the coronoid process in Dimetrodon compared with other quadrupeds , led to an increase in the length of the moment of external muscular and thus greater bite force. The construction of the inner ear and vestibular system of Dimetrodon were described by Case, but he drew no conclusions about the possible function of these organs.

In a 2001 study on the biomechanics of the dinosaur Albertosaurus's teeth, William L. Abler observed that examined Dimetrodon teeth possessed serrations so fine they resembled a crack in the tooth. Though These voids, termed an ampulla, would hinder the ability of the "crack" formed by the serration to propagate through the tooth. Dimetrodon was found to lack adaptations for preventing "crack" propagation

Sail
The most striking feature of Dimetrodon has been the one meter long protrusions at the back of the spine, the spinous processes also referred to as spines. These spines have a wide base and then his long and slender with a pointed tip. It is believed that the spines were connected by a solid skin and thus formed a sail on the back.

Many suggestions have been made about the function of the sail; as camouflage among reeds while it waited for prey, for sexual display, or literally as a sail while swimming. Romer and Price first suggested that it served a mechanical function and strengthened the backbone, but Romer later realized that the sail had evolved with features strongly suggesting an early attempt at temperature regulation. In a study of the relationship between body temperature and blood pressure, Rodbard analyzed the evolution of thermoregulation, which he thought was a one possible function of the sails of Dimetrodon and Edaphosaurus. The spines of Dimetrodon have grooves on the base that were presumably ingested by blood vessels and thus ensured good bloodflow through the skin of the sail. The theory is that Dimetrodon was active in the early morning when the sun rose. The sail would be pointed towards the sun and would absorb heat allowing rapid warming. This allowed Dimetrodon to hunt at a time when other animals were not sufficiently warmed up and were slow. The sail increased body surface area by 50%.

According to calculations by Bramwell Fellgett, it took a 200 kg (440 lb) Dimetrodon approximately one and a half hours for its body temperature to go from 26 to 32 °C (79 to 90 °F) A study by Haack concluded that warming was slower than previously thought and that the process probably took four hours.[14] In order to cool its body in the hot midday sun, Dimetrodon turned its sail away from the sun, causing the heat to drain. The rapid warming using the sail give Dimetrodon an edge over larger animals, weighing over 55 kg. Smaller animals had higher body surface-to-mass ratio, making them hotter than Dimetrodon. The prey of Dimetrodon would therefore have been mostly large animals like Diadectes, Eryops and Ophiacodon. The changing climate during the Permian period, when the temperature increased, is a possible reason for the extinction of Dimetrodon since the sail meant no benefit over other animals and was rather a disadvantage due to its fragility.

Dinosaurs Live highlight 03: Deinosuchus

Deinosuchus is an extinct genus related to the alligator that lived 73 to 80 million years ago, during the late Cretaceous period. The name translates as "terrible crocodile" and is derived from the Greek deinos (δεινός), "terrible", and soukhos (σοῦχος), "crocodile". The first remains were discovered in North Carolina (United States) in the 1850s; the genus was named and described in 1909. Additional fragments were discovered in the 1940s and were later incorporated into an influential, though inaccurate, skull reconstruction at the American Museum of Natural History. Knowledge of Deinosuchus remains incomplete, but better cranial material found in recent years has expanded scientific understanding of this massive predator.

Although Deinosuchus was far larger than any modern crocodile or alligator—measuring up to 12 m (39 ft) and weighing up to 8.5 metric tons (9.4 short tons)—in overall appearance it was fairly similar to its smaller relatives. It had large, robust teeth that were built for crushing, and its back was covered with thick hemispherical osteoderms. One study indicates that Deinosuchus may have lived for up to 50 years, growing at a rate similar to that of modern crocodilians, but maintaining this growth over a much longer period of time.

Deinosuchus fossils have been found in ten U.S. states, including Texas, Montana, and many along the East Coast. Fossils have also been found in northern Mexico. It lived on both sides of the Western Interior Seaway, and was an opportunistic apex predator in the coastal regions of eastern North America. Deinosuchus reached its largest size in its western habitat, but the eastern populations were far more abundant. Opinion remains divided as to whether these two populations represent separate species. Deinosuchus was probably capable of killing and eating large dinosaurs. It may have also fed upon sea turtles, fish, and other aquatic and terrestrial prey.

Description
Despite its large size, the overall appearance of Deinosuchus was not considerably different from that of modern crocodilians. It had an alligator-like broad snout, with a slightly bulbous tip. Each premaxilla contained four teeth, with the pair nearest to the tip of the snout being significantly smaller than the other two. Each maxilla (the main tooth-bearing bone in the upper jaw) contained 21 or 22 teeth. The tooth count for each dentary (tooth-bearing bone in the lower jaw) was at least 22. All the teeth were very thick and robust; those close to the rear of the jaws were short, rounded, and blunt. They appear to have been adapted for crushing, rather than piercing. When the mouth was closed, only the fourth tooth of the lower jaw would have been visible.

Modern American alligators, with the strongest bite of any living animal, have a maximum force of 9,452 newtons (2,125 lbf). The bite force of Deinosuchus has been estimated to exceed 18,000 newtons (4,000 lbf).[1] Even the largest and strongest theropod dinosaurs, such as Tyrannosaurus, probably had a bite force inferior to that of Deinosuchus.

Deinosuchus had a secondary bony palate, which would have permitted it to breathe through its nostrils while the rest of the head remained submerged underwater. The vertebrae were articulated in a procoelous manner, meaning that they had a concave hollow on the front end and a convex bulge on the rear; these would have fit together to produce a ball and socket joint.[7][8] The secondary palate and procoelous vertebrae are advanced features also found in modern eusuchian crocodilians.

The osteoderms (scutes) covering the back of Deinosuchus were unusually large, heavy, and deeply pitted; some were of a roughly semispherical shape. Deep pits and grooves on these osteoderms served as attachment points for connective tissues. Together, the osteoderms and connective tissues would have served as load-bearing reinforcement to support the massive body of Deinosuchus out of water. Consequently, despite its bulk, Deinosuchus was probably almost as agile on land as its modern relatives.

Although there is some disagreement as to its exact size, the fossil remains are nonetheless sufficient to indicate that Deinosuchus was substantially larger than any modern crocodilian. Even the relatively low estimate provided by Erickson and Brochu suggests that the maximum weight reached by Deinosuchus exceeded that of currently living species by a factor of three to five. Deinosuchus has often been described as the largest crocodilian of all time, but some other crocodyliforms—including Purussaurus, Rhamphosuchus, and Sarcosuchus—may have equaled or exceeded it in size.

Dinosaurs Live highlight 02: Pterodactyl

Pterodactylus (from the Greek πτεροδάκτυλος, pterodaktulos, meaning "winged finger") is a genus of pterosaurs, whose members are popularly known as pterodactyls. It was the first to be named and identified as a flying reptile. Its fossil remains have been found primarily in the Solnhofen limestone of Bavaria, Germany, dated to the late Jurassic Period (early Tithonian), about 150.8-148.5 million years ago, though more fragmentary remains have been identified from elsewhere in Europe and in Africa. It was a carnivore and probably preyed upon fish and other small animals. Like all pterosaurs, the wings of Pterodactylus were formed by a skin and muscle membrane stretching from its elongated fourth finger to its hind limbs. It was supported internally by collagen fibres and externally by keratinous ridges.

The name derives from the Greek words pteron (πτερόn, meaning 'wing') and daktylos (δάκτυλος, meaning 'finger') and refers to the way in which the wing is supported by one large finger.

Description
Pterodactylus is known from over 27 fossil specimens, and though most of those are juveniles, many preserve complete skeletons. The discovery of several specimens with well-preserved soft tissue traces has allowed scientists to faithfully reconstruct the life appearance of Pterodactylus. Pterodactylus was a relatively small pterosaur, with an estimated adult wingspan of about 1.5 meters (5 ft) in P. antiquus. Other "species" were once thought to be smaller. However, these smaller specimens have been shown to represent juveniles of Pterodactylus, as well as its contemporary relatives Ctenochasma, Germanodactylus and Gnathosaurus.[3]

The skulls of adult Pterodactylus were long and narrow with about 90 large, conical teeth. The teeth extended back from the tips of both jaws, and became smaller farther away from the jaw tips (unlike some relatives, where teeth were absent in the upper jaw tip and were relatively uniform in size). The teeth extended farther back into the jaw than in close relatives, as some were present below the front of the nasoantorbital fenestra, the largest opening in the skull. Unlike related species, the skull and jaws were straight, not curved upwards.[4] A small, hooked beak was present in the very tips of the jaws, with both upper and lower hook no larger than the teeth that surrounded them.

The neck was long, and covered in long, bristle-like pycnofibres. A throat pouch extended from about the middle of the lower jaw to the upper part of the neck.

Pterodactylus, like related pterosaurs, had a crest on its skull composed mainly of soft tissues. In adult Pterodactylus, this crest extended between the back edge of the antorbital fenestra (the largest opening in the skull) and the back of the skull. The back of the crest extended upward into a backward-curving cone-shaped structure. The crest was composed mainly of long, hardened fibres (twisted together in a spiral pattern inside the conical part of the crest), and covered in scales. In at least one specimen of P. longicollum, the crest had a short bony base, also seen in related pterosaurs like Germanodactylus. Crests have only been found on large, fully adult specimens of Pterodactylus, indicating that this was a display structure and only developed when individuals reached maturity.

The wings were long, and the wing membranes appear to have lacked the furry covering of pycnofibres present in some other pterosaurs (such as Pterorhynchus and Jeholopterus). The wing membrane extended between the fingers and toes as webbing, and a uropatagium (secondary membrane between the feet and tail) was present, as well as a propatagium (membrane between the wrist and shoulder).[6] Both the finger and toe claws were covered in keratin sheaths that extended and curved into sharp hooks well beyond their bony cores.

Dinosaurs Live highlight 01: Apatosaurus

Apatosaurus, also known by the popular but scientifically deprecated synonym Brontosaurus, is a genus of sauropod dinosaur that lived from about 154 to 150 million years ago, during the Jurassic Period (Kimmeridgian and early Tithonian ages). It was one of the largest land animals that ever existed, with an average length of 23 m (75 ft) and a mass of at least 23 metric tons (25 short tons). The composite term Apatosaurus comes from the Greek names apate (ἀπάτη)/apatelos (ἀπατηλός) meaning "deception"/"deceptive" and sauros (σαῦρος) meaning "lizard"; thus, "deceptive lizard".

Description
Apatosaurus was a large, long-necked quadrupedal animal with a long, whip-like tail. Its forelimbs were slightly shorter than its hindlimbs. One measurement places the total length of the species Apatosaurus louisae at 22 m (72 ft).

It was roughly the weight of four elephants. One specimen of A. excelsus was estimated to weigh 25,952 kg (57,210 lb); estimates for A. louisae were 20,600 kg (45,000 lb) and 22,407 kg (49,400 lb). Other estimates of the body mass of adult Apatosaurus species range from 18,000 kg (40,000 lb) to 35,000 kg (77,000 lb). A microscopic study of Apatosaurus bones concluded that the animals grew rapidly when young and reached near-adult sizes in about 10 years.

The skull was small in comparison with the size of the animal. The jaws were lined with spatulate teeth, which resembled chisels, suited to an herbivorous diet. Like other sauropods, the vertebrae of the neck were deeply bifurcated; that is, they carried paired spines, creating a wide and deep profile for the neck. The apparently massive neck was, however, filled with an extensive system of weight-saving air sacs. Apatosaurus, like its close relative Supersaurus, is notable for the incredibly tall spines on its vertebrae, which make up more than half the height of the individual bones. Also unusual among diplodocids is the shape of the tail, which is comparatively thin in breadth and short in height, a profile caused by the vertebral spines decreasing in height rapidly the farther they are from the hips. Apatosaurus also had very long ribs compared to most other diplodocids, giving it an unusually deep chest. The limb bones were also very robust.

Classification and Species
Apatosaurus is a member of the family Diplodocidae, a clade of gigantic sauropod dinosaurs. The family includes some of the longest creatures ever to walk the earth, including Diplodocus, Supersaurus, Suuwassea, and Barosaurus. Within the subfamily Apatosaurinae, Apatosaurus may be most closely related to Suuwassea, Supersaurus and Eobrontosaurus.

In 1877, Othniel Charles Marsh published the name of the type species Apatosaurus ajax. He followed this in 1879 with a description of another, more complete specimen, which he thought represented a new genus and species, which he named Brontosaurus excelsus. In 1903, Elmer Riggs re-examined the fossils. While he agreed with Marsh that Brontosaurus excelsus was likely a distinct species, he also noted many similarities between B. excelsus and A. ajax, and decided that both should be placed in the same genus. Riggs re-classified the species as Apatosaurus excelsus. Almost all paleontologists since Riggs published his opinions have agreed that the two species should be classified together in a single genus. According to the rules of the ICZN (which governs the scientific names of animals), the name Apatosaurus, having been published first, had priority as the official name; Brontosaurus is considered a junior synonym and has therefore been discarded from formal use.

Apatosaurus ajax is the type species of the genus, and was named by the paleontologist Othniel Charles Marsh in 1877 after Ajax, the hero from Greek mythology. It is the holotype for the genus and two partial skeletons have been found, including part of a skull. Apatosaurus excelsus (originally Brontosaurus) was named by Marsh in 1879. It is known from six partial skeletons, including part of a skull, which have been found in the United States, in Colorado, Oklahoma, Utah, and Wyoming. Apatosaurus louisae was named by William Holland in 1915 in honor of Mrs. Louise Carnegie, wife of Andrew Carnegie who funded field research to find complete dinosaur skeletons in the American West. Apatosaurus louisae is known from one partial skeleton which was found in Utah in the United States. Apatosaurus parvus was originally known as Elosaurus parvus, but was reclassified as a species of Apatosaurus in 1994.

Dinosaurs Live intro 02: Order of Dinosaurs

Saurischia and Ornithischia are the two orders, or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure. Saurischians ('lizard-hipped') are distinguished from the ornithischians ('bird-hipped') by retaining the ancestral configuration of bones in the hip.


SAURISCHIA
Saurischians are distinguished from ornithischians by their three-pronged pelvic structure, with the pubis pointed forward. The ornithischians' pelvis is arranged with the pubis rotated backward, parallel with the ischium, often also with a forward-pointing process, giving a four-pronged structure.

All carnivorous dinosaurs (the theropods) are saurischians, as are one of the two primary lineages of herbivorous dinosaurs, the sauropodomorphs. At the end of the Cretaceous Period, all non-avian saurischians became extinct. This is referred to as the Cretaceous-Tertiary extinction event. Avians (modern birds), as direct descendants of one group of theropod dinosaurs, are considered to be a sub-clade of saurischian dinosaurs in phylogenetic classification.

In his paper naming the two groups, Seeley reviewed previous classification schemes put forth by other paleontologists to divide up the traditional Order Dinosauria. He preferred one that had been put forward by Othniel Charles Marsh in 1878, which divided dinosaurs into four Orders: Sauropoda, Theropoda, Ornithopoda, and Stegosauria (these names are still used today in much the same way to refer to suborders or clades within Saurischia and Ornithischia).

Seeley, however, wanted to formulate a classification that would take into account a single primary difference between major dinosaurian groups based on a characteristic that also differentiated them from other reptiles. He found this in the configuration of the hip bones, and found that all four of Marsh's orders could be divided neatly into two major groups based on this feature. He placed the Stegosauria and Ornithopoda in the Ornithischia, and the Theropoda and Sauropoda in the Saurischia. Furthermore, Seeley used this major difference in the hip bones, along with many other noted differences between the two groups, to argue that "dinosaurs" were not a natural grouping at all, but rather two distinct orders that had arisen independently from more primitive archosaurs.[2] This concept that "dinosaur" was an outdated term for two distinct orders lasted many decades in the scientific and popular literature, and it was not until the 1960s that scientists began to again consider the possibility that saurischians and ornithischians were more closely related to each other than they were to other archosaurs.

ORNITHISCHIA
The ornithischian hip structure is superficially similar to that of birds, which led Seeley to name them "bird-hipped dinosaurs," though he did not propose any specific relationship with birds. He termed saurischians "lizard-hipped" dinosaurs because they retained the ancestral hip anatomy also found in modern lizards.

However, as later study revealed, the hip structure possessed by modern birds actually evolved independently from the "lizard-hipped" saurischians (specifically, a sub-group of saurischians called the Maniraptora) in the Jurassic Period. In this example of convergent evolution, birds developed hips oriented similar to the earlier ornithischian hip anatomy, in both cases possibly as an adaptation to a herbivorous or omnivorous diet.

This division, which has generally been accepted, is based on the evolution of the pelvis into a more bird-like structure (although birds did not descend from these dinosaurs), details in the vertebrae and armor and the possession of a 'predentary' bone. The predentary is an extra bone in the front of the lower jaw, which extends the dentary (the main lower jaw bone). The predentary coincides with the premaxilla in the upper jaw. Together they form a beak-like apparatus used to clip off plant material.

The ornithischian pubis bone points downward and toward the tail (backwards), parallel with the ischium, with a forward-pointing process to support the abdomen. This makes a four-pronged pelvic structure. In contrast to this, the saurischian pubis points downward and toward the head (forwards), as in ancestral lizard types. Ornithischians also had smaller antorbital fenestrae (holes in front of their eye sockets) than did saurischians, and a wider, more stable pelvis. A bird-like pubis arrangement, parallel to the vertebral column, evolved independently three times in dinosaur evolution, namely in the ornithischians, in the therizinosauroids and in bird-like dromaeosaurids.

Ornithischians shifted from bipedal to quadrupedal posture at least three times in their evolutionary history and have been shown to have been capable of adopting both postures early in their evolutionary history.

Dinosaurs Live intro 01: Description of Dinosaurs

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period (about 230 million years ago) until the end of the Cretaceous (about 65 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of all non-avian dinosaurs at the close of the Mesozoic era. The fossil record indicates that birds evolved within theropod dinosaurs during the Jurassic period. Some of them survived the Cretaceous–Paleogene extinction event, including the ancestors of all modern birds. Consequently, in modern classification systems, birds are considered a type of dinosaur—the only group which survived to the present day.

Dinosaurs are a varied group of animals. Birds, at over 9,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs. Dinosaurs are represented on every continent by both extant species and fossil remains. Some are herbivorous, others carnivorous. Many dinosaurs have been bipedal, and many extinct groups were also quadrupedal, and some were able to shift between these body postures. Many species possess elaborate display structures such as horns or crests, and some prehistoric groups even developed skeletal modifications such as bony armor and spines. Avian dinosaurs have been the planet's dominant flying vertebrate since the extinction of the pterosaurs, and evidence suggests that all ancient dinosaurs built nests and laid eggs much as avian species do today. Dinosaurs varied widely in size and weight; the smallest adult theropods were less than 100 centimeters (40 inches) long, while the largest sauropods could reach lengths of almost 50 meters (165 feet) and were several stories tall.

Although the word dinosaur means "terrible lizard," the name is somewhat misleading, as dinosaurs were not lizards. Rather, they were a separate group of reptiles with a distinct upright posture not found in lizards. Through the first half of the 20th century, most of the scientific community believed dinosaurs were sluggish, unintelligent, and cold-blooded. Most research conducted since the 1970s, however, has indicated that dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction, and many groups (especially the carnivores) were among the most intelligent organisms of the time.

Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons or replicas have been major attractions at museums around the world, and dinosaurs have become a part of world culture. Their diversity, the large sizes of some groups, and their seemingly monstrous and fantastic nature have captured the interest and imagination of the general public for over a century. They have been featured in best-selling books and films such as Jurassic Park, and new discoveries are regularly covered by the media.