Plagiolophus (mammal)

Plagiolophus (Ancient Greek: πλάγιος (oblique) + λόφος (crest) meaning 'oblique crest') is an extinct genus of equoids belonging to the family Palaeotheriidae. It lived in Europe from the middle Eocene to the early Oligocene. The type species, P. minor, was described in 1804 by the French naturalist Georges Cuvier based on fossils from the Paris Basin, France, including a now lost skeleton. Cuvier assigned the species to the genus Palaeotherium, and in 1847, Auguste Pomel assigned it to the new subgenus Plagiolophus. Plagiolophus was promoted to genus rank by subsequent palaeontologists and today includes as many as seventeen species. As proposed by the French palaeontologist Jean A. Remy in 2004, it contains three subgenera: Plagiolophus, Paloplotherium, and Fraasiolophus.

Plagiolophus is a derived (evolutionary advanced) member of its family with tridactyl (three-toed) forelimbs and hindlimbs. It has longer postcanine diastemata (gaps between teeth) compared to Palaeotherium and brachyodont (low-crowned) dentition that over time became more hypsodont (high-crowned) in response to climatic trends. It is also defined by an elongated facial region, a deep nasal notch, and orbits (eye sockets) that are more positioned backwards compared to those of Palaeotherium. Plagiolophus, as a species-rich genus, had a wide body mass range from less than 10 kg (22 lb) in the smallest species P. minor to over 150 kg (330 lb) in the largest species P. javali. Some species were stockier (P. annectens, P. fraasi, P. javali) while others were lightly built and had cursorial (running) adaptations (P. minor, P. ministri, P. huerzeleri).

Plagiolophus and other members of the Palaeotheriinae likely descended from the earlier subfamily Pachynolophinae during the middle Eocene. Western Europe, where Plagiolophus was mainly distributed, was an archipelago that was isolated from the rest of Eurasia and therefore showed high levels of endemism. While many species had short temporal ranges, P. minor was long-lasting to the extent that researchers observed changes in its dietary habits over time. P. minor is thought to have consumed less hard foods (fruits, seeds) and to have become more specialized but less selective towards tough, abrasive, and older leaves in response to environmental trends in the late Eocene to early Oligocene. Its dietary habits would have allowed it to occupy a different ecological niche from other palaeotheres like Palaeotherium and Leptolophus. Plagiolophus was diverse for much of its evolutionary history and survived far past the Grande Coupure extinction event, likely because some of its species were well-adapted towards major environmental trends as a result of their dietary changes and cursorial nature. It was able to adapt to more seasonal climates after the Grande Coupure and coexisted with immigrant faunas from the faunal turnover event. Its eventual extinction during the later early Oligocene marked the complete extinction of the Palaeotheriidae.