Description[edit]

Adult (25-year-old) and juvenile (2-year-old) with a human for scale

Although smaller than Tyrannosaurus, Tarbosaurus was one of the largest tyrannosaurids. The largest known individuals were between 10 and 12 m (33 and 39 ft) long.^[1] The mass of a fully grown individual is considered comparable to or slightly smaller than Tyrannosaurus, often estimated to be around 4–5 metric tons.^[2][3]

The largest known Tarbosaurus skull is more than 1.3 m (4.3 ft) long, larger than all other tyrannosaurids except Tyrannosaurus.^[4] The skull was tall, like that of Tyrannosaurus, but not as wide, especially towards the rear. The unexpanded rear of the skull meant that Tarbosaurus eyes did not face directly forwards, suggesting that it lacked the binocular vision ofTyrannosaurus. Large fenestrae (openings) in the skull reduced its weight. Between 58 and 64 teeth lined its jaws, slightly more than in Tyrannosaurus but fewer than in smaller tyrannosaurids like Gorgosaurus and Alioramus. Most of its teeth were oval in cross section, although the teeth of the premaxilla at the tip of the upper jaw had a D-shaped cross section. This heterodonty is characteristic of the family. The longest teeth were in the maxilla (upper jaw bone), with crowns up to 85 millimeters (3.3 in) long. In the lower jaw, a ridge on the outer surface of the angular bone articulated with the rear of the dentary bone, creating a locking mechanism unique to Tarbosaurus and Alioramus. Other tyrannosaurids lacked this ridge and had more flexibility in the lower jaw.^[5]

Restoration of an adult and subadultTarbosaurus next to a human

Tyrannosaurids varied little in body form, and Tarbosaurus was no exception. The head was supported by an S-shaped neck, while the rest of the vertebral column, including the long tail, was held horizontally. Tarbosaurushad tiny forelimbs, proportionably to body size the smallest of all members of the family. The hands had two clawed digits each, with an additional unclawed third metacarpal found in some specimens, similar to closely related genera. Holtz has suggested that Tarbosaurus also has a theropod reduction of fingers IV-I "developed further" than in other tyrannosaurids,^[6] as the second metacarpal in the Tarbosaurus specimens he studied is less than twice the length of the first metacarpal (other tyrannosaurids have a second metacarpal about twice the length of the first metacarpal). Also, the third metacarpal in Tarbosaurus is proportionally shorter than in other tyrannosaurids; in other tyrannosaurids (like Albertosaurus and Daspletosaurus), the third metacarpal is often longer than the first metacarpal, while in the Tarbosaurus specimens studied by Holtz, the third metacarpal is shorter than the first.^[4]

In contrast to the forelimbs, the three-toed hindlimbs were long and thick, supporting the body in a bipedal posture. The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips.^[1][4]

Ammonoidea

From Wikipedia, the free encyclopedia

"Ammonite" redirects here. For other uses, see Ammonite (disambiguation).

Ammonites Temporal range: 400–66 Ma PreЄ Є O S D C P T J K Pg N Devonian – Cretaceous
Artist's reconstruction of Asteroceras
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	†Ammonoidea Zittel, 1884
Orders and Suborders
See text

Ammonites /ˈæmənaɪts/ are an extinct group of marine invertebrate animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs are more closely related to living coleoids (i.e., octopuses, squid, andcuttlefish) than they are to shelled nautiloids such as the living Nautilus species. The earliest ammonites appear during the Devonian, and the last species died out during the Cretaceous–Paleogene extinction event.

Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically spiraled and nonspiraled forms (known as heteromorphs).

The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Pliny the Elder (d. 79 AD near Pompeii) called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing ram's horns.^[1] Often the name of an ammonite genus ends in -ceras, which is Greek(κέρας) for "horn".

Monospecific assemblages[edit]

Artist's reconstruction of O. regulare

These orthocone cephalopods are conspicuous in the fossil record for their occasional but persistent appearances in monospecific assemblages dense enough to form individual beds of limestone that often are regional key beds. Limestones that consist of monospecific assemblages of orthocone cephalopods are known as either cephalopod beds, cephalopod limestones, nautiloid limestones, or Orthoceras limestones in the geological literature. Based on studies of size distributions of the orthocone shells, it had been argued that these assemblages likely represent post-mating mass deaths, as are common among modern cephalopods (though not modern nautiloids) and indeed among many semelparous species. More recent taphonomic and sedimentologic studies argue that such limestones are the result of the gradual accumulation of orthocone shells over an extended period of time in areas with very low rates of sedimentation and strong bottom currents. These assemblages, are known mostly from Ordovician rocks but do occur later as well, at least into the Devonian. Well-known examples occur in Morocco, Scandinavia, the Alps, and Iowa (USA).

The Baltic island of Öland off the southern coast of Sweden has many quarries that yield orthocone nautiloids of great beauty. For centuries Öland has supplied greater Europe with material for floors, stairs and grave stones. This hard limestone is durable and the fossil inclusions make it very desirable. Occasionally the chambers of the fossil shells are colored differently. The ground water minerals that percolated the strata during diagenesisdetermines the color. Greens and browns are most common.

Orthoceras grew to a length of 15 centimeters (6 inches) and fed upon small animals.

Description[edit]

Pteranodon pieces are extremely well represented in the fossil record, allowing for detailed descriptions of their anatomy and analysis of their life history. Over 1,000 specimens have been identified, though less than half are complete enough to give researchers good information on the anatomy of the animal. Still, this is more fossil material than is known for any other pterosaur, and it includes both male and female specimens of various age groups and, possibly, species.^[2]

Size[edit]

Size of P. longiceps male (green) and female (orange) compared with a human

Adult Pteranodon specimens from the two major species can be divided into two distinct size classes. The smaller class of specimens have small, rounded head crests and very wide pelvic canals, even wider than those of the much larger size class. The size of the pelvic canal probably allowed the laying of eggs, indicating that these smaller adults are females. The larger size class, representing male individuals, have narrow hips and very large crests, which were probably for display.

Adult male Pteranodon were among the largest pterosaurs, and were the largest flying animals known until the late 20th century, when the giant azhdarchid pterosaurs were discovered.^[2]The wingspan of an average adult male Pteranodon was 5.6 metres (18 ft). Adult females were much smaller, averaging 3.8 metres (12 ft) in wingspan. The largest from the Niobrara Formation measured 6.25 metres (20.5 ft) from wingtip to wingtip. An even larger specimen is known from the Pierre Shale Formation, with a wingspan of 7.25 metres (23.8 ft), though this specimen may belong to the distinct genus and species Geosternbergia maysei.^[2] While most specimens are found crushed, enough fossils exist to put together a detailed description of the animal.

Methods used to estimate the mass of large male Pteranodon specimens (those with wingspans of about 7 meters) have been notoriously unreliable, producing a wide range of estimates from as low as 20 kilograms (44 lb) and as high as 93 kilograms (205 lb). In a review of pterosaur size estimates published in 2010, researchers Mark Witton and Mike Habib demonstrated that the latter, largest estimates are almost certainly incorrect given the total volume of a Pteranodon body, and could only be correct if the animal "was principally comprised of aluminium."^[3] Witton and Habib considered the methods used by researchers who obtained smaller mass estimates equally flawed. Most have been produced by scaling modern animals such as bats and birds up to Pteranodon size, despite the fact that pterosaurs have vastly different body proportions and soft tissue anatomy from any living animal.^[3]

Skull and beak[edit]

Skull and beak of specimen AMNH 7515

Unlike earlier pterosaurs such as Rhamphorhynchus and Pterodactylus, Pteranodon had toothless beaks, similar to those of birds. Pteranodon beaks were made of solid, bony margins that projected from the base of the jaws. The beaks were long, slender, and ended in thin, sharp points. The upper jaw was longer than the lower jaw. The upper jaw was curved upward; while this normally has been attributed only to the upward-curving beak, one specimen (UALVP 24238) has a curvature corresponding with the beak widening towards the tip. While the tip of the beak is not known in this specimen, the level of curvature suggests it would have been extremely long. The unique form of the beak in this specimen led Alexander Kellner to assign it to a distinct genus, Dawndraco, in 2010.^[4]

The most distinctive characteristic of Pteranodon is its cranial crest. These crests consisted of skull bones (frontals) projecting upward and backward from the skull. The size and shape of these crests varied due to a number of factors, including age, sex, and species. Male Pteranodon sternbergi, the older species of the two described to date (and sometimes placed in the distinct genus Geosternbergia), had a more vertical crest with a broad forward projection, while their descendants, Pteranodon longiceps, evolved a narrower, more backward-projecting crest.^[1] Females of both species were smaller and bore small, rounded crests.^[5] The crests were probably mainly display structures, though they may have had other functions as well.^[6]

Skeleton[edit]

Other distinguishing characteristics that set Pteranodon apart from other pterosaurs include narrow neural spines on the vertebrae, plate-like bony ligaments strengthening the vertebrae above the hip, and a relatively short tail in which the last few vertebrae are fused into a long rod.^[7] The entire length of the tail was about 3.5% as long as the wingspan, or up to 25 centimetres (9.8 in) in the largest males.^[7]

Description[edit]

Albertosaurus with a human for scale

Albertosaurus was smaller than some other tyrannosaurids, such as Tarbosaurus and Tyrannosaurus. Typical Albertosaurus adults measured up to 9 metres (30 feet) long,^[1][2] while rare individuals of great age could grow to be over 10 metres (33 feet) long.^[3] Several independent mass estimates, obtained by different methods, suggest that an adultAlbertosaurus weighed between 1.3 tonnes (1.4 short tons)^[4] and 1.7 tonnes (1.9 tons).^[5]

Albertosaurus shared a similar body appearance with all other tyrannosaurids. Typically for a theropod, Albertosaurus was bipedal and balanced the heavy head and torso with a long tail. However, tyrannosaurid forelimbs were extremely small for their body size and retained only two digits. The hind limbs were long and ended in a four-toed foot on which the first digit, called the hallux, was short and did not reach the ground. The third digit was longer than the rest.^[2] Albertosaurus may have been able to reach walking speeds of 14−21 kilometres per hour (8−13 miles per hour).^[6] At least for the younger individuals, a high running speed is plausible.^[7]

Skull and teeth[edit]

Skull cast at the Geological Museum in Copenhagen

The massive skull of Albertosaurus, which was perched on a short, S-shaped neck, was approximately 1 metre (3.3 feet) long in the largest adults.^[8] Wide openings in the skull (fenestrae) reduced the weight of the head while also providing space for muscle attachment and sensory organs. Its long jaws contained, both sides combined, 58 or more banana-shaped teeth; larger tyrannosaurids possessed fewer teeth, Gorgosaurus at least sixty-two. Unlike most theropods, Albertosaurus and other tyrannosaurids were heterodont, with teeth of different forms depending on their position in the mouth. The premaxillary teeth at the tip of the upper jaw, four per side, were much smaller than the rest, more closely packed, and D-shaped in cross section.^[2] Like with Tyrannosaurus, the maxillary (cheek) teeth of Albertosaurus were adapted in general form to resist lateral forces exerted by a struggling prey. The bite force of Albertosaurus was less formidable, however, with the maximum force, by the hind teeth, reaching 3,413 Newtons.^[9] Above the eyes were short bony crests that may have been brightly coloured in life and used in courtship to attract a mate.^[10]

Reconstruction

William Abler observed in 2001 that Albertosaurus tooth serrations resemble a crack in the tooth ending in a round void called an ampulla.^[11] Tyrannosaurid teeth were used as holdfasts for pulling meat off a body, so when a tyrannosaur pulled back on a piece of meat, the tension could cause a purely crack-like serration to spread through the tooth.^[11] However, the presence of the ampulla distributed these forces over a larger surface area, and lessened the risk of damage to the tooth under strain.^[11] The presence of incisions ending in voids has parallels in human engineering. Guitar makers use incisions ending in voids to, as Abler describes, "impart alternating regions of flexibility and rigidity" to the wood they work with.^[11] The use of a drill to create an "ampulla" of sorts and prevent the propagation of cracks through material is also used to protect aircraft surfaces.^[11] Abler demonstrated that a plexiglass bar with incisions called "kerfs" and drilled holes was more than 25% stronger than one with only regularly placed incisions.^[11] Unlike tyrannosaurs, ancient predators like phytosaurs and Dimetrodon had no adaptations to prevent the crack-like serrations of their teeth from spreading when subjected to the forces of feeding.^[11]