Amphitherium

Abstract: from the Lower Cretaceous of the United Kingdom and Morocco. The analysis of several mammalian teeth exhibiting peramurid characters and recently obtained from the Early Cretaceous localities of Durlston Bay (England) and Anoual (Morocco) led to the identification of three new taxa (Minimus richardfoxi n.g., n.sp., Magnimus ensomi n.g., n.sp. and Afriquiamus nessovi n.g., n.sp.) and to the reexamination of Peramura McKenna, 1975 (definition, contents, validity). It appears that this group of therian mammals, whose molars establish a morphological link between those of primitive Symmetrodonta and those of tribosphenic mammals, does not in fact show any exclusive synapomorphy, at least in our present state of knowledge; we are confronted, rather, with a few representatives of an evolutionary grade whose vast distribution in space and time suggests an even wider diversification, as well as an early origin for the pretribosphenid line. Moreover it is shown that Amphitherium is closer to tribosphenids than to dryolestoids, while Vincelestes does not qualify as a pretribosphenic mammal. We also discuss the question of the metacone, the stylocone and of the lingual cingulum on the upper molars of Peramus, to conclude that the ancestors of this genus did not go through a dryolestoid stage: the metacone is considered homologous to that of tinodontids, so that its lingual situation is in fact a primitive character. Finally it is concluded that it was the individualization of a hypoconid and a distal metacristid on the lower molars, hence the introduction of a different masticatory mode, that created, early in the pretribosphenid line, the conditions favorable to the later elaboration of a protocone on the upper molars; but the primitive stage of this cusp remains unknown.

Abstract: Four fragmentary mandibles from the Stonesfield Slate facies, Taynton Limestone Formation, Middle Bathonian (Middle Jurassic), England, represent two species of AmphitheriumA prevostiiA rixoni sp nov Both species had five lower premolariform teeth The composite formula for the lower dentition of Amphitherium appears to have been I/4, C/1, P/5, M/6–7 The seventh molar is not present in the presumably oldest individual, and its presence is regarded as probably an individual variation The mosaic evolution of patterns of differentiation of the postcanine dentition from nonmammalian cynodonts to modern therian mammals is reviewed It is concluded that Amphitherium probably had reached the grade of modern therians in the division between diphyodont premolars and monophyodont molars The common ancestor of Amphitherium and zatheres probably had lost the primitive pattern of posterior shift of the postcanine dentition, which appears to have consisted of five premolars and four or possibly five molars In Amphitherium the number of molars probably was secondarily increased

Abstract: From the Kirtlington Bone Bed of England (upper Bathonian), some 700 mammalian specimens, essentially isolated teeth or fragments, have been recovered by British workers; all orders of mammals known for that period are represented. About two thirds of the holotherian molars studied in this paper are attributed to the genus Palaeoxonodon Freeman, 1976, known so far by two lower molars of the type species and a few attributed upper molars. A new species, P. freemani n. sp., also based on lower molars, is proposed. A number of additional upper molars are referred to this genus; they indicate that the latter is not a “peramuran” as previously thought, but an amphitheriid, very close to the genus Amphitherium Blainville, 1838, itself, also known until now by lower dentitions only. Knowledge of amphitheriid upper molars contributes some new light to the discussion of the phylogenetic relationships of this family. In addition, a new taxon of uncertain ordinal position, Kennetheredium leesi n. gen., n. sp., is defined on lower molars. Finally, some teeth could not be precisely identified and are referred to Symmetrodonta indet., Dryolestida indet., Trechnotheria indet. and ?peramurid.

Abstract: The Middle Jurassic was a key interval of mammalian evolutionary history that witnessed the diversification of the therian stem group. Great Britain has yielded a significant record of mammalian fossils from this interval, represented by numerous isolated jaws and teeth from the Bathonian of Oxfordshire and the Isle of Skye. This record captures a key period in early cladotherian evolution, with amphitheriids, peramurans and ‘stem zatherians’ displaying intermediate talonid morphologies that document the evolutionary assembly of tribosphenic molars. We present a mandible with near-complete dentition from the late Bathonian (c. 167.4–166.5 Ma) Kilmaluag Formation, near Elgol, Skye, representing the amphitheriid Palaeoxonodon ooliticus, previously known only from isolated teeth. The specimen sheds new light on the taxonomic diversity of British Middle Jurassic stem therians, as the morphological variation within the preserved tooth row encompasses that previously ascribed to three distinct species within two genera: Palaeoxonodon ooliticus, P. freemani and Kennetheridium leesi. Thus, both P. freemani and K. leesi are subjective junior synonyms of P. ooliticus. The dental formula of P. ooliticus (i4:c1:p5:m5) is intermediate between the primitively larger postcanine count (p5:m6–7) of Amphitherium and the reduced number in peramurans and tribosphenidans (p5:m3). Phylogenetic analyses of P. ooliticus generally confirm a close affinity with Amphitherium, but highlight the lack of strong empirical support for hypothesized patterns of divergences among early cladotherians.