WHAT IS HISTOLOGY?

The exterior of fossilized bones provide the raw data upon which we base our understanding of how dinosaurs looked, behaved, and were related to each other. The microscopic, internal architecture of fossilized bones (bone histology) records the entire history of growth throughout the life of the individual, and provides important clues to the growth rates, longevities, and growth strategies of dinosaurs. Bone histology breathes new life into these fossils , and offers a rigorous, testable means of addressing hypotheses of dinosaur life history.

Studying Dinosaur Growth from the Inside Out

Bone histology reflects the effects of growing slowly or rapidly. Although we tend to think of bones as static and solid structures, they are actually dynamic structures that grow and change throughout the life of an animal. Bones are made up of minerals, proteins, blood vessles, nerves, and cells that impart both hard and "soft" tissue components and make them slightly flexible.

The arrangement of all of these structures provides clues to the rate at which a bone is growing - in general, the more organized a bone's microscopic structure is, the slower it grows. Sometimes, bones stop growing when it gets cold outside or resources are not readily available.

How Do We Study Growth in Living Animals?

In living animals, studying growth rates is done by measuring how mass changes as an organism gets older. (We all know that as you get older, you generally get bigger.) In most living animals, growth patterns over the course of life can be represented by an S-shaped curve. (IN a figure caption, describe what's happening at various points in a graph). An initial slow growth period is followed by a rapid growth stage in which most mass is obtained. After this rapid stage comes the stationary stage, in which growth rates are greatly reduced or growth completely ceases. In order to compare growth rates among animals of different sizes (i.e., mice and elephants), we look at the exponential stage, when growth is fastest (graph showing differences).

Getting S-Shaped Growth Curves in Dinosaurs

Most dinosaurs (except modern birds) have been extinct for 65 million years. Because we'd like to compare dinosaur growth rates with those of living animals and can't directly observe changes over time, we need another method of calculating whole-body growth rates for extinct animals. A special method that combines bone histology with mass calculations gives us the necessary information for interpreting the growth rates of extinct dinosaurs.

Histology provides us with a means of quantifying the ages of dinosaurs, either through counting structures called LAGs that, like the rings in trees, mark annual cycles of growth, or through calculation of longevity through the application of rates obtained from bone vascular and fiber composition. We look at these features in the same bone of sequentially bigger animals, and are able to place an age with masses calculated for each of the bones in the sample. Thus we construct an S-shaped growth curve for dinosaurs that can be compared with other living animals.

Growth Curve

Growth curve for Spermophilus richardsonii, a species of ground squirrel (modified from Zullinger et al., 1984). Early in life, growth is relatively slow. This slow period is followed by the exponential growth stage midway through development, when maximum growth rates are obtained and most mass is accrued. Development ends with a stationary phase, when growth slows or stops. Often, this growth slow down coincides with the onset of sexual maturity and the attainment of adult body size.

Bone periodicity

The periodicity of bone growth. A, cortical bone with lamellar-zonal, poorly vascularlized zones of active growth punctuated by numerous Lines of Arrested Growth (LAG) indicating periods of dramatic decrease in bone deposition. B, cortical bone of highly vascularized, fibrolamellar bone punctuated by one LAG. C, Zonal bone not punctuated by LAG, but demarcated by a regular decrease in the size and density of primary vascular canals.

Bone fiber

Bone fiber organization. A, Endosteal, lamellar zonal bone presents a parallel alignment of hydroxyapatite crystals and collagen fibers, and results in a striated texture under polarized light; B, Periosteal, parallel-fibered bone and flattened, circumferential vascular canals; and C, Periosteal, woven-fibered bone, with collagen fibers and hydroxyapatite crystals arranged haphazardly. This example includes several scattered secondary osteons.