How do we tell if a fossil is a juvenile or an adult if multiple sizes of the species' fossils have not been found?

Hi Keaghlan,

It can be tricky to identify a juvenile in the fossil record and it will depend on the type of organism.

That said, there are some characteristics that may suggest that a specimen is juvenile. In tetrapods the bones of juveniles tend to have unfused sutures between them, they have relatively large eyes and short snouts and in some there will be indicators in cross-sections of the bone, which may only show rapid growth features without any (or with few) growth lines or areas of bone remodelling - which is something that happens in older animals. These sorts of features can provide clues that something is juvenile.

A range of techniques can be applied to determine age of death in invertebrates. In ammonoids, it is generally accepted that when the spacing of the divisions between chambers (septal spacing) decreases then this indicates the animal is mature. In some groups of ammonoids the adults modify the body chamber.

With corals and bivalves it is possible to use a range of techniques based on growth lines, just like tree rings, to estimate age but this does not always distinguish between reproductive adults and large juveniles.

The final general method is to make an inference about adult body-size or modifications based on close living relatives.

"Hope is a duty from which palaeontologists are exempt."
David Quammen

Thin sections of bones can show if there is very rapid growing bone tissue (typical for immature animals) or what is termed an "external fundamental system", very slow growing dense bone that is typical for fully grown animals.

Furthermore, many animals show +/- yearly LAGs (lines of arrested growth) in their bones. The early ones are lost later in life due to bone modelling, but one can count what's preserved and make an inference on what's missing. Additionally, the bone remodelling itself tells you if an animals is very young or very old.