Thereuodon cf. taraktes (Sigogneau-Russell & Ensom, 1998)

Allain, Ronan, Vullo, Romain, Rozada, Lee, Anquetin, Jérémy, Bourgeais, Renaud, Goedert, Jean, Lasseron, Maxime, Martin, Jeremy E., Pérez-García, Adán, Fabrègues, Claire Peyre De, Royo-Torres, Rafael, Augier, Dominique & Bailly, Gilles, 2022, Vertebrate paleobiodiversity of the Early Cretaceous (Berriasian) Angeac-Charente Lagerstätte (southwestern France): implications for continental faunal turnover at the J / K boundary, Geodiversitas 44 (25), pp. 683-752 : 724-725

Thereuodon cf. taraktes

( Fig. 32 G-I)

DESCRIPTION

An isolated tooth crown ( Fig. 32 G-I) is assigned to the genus Thereuodon of the monogeneric family Thereuodontidae , on the basis of several characters (Sigogneau-Russell 1989; Sigogneau-Russell & Ensom 1998; Kielan-Jaworowska et al. 2004): it is labio-lingually narrow, with a great development of the parastylar lobe, forming almost a basin ( Fig. 32I ); a small vertical concavity can be seen on the anterior face of the paracone; the stylocone is well-developed ( Fig. 32G ); cusp D occurs just behind the stylocone, although it is heavily abraded on this specimen ( Fig. 32G ); a vertical ridge, the mediocrista, links the paracone to the cusp D, although it is weakly pronounced ( Fig. 32I ). Thus, three transversely narrow basins are present in occlusal view. Moreover, the paracone is high compared to the labial wall and labially but not posteriorly recurved ( Fig. 32G, I ). This crown is well-preserved, lacking only the metastyle.

Two species are known for the genus Thereuodon : the type-species T. dahmanii (Sigogneau-Russell 1989) from the Jurassic/Cretaceous transition of the Ksar Metlili Formation, Morocco, and T. taraktes (Sigogneau-Russell & Ensom 1998) , from the Early Cretaceous (Berriasian) Purbeck Group, England. In the Angeac-Charente specimen, the great protrusion of the parastylar cusp and the weakly marked mediacrista are reminiscent of T. taraktes , but the well-developed metacone is close to what is observed in T. dahmanii . The stylocone is closer to cusp D and less sharp than in T. dahmanii , but this conformation is similar to that of T. taraktes . Finally, as in T. taraktes but unlike T. dahmanii , there is no cuspule “c”. In conclusion, we tentatively assign this specimen to Thereuodon cf. taraktes .

Sigogneau-Russell (1989) and Sigogneau-Russell & Ensom (1998) considered the teeth they referred to the two species of Thereuodon as “symmetrodontan” permanent molars. However, the upper deciduous premolars of Nanolestes drescherae , a stem-Zatheria from the Late Jurassic of Guimarota, Portugal ( Martin 1999, 2002), show some similarities with the teeth of Thereuodon . Thus, the DP?3-5 of N. drescherae share with the teeth of Thereuodon an obtuse-angled trigone, a low and recurved paracone, and a trigone basin divided by a ridge connecting stylocone and paracone. These characters are diagnostic of Holotheria, a clade that include the last common ancestor to Kuehneotherium and Theria, and all of its descendants ( Hopson 1994; Wible et al. 1995). Martin (2002) thus reinterpreted the teeth of Thereuodon as holotherian, probably zatherian upper deciduous premolars. Moreover, Bonaparte (1990) and Sigogneau-Russell & Ensom (1998) observed a gross resemblance between the teeth of Thereuodon and the upper cheek teeth of Barberenia , from the Late Cretaceous of Argentina. These were subsequently identified by Martin (1999) as upper deciduous premolars, probably belonging to the “dryolestoidean” Brandonia from the same formation, an opinion then followed by Bonaparte (2002). Thus, it seems that Thereuodon should be considered as a stem-Zatheria ( Martin 2002). However, from a nomenclatural point of view, the fact that Thereuodon is known only by milk teeth does not invalidate the genus, which is why we use it here to precise the identification of this specimen.

Figures

FIG. 32. — Mammal teeth from Angeac-Charente: A, B, premolariform or molariform tooth of Gobiconodon? sp. (ANG M-21) in occlusal (A) and labial (B) views; C, D, premolariform tooth of Mammalia indet. (ANG M-34) in lingual (C) and labial (D) views; E, F, left lower molar of Triconodon sp. (ANG M-02) in lingual (E) and occlusal (F) views; G-I, left upper molar of Thereuodon cf. taraktes (ANG M-23) in labial (G), lingual (H) and occlusal (I) views. Scale bar: 500 µm.