During intensive fieldwork in Oman, in October 2013, we surveyed the easternmost tip of the Rub Al Khali desert (Fig. 2). Individual transects were carried out by five observers and collecting was conducted manually. We collected two specimens of P. khobarensis in two different localities near the border between Oman and Saudi Arabia, in an area of salt-encrusted flats and interdune sabkha (Table 1 and Fig. 1c). The two vouchers collected are housed at Salvador Carranza’s reptile collection at the Institute of Evolutionary Biology, Barcelona, Spain.

Data for the updated distribution map were compiled from Gallagher (1971), Osborne (1994), Martens (1996), Meinig and Kessler (1998), Cunningham (2000), Dakhteh et al. (2007), Fujita and Papenfuss (2011), Gardner (2013), Valdeón et al. (2013). The map was produced by representing coordinates from literature records and by georeferencing figures and extracting point coordinates with ArcGIS 10.0 (ESRI 2010).

A total of five individuals, the two newly collected ones and three additional specimens from the extremes of the species’ range (Fig. 2), were analyzed for variation in the mtDNA. Genomic DNA was extracted from ethanol-preserved tongue tissue samples from the newly collected specimens using the SpeedTools Tissue DNA Extraction kit (Biotools, Madrid, Spain). The mtDNA marker 12S rRNA gene was partially amplified using primers and conditions from Metallinou et al. (2012). Amplified fragments were sequenced for both strands and chromatograph contigs were assembled in Geneious v. R6 (Biomatters Ltd.). The online version of MAFFT v.6 (Katoh and Toh 2008) was used for sequence alignment, applying parameters by default. A median-joining haplotype network was constructed using the Fluxus Phylogenetic Network Analysis software v.4.612 (Bandelt et al. 1999; http://www.fluxus-engineering.com). Uncorrected p-distances between individuals were calculated with MEGA 5 (Tamura et al. 2011).

A series of morphological measurements were performed on the same five individuals, as well as three additional specimens from the locality in coastal Oman belonging to the field series of S. Carranza (Table 2 and Fig. 2) Measurements were taken by the first author on the right side of each specimen (unless defective), using a digital caliper with accuracy to the nearest 0.01 mm. Specimens were sexed by observing presence or absence of hemipenal bulges in adult specimens and measurements were performed as follows: snout-vent length (SVL) measured from tip of snout to vent; head length (HL), measured tip of snout to posterior ear opening border; head width (HW), measured dorsally as the distance between the eyes excluding the eyelid; transverse eye diameter (ED); forearm length (FL), from base of palm to elbow; arm length (AL), from elbow to the insertion of the forelimb on the posterior side; tibia length (BL), measured from base of foot to knee; femur length (ML), measured from knee to the insertion of the hind limb on the posterior side; tail length (TL), from vent to tip of tail; number of upper labial scales (ULS) and number of lower labial scales (LLS).

Analysis of the mitochondrial 12S marker revealed that both newly collected specimens share the same haplotype. Along a 380-bp alignment, there are 3 differences compared to sample IBE-S7746 from Barr Al-Hickman, in coastal Oman (M196 in Metallinou et al. 2012) (0.6% p-distance) and 2 differences compared to either one from Kuwait BEV.10039 and BEV.10040 (M16 and M37 in Metallinou et al. 2012, respectively) (0.3% p-distance). The overall genetic variability based on this marker was 0.4%.

The mean SVL of the specimens measured was 53.87 mm (50.71–61.37, N=7), and did not differ for males (53.92, 51.71–61.37, N=5) and females (53.76, 50.72–56.80, N=2). Tail length measured between 71.1 and 95.1% of SVL.

The Rub Al Khali is the largest desert in Arabia, the largest sand desert in the world and one of the driest (Garzanti et al. 2003, Vincent 2008). It extends across Saudi Arabia with its southern and eastern edges reaching Yemen, Oman and the U.A.E. In Oman, accessibility to this region is limited, and this is mirrored in the paucity of reptile records available, contrasting with the more abundant records from the U.A.E. in areas of similar ecological characteristics (Gardner 2013). We surveyed the easternmost tip of this desert in Oman on the night of 7 October 2013 and we collected two specimens of P. khobarensis in two different localities near the border between Oman and Saudi Arabia (Fig. 2 and Table 1): a male specimen (voucher code IBE-CN7611) (Fig. 1a, b) north of Hasirah oil field, and a juvenile specimen (IBE-CN8073) about 13 km by air east of Sahmah oil filed. In the first locality, the habitat was exclusively salt-encrusted flats, and P. khobarensis was the only species encountered during a 40-minute survey carried out by 5 observers. In the second locality, there was a succession between salt flats and sand dunes (Fig. 1c), with P. khobarensis found on the former and Stenodactylus arabicus on the latter. These records constitute the first inland records of P. khobarensis from Oman and are located almost 250 km from both the eastern records in inland U.A.E. and those in eastern coastal Oman (Fig. 2). The finding of P. khobarensis in this area indicates that its presence in Oman is most probably underestimated, due to the aforementioned difficulty of access to large parts of the inland deserts.

Interestingly, the low variability of the mtDNA observed with the sequenced marker (12S) indicates that there is probably connectivity between populations across its distribution range and corroborates the hypothesis that this species inhabits larger inland areas. Indeed, coastal and inland sabkhas are abundant in eastern Saudi Arabia (Barth 2002) and continental sabkhas are commonly found at the interdune corridors of north-eastern Rub Al Khali (Edgell 2006). This low variability observed in P. khobarensis contrasts with the much higher values observed in some members of the closely related genus Stenodactylus, as calculated based on specimens distributed across similar ranges in Metallinou et al. (2012) and Metallinou and Carranza (2013). Intraspecific variability ranged from 0.9% in S. leptocosymbotes, to 1.6% in S. arabicus and 2.9% in S. doriae.

Based on the measurements performed on voucher specimens in this study, Pseudoceramodactylus khobarensis is shown to have substantially higher maximum SVL than previously documented. One female reached 56.80 mm (BEV.10040) and one male 61.37 mm (IBE-CN7611), both exceeding SVL of the largest specimens measured by Haas (1957) and Arnold (1980). Moreover, numbers of labial scales presented herein (upper 10–13, lower 9–12) seem to be slightly different from the counts given for the type series by Haas (1957) (9/10 upper, 8/9 lower labials) but are in perfect agreement with those by Arnold (1980) who included one specimen from the original description in his examined material. Therefore, this situation can probably be attributed to observer-related discrepancies, rather than an actual difference in counts of this meristic variable.

Pseudoceramodactylus khobarensis is a remarkable desert reptile in that it is the only lizard habitually found on sabkha substrate (Arnold 1977), a habitat almost devoid of vegetation due to extraordinary salinity (König 2012). The species is widespread and classified by the International Union for Conservation of Nature (IUCN) as Least Concern, but a decreasing population trend is observed in parts of its range due to ongoing significant habitat loss through coastal development, especially in the U.A.E. (Sharifi et al. 2012). In this way, it is important to document the species’ distribution and understand its ecological requirements at national and regional scales in order to prevent imperilment in larger parts of its range.