Skeleton of Giraffatitan, with sections of the vertebral column labeled; image Courtesy of Sauropod Vertebra Picture of the Week

The vertebral column is divided into four sections in dinosaurs and their relatives:

• Cervical: the neck
  
• Dorsal: the back
  
• Sacral: the hips
  
  • (Sometimes the sacral vertebrae are fused into a single unit, called a sacrum)
• Caudal: the tail
  

Labeled portions of the skeleton of Camarasaurus; image Courtesy of Sauropod Vertebra Picture of the Week

Attached to the cervical and dorsal vertebrae are ribs (one on each side). Sacral ribs also exist, but are often fused to the pelvic girdle (see below). Instead of ribs, caudal vertebrae have chevrons, single bones which protect the nerves and blood vessels that run underneath the caudal centra.

Ventral to the guts of dinosaurs and many other land vertebrates are gastralia (singular gastralium), or "belly ribs". In dinosaurs they are one or two pieces per side.

Some dinosaurs have dermal ossifications or scutes: bones in the skin of the animal used for armor.

Bones and Landmarks of the Appendicular Skeleton
The appendicular skeleton is comprised of the limbs and their girdles (bones that attach the limbs to the axial skeleton.

The Pectoral Girdle The forelimb is attached to the dorsal part of the axial skeleton by the pectoral girdle. The pectoral girdle is composed of the following bones:

Left lateral view of the left pectoral girdle and forelimb of Camarasaurus; image Courtesy of Sauropod Vertebra Picture of the Week

Right lateral view of right pectoral girdle and forelimb of Centrosaurus; image courtesy of The Open Dinosaur Project

The Forelimb

• Carpals: various small bones of the wrist. The wrist as a whole is called the carpus (pl. carpi)
  
• Metacarpals: the long bones of the palm of the hand. These are numbered I-V, with I being the medialmost (the one to which the thumb is attached) and V being the lateralmost (the one to which the pinky is attached). All the metacarpals as a unit are called the metacarpus (pl. metacarpi)
  
• Digits: fingers. Digits are numbered I-V as above, with I being the thumb, and V being the pinky. Digits are composed of individual finger bones or phalanges (singular phalanx). The distalmost, claw- or hoof-bearing phalanx is called the ungual

Anterior view of right manus of Centrosaurus; image courtesy of The Open Dinosaur Project

The Pelvic Girdle

Left lateral view of the left pelvic girdle and hindlimb of Camarasaurus; image Courtesy of Sauropod Vertebra Picture of the Week

The hindlimb is attached to the sacral part of the axial skeleton by the pelvic girdle (aka the pelvis (pl. pelves) or "hips"). The pelvic girdle is composed of three bones on each side:

Right lateral view of right pelvic girdle and hindlimb of Centrosaurus; image courtesy of The Open Dinosaur Project

The Hindlimb Note that the structure of the hindlimb is very similar to that of the forelimb.

• Tarsals: various small bones of the ankle. The wrist as a whole is called the tarsus (pl. tarsi. Two tarsals of importance in dinosaurs are the two proximal tarsals, the astragalus (pl. astragali) and calcaneum (pl. calcanea), which fit onto the distal ends of the tibia and fibula
  
  Incidentally, dinosaurs and their closest relatives lack a heel (which is formed in other land vertebrates by a backwards projection of the calcaneum)
• Metatarsals: the long bones of the body of the foot. These are numbered I-V, with I being the medialmost (the one to which the big toe is attached) and V being the lateralmost (the one to which the little toe is attached). All the metacarpals as a unit are called the metatarsus (pl. metatarsi). Unlike humans and bears, but like cats and dogs and horses (and birds...), dinosaurs held their metatarsi upright, so that their ankles did not normally touch the ground
  
• Digits: toes. Digits are numbered I-V as above, with I being the big toe, and V being the little toe. Digits are composed of individual finger bones or phalanges (singular phalanx). The distalmost, claw- or hoof-bearing phalanx is called the ungual

Excellent overview of dinosaur anatomy, and here, and here as well.

TAXONOMY
Taxon (pl. taxa): a named group of organisms.

Traditionally, each culture had its own name for the animals, plants, and other organisms in their region. But EACH culture had its own set of names, so the same type of animal might have many different names. During the 1600s and 1700s, methods were proposed for a formal scientific set of names.

Carl von Linne' (Linnaeus) developed a universal set of rules in the Systema Naturae ("System of Nature") in 1758; later workers added and modified the system (primarily with the addition of new "ranks").

Some of the Linnean rules:

• All names are in Latin or Greek, or are modified into Latin form;
• Each name must be unique;
• All names are fit into a nested hierarchy (species into genera, genera into families, and so forth);
• In traditional Linnean taxonomy, there is a set of official ranks (from smallest to largest, species, genus, family, order, class, phylum) (later workers added additional intermediate ranks, such as tribes, subfamilies, superfamilies, subphyla, etc.);
  • The primary unit is the species (pl. species):
    • Refers to a "specific" kind of organism
    • Definition of a "species" varies from biologist to biologist; some definitions ("naturally occurring interbreeding populations") cannot be tested for fossils!
      • More about species below
    • Each species has a type specimen accessioned in an appropriate institution (museum, zoological or botanical garden, or other such collection);
    • Whoever describes the type specimen of a new species has the right to name that new species (following the rules below);
  • The next higher unit, the genus (pl. genera) is composed of one or more species
    • Refers to a more "generic" category than species
    • Definition of a "genus" is problematic as well, since it is composed of one or more "species";
    • Each genus has a type species: all other species are assigned to the genus based on their similarity to the type species;

Linnean taxonomy has its own special set of grammatical rules:

• Genera have one word names (e.g., Panthera, Homo, Ginkgo, Tyrannosaurus);
  • The genus name is always Capitalized and italicized (or underlined if you don't have access to italics);
• Species have two word names, the first part of which is the same as the genus name (e.g., Panthera leo, Homo sapiens, Ginkgo biloba, Tyrannosaurus rex)
  • The genus name is ALWAYS capitalized, the second part ("trivial nomen") is ALWAYS in lower case, and the name is ALWAYS italicized or underlined;
  • Species names can be abbreviated by using only the first letter of the genus name, followed by a period (NEVER by a hyphen): H. sapiens and T. rex are correct; H. Sapiens or T-Rex are WRONG!! (Subtle hint: do not use the incorrect form on your homework or tests);
• All taxon names other than species have one word names, which are capitalized; all taxon names other than genera and species are in roman letters (i.e., they are never italicized/underlined): Dinosauria, Tyrannosauridae, Animalia; not Dinosauria, tyrannosauridae, or animalia.
• Taxon names of whatever "rank" have some etymology (derivation) (that's true of all words, really). Sometimes the name might be descriptive (e.g., "Triceratops horridus", the "roughened three-horned face") or it might honor a place of discovery (e.g., "Albertosaurus", found in the Canadian Province of Alberta) or some individual (e.g., "Diplodocus carnegii", after billionaire Andrew Carnegie who's funding supported the expedition and museum which found this species). But the name can be inaccurate (e.g., "Basilosaurus"--Emperor Reptile--is a whale, not a reptile!) but if the name was formed obeying the rules of taxonomy, that inaccurate descriptor is fine.

Because there is disagreement about the features used to define a particular species or genus, different biologists and paleontologists will sometimes disagree about which specimens belong in a particular species, and which species belong in a particular genus (and so forth).

• Taxonomists who consider a particular set of specimens to represent many taxa are called splitters; those who consider a particular set to represent few taxa are called lumpers;
• If a taxonomist feels that some specimens of a genus belong to an as-yet unnamed species, they can split these specimens off as a new species (which a new type specimen);
• On the other hand, if a taxonomist considers that two previously named species are not distinct enough from each other to truly be distinct species (that is, the taxonomist regards the two names as synonyms), they may lump them together:
  • In these cases, the Rule of Priority is used: whichever of the names was published first, even if only by days, is the name that must be used;
• The same case applies to genera: if two genera are thought to represent the same genus, the first named genus name is the one that is used.

For those interested in a website concerning some unusual Linnean species names, click here.

SPECIES
What is a species? Above we see the rules for these names, but it doesn't tell us about what it is being named.

Linnaeus' "species" were taxa like lions, tigers, black bears, etc. These were assemblages of individuals that share certain attributes:

• Similar appearance
• Similar habits and behaviors
• Similar habitats

Darwin did not regard species as a distinct "kind" of biological entity. Instead, he considered them as essentially the same thing as goegraphic or stratigraphic variations (see these below), but ones in which extinction has removed the intermediate forms that otherwise would blend into the closest living relative group.

20th Century biologist Ernst Mayr (and most contemporary biologists) formalized their definition of a species as a "naturally occurring populations that interbreed and produce viable fertile offspring".

But there are some problems with this. For one: hybrids (crosses between two separate species) do occur naturally, and many of these are actually fertile! And for paleontologists: we can't test interfertility between populations because they are dead!

So we are stuck looking only at shapes (and in fact, only the shapes of those hard parts that survive fossilization).

The question then becomes: how different do two individuals, or two populations, have to be for us to consider them different species? This is actually a terribly difficult question even with living organisms!! There are several sources of variation:

• Sexual dimorphism: different sexes are different sizes and shapes and have different structures
• Ontogenetic (growth): babies look different from juveniles look different from subadults look different from adults (can be even more extreme in animals that undergo metamorphosis, like amphibians and many insects)
• Geographic: populations in different regions might have slightly different sizes, color patterns, proportions, behaviors, etc. For example, some biologists consider the populations of ourangutans, tigers, African elephants, etc. as distinct species; others simply regard them as regional variants
• Stratigraphic: lineages (ancestor and descendant populations) may shift in some traits or characteristics over time
• Individual: one of the great "discoveries" of Darwin and Wallace, the recognition that no two individuals in a population are identical! (Before them, many people thought that there existed the perfect "type" of each kind of organism, and all variation is degeneration from that perfection. Darwin and Wallace showed that the variation is the reality)

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Last modified: 9 September 2011