Skeletochronology is a technique used to determine the individual,
chronological
Chronology (from Latin , from Ancient Greek , , ; and , ''wikt:-logia, -logia'') is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence of events. It is also "the deter ...
ages of
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s by counting
lines of arrested, annual growth, also known as LAGs, within
skeletal
A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fram ...
tissues.
Within the annual bone growth specimens, there are broad and narrow lines. Broad lines represent the growth period and narrow lines represent a growth pause.
These narrow lines are what characterises one growth year, therefore make it suitable to determine the age of the specimen.
Not all bones grow at the same rate and the individual growth rate of a bone changes over a lifetime,
therefore periodic growth marks can take irregular patterns. This indicates significant chronological events in an individual's life. The use of bone as a
biomaterial
A biomaterial is a substance that has been Biological engineering, engineered to interact with biological systems for a medical purpose – either a therapeutic (treat, augment, repair, or replace a tissue function of the body) or a Medical diag ...
is useful in investigating structure-property relationships.
In addition to current research in skeletochronology, the ability of bone to adapt and change its structure to the external environment provides potential for further research in
bone histomorphometry in the future.
Amphibian
Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
s and
Reptile
Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s are commonly aged determined, using this method, because they undergo discrete annual activity cycles such as winter
dormancy
Dormancy is a period in an organism's Biological life cycle, life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolism, metabolic activity and therefore helps an organism to conserv ...
or
metamorphosis
Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...
, however it cannot be used for all species of bony animals.
The different environmental and
biological
Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, origin, evolution, and distribution of ...
factors that influence bone growth and development can become a barrier in determining age as a complete record may be rare.
Method
The extraction and study of bone tissue varies depending on the taxa involved and the amount of material available. However, skeletochronology best focuses on LAGs that encircle the entire shaft in a ring form and have a regular pattern of deposition.
These growths show a repeated pattern, 'described mathematically as a time series'.
The tissues are divided using a microtome, stained with
haematoxylin
Haematoxylin American and British English spelling differences#ae and oe, or hematoxylin (), also called natural black 1 or Colour Index International, C.I. 75290, is a chemical compound, compound extracted from wood#Heartwood and sapwood, heart ...
to be then viewed under a microscope.
The analysis is frequently performed on dry bones with the additional application of alcohol or congelated preservation if needed, as the aim is to enhance the optical contrast which results from different physical properties to light.
It is important to consider potential problems when selecting particular bones to study. If there is a weak optical contrast, it makes counting the arrested growth rings difficult and often inaccurate. There is also a possible presence of additional growth marks that are created to supplement weaker areas of growth.
In these circumstances, alternative bones must be considered that may present more accurate data. Another case is the doubling of lines of arrested growth where two closely adjacent twin lines can be seen. However, when the pattern is widespread for several age classes in that species, then the twin LAGs can be counted as a single year growth.
The most common issue to arise is the destruction of bone from biological processes, most frequently discovered in
mammal
A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s and
Bird
Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s. This causes age to be significantly underestimated. Over the lifespan of an individual, bone is constantly being reconstructed as specialised cells remove and deposit bone leading to a constant renewal of the bone material.
The continuous resorption and deposition leaves gaps in the record of growth and missing bone tissue is a case at any stage of a vertebrate's life cycle; 'complete specimens that allow precise identification are extremely rare'.
[
Therefore, to account for any missing bone tissues in a specimen, retrocalculation of skeletal age is to be completed.
Three approaches can be identified in retro calculating.]