A dwarf walrus from the Miocene of Baja California Sur, Mexico

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.


Introduction
The evolutionary history of odobenids is intimately tied with the North Pacific, with the group originating in the region during the early Miocene [1,2]. As such, their fossil record offers unique insight into their ancient diversity and evolutionary patterns over time [1][2][3][4]. During their early history odobenids occurred sympatrically with otariids and desmatophocids, with the latter having larger body sizes and presumably occupying higher trophic levels [1][2][3][4][5][6]. This pattern shifted after the mid-late  Miocene extinction of desmatophocids, leading to a greater diversity of odobenids and an overall increase in body size within this group, peaking during the latest Miocene-early Pliocene [3,4]. These multispecies communities were then composed of otariids and usually two or more species of large (greater than or equal to 2.5 m in body length) odobenids [3,4].
Here, we describe a new odobenid from mid-late Miocene deposits in Baja California Sur, Mexico. The new taxon is characterized by its small body size, representing a case of dwarfism in a group that is otherwise characterized by an increase in body size over time [3,7].

Phylogenetic analysis
For the phylogenetic analysis, we used the matrix of Tanaka & Kohno [8] as modified by Velez-Juarbe [9], by adding UABC FCM 0072 (see electronic supplementary material). All characters were equally weighted and unordered. A Bayesian inference analysis was performed using MrBayes [10] using the following parameters: mcmc = 3 000 000, saplefreq = 1000, printfreq = 1000, starttree = random. The heuristic analysis was performed using PAUP* [11] by doing a heuristic search with 1000 replicas; statistical support was obtained by doing 1000 bootstrap replicas.      [5,20,26,27]). The posterior surface of the calcaneal tuber is transversely concave with a nearly rounded outline. Proximally, just distal to the posterolateral corner of the calcaneal tuber is a prominent, subconical rugosity (herein termed calcaneofibulare tuberosity), probably marking the attachment of the calcaneofibulare ligament; a similarly positioned, but much lower rugosity is also present in Proneotherium repenningi and observed specimens of Neotherium mirum (LACM 51077 and LACM 127997) [26,27]. Dorsally, the ectal facet is elongated, broadly convex, with its posterior half oriented medially, while its distal half faces anterodorsally, similar to that of Neotherium mirum and Proneotherium repenningi [26,27]. Distomedial to the ectal facet, the sustentaculum tali projects as far medially as the medial tuberosity; the sustentacular facet faces dorsally and slightly anteriorly, forming a shallowly concave, rounded surface. As in Proneotherium repenningi, the sustentacular facet is continuous distally with an accessory facet that reaches the dorsomedial corner of the cuboid facet [27]. The sustentacular facet and the ectal facet are divided by a relatively shallow sulcus calcanei. Distolaterally the peroneal tubercle forms an elongated shelf; it has a shallow sulcus on its dorsal surface and another one that extends obliquely along its lateral surface. Anteriorly, the cuboid facet is shallowly concave and has a round outline; it forms an angle of approximately 70°relative to the long axis of the bone. Ventromedial to the cuboid facet is a prominent, knob-like, anterior tubercle. We are confident that LACM 60914 represents an odobenid, as it has a prominent medial tuberosity, which can be considered as an odobenid synapomorphy [5,20]. The overall morphology of LACM 60914 is similar to that of Neotherium mirum, differing mainly, from this and other odobenids in the enlarged attachment of the calcaneofibulare ligament. The Tortugas calcaneum is close in size to the smallest calcaneum of Neotherium mirum examined (LACM 127997; table 2) and to the lectotype of Neotherium mirum, and notably smaller than that of Proneotherium repenningi [26][27][28]. Size differences among specimens of Neotherium mirum are thought to represent sexual dimorphism; with female individuals being smaller than their male counterparts, it is possible that LACM 60914 represents a male individual of Nanodobenus [5,6,9]. However, because multiple pinnipeds, including more than one odobenid, are supposed to be present in the Tortugas Fm. [15], and lack of overlap with the type, we tentatively assign LACM 60914 to Nanodobenus arandai.

Results
The Bayesian inference tree and the consensus tree obtained from the heuristic search (336 most parsimonious trees, 300 steps long, with ci = 0.493 and ri = 0.724) had very similar topologies, with the former having better resolution among the more derived taxa ( figure 4). The topology closely resembles that of recent works on odobenids [8,9,21]. Nanodobenus falls in a polytomy with Pseudotaria muramotoi, Pelagiarctos spp. and Archaedobenus akamatsui. The lack of resolution is probably due to incompletely known taxa such as Pelagiarctos spp. and Nanodobenus, which can only be scored for 35% and 28% of the characters; this was part of the reason Pelagiarctos was not included in some analyses [8]. Nevertheless, Nanodobenus arandai shares unique features with basal and derived odobenids as described above. Reduction/fusion of the post-canine root lobes happened multiple times in odobenids and in this respect Nanodobenus resembles some specimens that have been referred to as Imagotaria spp.

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 [3,4,7,31]. 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) [3,4,7,32]. Nanodobenus arandai seems to be the exception to this trend. With a body length estimate of 1.65 m ( figure 4 and table 3 [3,4,29]; body size estimates from table 3 and outlines modified from Berta et al. [2] and Lydersen [30]. Numbers in nodes represent posterior probability (in bold) and bootstrap values.
assemblage ( figure 4 [3, fig. 7]). 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 ( figure 4 and table 3) [3,5]. 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 (figure 4) [5,15]. Other pinnipeds are known from the Tortugas Formation in the study area [15], 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. [7], greatly contrasting with that of coeaval odobenids and even more with the slightly younger Pontolis magnus ( figure 4 and table 3). Interestingly, this mid-late Miocene   [7].
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 [34], and the occurrence of several small phocids in the North Sea and Paratethys [35].