The evolution of blanid amphisbaenians (Mediterranean worm lizards) is mainly inferred based on molecular studies, despite their fossils are common in Cenozoic European localities. This is because the fossil record exclusively consists in isolated elements of limited taxonomic value. We describe the only known fossil amphisbaenian skull from Europe – attributed to Blanus mendezi sp. nov. (Amphisbaenia, Blanidae) – which represents the most informative fossil blanid material ever described. This specimen, from the Middle Miocene of Abocador de Can Mata (11.6 Ma, MN7+8) in the Vallès-Penedès Basin (Catalonia, NE Iberian Peninsula), unambiguously asserts the presence of Blanus in the Miocene of Europe. This reinforces the referral to this genus of the previously-known, much more incomplete and poorly-diagnostic material from other localities of the European Neogene. Our analysis – integrating the available molecular, paleontological and biogeographic data – suggests that the new species postdates the divergence between the two main (Eastern and Western Mediterranean) extant clades of blanids, and probably precedes the split between the Iberian and North-Western African subclades. This supports previous paleobiogeographic scenarios for blanid evolution and provides a significant minimum divergence time for calibrating molecular analyses of blanid phylogeny.

ACM localities are situated in the area of els Hostalets de Pierola, which displays thick Middle to Late Miocene alluvial sequences. They were deposited in distal-to-marginal, inter-fan zones of the coalescing alluvial fan systems of els Hostalets de Pierola and Olesa [42] . More than 250 localities have been defined along the ACM composite series (ca. 250 m in thickness), which can be accurately dated based on lithostratigraphic, magnetostratigraphic and biostratigraphic correlation [34] , [35] , [41] , [43] . The whole series spans from ca. 12.5 to 11.4 Ma [43] , whereas locality ACM/C8-A4 (from which all the remains reported in this paper come from) is correlated to subchron C5r.2 n, with an interpolated age of 11.6 Ma (late Aragonian, close to the Middle to Late Miocene boundary).

The fossil remains described in this paper come from Abocador de Can Mata (ACM) [33] – [35] . This stratigraphic series is situated in the Vallès-Penedès Basin (NE Iberian Peninsula) – a NNE-SSW-oriented half-graben limited by the Littoral and Pre-littoral Catalan Coastal Ranges, which was generated by the rifting of the NW Mediterranean region during the Neogene [36] – [39] . Except for some Early and Middle Miocene shallow marine and transitional sequences, most of the basin infill consists of marginal alluvial fan sediments with a rich fossil record of Early, late Middle and Late Miocene terrestrial vertebrates [40] , [41] .

The fossorial adaptations of amphisbaenians [2] are reflected in their cranial and postcranial osteology, thus facilitating their recognition in the fossil record, even if only disarticulated material is available. In Europe, the presence of a single family (Blanidae), at least regarding the Neogene, enables an easy identification at least at this level. Most findings consist in vertebrae or, more rarely, isolated tooth-bearing skull bones. The former, given the uniformity in postcranial anatomy of amphisbaenians [19] , do not enable an attribution below the family level; the latter, in turn, display a rather uniform morphology from the Oligocene onwards (only members of Blanidae are represented) and provide restricted taxonomic information. Such a morphologic homogeneity, coupled with the high intraspecific variability inferred from some extant species, hinders the identification at the species level of most isolated fossil remains. The much more informative, but tiny and fragile, skulls of amphisbaenians are only rarely preserved. Thus, although some crania are known from the Cenozoic of North America [20] – [24] and Africa [25] , in Europe a single cranial specimen from a putative stem amphisbaenian is known from the Eocene [9] . This preservational bias explains why, for extinct blanids, only three species of two different genera are currently recognized (on the basis of lower jaws): Palaeoblanus tobieni, from MP27-MN13 of France, Germany, Italy and Spain [26] – [28] ; Blanus antiquus, from the MN3–MN6 of Austria and Germany [29] ; and Blanus gracilis, from the MN2–MN4 of the Czech Republic and Italy (and, with doubts, from the MN7+8 of Romania) [30] – [32] .

The divergence of the various amphibaenian extant clades has been mainly related to vicariance events [5] , [14] . Intercontinental oceanic dispersal events might have also occurred, as indicated by the purported sister-taxon relationship between the Mediterranean Blanidae and the Caribbean Cadeidae [13] , although more recent results indicate that such relationship is uncertain [8] . With regard to Mediterranean worm lizards, molecular data consistently distinguish three extant clades of current disjunct distribution [15] : an Eastern Mediterranean clade (Blanus strauchi) [16] ; an Iberian one (Blanus cinereus, and possibly the recently described cryptic species Blanus mariae – but see ref. [17] ); and a North-Western African one (Blanus mettetali and Blanus tingitanus). Molecular evidence indicates that Iberian and North-Western African clades are more closely related to each other, with the Eastern Mediterranean clade having diverged first [12] . Molecular estimates of the divergence time among clades is mainly based on paleobiogeographic assumptions [12] , due to the restricted information provided by the European fossil record of amphisbaenians, in spite of its relative abundance throughout the European Cenozoic [18] .

Amphisbaenians (worm lizards) constitute a poorly understood clade of burrowing and usually completely limbless squamates [1] , [2] . Both molecular [3] – [8] and paleontological data currently indicate that amphisbaenians are the sister-taxon of lacertids, so that the former's limbless condition evolved independently from snakes. Amphisbaenians and lacertids probably diverged during the Late Cretaceous [9] , although worm lizards are only undoubtedly recorded from the Paleogene onwards [10] . Among the 150–190 species of extant amphisbaenians [2] , [11] , most of them inhabit the southern continents (Afro-Arabia and South America), and only a few species are distributed in the Mediterranean region. Apart from Trogonophis wiegmanni (Trogonophidae), all extant Mediterranean amphisbaenians are included in the genus Blanus – previously allocated to the Amphisbaenidae, but currently included into a more basal family of their own, the Blanidae, both on the basis of molecular and morphologic evidence [2] , [8] , [12] , [13] .

The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “ http://zoobank.org/ ”. The LSID for this publication is: urn:lsid:zoobank.org:pub: 062AC1C9-86C7-4271-B7A4-056F1DBA52A4. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central, LOCKSS.

IPS60464 was scanned on a GE phoenix v|tome|x s180 (GE Measurement & Control Solutions, Hanover, Germany) at the American Museum of Natural History (AMNH) using a nanofocus X-ray tube with the following parameters: voltage 105 kV and current 70 mA and a magnification of 15.86723491. We obtained 1100 slices with slice thickness of 0.2 mm and a pixel size of 0.01260459 mm. The raw data were imported to VG Studio Max 2.1 and exported to Avizo 7.0 for analysis, segmentation, and visualization. We segmented each bone slide by slide and deleted the covering crust and the infilling matrix present in the original fossil by considering the different densities of bone, crust and sediment in Avizo 7.0.

No permits were required to carry out this study, since the described fossil specimens (see catalog numbers below) are adequately curated at Institut Català de Paleontologia Miquel Crusafont. The fossils were recovered by Josep M. Méndez, a technician of this institution, by screen-washing sediments previously excavated in 2011, in the course of a paleontological excavation directed by one of the authors (Josep M. Robles), under a permit (437 K121 N352 2011-1/6509) issued by the Servei d'Arqueologia i Paleontologia of the Generalitat de Catalunya (Catalan local government).

Regarding extinct taxa, the new species differs from B. antiquus in the larger size and more heterodont dentition (greater variability in the height and robustness of the teeth); and from B. gracilis, in the much larger size, the more robust tooth-bearing bones and teeth, and the more closely packed teeth. The new species also differs from all extant Blanus spp. in the larger size and – as far as it can be ascertained for those species for which cranial osteology is known (B. cinereus and B. strauchi) – in the longer nasal process of the premaxilla, the relatively longer frontals compared to the rest of the skull, the more straighter suture between the frontals, the more developed frontal articular facet for the maxilla and prefrontal, and the presence of a longer and posterodorsally directed maxillary orbital process. Additionally, the new species further differs from B. strauchi in the less protruding snout lacking a ventrally-projected proximal tip of the premaxilla, as well as in the stouter teeth; and from both B. cinereus and B. strauchi, in the stronger interdigitation of the frontoparietal suture. The paracotylar tubercles of the cervical and trunk vertebrae are unknown in the rest of Blanus spp., but a similar structure might be present in B. gracilis.

Large-sized species of Blanus with a slightly protruding snout. Dentition heterodont, with robust pleurodont teeth (seven premaxillary, five maxillary, eight dentary), the first dentary tooth being smaller than the third one. Tooth-bearing bones robust. Nasal process of the premaxilla long. Frontals long relative to the skull, with an almost straight suture between them, and a well-developed facet for articulation with the maxilla and the prefrontal; frontoparietal suture strongly interdigitated. Long, acuminated and medially-directed orbital process present in the maxilla. Premaxilla anteriorly (not ventrally) projected. Cervical and anterior trunk vertebrae with paracotylar tubercles.

(A–C) Premaxilla in left lateral (A), dorsal (B), and ventral (C) views. (D–G) Right maxilla in labial (D), lingual (E), dorsal (F) and ventral (g) views. (H, I) Right nasal in dorsal (H) and ventral (I) views. (J, K) Right vomer in dorsal (J) and ventral (K) views. (L, M) Right palatine in dorsal (L) and ventral (M) views. (N, O) Right ectopterygoid in labial (N) and anterior (O) views. (P, Q) Right pterygoid in dorsal (P) and ventral (Q) views. (R–U) Right frontal in dorsal (R), ventral (S), lateral (T) and medial (U) views. (V, W) Right prefrontal in lateral (V) and medial (W) views. (X–Z) Parietal/otic-occipital complex/parabasisphenoid in dorsal (X), right lateral (Y) and ventral (Z) views. (AA, AB) Left quadrate in lateral (AA) and medial (AB) views. Colors correspond to those in figure 1 . Abbreviations: app, apical process of parietal; appr, alar process of prootic; bps, basipterygoid process; cec, cephalic condyle of quadrate; chp, choanal process of vomer; chv, choanal vault; cp, cultriform process of parabasisphenoid; dcr, dorsal crest of quadrate; dp, descending process of frontal; epm, ectopterygoid process of maxilla; epp, ectopterygoid process of palatine; fnpp, frontal facet for the nasal process of premaxilla; fpm, frontal process of maxilla; fpn, frontal process of nasal; fpp, frontal process of prefrontal; fps, frontoparietal suture; fvo, fenestra vomeronasalis; fvp, facet for palatine vomerine process; Gf, Gasserian foramen; hf, hypoglossal foramen; lptp, lateral pterygoid process of ectopterygoid; mac, mandibular condyle of quadrate; mafa, ectopterygoid facet for the articulation of the ectopterygoid process of maxilla; map, median articular plane; mfo, maxilla labial foramina; mfp, maxillary facial process of nasal; mp, maxillary process of palatine; mpf, maxillary process of frontal; mpp, maxillary process of prefrontal; mrp, maxillary rostral process of nasal; mt, maxillary teeth; mptp, medial pterygoid process of ectopterygoid; mf, frontal facet for maxilla and prefrontal; nc, nasal chamber; np, nassal process of premaxilla; npk, nasal process of premaxilla keel; occ, occipital condyle; oocl, otic-occipital lapet; op, orbital process of maxilla; osp, ventral process of frontal; pa, parietal; paf, frontal facet for parietal; pfa, facet of frontal for the nasal process of premaxilla; pbs, parabasisphenoid; pff, frontal facet for prefrontal; pfp, prefrontal process of maxilla; pmf, premaxilla foramina; pmp, premaxillary process of nasal; pmt, premaxillary teeth; pp, palatal process of premaxilla; ptfa, ectopterygoid facet for pterygoid; ptp, pterygoid process of palatine; pvp, posteroventral process of quadrate; qp, quadrate process of pterygoid; rpm, rostral process of maxilla; rpv, rostral process of vomer; saf, superior alveolar foramen; ss, supradental shelf of maxilla; tp, transverse process of pterygoid; vf, vagus foramen; vlp/X, ventrolateral process/“element X”; vp, vomerine process of palatine. Scale bar equals 2 mm.

Both cervical and trunk vertebrae are preserved ( Fig. S1 ). They are all procoelous. The cervical segment is represented by four fragmentary vertebrae encrusted by a concretion that keeps them together ( Fig. S1A –D). Their morphology is barely visible, but the referral to an amphisbaenian is supported by the following features: neural arch without neural spine; presence of a hint of prezygapophyseal processes; large and protruding synapophyses; and centra proportionally very short and narrow, slightly convex ventrally, and provided of a small hypapophysis. Moreover, the cervical vertebrae have neural arches with a truncated posterior tip and small paracotylar tubercles well separated from the large synapophyses. The remaining 45 isolated trunk vertebrae represent all trunk sectors and display a variety of morphologies and length ( Fig. S1E –X). These vertebrae are rather large, with a centrum length (from the ventral edge of the cotyle to the posterior tip of the condyle) varying from 2.0 to 3.1 mm ( Fig. S1E –X). Anterior trunk vertebrae are characterized by being wider and shorter than the posterior ones, with a taller neural arch and at least a hint of paracotylar foramina. Trunk vertebrae are otherwise characterized by the following morphology. In dorsal view, the prezygapophyses are prominent and developed in anterolateral direction; the prezygapophyseal facets are roundish or vaguely drop-shaped; the prezygapophyseal processes are small and stout (preserved only in few cases); the interzygapophyseal constriction is distinctly developed; the anterior edge of the neural arch is convex, whereas the posterior edge is notched (the median notch is delimited on both sides by a small convexity); the dorsal surface of the arch is thickened in the area surrounding such median notch, forming in some cases a ridge with the shape of an inverse V; the neural spine is absent, but a sort of sagittal ridge is developed in all the cases. In ventral view, the lower rim of the cotyle is regularly concave and posteriorly placed as compared to the dorsal rim; the most anterior trunk vertebrae show small paracotylar tubercles, which are regularly absent in the other vertebrae; the prezygapophyses are anterolaterally directed and show at least a hint of their process also in the cases in which they are not visible in dorsal view; the synapophyses are roundish and laterally protruding; the centrum is variably eleongated (especially in the most posterior vertebrae); the ventral surface of the centrum is rather flat and well delimited by straight or slightly concave lateral edges; two foramina pierce the ventral surface of the centrum in its anterior quarter; the cotyle surface is only minimally visible; the postzygapophyseal facets are elongated and drop-shaped. In lateral view, the neural spine is regularly absent; the dorsal edge of the neural arch can be variably concave – more concave in the anterior vertebrae, nearly straight in the most posterior ones – but is often flat close to the posterior edge (where the above-described V-shaped ridge is developed); the synapophyses are massive and globular; there are no lateral foramina; the boundary between the lateral and ventral surface is neat and corresponds to the ventral edge (there is no gradually sloping lateral surface); the dorsal edge of the dorsoventrally depressed condyle is placed much more anteriorly than the ventral edge. In anterior view, the cotyle is distinctly dorsoventrally depressed, oval with a nearly straight ventral rim; the neural canal is generally small and triangular; the dorsal edge of the neural arch is distinctly convex and in some cases tectiform and apically pointed; the zygosphene is regularly absent; the prezygapophyseal facets are distinctly tilted in dorsolateral direction; the synapophyses are massive and laterally protruding. In posterior view, the shape of the condyle matches that of the cotyle; the neural canal is wider than in anterior view; the posterior edge of the neural arch is markedly depressed and medially flat or nearly so; there is no evidence of zyganthra, but in some cases the dorsal surface of the medial edge of the postzygapophyseal facet delimits a small concavity along with the ventral surface of the neural arch; the ventral edge of the postzygapophyseal facets is tilted in dorsolateral direction.

The right lower jaw ( Figs. 2B and 4A–D ) is complete and in articulation with the quadrate. The dentition is pleurodont and closely packed. The dentary ( Fig. 4A–E ), short and robust, bears eight teeth: the third tooth is the largest, whereas the fourth and the last ones are the smallest. The first tooth is not particularly enlarged, especially when compared to the third, which is clearly the largest. The symphysis shapes a marked angle with the ventral border of the dentary, which is roughly straight, only with a slightly convex central region. The subdental shelf has a high and rather rounded lingual surface ( Fig. 4E ). The Meckelian canal is open throughout all of its length ( Fig. 4E ), although it is posteriorly covered by a rather large splenial preserved in anatomical connection ( Fig. 4A ). A fused intramandibular septum (note that the homology of this element with those of anguids has been called into question, and it has been regarded as absent in other amphisbaenians [45] )( Fig. 4E ), covered by the anterior process of the coronoid and the anterior portion of the surangular/articular, closes the region between the posteroventral margin of the subdental shelf and the dorsal margin of the Meckelian canal. There are three large labial foramina situated at the levels between the first and second tooth, between the third and fourth, and under the sixth ( Fig. 4B ) Posteriorly, the dentary bears three different structures ( Fig. 4B, E ): a dorsally-positioned coronoid process, which is higher than wide and rather long; a surangular process that reaches a slightly more posterior position; and an angular process that marks the posterior-most point of the dentary. The postdentary region ( Fig. 4A–D ) is shorter than the dentary, but not as reduced as in other amphisbaenians, such as for example Diplometopon [44] . In contrast to most amphisbaenians [53] , the postdentary bones do not constitute a compound bone ( Fig. 4A–D ). The splenial and the angular can be distinguished, but the articular and surangular are more difficult to separate in the CT scan, suggesting they probably represent a compound bone. The retroarticular process ( Fig. 4A–D ) is present, posteriorly directed, and not enlarged. The lower jaw has a dorsally-arched postdentary ventral region ( Fig. 4A, B ).

Both quadrates are preserved ( Figs. 2A–F and 3AA, AB), the right one in articulation with the lower jaw ( Fig. 2A, B ). They are robust, and their dorsal articulation contacts the otic capsule, whereas a reduced mandibular condyle articulates with the lower jaw ( Figs. 2A, B and 3AA, AB). The presence or absence of the squamosal is difficult to ascertain, but this is not unexpected, as this bone is barely identifiable even in extant specimens, ant the same applies to the epipterygoid.

The orbit is formed by a small anterior portion of the parietal and the tabulosphenoid, the lateral margin of the frontal, the prefrontal, a small posterior portion of the maxilla and the dorsal margin of the ectopterygoid ( Fig. 2A, B ). The tabulosphenoid only preserved on the left side, is a paired (or unpaired but broken in its midline) element situated dorsally from both the palatine and pterygoid; it contacts anteriorly with the posteroventral margin of the descending process of the frontal ( Fig. 2C, D ). It is possible that the parabasisphenoid is co-ossified ( Figs. 2D and 3Z ), although this bone sometimes appears disarticulated in Blanus specimens (this could also be related to a younger ontogenetic age of the accessed specimens). The orbital rim is incomplete posteriorly, due to the lack of a jugal ( Fig. 2B ).

The occipital condyle is bicipital, and connects to the basioccipital plate through a rather wide neck. The foramen magnum is bordered by the exoccipitals and supraoccipital, the latter presenting a wide dorsoposteriorly positioned notch almost reached by the posterior margin of the parietal. The alar process of the prootic is rather long, and the paroccipital processes are laterally oriented. Although some additional elements have been identified (e.g. vagus foramen, ventrolateral process/“element X”, hypoglossal foramen), the description of their morphology is precluded by the poor preservation of the region and/or a lack of resolution of the CT-Scan.

The unpaired parietal ( Figs. 2A–C and 3X–Z ) is long, more than twice the length of the frontals. It displays a dorsal protuberance ( Figs. 3X ) that marks the beginning of what might represent an incipient sagittal crest – in fact, the latter is observable in the CT sections, in spite of not being clearly expressed on the surface. The lateral walls of the parietal are vertically developed, being closed by the frontals anteriorly, the tabulosphenoid (sensu [54] , orbitosphenoid of [44] ) anteroventrally, the parabasisphenoid ventrally, and the otic-occipital complex posteroventrally ( Figs. 2C, D and 3Y ).

The frontals are paired, with a rather straight suture between them, and a strongly interdigitated suture with the parietal ( Figs. 2A and 3R–U ). These bones are almost three times longer than wide, and the long nasal process of the premaxilla precludes the dorsal contact between the two frontals for at least one third of their length ( Figs. 2A and 3R, U ). However, the frontals are in contact below the nasal process of the premaxilla, and have a well-marked facet to receive it ( Fig. 3R, U ). Posteroventrally, they show strong and ventrally-directed (descending) processes ( Fig. 3S–U ), which meet each other in the midline and contact the tabulosphenoid posteriorly ( Fig. 2D ). The suture between the frontal and the nasal, which has been slightly displaced below the frontal, is arched ( Fig. 2A ). The frontal contacts the maxilla, separating the large prefrontal from the nasals ( Fig. 2A, B ). The dorsolateral surface of the frontals bears a marked facet for articulation with the maxilla and prefrontal ( Figs. 2A and 3R ).

The maxilla ( Figs. 2 , 3D–G ), only preserved on the right side, bears five robust and only weakly curved teeth, the second one being the largest, and the first one the smallest. The reduction of the first maxillary and the most lateral premaxillary teeth allows for the necessary space to accommodate the enlarged third dentary tooth when the mouth is closed ( Fig. 2B ). Distalwards from the second tooth, there is a reduction in maxillary tooth height. The supradental shelf is wide, and the sulcus dentalis is apparently lacking or only slightly developed ( Fig. 3G ). The superior alveolar foramen is situated at the level of the distal margin of the last tooth ( Fig. 3E ). The maxilla contacts the premaxilla and the maxillary rostral process of the nasal through a rather wide and medially directed rostral process (premaxillary process of the maxilla in [45] ) ( Fig. 3D–G ), as well as the frontal and prefrontal bones in its dorsal and posterior margins, respectively ( Fig. 2A, B ). The orbital process, situated dorsolabially, is relatively long ( Fig. 3D–G ) compared to other Blanus species. The dorsal process (frontal process of [45] ) approaches the bifurcated condition seen in trogonophids [53] as well as B. cinereus and B. strauchi ( Fig. 5K–N ), although in the former the prefrontal is absent [44] . The maxilla has a long posteroventrally positioned process (ectopterygoid process; Fig. 3D–G ), which lies ventrolaterally to the anterior extension of the ectopterygoid. Two large foramina pierce the maxilla at the level of the posterior edge of the second and fourth tooth ( Fig. 3D ).

The right septomaxilla ( Fig. 2C ) is present and appears rather simple in structure, although it should be taken into account that some processes formed by thin bone may have been either not preserved or artificially deleted during the CT-scan processing. This is supported by the fact that the septomaxilla does not contact the surrounding bones, whereas it should be in contact with the premaxilla, maxilla and/or nasal. The general ventrally convex shape of the septomaxilla, however, agrees with that of Blanus cinereus according to the material figured in the literature [47] and examined in the comparative sample.

The azygous premaxilla ( Fig. 3A–C ) bears seven tooth positions; the central one is greatly enlarged, and all of them are robust and cylindrical. This is evident even considering the poor preservation of the central tooth and the right lateral teeth being broken at different levels. It is not possible to discern whether the lateral teeth were much shorter than the others, but a moderate decrease in size is suggested by the CT sections. The nasal process of the premaxilla is broad and very long, slightly tapering dorsally and with a subtle waisting at its base. The inner surface of the nasal process is provided with a prominent and long medial keel ( Fig. 3C ). The anterior external surface is pierced by two large foramina having their exit on the inner side (longitudinal canal of [45] ). The nasal process of the premaxilla precludes the dorsal contact between the nasals and that of the frontals in their anterior third ( Fig. 2A ). The poorly developed palatal process laterally contacts the rostral process of the maxilla. The palatal process probably contacted the vomer in its original position, but displacement or incomplete preservation of the latter results in the lack of contact in the fossil ( Fig. 2D ). The supradental platform is horizontal and thin, and displays a central notch.

Both the neck and anterior trunk vertebrae ( Fig. S1 ) show the typical amphisbaenian morphology (i.e., dorsoventrally flattened and without neural spine) (e.g. [51] ). They are however further characterized by the presence of paracotylar turbercles, which are unknown from other Blanus spp. The largest vertebrae of the new species, in agreement with skull size, are slightly larger than the largest Neogene Blanus vertebrae reported so far [50] , [51] , also much larger than those of extant species – at least regarding B. cinereus and B. strauchi (AB pers. obs.), since these are unknown for B. mettetali or B. tingitanus, although these two species are reported to be smaller than B. cinereus [52] .

The lower jaw ( Figs. 1B and 3A–D ) displays the typical blanid configuration [2] ; the dentary ( Fig. 4E ), due to its heterodonty, is clearly distinct from those of both Palaeoblanus tobieni and Blanus antiquus, which display a homodont dentition (see figures in refs. [29] , [50] ) mainly regarding tooth height and robustness. The dentary of IPS60464 is much larger than those of B. gracilis, B. strauchi and B. cinereus, but only slightly larger than those of B. antiquus and P. tobieni ( Fig. 6 ).

IPS60464 is an almost complete skull (11.3 mm in length) that includes the right lower jaw in articulation ( Figs. 1 and 2 ). The specimen is exceptionally well preserved, including all unpaired elements, whereas all paired bones are represented at least in one side ( Figs. 2 and 3 ). The skull is however covered by a carbonate concretion that obscures most of its external morphology. It also displays a matrix infilling that precludes the observation of the palate, the inner surfaces of the skull roof and the lingual surfaces of the lower jaw. The small size and fragility of the specimen precluded mechanical preparation, so its description is based on computed tomography (CT) scans. The latter not only revealed the external morphology, but further granted access to the internal cranial morphology (otherwise unobservable), thereby enabling the description of isolated bones and their joint surfaces. A description of the skull and vertebrae ( Fig. 2 and Fig. S1 ) is provided below, followed by comparisons with fossil and extinct blanids.

Results and Discussion

The fossil record of Mediterranean worm lizards Although amphisbaenians are abundant in Paleogene and Neogene localities from Europe, the usually fragmentary nature of the material hinders their identification. The basal phylogenetic position of the Blanidae among the Amphisbaenia points to a long fossil history for the former [3]–[8]. It is therefore possible that blanids were already present in Europe at least by the late Eocene, as suggested by some fossils attributed to Blanosaurus and less certainly to Blanus [55], [56]. Part of the Paleogene material previously referred to indeterminate amphisbaenids [55], [57], [58] is better attributed to indeterminate blanids [10], because the genus Blanus (to which similarities have been pointed) is no longer included in the former family [2]. The only exception regarding the incompleteness of the material is the articulated skeleton of Cryptolacerta from Messel (Germany), interpreted as a stem worm lizard [9]. Also on the basis of fragmentary remains, amphisbaenians other than blanids are present in Paleocene localities from Belgium and France in the form of Polyodontobaena and Camptognathosaurus, both included in the recently described family Polyodontobaenidae [56]. Moreover, uncertainties remain with regard to the attribution of several taxa. Thus, Campinosaurus woutersi – initially described as an anguimorph [59] and later argued to be an amphisbaenian [55], [10] – may not belong to this group, because the tooth count and morphology of the dentary both indicate scincoid affinities [60]. Even more problematic is the purported record in the early Eocene of France [10] of the North American genus Anniealexandria, with important paleobiogeographic implications. Such a referral is doubtful [60], because it is based on the presence of nine dentary teeth – a diagnostic character of this genus [10], which is seldom present in other genera. There is however some variability in the number of tooth positions among extant amphisbaenians. Thus, a count of nine dentary teeth has been also reported for several species of Amphisbaena, such as Amphisbaena fuliginosa [61], and it is also observable in the Amphisbaena alba figured in the literature [62], [63]. We further report a posteriorly located ninth small tooth in an extant Blanus strauchi dentary from Vic Siirt (Turkey) in the S. Bailon personal collection. In Amphisbaena alba, the replacement and the replaced tooth sometimes coexist [64], so that apparently increased counts (from typically eight to nine dentary teeth) might be related to the temporary retention of an old replaced tooth with the new, replacement one. It is also possible that ontogenetically older specimens, possessing longer dentaries, might accommodate a larger number of teeth. Although this should be confirmed through the study of ontogenetic series, dental counts are likely to be related to ontogenetic stage, so that adult, large squamate individuals would give more reliable tooth counts [65]. Unfortunately, the ontogenetic stage is usually difficult to ascertain from fossil specimens. Given these considerations, the referral of European material to the North-American genus Annialexandria might be incorrect [60], being alternatively attributable to an indeterminate amphisbaenian (?Blanidae) with nine dentary teeth. The taxonomic status of other amphisbaenian genera from the Paleogene of Europe is also unclear. Omoiotyphlops priscus, from the Phosphorites du Quercy (Eocene or Oligocene from France) [66], is currently considered a nomen dubium, because it is based on few, undiagnostic vertebrae [55], [67]. Louisamphisbaena ferox from Grisolles (latest middle Eocene, France), in turn, is arguably a blanid [10], but the taxonomic validity of this genus is unclear, since the reported presence of a second curved tooth in the maxilla and the widely spaced teeth in the dentary do not enable a clear-cut distinction from Blanus. Moreover, no comparison to Palaeoblanus tobieni was made in the original description, despite sharing with the latter an enlarged first tooth – although Louisamphisbaena certainly lacks other characters of Palaeoblanus. Among the late Paleogene amphisbaenians, the monotypic blanid genus Palaeoblanus [50] is more clearly diagnosable than the other above-mentioned genera. This genus, originally described from the Miocene of Germany [50], has been also identified from the late Oligocene and Miocene of France, Germany, Italy and Spain [26]–[28], [68]. Palaeoblanus was not included in the Blanidae when the family was erected [2]. This is probably due to the poorly informative material referred to Palaeoblanus and the uncertainty of this distinctiveness of this genus from Blanus, rather than to any evidence against Palaeoblanus belonging to this family. Dentaries of Palaeoblanus possess a distinctly larger first tooth [26], [50], a more homogeneous and blunt dentition, and a more rounded symphysis than species of Blanus. On the basis of these features, we therefore support the distinct generic status of Palaeoblanus. At the same time, we support the ascription of Palaeoblanus to the Blanidae, thus representing the only extinct blanid genus recorded from the Neogene. A potential, currently unnamed, second species of Palaeoblanus has been reported from the Middle Miocene of Sandelzhausen (Germany) [68], based on the divergence of the lateral teeth. Such feature is however doubtful, because we found several specimens of B. cinereus (e.g., MDHC 156) with the same morphology – which is variable intraspecifically, and hence of no taxonomic value for diagnosing species. Moreover, the features purportedly justifying the referral of this material to Palaeoblanus – the proportion of the lateral teeth and the relatively larger size of the premaxillary foramina [68] – are insufficient to discount an alternative attribution to Blanus of the Saldenzhausen blanid material, which is best referred to as Blanidae indet. The French records at Mas de Got and Pech Desse [30] correspond to a large form with homodont, blunt teeth, most probably representing MP22 and MP28 records of Palaeoblanus. Besides Palaeoblanus tobieni, only two extinct species of Blanus – B. antiquus and B. gracilis, from several German and Czech localities [29], [30] – are recognized in the Miocene. Even though similarities with the extant genus Blanus were noted, Blanus gracilis was originally attributed to a different genus, Omoiotyphlops [30], which is currently well established as a junior synonym of Blanus [69]. In fact, B. gracilis and B. antiquus have been considered synonymous by some authors [26], [69], in which case the nomen B. gracilis would have priority [31]. However, the smaller size, slenderer dentary and teeth, greater interdental space, and more heterodont dentition of B. gracilis compared to B. antiquus support their different species status. As it is evident in the corresponding drawings of Figure 6, B. strauchi and B. gracilis are much more similar to each other than to either B. cinereus or B. antiquus. Material from Sansan [69] clearly shows that two different forms are present in the same locality. Although similarities to B. gracilis and B. antiquus were noted for the smaller form, referred to Blanus sp. [69], in fact it shows greater similarities (mainly regarding the robust, heterodont and closely-packed dentition as well as the robustness of the dentary) with B. mendezi sp. nov. The slightly larger form (dentary length of 7 mm), left unassigned at the genus level, resembles instead Palaeoblanus (blunt crowns, rather homodont dentition, and rather rounded symphysis) [69]. There is no morphologic evidence that the above-mentioned extinct species of Blanus already belong to any of the several clades identified by molecular studies among the extant taxa [12], [49]. In contrast, fossil remains from Pliocene, Pleistocene and Holocene deposits of Western Europe (mainly Iberian Peninsula and Southern France) have been attributed to the extant B. cinereus [51], [70], [71]. Material from the latest Pliocene of Casablanca (Morocco), in turn, was referred to Blanus sp. [72]. Given that this locality is comprised within the present distribution range of B. mettetali, and very close to that of B. tingitanus, it is likely that these remains belong to one of the two extant species of the North-Western African clade. The same situation applies to the Pliocene record of an indeterminate amphisbaenian from Turkey [73], which might potentially belong to B. strauchi – or to an extinct species closely related to the latter from the Eastern clade.

The amphisbaenian fossil record in the Iberian Peninsula According to the available literature, amphisbaenian fossil remains from the Iberian Peninsula are not particularly abundant. However, if it is taken into account that Paleogene and Neogene herpetofaunas from this area remain understudied, this fact seems to be largely a sampling artifact that does not reflect a real absence. With regard to the Paleogene, amphisbaenians have been described from the Early Eocene of Silveirinha [58], the late Eocene of Sossís [60] and the Oligocene of Montalbán [74]. Despite the rather fragmentary nature of the described remains, there is no clear evidence that these Paleogene specimens belong to an amphisbaenian group other than the Blanidae [60]. The Iberian Neogene record is substantially better than that from the Paleogene, although the material described so far is quite scarce. Miocene amphisbaenian remains have been reported from the Early Miocene of Córcoles, the Middle Miocene of Tarazona de Aragón, and the late Miocene of Can Missert, Los Valles de Fuentidueña, Viladecavalls, Can Llobateres and Bacochas, among other localities [27], [75], [76]. The possible presence of an amphisbaenian skull was reported decades ago from Viladecavalls [75], but the specimen was never described and it is currently lost (we were unable to locate it among the collections of the ICP). Amphisbaenian records from Iberian Plio-Pleistocene localities are more numerous [51], [70], [71], [77], including currently undescribed material [27]. In Iberia, Palaeoblanus has been reported from several Early to Middle Miocene localities (MN3–MN6) [27], but material has never been described or figured. The numerous blanid records from Spain (and France) reported in ref. [27], mainly based on undescribed material, show that Miocene remains are generally attributed to either Blanus sp. or Palaeoblanus sp., whereas the Plio-Pleistocene material is customarily attributed to B. cinereus. This higher taxonomic resolution for the more recent material is not attributable to a better knowledge (more complete preservation and/or higher number of recovered specimens), but related to the fact that researchers are more cautious when referring Miocene material to an extant species. The referral of Plio-Pleistocene Iberian remains to B. cinereus is further complicated by the recent description, mostly on molecular grounds, of the cryptic sibling species B. mariae, which would be morphologically very similar to B. cinereus [49]. Besides molecular differences, B. mariae has been reported to display a slightly larger size and some external morphologic differences compared to B. cinereus, but further research is required to confirm the distinct taxonomic status of the former as a distinct species instead of a subspecies of the latter – especially because it is unknown whether such differences are maintained or not in their contact zone [17]. The currently lack of osteological data for B. mariae seriously hinders the identification of Plio-Pleistocene Iberian blanids at the species level.

General discussion Blanus mendezi sp. Nov. The described skull, IPS60464, represents the most informative blanid fossil material ever described. Both the general configuration of the skull and the dental morphology of IPS60464 are in accordance with those of extant blanids, represented by the single extant genus Blanus. Similarities include: the tooth count (premaxilla: seven; maxilla: five; dentary: eight); the morphology, proportions and arrangement of skull bones (see above); and the shape and arrangement of the sutures – for a description of the cranial osteology of B. cinereus and B. strauchi, see ref. [47] and also below. Truncated nasals such as those displayed by IPS60464 (Fig. 3H, I) are the only diagnostic cranial features of blanids reported in the literature [2]. This character is unknown for fossil purported blanids, so that the ascription of isolated fossil material to this family has been mostly based on its overall similarity with the extant species of Blanus. Despite a recognized similarity to the genus Blanus, Palaeoblanus has not been formally referred to Blanidae – it was not mentioned in the erection of the family [2], and it was referred to the Amphisbaenidae by other authors [26], [27]. In contrast, we refer Palaeoblanus to the Blanidae on the basis of dentary morphologic similarities. IPS60464 differs from the extinct Palaeoblanus in lacking an enlarged first dentary tooth, and in displaying a heterodont and pointed dentition as well as a marked angle at the symphyseal level. These features allow an unambiguous attribution to the extant genus Blanus. IPS60464 therefore unambiguously confirms the presence of this genus in the European Miocene. Moreover, as stated in the differential diagnosis above, the described cranial material differs from the two previously-described extinct species of this genus (B. antiquus and B. gracilis) – known from somewhat older localities [29], [30] – and also from extant Blanus spp. Besides the larger size of the former, differences include several dentognathic and/or cranial features (skull proportions, the shape of some sutures, and various morphologic details of the premaxilla, maxilla, frontals, nasals and dentary), thereby requiring the erection of the new species, Blanus mendezi sp. nov. (see diagnosis above). A more detailed evaluation of the taxonomic status of previously known fossil blanid species is precluded by their incomplete preservation. Thus, whereas tooth-bearing bones easily allow the discrimination between the monotypic genus Paleoblanus and Blanus spp., differences in this regard among Blanus species are subtler. Accordingly, the taxonomic status of both B. antiquus and B. gracilis should be subject to further scrutiny when more complete (cranial) remains become available, although they can be distinguished from B. mendezi on the basis of available evidence. With regard to extant species of this genus, a more detailed diagnosis of B. mendezi is also precluded – not by the morphology preserved in the holotype of the new species, but rather by the partial current knowledge on the osteology of living taxa. Thus, although the cranial morphology of extant amphisbaenians has been reported in several studies [47], [78], only that of B. cinereus and B. strauchi among extant blanids have been described in some detail [47]. In spite of this fact, the material described here sheds new light in the evolution of Mediterranean worm lizards from both phylogenetic and paleobiogeographic viewpoints.