A burrowing frog from the late Paleocene of Mongolia uncovers a deep history of spadefoot toads (Pelobatoidea) in East Asia

Fossils are indispensible in understanding the evolutionary origins of the modern fauna. Crown-group spadefoot toads (Anura: Pelobatoidea) are the best-known fossorial frog clade to inhabit arid environments, with species utilizing a characteristic bony spade on their foot for burrowing. Endemic to the Northern Hemisphere, they are distributed across the Holarctic except East Asia. Here we report a rare fossil of a crown-group spadefoot toad from the late Paleocene of Mongolia. The phylogenetic analysis using both morphological and molecular information recovered this Asian fossil inside the modern North American pelobatoid clade Scaphiopodidae. The presence of a spade and the phylogenetic position of the new fossil frog strongly support its burrowing behavior. The late Paleocene age and other information suggestive of a mild climate cast doubt on the conventional assertion that burrowing evolved as an adaptation to aridity in spadefoot toads. Temporally and geographically, the new fossil provides the earliest record of Scaphiopodidae worldwide, and the only member of the group in Asia. Quantitative biogeographic analysis suggests that Scaphiopodidae, despite originating in North America, dispersed into East Asia via Beringia in the Early Cenozoic. The absence of spadefoot toads in East Asia today is a result of extinction.

). Gene sequences were downloaded from Genbank s2 and aligned using MUSCLE s3 under default settings. For Leptolalax, Xenophrys, Xenopus, Hyla, Leptobrachium, Rana and Meristogenys, because different species were sequenced across different genes, we merged the gene data of different species into the same genus in our analysis. The analysis was performed under Maximum Parsimony criterion using POY 4.1.2 s4 . Tree searching methods include tree building, swapping using TBR, perturbation using ratchet, and tree fusing. Besides the combined analysis as shown in the paper, we also ran a morphology-only analysis. It recovered 106 MPTs and a strict consensus (Supplementary Figure S1) that, while had less overall resolution than the combined analysis, did also recover a monophyletic Pelobatoidea and Prospea as the sister taxon to Spea. A few differences between the morphological tree and the combined tree include: 1) the morphological tree does not resolve the root node of Pelobatoidea, leaving a polytomy among Pelobatidae, Scaphiopodidae, Megophryidae and Pelodytidae; 2) it supports a monophyletic relationship of Pelobates syriacus and Pelobates fuscus, which is not supported by the combined analysis; 3) it supports a monophyletic relationship of Alytes and Discoglossus, which is not supported by the combined analysis.
Biogeographic Analyses. We performed the S-DIVA reconstruction and Bayesian MCMC analyses using RASP 24 . Two analyses bear similar results, so we only show the results from the Bayesian analysis. 55 MPTs were used to generate a "condensed tree" in RASP, which was subsequently used for both analyses.  Morphological Character List 1. Shape of the skull in dorsal aspect: skull apparently wider than long (0); or roughly as long as wide, or longer (1).

Remarks: Modified from [19] (character 1) and [20] (character 1). The length of
the skull is measured from the tip of snout to the foraman magnum, and the width is measured from its widest part, usually at the angle of jaws. In Triadobatrachus the skull is wider than long, and this is considered as the primitive condition.

Remarks: Modified from [19] (character 3) and [20] (character 2).
Triadobatrachus has low irregular rugosities on the anterior part of the frontoparietal, but not on other dermal roofing bones. Extensive sculpture of dermal skull roof can be seen in Pelobates as having a pitted pattern, and in Scaphiopus as having a grooved pattern.

Remarks: Modified from [20] (character 3). Triadobatrachus is reconstructed to
have two nasals with a medial articulation, and this is considered to be primitive. Condition 1 is seen in living leiopelmatids and pelodytids, and condition 2 is seen in Xenopus. 4. Anterolateral margin of nasal: nasal with a concave anterolateral margin for embracing the narial opening (0); or nasal more circular, with essentially a straight anterolateral margin, not embracing the narial opening (1).

Remarks: Modified from [19] (character 5). The polarity is tentative, because
Triadobatrachus has no anterior part of nasal preserved. In Jurassic frog Vieraella and Notobatrachus, the nasal has a concave anterolateral margin, so this is considered primitive. Condition 1 is seen in most pelobatoids. 5. Distinct rostral process of nasal: present (0); or absent (1).

Remarks: Modified from [20] (character 5). When present, it is a moderately
developed process extending anteriorly towards the premaxilla along the midline 19 and above the septum nasi. Polarity of this character is tentative, because Triadobatrachus has no anterior part of nasal preserved. Distinct rostral process of nasal is seen in Vieraella and Notobatrachus, so it is considered as the primitive condition. 10. Dorsal exposure of frontoparietal fontanelle: fontanelle not exposed (0); exposed 50% of its length or less (1); or exposed more than 50% of its length (2).

Remarks: Modified from [19] (character 12) and [20] (character 25).
Triadobatrachus has palatine retained as a discrete bone. This is considered as the primitive condition. Neusibatrachus is coded as absent s10 . Neobatrachians possess the palatine as a discrete element. 34. Posterolateral alae of parasphenoid: anteroposterior width of alae equal or greater than one-third distance between lateral ends (0); or width narrower than one-third distance between lateral ends (1); or alae absent (2).   Triadobatrachus has free ribs on all its presacrals and this is considered as the primitive condition.

Remarks
52. Length of transverse process: transverse process of verbetra II longest, or of equal length of III and IV (0); or transverse process of vertebra III longest (1); or that of IV longest (2). 57. Relative length of urostyle: shorter than combined length of presacral vertebrae (0); or as long or longer than combined length of presacral vertebrate (1).

Remarks:
Modified from [13] (character 46). Triadobatrachus has a short tail, with length significantly shorter than the combined length of the presacral vertebrae. In case of a fused urostyle, it is shorter than the combined length of presacral vertebrae in fossils such as Vieraella and Notobatrachus. So a short