ORNITHOCHEIRUS: THE T-REX OF THE SKIES

Ornithocheirus (from Greek "ορνις", meaning bird, and "χειρ", meaning hand) was a pterosaur from the Cretaceous period of Europe and South America. Based on poor fossil material, the genus has caused enduring problems of zoological nomenclature.

Fossil remains currently classified as Ornithocheirus have been recovered mainly from the Cambridge Greensand of England, dating to the beginning of the Albian stage of the late Cretaceous period, about 110 million years ago.Additional fossils from the Santana Formationof Brazil, dating to 112-108 million years ago, have been classified as species of Ornithocheirus.The original material of Ornithocheirus simus, recovered from England, indicates a mid-sized species with a wing span of 2.5 m (8 ft). Referred specimens attributed to Ornithocheirus simus(alternately called Criorhynchus simus) can reach 5 m (16 ft). Tropeognathus mesembrinus is also usually considered a part of the Ornithocheirus genus as O. mesembrinus, and reached over 6 m (20 ft) in wingspan.Both O. simus and O. mesembrinus bore distinctive convex "keeled" crests on their snouts.The upper crests arose from the snout tip and extended back to the nostril. An additional, smaller crest projected down from the lower jaw at the symphysis ("chin" area). While many ornithocheirids had a small, rounded bony crest projecting from the back of the skull, this was particularly large and well-developed in Ornithocheirus.Unlike the related Anhanguera and Coloborhynchus, which had an expanded rosette of teeth at the jaw tips, Ornithocheirus had straight jaws that narrowed toward the tip. Also unlike related pterosaurs, the teeth of Ornithocheirus were mostly vertical, rather than set at an outward-pointing angle. They also had fewer teeth than related species.The type specimen of Ornithocheirus simus is represented only by a broken piece of the upper jaw tip. While it does preserve several characteristic features of Ornithocheirus, it is nearly identical to comparable bones in o. mesembrinus, making clear distinction between these two species impossible.During the 19th century, in England many fragmentary pterosaur fossils were found in the Cambridge Greensand, a layer from the early Cretaceous, that had originated as a sandy seabed. Decomposing pterosaur cadavers, floating on the sea surface, had gradually lost individual bones that sank to the bottom of the sea. Water currents then moved the bones around, eroding and polishing them, until they were at last covered by more sand and fossilised. Even the largest of these remains were damaged and difficult to interpret. They had been assigned to the genus Pterodactylus, as was common for any pterosaur species described in the early and middle 19th century.

A DYNASTY OF TYRANTS: DASPLETOSAURUS

Daspletosaurus meaning "frightful lizard) is a genus of tyrannosaurid theropod dinosaur that lived in western North America between 77 and 74 million years ago, during the Late Cretaceous Period. Fossils of the only named species (D. torosus) were found in Alberta, although other possible species from Alberta and Montana await description. Including these undescribed species, Daspletosaurus is the most species-rich genus of tyrannosaur.Daspletosaurus is closely related to the much larger and more recent Tyrannosaurus. Like most known tyrannosaurids, it was a multi-ton bipedal predator equipped with dozens of large, sharp teeth. Daspletosaurus had the small forelimbs typical of tyrannosaurids, although they were proportionately longer than in other genera.As an apex predator, aspletosaurus was at the top of the food chain, probably preying on large dinosaurs like the ceratopsid Centrosaurus and the hadrosaur Hypacrosaurus. In some areas, Daspletosaurus coexisted with another tyrannosaurid,Gorgosaurus, though there is some evidence of niche differentiation between the two. While Daspletosaurus fossils are rarer than other tyrannosaurids, the available specimens allow some analysis of the biology of these animals, including social behavior, diet and life history.While very large by the standard of modern predators, Daspletosaurus was not the largest tyrannosaurid. Adults could reach a length of 8–9 meters (26–30 ft) from snout to tail.Mass estimates have centered around 2.5 tonnes (2.75 short tons), but have ranged between 1.8 tonnes (2 tons) and 3.8 tonnes (4.1 tons).Daspletosaurus had a massive skull that could reach more than 1 meter (3.3 ft) in length. The bones were heavily constructed and some, including the nasal bones on top of the snout, were fused for strength. Large fenestrae (openings) in the skull reduced its weight. An adult Daspletosaurus was armed with about six dozen teeth that were very long but oval in cross section rather than blade-like. Unlike its other teeth, those in the premaxilla at the end of the upper jaw had a D-shaped cross section, an example of heterodonty always seen in tyrannosaurids. Unique skull features included the rough outer surface of the maxilla (upper jaw bone) and the pronounced crests around the eyes on the lacrimal,postorbital, and jugal bones. The orbit (eye socket) was a tall oval, somewhere in between the circular shape seen in Gorgosaurus and the 'keyhole' shape ofTyrannosaurus.The type specimen of Daspletosaurus torosus (CMN 8506) is a partial skeleton including the skull, the shoulder, a forelimb, the pelvis, a femur and all of the vertebrae from the neck, torso and hip, as well as the first eleven tail vertebrae. It was discovered in 1921 by Charles Mortram Sternberg, who thought it was a new species ofGorgosaurus. It was not until 1970 that the specimen was fully described by Dale Russell, who made it the type of a new genus, Daspletosaurus, from the Greek stems δασπλητo-/daspleto- ('frightful') and σαυρος/sauros('lizard'). The type species is D. torosus, which is Latin for 'muscular' or 'brawny.' Aside from the type, there is only one other well-known specimen, a complete skeleton discovered in 2001. Both specimens were recovered from the Oldman Formation in the Judith River Group of Alberta. A specimen from the younger Horseshoe Canyon Formation in Alberta has been reassigned to Albertosaurus sarcophagus. The Oldman Formation was deposited during the middle Campanian stage of the Late Cretaceous, from about 77 to 76 Ma (million years ago).

A DYNASTY OF TYRANTS: GORGONOSAURUS

Gorgosaurus meaning "fierce lizard" is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 76.5 and 75 million years ago. Fossil remains have been found in the Canadian province of Alberta and possibly the U.S. state of Montana. Paleontologists recognize only the type species, G. libratus, although other species have been erroneously referred to the genus.Like most known tyrannosaurids, Gorgosaurus was a bipedal predator weighing more than a metric ton as an adult; dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was most closely related to Albertosaurus, and more distantly related to the larger Tyrannosaurus. Gorgosaurus and Albertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts consider G. libratusto be a species of Albertosaurus; this would make Gorgosaurus a junior synonym of that genus.Gorgosaurus lived in a lush floodplain environment along the edge of an inland sea. An apex predator, it was at the top of the food chain, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Though these animals were roughly the same size, there is some evidence of niche differentiation between the two. Gorgosaurus is the best-represented tyrannosaurid in the fossil record, known from dozens of specimens. These plentiful remains have allowed scientists to investigate its ontogeny, life history and other aspects of its biology.Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, closer in size toAlbertosaurus and Daspletosaurus. Adults reached 8 or 9 meters (26 to 30 ft) from snout to tail.Paleontologists have estimated full-grown adults to weigh more than 2.4 tonnes (2.7 short tons). The largest known skull measures 99 centimeters (39 in) long, just slightly smaller than that of Daspletosaurus.As in other tyrannosaurids, the skull was large compared to its body size, although chambers within the skull bones and large openings (fenestrae) between bones reduced its weight. Albertosaurus and Gorgosaurus share proportionally longer and lower skulls than Daspletosaurus and other tyrannosaurids. The end of the snout was blunt, and the nasal and parietal bones were fused along the midline of the skull, as in all other members of the family. The eye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera. A tall crest rose from the lacrimal bone in front of each eye, similar to Albertosaurus and Daspletosaurus. Differences in the shape of bones surrounding the brain set Gorgosaurusapart from Albertosaurus.

A DYNASTY OF TYRANTS: ALECTROSAURUS

Alectrosaurus   is a genus of tyrannosaurid theropod dinosaur from the Late Cretaceous Period of Inner Mongolia. It was a bipedal carnivore with a body shape similar to its much larger relative,Tyrannosaurus rex. Alectrosaurus was much smaller though, most likely less than five meters (17 ft) long.

The generic name Alectrosaurus can also be translated as "alone lizard," and is derived from the Greek words alektros("unmarried") and sauros ("lizard"). At the time of its discovery, it was unlike any other Asian carnivore known. There is one named species (A. olseni), which is named in honor of George Olsen, who discovered the first specimens in 1923 on the thirdAmerican Museum of Natural History expedition to Mongolia. Both genus and species were named by American paleontologist Charles Gilmore in 1933. In a 2001 study conducted by Bruce Rothschild and other paleontologists, 23 foot bones referred to Alectrosaurus were examined for signs of stress fracture, but none were found.The holotype (AMNH 6554), or name-bearing specimen, of Alectrosaurus is a hind limb discovered in the Iren Dabasu Formation of the Inner Mongolia Autonomous Region (Nei Mongol Zizhiqu) of the People's Republic of China. Forelimb material in the type specimen was later found to be misidentified therizinosauroid remains.The age of this geologic formation is not clear, but is commonly cited as the Campanian stage of the Late Cretaceous Period, about 83 to 74 million years ago.

More material, including comparable hind limb material as well as skull and shoulder elements, has been referred to Alectrosaurus. These fossils were found in the Bayan Shireh Formation of Outer Mongolia, a formation which is also of uncertain age. It may possibly extend into the early Campanian, but recent estimates suggest it was deposited from Cenomanian through Santonian times. Iren Dabasu and Bayan Shireh dinosaur faunas are similar, but van Itterbeecka et al. claimed that the Iren Dabasu is probably Campanian-Maastrichtian in age and possibly correlated with the Nemegt Formation, so it is not surprising that a species ofAlectrosaurus would be found there.

Furthermore, several more partial skeletons may have been found in both Inner and Outer Mongolia. These remain undescribed as of early 2007.Alectrosaurus is undoubtedly a tyrannosauroid, but due to its fragmentary nature, there is presently very little confidence in restoring its relationships with other tyrannosauroids and many recent cladistic analyses have omitted it altogether. One study recovered Alectrosaurus at no less than eight equally parsimonious positions in a tyrannosauroid cladogram.Some paleontologists have considered Alectrosaurus olseni to be a species of Albertosaurus.

BEAR DOG

Amphicyonidae is an extinct family of large terrestrial carnivores belonging to the suborder Caniformia (meaning "dog-like") and which inhabited North America, Europe, Asia, and Africa from the Middle Eocene subepoch to the Pleistocene epoch 46.2—1.8 Mya, existing for approximately  million years.Amphicyonids, often referred to as "bear 

dogs", crossed from Europe to North America during the Miocene epoch and are considered an Old World taxon. The earliest to appear is the (rather large)Ysengrinia (30—20 Mya), followed by Cynelos (24—7 Mya) and Amphicyon (23—5 Mya). These animals would have followed ungulates and other mammals to the New World for a period of approximately 7 million years. The New World amphicyonids of the subfamilies Daphoeninae (42-16 Mya) and Temnocyoninae (33-20 Mya) coexisted with the Old World counterparts. Note that the (often similar looking) members of the family Hemicyonidae are often called "bear-dogs" as well (although they are increasingly referred to as "dog-bears" to avoid confusion).While amphicyonids have traditionally been viewed as closely related to ursids (bears), some evidence suggests that they may instead be basal caniforms. (Hunt, 2004b). They were about as tall as the American black bear and were most likely ambushers because their legs were made for short, sudden bursts of speed. Bear-dog also nested their young in underground burrows.During the early Miocene, a number of large amphicyonids migrated from Eurasia into North America. These taxa belong to the Old World amphicyonid sub-family Amphicyoninae. The earliest to appear is the large bear dog Ysengrinia Ginsburg, followed by Cynelos Jourdan, and then by Amphicyon. This influx of amphicyonines, accompanied by other Old World ungulates and small mammals, indicates a prolonged interval (from 23 to 16.5 Ma) of faunal exchange between Asia and North America in the early Miocene, using the trans-Beringianroute.New World daphoenines (Daphoenodon, Borocyon) and temnocyonines coexisted with Old World amphicyonines
 (Ysengrinia, Amphicyon, Cynelos) 23.7-17.5 million years ago. These are the largest terrestrial carnivorans 50 kilograms (110 lb) to 200 kilograms (440 lb) that evolved on the North American continent up to this time. The immigrant 

amphicyoninesYsengrinia, Cynelos and Amphicyon appear at 23, 19.2, and 18.8 Ma, respectively, and herald the beginning of a Eurasian amphicyonine migration into North America that continued into the mid-iocene.Amphicyonids were as small as 5 kilograms (11 lb) and as large as 100 to 600 kilograms (220 to 1,300 lb) and evolved from wolf-like to bear-like.The diet of the amphicyonids was fully carnivorous as opposed to hypercarnivorous to mesocarnivorous in Canidae.