Plagiolophus ( (oblique) + (crest) meaning ) 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 in the smallest species P. minor to over 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.
Taxonomy Research history Early history
In 1804, the French naturalist Georges Cuvier, having established the genus Palaeotherium and some of its species (P. medium and P. magnum), recognized a third species, Palaeotherium minus, based on some postcranial fossils from the gypsum quarries of the outskirts of Paris (known as the Paris Basin), although he did not elaborate further on them. In a later journal of the same year, he described a nearly completely skeleton from the quarries of the French commune of Pantin, originally found by the French naturalist Auguste Nicholas de Saint-Genis. According to Cuvier, the quarry workers previously thought the skeleton to be of a ram, and it was presented as such in public newspapers. The French prefect Nicolas Frochot later acquired it and brought it to the National Museum of Natural History, France, where Cuvier was then able to observe that it must have been a skeleton of a Palaeotherium species. He noted that the majority of the fossil bones were detached from others and/or damaged but that postcranial elements such as scapulae, humeri, femora, vertebrae, and ribs were found. The naturalist also provided a figure of the skeleton within the journal. Of note is that the skeleton, confirmed to be of a pregnant female, has since been lost.165
In 1812, Cuvier published his drawing of a skeletal reconstruction of P. minus based on known fossil remains of the species including the mostly complete skeleton. He also suggested theoretical lifestyles of several Palaeotherium species. In particular, he suggested that P. minus resembled a tapir, was smaller than a sheep, and was cursorial (adapted for running) based on the slender morphologies of its leg bones. Such a behaviour and small size would have differed from other species of Palaeotherium, several of which according to Cuvier had stockier limbed bone builds. He also identified that P. medium, P. magnum, P. minus, P. crassum, and P. curtum were all tridactyl, or three-toed.
"P. minus" (= Plagiolophus minor) was amongst the fossil mammal species represented as sculptures in the Crystal Palace Dinosaurs attraction in the Crystal Palace Park in the United Kingdom, open to the public since 1854 and constructed by English sculptor Benjamin Waterhouse Hawkins. The Plagiolophus sculpture is smaller than the P. magnum and P. medium sculptures and is in a sitting position unlike the other two. The models' resemblances to tapirs reflected early perceptions that the palaeothere species resembled them in body plan appearances. Despite this, the sculptures differ from living tapirs in several ways, such as shorter plus taller faces, higher eye positions, slender legs, longer tails, and the presence of three toes on the forelimbs unlike the four toes of the forelimbs of tapirs.
Hawkins and other workers seemingly used Cuvier's research for reference to the anatomy of P. minor and reproduced its size and proportions accordingly. The P. minor sculpture, sheep-sized, originally had a short head that probably measured about in length and had pointed ears, large eyes, long lips, a stocky proboscis, a muscular neck, and a short plus slender tail. It looks similar to the P. medium sculpture overall but lacks skin details. Although the original head's form is poorly known, it appeared to have been longer and more robust than that of P. medium. Within the later half of the 20th century, the original head was lost and replaced with a head cast of P. medium. Because the size and form of the head made it difficult to attach to the P. minor body normally, the back portion of the cranium was removed and the neck lengthened. This resulted in the sculpture appearing to look forward instead of upwards like before. The P. minor sculpture lost its head twice more, once recently in 2014 when its head was tossed into a lake of the Crystal Palace by an unknown criminal and had to be recovered.
Naming and later 19th century research history
In 1846, French palaeontologist Auguste Aymard recorded a mandible with teeth from the French department of Haute-Loire, noting that it was the approximate size of that of Palaeotherium curtum but had different molar morphologies from it and the small-sized "P. minus", establishing the name P. ovinum. In 1847, the French palaeontologist Auguste Pomel erected the Palaeotherium subgenus Plagiolophus, which he reclassified P. minus to. The genus name derives from the Ancient Greek words ("oblique") and ("crest") meaning "oblique crest".
British palaeontologist Richard Owen in 1848 wrote about a nearly complete lower jaw with both deciduous (or milk teeth) and permanent dental sets that was uncovered from the Eocene beds of Hordle, England, by Alexander Pytts Falconer, observing that it had one less premolar for a total of three of them than in other species of Palaeotherium and erecting the genus Paloplotherium and the species Paloplotherium annectens based on the mandible. He then described a cranium belonging to Paloplotherium that similarly had nearly complete dentition but evolutionarily lost a premolar. After comparing the dentition to those of both Palaeotherium and Anoplotherium, he determined that the dentition of Paloplotherium was similar to that of the former but differed mainly by the absence of the first premolar. He wrote that the permanent dental formula of Paloplotherium is for a total of 40 teeth. Paloplotherium derives from the Ancient Greek words ("ancient"), ("arms"), and ("wild beast") meaning "ancient armed beast".
In 1852, German palaeontologist Christian Erich Hermann von Meyer, recognizing Plagiolophus as a distinct genus and emending Plagiolophus minus to Plagiolophus minor, erected the species P. fraasi based on fossils from the German locality of Frohnstetten originally found by Oscar Fraas. Fraas had studied fossils of palaeotheres from Frohnstetten since 1851, assembling a complete skeleton of P. minor using fossils from there in 1853. In 1853, Pomel listed in the genus Plagiolophus multiple previously recognized species, namely P. ovinus (reclassified from Palaeotherium and emended from P. ovinum), P. minor, and P. annectens (by extent synonymizing Paloplotherium with Plagiolophus). He also erected another species P. tenuirostris. In 1862, Swiss palaeontologist Ludwig Ruetimeyer established P. minutus based on dental remains from the Swiss locality of Egerkingen.
Not all taxonomists agreed on Paloplotherium as a synonymous genus. In 1865 for example, French palaeontologist Jean Albert Gaudry recognized Paloplotherium as valid genus instead of Plagiolophus, grouping P. minor, P. ovinus, and P. annectens into it and erecting another species P. codiciense. In 1869, Swiss palaeontologists François Jules Pictet de la Rive and Aloïs Humbert wrote that Palaeotherium, Paloplotherium, and Plagiolophus were all valid genera and erected two species for the latter: P. siderolithicus using fossil molars from a museum collection and P. valdensis based on a mandible that was smaller in proportion than that of P. minor. In 1877, French naturalist Henri Filhol erected Paloplotherium Javalii based on fossil jaws including that from the fossil collection of the French official Ernest Javal, who he named the species after. Ruetimeyer in 1891 erected another species Paloplotherium magnum, stating that its size would have been that of Palaeotherium magnum.
In addition to the now-lost skeleton studied originally by Georges Cuvier in 1804, several other incomplete skeletons have been described by other palaeontologists. Skeletal elements were figured by Cuvier and later Blainville in 1839–1864, and the latter naturalist also figured skeletal elements from the French commune of Monthyon surrounding the skeleton whose whereabouts are also unclear.165 P. minor is also known from another assembled skeleton that was originally documented by Fraas in the later 19th century, although Stehlin referenced that Fraas paid little attention to studying the limb bones.
20th- and 21st-century taxonomy