Dinosaur bones were living tissues that changed as the animal grew, not just hard supports for movement. When paleontologists slice a fossil bone very thinly and study it under a microscope, they can see patterns that record growth over time. These patterns often look like tree rings and can help scientists estimate age, growth rate, and life history.
Growth rings matter because they turn fossils into biological records, not just shapes in rock.
The rings form because bone growth speeds up and slows down during different seasons or life stages. Fast growth usually creates more widely spaced, porous bone tissue, while slow growth can leave narrow lines called lines of arrested growth. By counting and measuring these features, scientists can compare juveniles, adults, and different dinosaur species.
This evidence helps reveal whether a dinosaur grew quickly like many birds and mammals or more slowly like many reptiles.
Key Facts
- Lines of arrested growth, or LAGs, are dark rings that often mark pauses or slowdowns in bone growth.
- Estimated age can be found by counting annual growth marks: age ≈ number of yearly LAGs.
- Growth rate can be estimated with growth rate = change in body size / change in time.
- Wide spaces between rings usually indicate faster growth during that period.
- Bone histology is the study of microscopic bone structure, including growth rings, blood vessel spaces, and tissue organization.
- Some inner growth rings may be destroyed as the bone reshapes itself, so age estimates may need correction.
Vocabulary
- Growth ring
- A visible band in bone tissue that records a period of growth, often linked to seasonal changes.
- Line of arrested growth
- A narrow dark line in bone that marks a time when growth slowed greatly or stopped.
- Bone histology
- The microscopic study of bone tissues and structures to learn how an animal grew and lived.
- Cortex
- The dense outer wall of a bone where many growth rings and blood vessel traces can be preserved.
- Medullary cavity
- The hollow inner space of a long bone that often held marrow and may erase older growth records as it expands.
Common Mistakes to Avoid
- Counting every visible line as exactly one year is wrong because some marks can be extra stress lines or missing due to bone remodeling.
- Assuming all dinosaurs grew at the same rate is wrong because growth patterns varied by species, size, environment, and life stage.
- Ignoring the spacing between rings is wrong because ring width can show changes in growth speed, not just age.
- Treating fossil bone as dead, unchanging rock from the start is wrong because the original bone was living tissue before it fossilized.
Practice Questions
- 1 A dinosaur bone cross-section shows 9 clear yearly LAGs. If no inner rings were lost, what is the estimated age of the dinosaur at death?
- 2 A juvenile dinosaur increased from 80 kg to 260 kg over 4 years. What was its average growth rate in kg per year?
- 3 Two dinosaur bones have the same number of growth rings, but one has much wider spacing between the rings. Explain what this suggests about the growth history of the two dinosaurs.