A fossil bone can preserve traces of an animal’s growth long after its outward shape has stopped changing. Under a microscope, layers of tissue, old blood-vessel spaces and fine lines may reveal rapid construction, rebuilding and pauses in deposition. Reading those traces is more difficult than counting rings on a calendar.
Juan Marcos Jannello and Anusuya Chinsamy demonstrated the problem in a 2023 study of extinct giraffids from Langebaanweg, South Africa. They examined thin sections of 20 limb bones assigned to Sivatherium hendeyi and Giraffa cf. Giraffa jumae. The bones recorded interrupted growth, but the number of marks did not provide a straightforward age for the animals. Some juvenile material retained more marks than adult material.
The result opens a window onto how these animals grew. It also shows why an attractive interpretation—one visible line equals one year—needs evidence of its own.
Looking inside the museum specimens
The fossils came from the collections of Iziko Museums of South Africa. They represented different limb elements, including femora, tibiae and the elongated bones of the lower limbs. Nine were assigned to the Giraffa form and eleven to Sivatherium.
The notation “cf.” in the Giraffa identification preserves uncertainty: the material was compared with Giraffa jumae, rather than treated as an identification without qualification. The study concerned two fossil forms from the same locality, not a continuous series of ancestors leading to living giraffes.
Because making thin sections removes fossil material, the researchers preferentially sampled incomplete or fragmented specimens. They embedded pieces in resin, cut them near the middle of the shaft where possible, and polished them until light could pass through. Ordinary and polarized light then revealed the arrangement of the bone tissue.
The distinction between juvenile and adult specimens came from skeletal anatomy, especially whether preserved bone ends had fused. It was not based on already knowing the animals’ ages in years. The sample included juvenile and adult material, but it did not evenly cover every stage of growth or every skeletal element.
Fast construction and later rebuilding
The sections showed a familiar broad pattern of mammalian growth. Juvenile bone included highly vascularized tissue associated with rapid deposition. The many spaces for blood vessels are part of that tissue’s organization, rather than holes produced simply by fossil damage.
In more mature material, parts of the cortex showed slower-deposited tissue and extensive remodeling. During life, bone can be removed and replaced. Under the microscope, the replacement structures interrupt the earlier fabric, leaving a record that has been revised as well as accumulated.
That distinction becomes crucial when trying to read an animal’s past. A bone is not a passive notebook in which every early entry remains intact. Later remodeling can remove evidence of earlier growth, and different parts of a skeleton need not preserve the same history equally well.
Jannello and Chinsamy found considerable variation among the sampled bones. Their descriptions trace changes from the inner cortex toward the outer margin and distinguish original tissue from later reconstruction. Those observations give the growth lines their context; a line counted without that surrounding anatomy would be much harder to interpret.
A pause does not announce its cause
Among the features recorded were lines of arrested growth, often abbreviated LAGs. These mark interruptions in bone deposition. The researchers also distinguished other structures, including bands associated with slower growth and lines associated with remodeling, so that superficially similar marks would not all be treated as the same event.
Some growth marks occurred close together in pairs or triplets. The authors considered that such closely associated lines could belong to one episode rather than several independent annual cycles. The arrangement of the marks therefore mattered as much as their raw number.
One striking result was that a juvenile Sivatherium lower-limb specimen preserved repeated growth interruptions, exceeding the count in some adult specimens. This did not make the juvenile older than those adults. It showed that the surviving marks could not be read as a simple, universally preserved age counter.
There are at least two issues to separate. Earlier marks may have disappeared as an adult bone remodeled. In addition, the original interruptions may not have occurred exactly once each year. Either problem can break a direct conversion from a line count to an age.
Stress is an interpretation to test
The authors proposed that some of the fossil giraffids’ growth interruptions reflected stressful periods, possibly related to changing environmental conditions or reduced food availability. Other research on the locality supplied reasons to consider such a possibility, and the bone sections showed that growth had not always proceeded uninterrupted.
The sections did not identify a particular drought, fire or flood responsible for an individual line. Nor did the researchers have a known-age series of these extinct animals with which to demonstrate the timing of mark formation. Their environmental explanation is an interpretation of the pattern, not a measured cause attached to each mark.
A study of living giraffes points to the same problem of preservation. In her 2020 master’s thesis, Caitlin Caryl Danielle Smith found that growth-mark numbers and positions varied among limb bones within the same animal. Some bones were so extensively remodeled that no growth marks could be found. Which bone survives can therefore affect the history a researcher is able to read.
Smith assigned growth stages mainly from body mass, with uncertainty between subadult and adult categories. Her study did not establish that one mark formed each year. She called for more juveniles of known age to test when growth marks begin to appear and whether they can reliably reveal age.
The history that survives
The sample was small and unevenly distributed across growth stages. The researchers therefore did not present a statistical reconstruction of species-wide growth schedules or a precise age at maturity. More fossil material spanning different stages would be needed to test how representative the observed patterns are.
Even within those limits, the bones supplied information that external measurements alone could not. They recorded rapid early deposition, later rebuilding and repeated interruptions. Our giraffe science library often follows evolution through the shapes of bones; this study follows biological history within the tissue itself.
Its most useful lesson is that a fossil growth mark is evidence of an event before it is evidence of a date. Understanding how that event was recorded—and what subsequent remodeling removed—is essential before turning the mark into a birthday.
Sources
Juan Marcos Jannello and Anusuya Chinsamy (2023). “Osteohistology and palaeobiology of giraffids from the Mio–Pliocene Langebaanweg (South Africa).” Journal of Anatomy 242, 953–971. Full paper.
Caitlin Caryl Danielle Smith (2020). “Giraffa camelopardalis: limb bone histology through ontogeny.” Master’s thesis, University of Cape Town. University repository and full thesis.
