Barytherium, Andrews, 1901

Zouhri, Samir, Gingerich, Philip D., Khalloufi, Bouziane, Bourdon, Estelle, Adnet, Sylvain, Jouve, Stéphane, Elboudali, Najia, Amane, Ayoub, Rage, Jean-Claude & Tabuce, Rodolphe, 2021, Middle Eocene vertebrate fauna from the Aridal Formation, Sabkha of Gueran, southwestern Morocco, Geodiversitas 43 (5), pp. 121-150 : 141-143

? Barytherium sp.

EXAMINED MATERIAL. — FSAC Bouj-380a, 380b, and 380c, dental fragments.

REMARKS

Zouhri et al. (2018) mentioned fragmentary dental remains of undetermined proboscideans in Gueran fauna (Laazri locality). Combining light and SEM microscopy, we here studied these dental fragments (FSAC Bouj-380a, 380b, and 380c) to describe the enamel microstructure and propose a systematic assignment. Following the protocol detailed in Tabuce et al. (2017), we realized and analyzed a vertical section for the three specimens, which reveal a similar enamel microstructure. From the enamel dentine junction (EDJ) to the outer enamel surface (OES), the specimens present a one-layered Schmelzmuster [the three-dimensional arrangement of the different enamel types in one tooth ( Koenigswald & Sander, 1997)] formed by thick bundles of prisms that decussate in all directions; this enamel type is the so-called 3D enamel, a structure known only in proboscideans. In some zones of the outer part of the enamel layer, the vertical component of the decussation is attenuated, evoking Hunter-Schreger bands (HSB).

3D enamel is documented in several Paleogene proboscidean species: Numidotherium koholense Jaeger, 1986 (sampled from the early Eocene of El Kohol, Algeria, Bertrand 1988 and Tabuce et al. 2007), Numidotherium sp. (sampled from the late Eocene of Ad-Dakhla, Morocco, Adnet et al. 2010), Arcanotherium savagei ( Court, 1995) ( Court 1995; sampled from the?late Eocene of Dur At-Talah Escarpment, Libya, Tabuce et al. 2007), Barytherium grave Andrews, 1901 (sampled from Dur At-Talah Escarpment, Libya, Bertrand 1988), and Omanitherium dhofarensis Seiffert, Nasir, Al-Harthy, Groenke, Kraatz, Stevens & Al-Sayigh, 2012 (sampled from the earliest Oligocene of Thaytiniti 2, Oman; Tabuce unpublished data).

Among these five species, only Arcanotherium savagei differs from the proboscidean from Laazri by a three-layered Schmelzmuster with 3D enamel only limited to the inner zone, overlain by HSB then radial enamel in the outer zone. Such a complex Schmelzmuster also characterizes all Neogene elephantoids (mammutids, gomphotheres, stegodonts, and elephants) and in a lesser degree Palaeomastodon beadnelli Andrews, 1901 which developed slightly irregular HSB in the inner zone, evoking 3D enamel ( Koenigswald et al. 1993). As a result, similar to the proboscidean from Laazri, only Numidotherium koholense , Numidotherium sp. from Ad-Dakhla, Barytherium grave , and Omanitherium dhofarensis present a one-layered Schmelzmuster formed by 3D enamel.

In addition, the HSB-like structures that occur in places in the outer part of the enamel the proboscidean from Laazri were only mentioned in Numidotherium koholense and Numidotherium sp. from Ad-Dakhla ( Tabuce et al. 2007; Adnet et al. 2010). However, the supposed lack of such HSB-like structures in Barytherium grave must be taken with caution due to the unique published macroscopic analysis (no SEM data available) for this species ( Bertrand 1988). Interestingly, in his unpublished Ph.D. thesis, Bertrand (1989: pl. 18D) figured a view of the outer part of the enamel of Barytherium grave in which HSB-like structures are clearly visible. To conclude, Barytherium and Numidotherium present the same enamel microstructure as the proboscidean from Laazri.

To complete the observations, we measured the molar enamel thickness in Barytherium Andrews, 1901 , Numidotherium Jaeger, 1986 , and Omanitherium Seiffert, Nasir, Al-Harthy, Groenke, Kraatz, Stevens, Al-Sayigh, 2012 . Comparison with the proboscidean remains from Laazri reveals interesting results. The great enamel thickness of FSAC Bouj-380a (± 3.4 to 4.6mm), Bouj-380b (± 2.6 to 2.9 mm) and Bouj- 380c (± 3.1 to 4.2 mm) approaches the rare available data for Barytherium grave (± 2 mm, ± 3.1 mm, ± 2.5 mm; plate 18A-C in Bertrand 1989). Conversely, molars of Numidotherium sp. from Ad-Dakhla and Omanitherium dhofarensis have thinner enamel thickness (± 1 mm and ± 0.7 mm, respectively). Molars of Numidotherium koholense have also thinner enamel thickness (± 2.4 to 3.1 mm for the M3, the largest molar).

To conclude, enamel microstructure and thickness strongly favor an assignment to Barytherium for the proboscidean from Laazri. The presence of Barytherium in the Aridal Formationat of Gueran, if confirmed, would indicate that this genus occurred as early as the Bartonian. So far, this genus was only known by its type species, Barytherium grave , originally described from the late Eocene (Priabonian) of the Fayum depression and then from the Dur At-Talah Escarpment, a locality which is still poorly constrained in age between Bartonian to Priabonian ( Tabuce et al. 2012; Sallam & Seiffert 2016; Longrich 2017). Interestingly also, Gingerich & Cappetta (2014) mentioned a possible Barytherium -sized proboscidean from the early middle Eocene (Lutetian) of Togo.