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Nanodobenus arandai
Nanodobenus arandai 

Temporal range: Miocene

Scientific classification
Kingdom:  Animalia
Phylum:  Chordata
Class:  Mammalia
Order:  Carnivora
Clade:  Pinnipedia
Superfamily:  Otarioidea
Family:  Odobenidae
Genus:  †Nanodobenus
Species:  Nanodobenus arandai

Nanodobenus arandai is a relative of the walrus. The type species is N. arandai.

Journal Reference:
Jorge Velez-Juarbe and Fernando M. Salinas-Márquez. 2018. A Dwarf Walrus from the Miocene of Baja California Sur, Mexico. Royal Society Open Science. DOI: 10.1098/rsos.180423

Here, we describe the odobenid Nanodobenus arandai gen. et sp. nov., based on a nearly complete left mandible from the mid to late Miocene Tortugas Formation in Baja California Sur. Nanodobenus is distinguished among odobenids by displaying a unique combination of plesiomorphic and derived characters, such as narrow mandibular symphysis, well-developed genial tuberosity, bilobed canine and p2 roots, bulbous post-canine teeth with the paraconid, protoconid and hypoconid, and smooth lingual cingula. Moreover, it is characterized by its small adult body length, which is estimated at about 1.65 m. Throughout the Miocene–Pliocene odobenids are characterized by an increase in body size, especially after the extinction of desmatophocids in the late Miocene. The small size of Nanodobenus departs from this trend, demonstrating that there was greater size disparity among odobenids in the mid–late Miocene than previously thought. It is hypothesized that Nanodobenus occupied a niche that was later on occupied by similar-sized otariids, such as Thalassoleon mexicanus, which occurs sympatrically with large odobenids in the overlying Almejas Formation.

[Image: Nanodobenus_arandai-novataax_2018-Velez-...rquez_.jpg]

Etymology. The name derives from the combination of ‘nano’, from the Latin ‘nanus’ which translates to dwarf, in reference to the small size of the specimen and estimated body size, combined with Odobenus, the genus of extant walrus, Odobenus rosmarus(Linnaeus, 1758), and the occasionally used suffix in odobenids (e.g. Archaeodobenus). The specific epithet honours Dr Francisco Aranda-Manteca (UABC) in recognition of his mentorship to the junior author and contributions to the knowledge of extinct marine vertebrates of Baja California and Baja California Sur.

Holotype. UABC FCM 0072, nearly complete left mandible, including p2–4. Collected by T. McMillan, c. 1987.

Type locality. Arroyo La Chiva (=Arroyo Tiburón [12–14]), Asunción, Baja California Sur, Mexico (figure 1).

[Image: Nanodobenus_arandai-novataax_2018-Velez-...quez_i.jpg]
Figure 2. Mandible and lower dentition of Nanodobenus arandai gen. et sp. nov. (UABC FCM 0072).
Mandible in lateral (a), medial (b) and occlusal © views. Lower dentition in labial (d), lingual (e) and occlusal (f) views. 
Abbreviations: c, lower canine; di, digastric insertion; gt, genial tuberosity; hyd, hypoconid; lc, lingual cingulum; p1–4, lower premolars 1–4; m1–2, lower molars 1–2; maf, masseteric fossa; mf, mental foramina; mnf, mandibular foramen; ms, mandibular sumphysis; pad, paraconid; prd, protoconid; wf, wear facet.

[Image: Nanodobenus_arandai-novataax_2018-Velez-...rquez_.jpg]

[Image: Nanodobenus_arandai-novataax_2018-Velez-...1rquez.jpg]
Figure 4. Time-calibrated strict consensus tree of Odobenidae and body size distribution. 
Species range from [Velez-Juarbe, 2017; Boessenecker & Churchill, 2018; Boessenecher et al., 2018]; body size estimates from table 3 and outlines modified from Berta et al. [2018] and Lydersen [2018]. Numbers in nodes represent posterior probability (in bold) and bootstrap values.

Discussion and conclusion

During the late Miocene–Pliocene, odobenids show a marked trend of increasing body size, similar to what is observed in other marine mammals. In odobenids this increase seems to have been possible due to a combination of factors, such as extinction of desmatophocids, increased marine productivity and exploitation of other feeding niches (e.g. benthic feeding). Nanodobenus arandai seems to be the exception to this trend. With a body length estimate of 1.65 m, it lived during a transitional time when desmatophocids or odobenids were the largest pinniped in any assemblage. Its size was actually closer to that of the early otariid Pithanotaria starri, which is known from California and was sympatric with large odobenids such as Imagotaria downsi and others. It is possible that Nanodobenus was occupying a niche similar to that of Pithanotaria, and was replaced by otariids such as Thalassoleon mexicanus, known from the overlying Almejas Formation, which occurs sympatrically with the larger odobenids Aivukus cedrosensis and Dusignathus santacruzensis. Other pinnipeds are known from the Tortugas Formation in the study area, and examination of the material housed at LACM confirms the presence of a small odobenid (LACM 60914, tentatively referred to Nanodobenus). However, the identity of other pinnipeds from that formation needs to be confirmed by examination of specimens in other institutions, although it is evident that multiple pinniped taxa were present.
Nanodobenus has a smaller body size to any other odobenid, and smaller than ancestral length (195 cm) estimated by Churchill et al., greatly contrasting with that of coeaval odobenids and even more with the slightly younger Pontolis magnus. Interestingly, this mid–late Miocene dwarfism seems to mirror the pattern seen in the southeastern Pacific with the occurrence of a dwarf seal in assemblages dominated by mid–large phocids, and the occurrence of several small phocids in the North Sea and Paratethys.

Attached Files
.pdf   A dwarf walrus from the Miocene of Baja California Sur, Mexico.pdf (Size: 1.01 MB / Downloads: 0)
[Image: wildcat10-CougarHuntingDeer.jpg]
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