A dwarf among giants: Titanohyrax tantulus (Hyracoidea, Mammalia) from the Eocene of Chambi, Tunisia and Glib Zegdou, Algeria
Résumé
The Hyracoidea was a successful mammalian order in Africa from the early Eocene to the mid-Miocene. Through the Eocene/Oligocene, hyracoids represented the dominant 'ungulate' group in terrestrial ecosystems. Among them, the genus Titanohyrax embraces very large species as well as a small one, T. tantulus, only known from the ?early/middle Eocene of Chambi, Tunisia. Recently, the generic affinity of this species has been disputed, implying a latest Eocene origin for Titanohyrax. Testing the competing hypotheses requires new data as most species of Titanohyrax are documented by few specimens. Based on new fossils from Chambi and CT scan analysis of the holotype, we revised T. tantulus. A complete skull from the coeval locality Glib Zegdou, Algeria, is also discussed. This analysis reveals that the holotype bears a part of dP3-4 and M1-2 rather than dP2-M1 as it was proposed. More importantly, derived dental characters, including a small compressed hypocone on P2-M3 and highly molarized P3-4 with a W-shaped ectoloph, are distinctive features of Titanohyracidae. Enamel microstructure also indicates affinities with the titanohyracids Antilohyrax and Afrohyrax. Titanohyrax tantulus and Antilohyrax also share a tall, narrow and vertically oriented supraoccipital and no diastemata between the anterior teeth. Moreover, the lack of postorbital bar, a well-developed metastylid on p3-m3, and broad spatulate lower incisors, that lacked tines or serrations, are diagnostic characters of Titanohyrax. As a result, there is no apparent reason to question the generic affinity of T. tantulus. However, the latter differs substantially from the other species of the genus, in harboring a suite of primitive characters: lower crowned cheek teeth, absence of mesostyle on P1-2, less labially expanded mesostyle and parastyle on P4-M3, more labially positioned paracone and metacone, blunt parastyle and mesostyle, longer preprotocrista and prehypocrista, and presence of a paraconular swelling on molars. Titanohyrax tantulus finally differs from late Eocene titanohyracids (Antilohyrax and Titanohyrax sp.) in having sharp supraorbital ridge and extensive sagittal crest, certainly also primitive cranial characters. To conclude, T. tantulus represents an early offshoot of the titanohyracids; this family thus testifies to the longest fossil record among hyracoids. It only disappeared during the early Miocene, likely due to competition with artiodactyls and perissodactyls. * Speaker
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