Why did larger dinosaurs evolve in South America?

One theory to explain the large South American animals involves locality. Scientists are excited about the possibility that environmental conditions differed greatly on all the continents, causing the animals to evolve even larger in South America because of that distinct environment. Just what those certain environmental conditions were is unknown. But if something in the environment truly caused distinct differences in the dinosaurs, there are probably many different types of dinosaurs still to be discovered all around the world. Only time will tell if scientists can determine the true reasons why the South American dinosaurs were giants of the Cretaceous world.

How many bones made up the average dinosaur skeleton?

Although the largest dinosaurs may have had a few more bones in their necks and tails, the number of bones in the average dinosaur was approximately 200.

In general, how do scientists decide the bone positions of dinosaur skeletons?

Determining where bones go in a dinosaur skeleton is not an easy task. Scientists have to compare every bone with other dinosaur skeletons, as well as with modern species of reptiles, and hope to find a skeleton in a death pose that was close to its living structure. Many times in the past, certain parts of a skeleton were put in the wrong place. For example, heads of certain dinosaurs have been put on the wrong skeleton, and the thumb spike of the Iguanodon was first interpreted as a nose spike.

The positions of bones in dinosaur skeletons are determined using what scientists call an anatomical direction system, and only includes what is internal (in other words, it is not based on external conditions). This system uses pairs of names to determine certain directions based on the average (or standard) posture of tetrapods, with the back up, belly down, head pointing forward, and all four legs on the ground.

Each pair of names denotes opposite directions similar to when we refer to north and south. Here are four examples of such paired names:

Anterior and posterior: The direction of anterior is toward the tip of the snout, while the posterior direction is toward the tip of the tail. This is analogous to front and back, respectively.

Dorsal and ventral: Dorsal means toward and beyond the spine, while ventral means toward and beyond the belly. These are analogous to up and down, respectively.

Medial and lateral: These are directions referenced to an imaginary plane located in the center of the body, running from tail to snout. Medial means closer to this central reference: lateral means farther out.

Proximal and distal: These are normally used to indicate directions in the limbs and sometimes the tail. Proximal means closer to the trunk or base of a limb, while distal means farther out from the trunk or from the base of the limb.

What are the major parts of a dinosaur skeleton?

A dinosaur skeleton is divided into two major parts: the skull and all the rest of the bones, which are normally referred to as the postcranium (posterior to the cranium). This postcranium can be further divided into the bones of the spine, trunk, and tail (axial skeleton), and the bones of the limbs and limb girdles (appendicular skeleton).

In general, what comprises a dinosaurs skull?

The skull of a dinosaur is made up of the teeth and all the bones in the head. These bones can occur in pairs, on opposite sides of the head, or singly (usually around the middle plane of the skull). There are two major sections of the skull. The upper part, or cranium, contains the braincase, nostrils, upper jaw, and eye sockets. The other section is the lower jaw, which is made up of the right and left lower jaws (mandibles).

Examples and locations of dinosaur skull bones include: the cheekbone (jugal) , located below the eye opening (orbit); the postorbital, a small bone located behind the eye opening; and the lacrimal, a bone that separates the eye opening and the opening forward of the eye.

There are several interesting details about dinosaur skulls. For example, there are more than 30 bones in the skull of a dinosaur. Most dinosaurs had unusually rigid joints between their skull bones called sutures (similar to sutures in a human skull). There were also kinetic skulls, such as the Allosaurus fragilis, in which several of the skull bones were joined but could still move, probably so they could stretch parts of the skull in order to gobble down extremely large chunks of meat.

We often think of dinosaur fossils being discovered like this one, with just about all the pieces perfectly preserved. In most cases, though, only incomplete skeletons are found, for various reasons (iStock).

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