Dinosaur Cancellous Bone

where R could be any chains of organic compounds that attach to amino acids, giving them distinctive formulas and names. Only 20 different amino acids, but in a myriad combinations, compose the millions of different proteins in any given animal body.

Because unaltered dinosaur soft tissues are typically not preserved, paleontologists must look to bones or eggshells for evidence of the dinosaurs' original amino acids and proteins. The proteins specifically involved in bone production are albumin, collagen, hemoglobin, and osteocalcin. Albumin and hemoglobin are proteins associated with blood circulating through a bone and help with facilitating biochemical reactions in an organism. Collagen and osteocalcin are structural proteins, meaning that they lend physical support to an organism's body and are typically fibrous.

Bone formation in modern vertebrates begins through structural proteins with the laying down of collagen fibers in a parallel, linear arrangement. This forms part of a flexible, non-ossified connective tissue called cartilage, which grows through cells called chondrocytes. The formation of cartilage is followed by biomineral-ization of dahllite in spaces between the collagen fibers. This process is performed by osteoblasts, which are bone cells partly composed of osteocalcin that later become osteocytes. Osteocytes ensure that the bone continues to function as living tissue. Crystallization takes place in both the center and the outer, middle part of the cartilaginous structure and replaces the cartilage with mineralized tissue. It also eventually opens spaces for marrow canals, which carry blood to the bones. The further development of mineralized tissue extends to either end of the originally cartilaginous structure and results in outward bone growth, from the development of an embryo through to adulthood.

Growth of bone eventually results in two types of bone, cancellous and compact. Cancellous bone has a "spongy" (porous) texture, where the pores (lacunae) are occupied by osteocytes during the life of the animal. Tiny canals called can-naliculi interconnect these lacunae. Cancellous bones have a density based on only 10% to 30% of the original biomineralized matter, which made for light yet strong bones in dinosaurs and other vertebrates. In contrast, compact bone was originally about 95% biomineralized matter and accordingly is more durable. In some cases, this type of bone is preferentially preserved among the body parts that were physically reworked after death of a dinosaur (Chapter 7). Within a single bone, the amount of cancellous versus compact bone can vary. For example, a limb bone has mostly compact bone on the diaphyses (shafts) whereas the epiphyses (wider ends) have more cancellous bone. A thin outer layer of compact bone protects this cancellous bone (Fig. 8.4).

FIGURE 8.4 Differences in proportions of cancellous and compact bone in the diaphyses and epiphyses (respectively) of a limb bone.

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