Phenacodontidae is an extinct family of large herbivorous mammals traditionally placed in the "wastebasket taxon" Condylarthra, which may instead represent early-stage perissodactyls. They lived from the late early Paleocene to early middle Eocene (about 60–50 million years ago) and their fossil remains have been found in North America and Europe. The only unequivocal Asian phenacodontid is Lophocion asiaticus.
Description These animals had a variety of body sizes, and could be as small as domestic cats (Tetraclaenodon and Ectocion) and as large as sheep (Phenacodus). The skull of phenacodontids is long and narrow, and equipped with a small braincase. The skeleton of phenacodontids show several primitive characteristics (the long and heavy tail for example) but also a number of advanced, Perissodactyla-like adaptations: Their long legs, for example, had five fingers, but the first finger showed a clear reduction, and in some forms (like Phenacodus) the fifth finger was reduced as well. Some species had tapir-like adaptations suggestive of the presence of a short proboscis or a strong prehensile lip. The teeth of phenacodontids, particularly in the latter forms, were quite specialized: The molars and premolars were equipped with low cusps that sometimes joined in ridges, similar to the condition found in some perissodactyls. Some forms, like Meniscotherium, had enlarged ridges. This adaptation is unusual for mammals as old as phenacodontids. Only a few other archaic mammals possessed teeth with similar structures, such as Pleuraspidotherium.
Evolution The phenacodontids evolved in the middle Paleocene in North America. Early forms were usually small; Tetraclaenodon, for example, was the size of a fox. Later forms were much larger and invaded Europe, although they never became as plentiful as in North America. Towards the beginning of the Eocene these animals slowly disappeared from the fossil record. Only a few forms survived into the middle Eocene: the Phenacodus in Europe and North America, Almogaver in Europe and Ectocion in North America. An exception to the scarcity of Eocene phenacodontids is the dog-sized genus Meniscotherium, whose fossils are very abundant.
Classification Phenacodontids have classically been included in the large group Condylarthra, now considered polyphyletic. In particular, the genus Phenacodus is often illustrated as a typical example of a "condylarth", due to the remarkable abundance of fossil remains.
Subfamily Phenacodontinae Cope, 1881 Genus Tetraclaenodon Scott, 1893 Tetraclaenodon floverianus Cope, 1890 Tetraclaenodon puercensis (Cope, 1881) Tetraclaenodon septentrionalis Thewissen, 1990 Genus Copecion Gingerich, 1989 Copecion brachypternus (Cope, 1882) Copecion davisi Gingerich, 1989 Genus Ectocion Cope, 1882 Ectocion cedrus Thewissen, 1990 Ectocion collinus Russell, 1929 Ectocion ignotum Novacek et al., 1991 Ectocion major (Patteron & West, 1973) Ectocion mediotuber Thewissen, 1990 Ectocion osbornianus (Cope, 1882) Ectocion parvus Granger, 1915 Ectocion superstes Granger, 1915 Genus Phenacodus Cope, 1873 Phenacodus bisonensis Gazin, 1956 Phenacodus condali (Crusafont i Villalta, 1955) Phenacodus grangeri Simpson, 1935 Phenacodus intermedius Granger, 1915 Phenacodus lemoinei Thewissen, 1990 Phenacodus magnus Thewissen, 1990 Phenacodus matthewi Simpson, 1835 Phenacodus primaevus Cope, 1873 Phenacodus teilhardi Simpson, 1929 Phenacodus trilobatus Cope, 1882 Phenacodus vortmani (Cope, 1880) Genus Lophocion Wang and Tong 1997 Lophocion asiaticus Wang and Tong 1997 Lophocion grangeri Bai et al. 2019 Subfamiliy Meniscotheriinae Cope, 1882 Genus Meniscotherium Cope, 1874 Meniscotherium chamense Cope, 1874 Meniscotherium tapiacitum Cope, 1882 Genus Orthaspidotherium Lemoine, 1878 Orthaspidotherium edwardsi Lemoine, 1878 Some phylogenetic analyses have revealed effective relationships between the various groups of "condylarths". One phylogeny suggests there may be close correlations between a clade containing proboscideans, hyracoids, perissodactyls and phenacodontids and another clade with Microhyus and the macroscelids. The clade including these forms would be analogous to the clade Taxeopoda, proposed in 1998. According to more recent views, instead of a monophyletic clade, the condylarths are better understood as an evolutionary grade that lead to the true ungulates. Phylogenetic analysis of 2012 recover phenacodontids as a series of sister taxa to the Altungulata clade. Later phylogenetic studies confirm that phenacodonts were most closely related to modern odd-toed ungulates.
Paleobiology The specialized teeth found in at least some phenacodontids seem to indicate a primary herbivorous lifestyle. The shape of the legs indicated that some phenacodontids (like Phenacodus) were swift runners.
See also Radinskya, a basal perissodactyl from the Paleocene of China
Notes
References Agusti, Jordi; Anton, Mauricio (2002). Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe. Columbia University Press. ISBN 0-231-11640-3. Archibald, J. D. (1998). "Archaic ungulates ("Condylarthra")". In Janis, C. M.; Scott, K. M.; Jacobs, L. L. (eds.). Evolution of Tertiary Mammals of North America. Vol. 1: Terrestrial Carnivores, Ungulates, and Ungulate-like Mammals. Cambridge University Press. pp. 292–331. Prothero, D. R.; Manning, E. M.; Fischer, M. (1988). "The phylogeny of the ungulates". In Benton, M. J. (ed.). The Phylogeny and Classification of the Tetrapods. Vol. 2: Mammals. Oxford, UK: Clarendon Press. pp. 201–234. Tabuce, R.; Coiffait, B.; Coiffait, P.E.; Mahboubi, M.; Jaeger, J.J. (2001). "A new genus of Macroscelidea (Mammalia) from the Eocene of Algeria: a possible origin for elephant-shrews". Journal of Vertebrate Paleontology. 21 (3): 535–546. doi:10.1671/0272-4634(2001)021[0535:angomm]2.0.co;2. S2CID 85833115. Tabuce, R.; Marivaux, L.; Adaci, M.; Bensalah, M.; Hartenberger, J.L.; Mahboubi, M.; Mebrouk, F.; Tafforeau, P.; Jaeger, J.J. (2007). "Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade". Proceedings of the Royal Society B. 274 (1614): 1159–1166. doi:10.1098/rspb.2006.0229. PMC 2189562. PMID 17329227. Thewissen, J. G. M. (1990). "Evolution of Paleocene and Eocene Phenacodontidae (Mammalia, Condylarthra)". Papers on Paleontology. 29. University of Michigan: 1–107. {{cite journal}}: Cite journal requires |journal= (help)
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