Canis arnensis, the Arno River dog, is an extinct species of canine that was endemic to Mediterranean Europe during the Early Pleistocene. The Arno River dog has been described as a small jackal-like dog. Its anatomy and morphology relate it more to the modern golden jackal (Canis aureus) than to the larger Etruscan wolf of that time. It is probably the ancestor of modern jackals.
Taxonomy
The fossil record for ancient vertebrates is composed of rarely occurring fragments from which it is often impossible to obtain genetic material. Researchers are limited to morphologic analysis, but it is difficult to estimate the intraspecies and interspecies variations and relationships that existed between specimens across time and place. Some observations are debated by researchers who do not always agree and hypotheses that are supported by some authors are challenged by others. Several species of Caninae from the Pleistocene of Europe have been described. Most of their systematic and phylogenetic relationships have not been resolved because of their similar morphology.
Upper Valdarno is the name given to that part of the Arno Valley situated in the provinces of Florence and Arezzo, Italy. The region is bounded by the Pratomagno mountain range to the north and east and by the Chianti Mountains to the south and west. The Upper Valdarno Basin has provided the remains of three fossil canid species dated to the Late Villafranchian era of Europe 1.9-1.8 million years ago that arrived with a faunal turnover around that time. The Swiss paleontologist Charles Immanuel Forsyth Major discovered two species in this region, these being the Falconer's wolf (Canis falconeri Forsyth Major 1877) that was later reclassified as Lycaon falconeri, and the smaller Etruscan wolf (C. etruscus Forsyth Major 1877). Forsyth Major did not publish a complete description of the Etruscan wolf, and later Domenico Del Campana worked on expanding Forsyth Major's descriptions when he recognized among the specimens a smaller, jackal-sized species. This he named the Arno River dog (C. arnensis Del Campana 1913) in honour of the nearby Arno River.
Canis senezensis
C. senezensis (Martin 1973) is represented by two maxillary bone fragments. This medium-sized canid was discovered in Senez, France and dated 2.1-2.0 million years ago. In 2011, a study compared all of the 55 Early Pleistocene wolf-like specimens found across Europe and found that their morphometric variation was no different than that of modern wolf populations, with their difference in size representing male and female specimens. However, the study proposed two lineages. One lineage is C. arnensis which includes C. accitanus and C. senezensis, and the other lineage being C. etruscus that includes C. appoloniensis.
Canis accitanus
A later study based on better-quality specimens of C. arnensis found the proportions and dental morphology of C. senezensis to be close and supported C. senezensis to be an early form of C. arnensis, however it disputed that C. accitanus was close to C. arnensis. Its taxonomic status remains disputed.
Lineage
The Arno River dog has been described as a small jackal-like dog because of the relative length of its upper molars M1 and M2. The Finnish paleontologist Björn Kurtén described it as coyote-like and not similar to the gray wolf (C. lupus) but similar to the early coyote-like C. priscolatrans. Kurten was uncertain if C. priscolatrans derived from C. lepophagus through C. arnensis, but believed that C. priscolatrans was a population of large coyotes that were ancestral to Rancholabrean and recent C. latrans. He noted that C. arnensis of Europe showed striking similarities to C. priscolatrans, and they could represent what once was a Holarctic population of coyotes.
In 1993, the Italian paleontologist Lorenzo Rook identified a new taxon dating from the end of the Villafranchian. It was found at the Mediterranean sites of Venta Micena, Pirro Nord, Le Vallonet, Cueva Victoria, Huescar-1, Colle Curti, Cúllar de Baza-1, L’Escale, Petralona, and the Israeli site of Oubeidiyah. The taxon was named Canis aff. arnensis as it was assessed as an advanced form of C. arnensis. In 1996, Rook and the Italian paleontologist Danilo Torre propose that during the Lower Pleistocene to Mid Pleistocene transition, Europe was home to two different lineages. In the Mediterranean areas existed the lineage of C. arnensis (primitive form) that gave rise to C. aff. arnensis (advanced form). In Central and northern Europe existed the lineage of C. etruscus that gave rise to C. mosbachensis.