Bones are not only important for structure and function in the vertebrate body, but the internal structure of the bone can reveal secrets about how an animal grows. The study of the internal microstructure of bone tissue is call bone histology ("histology" meaning the study of tissue). Four main factors affect how bones grow: the evolutionary history of an organism (also termed "phylogeny"), the growth stage of an organism (whether it is a juvenile, sub-adult, adult, etc.), the function of the bone being studied, and the animal's environment. Unique suites of microstructural characteristics have been used to analyze the effects of these four factors on bone tissue development.
The thin section of a chinstrap penguin, Pygoscelis antarcticus, humerus (Figure 1A) shows many of the microstructural features used to study bone growth. Vascular canals (Figure 1B, white arrow) carry blood and nutrients through the bone. The number and orientation of these vascular canals have been tied to evolutionary history, metabolism, growth rates, response to environmental stress, and functional support. Osteocytes (Figure 1B, yellow arrow) are remnants of the cells that lay down new bone tissue, so the shape and density of these cells have been correlated with growth dynamics as well. Other histological features like collagen fibers and the presence or absence of growth marks can also be related to the evolution, growth, and function of bones. On a larger scale, note the thickness of the bone and the smallness of the medullary cavity (the middle of the bone usually filled with marrow) – this is a characteristic typical of aquatic animals as their bones are denser to maintain buoyancy in the water. Because penguins do not fly like other birds, but are characterized by "underwater flight", their bones can be thicker and heavier to be more efficient under water.
Biologists can study the bone histology of a penguin to explore the questions of how fast the individual grew and who it was related to evolutionarily, and to look for effects of underwater flying, Antarctic winters, and certain behaviors like migration. Paleontologists can also look at the same histological features in fossil bones to ask similar questions: how fast did the animal grow? How old was it when it died? How did its bones function? How did the animal's environment affect its growth? Comparisons to similar organisms that are still alive today help paleontologists find answers.
Histological studies on bones has lead to an array of fascinating research and discoveries in paleontology and evolutionary biology, including:
Bone histology is a hot topic right now in paleontology so new avenues of research are constantly being explored as the field continues to grow. This penguin specimen, specifically, is being used in research studying the effects of bone function on microstructure in extinct and extant (living) theropod dinosaurs – including Tyrannosaurus rex, Allosaurus sp., and the modern emu and chinstrap penguin.