Bone Histology and Biogeochemistry

Histology is the study of tissues: how they are formed and how they function. Many tissues have been mentioned in this chapter and others: bones, teeth, cartilage, muscles, blood, organs, and skin. Because the most commonly preserved former tissues of dinosaurs are bone, whenever a paleontologist is discussing dinosaur histology they are referring to bone histology, with only a few exceptions. But of course, such a discussion necessarily should be accompanied by considering how those bones may have been changed since the dinosaur was alive.

Many dinosaur bones recorded growth lines, which are of two types: annuli and lines of arrested growth (also known by their acronym LAGs). Annuli are layers of bone fibers that were formed parallel to one another and do not show vascularization. These are detectable because of their contrast with surrounding vascularized (fibro-lamellar) bone (Fig. 8.5). LAGs are similar to annuli but are not as thick and correspondingly have fewer bone fibers. Based on studies of modern vertebrates, growth lines in general are a result of some temporary slowing (represented by annuli) or stoppage (represented by LAGs) of bone growth. The latter can correlate approximately

FIGURE 8.5 Annuli (unvascularized area) and lines of arrested growth (LAGs) in ornithopod limb bone.

FIGURE 8.5 Annuli (unvascularized area) and lines of arrested growth (LAGs) in ornithopod limb bone.

Bone Histology Lags

with a lack of growth each year. As a result, a minimum age can be estimated in any given individual dinosaur that has observable growth lines in its bones.

LAGs recorded in dinosaur bones represent periods of interrupted growth, which can be attributed to yearly cycles in growth that suggest ectothermy. However, LAGs also are present in some mammal bones. Subsequent research shows that a number of environmental factors, such as prolonged droughts or cold winters, can cause these features. Additionally, some dinosaurs have LAGs but others lack them, which means that they cannot be used as a universal indicator of thermoregulation. For those dinosaurs that have LAGs, and using the assumption that they represent annual growth lines, growth rates have been calculated for some dinosaur genera (Chapter 11). High growth rates should reflect endothermy, whereas slow growth rates are characteristic of ectothermy. The growth rates calculated for some dinosaurs are faster than those known for crocodiles, but slower than those of birds. Another interesting feature common to compact bone in dinosaurs is that it does seem highly vascularized. This feature is common in endotherms, but also has been seen in the bones of some endotherms.

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  • phyllis
    Is there a solution for an arested growth of a bone?
    8 months ago

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