By Colline Brassard and Trish Fleming. Dogs (Canis familiaris) are descendants of the grey wolf (Canis lupus). The earliest accepted dog remains date back to about 15,000 years ago. Although all dogs share this same ancestor, their life as human domesticates has led to considerable variation, and modern dogs are one of the most variable species on the planet, both in terms of size and proportions.
This tremendous variability is the result of rapid phenotypic changes in response to strong intentional selection by humans for key traits (e.g., for guarding, hunting or running), which has contributed to alterations in their functional morphology.
While the skull of fossils is often crushed or distorted, the mandible (lower jaw) is far more robust, and can survive in its original form. The shape of the mandible is influenced by the mechanical action of the jaw muscles that connect it to the skull, and mandible shape therefore reflects the animal’s diet as well as the overall shape of their head.
We used three-dimensional geometric morphometrics to describe the shape of 525 ancient dog mandibles from European archaeological sites ranging from 8,100 to 3,000 cal. BC, comparing these to a reference sample of modern dogs, wolves, and dingoes .
What did ancient European dogs look like?
For the first time, we demonstrated that prehistory dogs showed an important variability in jaw size and shape, suggesting they already varied a lot in form of their head, with ancient dog mandibles varying from very small, similar to some modern small dogs such as the pomeranian or dachshund, to larger sizes compatible with modern large dogs such as the husky, golden retriever or German shepherd.
The mandible size of ancient dogs was always much smaller than that of wolves, and on average corresponds to that of modern beagles.
Variability in mandible form suggests that the first dog wranglers did not exert strong selection on dogs for aesthetic or utilitarian reasons that might have constrained the anatomy of the lower jaw. As expected, no extreme forms were observed among the ancient dogs: the very brachycephalic (pitbull, amstaff, boxer, bulldog etc.) and very dolichocephalic (borzoi) modern dogs have no equivalent in ancient times.
We identified some unique mandible shapes among ancient dogs, with no equivalent in modern dogs. These unique shapes may be partly related to the changes in the genetic composition of dog populations, but the anatomical differences are located in areas of functional importance, suggesting these changes reflect differences in lifestyle and access to food.
The mandibles of ancient dogs show traits that allowed us to distinguish them from modern dogs. Differences in the curvature of the body of the mandible under the carnassial tooth and greater surface area for muscle attachment suggests greater importance of the role of the temporalis (jaw-closing) muscle in ancient dogs, which is likely related to diet of these dogs — it suggests that ancient dogs were feeding on more tough and hard foods than most modern dogs. Ancient dogs also showed traits suggesting that they had a greater bite force than modern dogs.
How do dingoes compare?
Modern dingoes have mandible shapes that place them in an intermediary position between wolves and modern dogs. The mandible shape of modern dingoes mostly groups them with modern and ancient wolves, but some are close to modern dogs and the mandible of one individual was most similar to ancient dogs.
The dingo was brought to Australia somewhere about 3,600 to 5,000 years ago. It lived in isolation until about 200 years ago when Europeans brought modern dogs onto the continent.
Dingoes have a carnivorous diet, with their principal diet being kangaroos and wallabies. They have recently been shown to have a single copy of the amylase gene , indicating their separation from the modern dog lineage prior to adaptation to an omnivorous diet.
 Brassard et al. 2022 Unexpected morphological diversity in ancient dogs compared to modern relatives Proceedings of the Royal Society B https://doi.org/10.1098/rspb.2022.0147
 Field et al. 2022 The Australian dingo is an early offshoot of modern breed dogs Science Advances 8(16)