Fossil Bone Histology

9780520273528This is a brand new book by Padian & Lamm, published just last month by University of California Press: Bone Histology of Fossil Tetrapods: Advancing Methods, Analysis, and Interpretation. I have not read it (although it seems like a state-of-the-art book), but thought this would be a good way to introduce a basic post on fossil bone histology.

Histology is a widespread method in biology. At its purest, it’s the practice of slicing structures thin enough that we can shine light through them, making them examinable under a microscope. More advanced histology can involve staining to make certain components stand out. One can also make serial thin sections, scan them digitally, and make 3D reconstructions that can be navigated through.

Histology is also used in geology – making thin sections of rocks is the most surefire way of identifying their mineral components. All three-dimensionally preserved fossils can also be studied histologically – see my post on the Herefordshire locality for examples of fossils that can only be studied by serial histology.

Histology of fossil bones has a long history, dating back at least to the 19th century. See the work of James Bowerbank (1848) for an example using pterosaur bones. Much information can be gleaned from bone histology. For another pterosaur example, de Ricqlès et al. (2000) used histology to find that pterosaurs had a fast metabolism and grew at rates more similar to birds than to reptiles.

Such insights can then lead to more informed hypothesising about the ecology and systematics of extinct animals. For example, histological analyses of theropod bones show that they had bones that they grew very rapidly and with a structure typical of today’s large birds (Erickson et al., 2001); this is one more piece of evidence for the dominant hypothesis of birds being theropods.

The detail that can be received from such analyses is considerable. Varricchio (1993) found that the troodontid dinosaur Troodon formosus reached its adult size in less than five years. The basis for such observations is the fact that bone is a living tissue that records the growth and life of the animal. Bone deposition can occur in seasonal cycles, in which case the histological section shows a tree ring-like pattern, or bone deposition can be continuous. The rapidity of bone deposition results in different bone microstructure, all of it preserved in fossils, allowing easy distinction between fast and slow growth. In essence, you can make a cross section of a bone and read the story of its animal from birth to death, like a timeline. Below you can try it for yourself, using a diagram ripped from Martin’s Introduction to the Study of Dinosaurs (2006, 2nd ed.).

histolog

Starting from the bottom, you have a dark brown line. This is a line of arrested growth (LAG), a line indicating that no growth happened for a certain period. Between the first two LAGs are two closely-packed layers of fibrolamellar bone, characterised by those large white blotches, which are canals. These form when growth rate is high, so fibrolamellar bone is a telltale sign of fast growth.

Between the second and third LAG are two vascularised layers, but there is a distinct space between the layers. This is called an annulus, and the lack of canals tells us that the rate of growth was low.

So, taken all together, what this particular section shows us, generally, is a cyclical growth pattern. LAG, followed by vascularisation, annulus, vascularisation, then another LAG. One could take LAGs to represent yearly lines (as with tree rings), and the alternating vascularisation to represent seasonal cycles of fast/slow growth, and then hypothesise as to how these patterns can emerge. Hypotheses can then be tested somewhat using Recent bones with known growth patterns.

One can then go further and examine differences between growth patterns in adults and juveniles, provided specimens of the same species are around. This allows us to elucidate life history patterns – is there a fast juvenile growth rate then arrest, or did the animal grow at the same rate until death?

I can only offer you this glimpse into this research area, and my aim was not only to introduce it, but to give you an idea of how all the conclusions we come to about dinosaurs, their physiologies, their lifestyles, their ecologies, etc. aren’t baseless speculations, but come from detailed examination and analysis of such things as slices of fossilised bone. As the old palaeontologist adage goes, “every fossil tells a story”.

References:

Bowerbank JS. 1848. Microscopical Observations on the Structure of the Bones of Pterodactylus Giganteus and other Fossil Animals. Quarterly Journal of the Geological Society 4, 2-10.

De Ricqlès AJ, Padian K, Horner JR & Francillon-Vieillot H. 2000. Palaeohistology of the bones of pterosaurs (Reptilia: Archosauria): anatomy, ontogeny, and biomechanical implications. Zoological Journal of the Linnean Society 129, 349-385.

Erickson GM, Rogers KC & Yerby SA. 2001. Dinosaurian growth patterns and rapid avian growth rates. Nature 412, 429-433.

Varricchio DJ. 1993. Bone microstructure of the Upper Cretaceous theropod dinosaur Troodon formosus. Journal of Vertebrate Paleontology 13, 99-104.

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