Dogs are known for being one of the most morphologically diverse species on the planet – that is, they come in a big range of shapes, sizes and appearances.

From the diminutive Chihuahua and Chinese crested dog to the mighty Great Dane, it can sometimes be hard to believe that all these animals are the same species – not to mention all descended from wolves.

A new study published in Proceedings of the Royal Society B has shed light on how far back in time these variations in dog size and shape have existed.  

The international research team, based mostly in France, analysed 525 ancient dog mandibles (lower jawbones) found in archaeological sites around western Europe and Romania. The samples dated from between approximately 8100 and 3000 BCE, or from the Mesolithic (Middle Stone Age) to the Bronze Age.

The study also included some archaeological wolf mandibles, as well as jawbones from modern wolves, dingoes and domestic dogs.

Why mandibles? For one, these bones are relatively common in the archaeological record, allowing for the assembly of a larger dataset. Dog mandibles are also a good indicator of overall skull shape – which is one of the most pronounced areas of variation in modern dog breeds. Think of a long-snouted borzoi compared to a compact bulldog.

The new study found considerable morphological diversity in their sample of ancient dogs as far back as the Neolithic (Late Stone Age) in Europe.  

However, none of the ancient dogs reached the extremes of size or skull shape seen in modern breeds – there was no Bronze Age equivalent of the Chihuahua, borzoi or pug.

The ancient dogs’ mandibles were consistently smaller than those of wolves, with an average size about the same as a modern beagle.

According to the study authors, their results suggest that dogs in this time period were not being placed under strong artificial selection by humans to favour specific aesthetic characteristics, or behaviours such as herding. Instead, factors such as the greater toughness and diversity of ancient dogs’ diets, or changes in demography as dogs and human migrated across the continent, may have influenced their skull shape.

Now that a baseline for ancient dog morphological diversity has been established, the researchers hope to use future work with genetic, isotopic or other archaeological data to better understand what caused the level of variation that they observed.

“This would allow more profound insights into how the changes in human societies were accompanied by changes in the morphology of dogs through time and space,” they write.

“Paleogenetic data might, for example, allow to test whether the acquisition of the ability to digest starch was accompanied by changes in mandible form.”