The Arabian Peninsula is comprised of diverse landscapes with massive deserts spanning across most of the interior, enclosed by mountain ranges on the margins of the peninsula (Edgell 2006), and is renowned for its harsh, hot, and arid climate. These diverse landscapes are often inhabited by specialized species/lineages displaying a wide array of unique adaptations to arid conditions. As the dominant inhabitants of arid areas, squamates became exemplary models to investigate biodiversity patterns, phylogeographic assessments, and ecological and evolutionary studies (Camargo et al. 2010). Squamate reptiles from the arid areas of Arabia have been recently shown to harbor more diversity than previously thought. In the past two decades, the growing number of taxonomic, biogeographic, and phylogenetic studies have drastically broadened our knowledge of the Arabian squamate fauna, often presenting many specialized deep lineages, as well as newly formed species. Some studies included revisions of geographically widely distributed genera (e.g., Carranza and Arnold 2012; Metallinou et al. 2012, 2015; de Pous et al. 2016; Tamar et al. 2016a, 2016b, 2018; Šmíd et al. 2013, 2015a; Simó-Riudalbas et al. 2019; Machado et al. 2021). Other studies focused on species complexes or identification and description of new taxa (e.g., Busais and Joger 2011; Metallinou and Carranza 2013; Vasconcelos and Carranza 2014; Machado et al. 2019; Tamar et al. 2019a, 2019b, 2019c; Šmíd et al. 2015b, 2017, 2023; Burriel-Carranza et al. 2023).

Within the agamid fauna of Arabia, the genus Pseudotrapelus Fitzinger, 1843 is restricted to the hilly and mountainous areas enclosing the Arabian Peninsula and the Red Sea (Fig. 1; Arnold 1980; Disi et al. 2001; Baha El Din 2006; Gardner 2013; Tamar et al. 2016b; Bar et al. 2021; Carranza et al. 2021; Šmíd et al. 2021). In previous years, taxonomic work on Pseudotrapelus has been hampered by the conservative and homogeneous morphology of its members. For many years authors followed a conservative approach treating most variations as a single species, P. sinaitus (Heyden, 1827), with several morphological forms or as a species complex (e.g., Anderson 1896, 1901; Arnold 1980; Fritz and Schütte 1988; Schätti and Gasperetti 1994; Baha El Din 2006; Sindaco and Jeremčenko 2008). Within the past decade, four new species have been described and one was resurrected (Melnikov and Pierson 2012; Melnikov et al. 2012, 2013a, 2015; Uetz et al. 2023). Currently Pseudotrapelus consists of six recognized species (Uetz et al. 2023): (i) P. aqabensis Melnikov, Nazarov, Ananjeva & Disi, 2012, from north western Saudi Arabia, through southern Jordan westwards to the Sinai Peninsula; (ii) P. chlodnickiiMelnikov et al., 2015, from Sudan and Egypt, including the Sinai Peninsula; (iii) P. dhofarensis Melnikov & Pierson, 2012, from the mountains of southern Oman and southern Yemen; (iv) P. jensvindumi Melnikov, Ananjeva & Papenfuss, 2013, endemic to the Hajar Mountains of Oman and the United Arab Emirates (UAE); (v) P. neumanni (Tornier, 1905), from the mountains of southern Yemen and southwestern Saudi Arabia; and (vi) P. sinaitus from the Sinai Peninsula in Egypt, eastwards through most of Jordan to southern Syria and northwestern Saudi Arabia (Fig. 1).

Unfortunately, the recent descriptions of four species were based on a small number of specimens, with no comprehensive comparisons among species, and genetic data that were based solely on the mitochondrial COI gene (Melnikov and Pierson 2012; Melnikov et al. 2012, 2013a, 2013b, 2015). This resulted in insufficient diagnostic characters to identify specimens or describe new species (see Tamar et al. 2016b and references therein). These studies thus created much biogeographic uncertainty and taxonomic confusion within Pseudotrapelus.

The comprehensive molecular phylogenetic study of Pseudotrapelus by Tamar et al. (2016b) provided much needed information on the genetic structure of the genus, and the phylogenetic relationships among its members using multiple loci, as well as on the distribution ranges of its species. Tamar et al. (2019a) later revealed cryptic diversity within the genus, with an unnamed lineage from central Saudi Arabia that is distinct in all genetic analyses performed, including both mitochondrial and nuclear species delimitation methods, and is phylogenetically close to P. sinaitus and P. chlodnickii. The absence of detailed morphological comparisons among Pseudotrapelus members, caused by the lack of specimens of this cryptic lineage, hindered its taxonomic evaluation at that time. In this study, we aim to provide an integrative taxonomic assessment of this lineage, including the addition of new specimens from central Saudi Arabia.

In addition to the two specimens reported in Tamar et al. (2019a), we collected two additional Pseudotrapelus individuals from the vicinity of Riyadh in Saudi Arabia (Fig. 1) during a targeted fieldwork in 2019. The specimens were deposited in the National History Museum Prague, Czech Republic (NMP-P6V; see voucher codes in the taxonomic section below and in Appendices 1–3).

The phylogenetic analyses of Pseudotrapelus using BI and ML methods based on the concatenated dataset yielded similar topologies, with most nodes well supported and all recognized species monophyletic (Fig. 2). The phylogenetic structure within Pseudotrapelus recovered two major clades. Clade I (Bayesian posterior probabilities/SH-aLRT/UFBoot support values 1/97.8/98; support values are given in the same order hereafter), though with unsupported internal topology in both the ML or BI analyses, included the recognized species P. chlodnickii and P. sinaitus, and the four samples from the vicinity of Riyadh as the new species described below. Clade II (support 1/98.9/99) included the four remaining species with a sister species relationship between P. jensvindumi and P. dhofarensis (support 0.95/21.3/55), and between P. neumanni and P. aqabensis (support 1/87.7/93), though the topological structure was not supported for the former relationship. In contrast to the concatenated dataset, the phylogenetic tree based on the single barcoding COI marker recovered mostly weak support for the topological structure, apart from the distinction of clade I with strong support (support 1/99.6/99). The BI and ML analyses of the COI dataset recovered similar unsupported topology, though in both methods all recognized species were monophyletic, as well as the new species described herein. The paratype of the new species was recovered in a distinct lineage and did not cluster with any type specimens of the other recognized species. The nuclear networks inferred for the phased c-mos and MC1R alignments (Fig. 3) exhibited no allele sharing between any of the species in the MC1R network, contrasting to the c-mos network, in which allele sharing was present among the phylogenetically close Arabian species of clade II, which may indicate incomplete lineage sorting rather than gene flow among species. The two allele networks yielded a similar pattern of distinct haplotypes for the species of clade I, including the new species described below. Mitochondrial uncorrected interspecific genetic distances among Pseudotrapelus species (Appendix 4) ranged between 2 and 8.6% in 16S and between 11–19.8% in ND4. The lowest distances are between P. aqabensis and P. neumanni (2.2% in 16S and 11% in ND4). The genetic distances between the new species described herein and the other recognized species of clade II ranged between 7.9 and 8.6% in 16S and between 18.2 and 19.8% in ND4, while the distance to the species of clade I, to which it belongs, ranged between 6.3 and 7.0% in 16S and between 15.3 and 15.6% in ND4. The uncorrected interspecific genetic distances among the species in the COI marker indicate that the new species described below presents mostly high distances from all other species (between 13.6% and 15.9%).

A previous phylogenetic study of the agamid genus Pseudotrapelus disclosed an old cryptic lineage in the Arabian Peninsula (Tamar et al. 2019a). That study, using species delimitation analyses and phylogenetic relationships among currently recognized species, highlighted the presence of genetic differentiation and existence of a yet undescribed taxon within this genus. In this study, we investigated this old diversification event that has resulted in the description of a new species within Pseudotrapelus. We used an integrative approach applying phylogenetic analyses with morphological comparisons to assess the interspecific diversity of these agamid species distributed in the mountainous regions of Arabia and northeast Africa. The inferred topology of the concatenated dataset was congruent across analyses and supports the current taxonomy (Tamar et al. 2016b, 2019a). We support the differentiation of a lineage occurring in the mountains of central Saudi Arabia described herein as P. tuwaiqensissp. nov., phylogenetically close to P. sinaitus and P. chlodnickii. According to our phylogenetic analyses and genetic distances, the genetic distinctiveness of P. tuwaiqensissp. nov. is evident at both the mitochondrial and nuclear levels, not clustering with any samples of other members of the genus, as well as owning private alleles in the two nuclear genes analyzed, suggesting no signal of gene flow. Tamar et al. (2019a) included two samples of P. tuwaiqensissp. nov. (the holotype and one paratype) and showed their distinctiveness using concatenated and species tree analyses, and mitochondrial- and nuclear based species delimitation analyses. These results further support the species status of P. tuwaiqensissp. nov.

The current distribution of P. tuwaiqensissp. nov. (specimens genetically identified) is known from around the city of Riyadh and is confined to the Tuwaiq Escarpment in Central Saudi Arabia (sightings are also known from the Ibex Reserve Protected Area located ca. 150 km south of Riyadh). The divergence time estimates of the clade comprising P. tuwaiqensissp. nov., P. sinaitus, and P. chlodnickii was approximately from the late Miocene to the Early Pliocene, around 6 million years ago (Tamar et al. 2019a). The aridification of Arabia, fluctuating climate, and the progression of sandy areas in the Arabian region during the Late Miocene (Edgell 2006; Preusser 2009) might have created distributional restrictions for the ancestral Pseudotrapelus populations, limiting them to the remaining rocky habitats. Although the current geographic distribution of P. tuwaiqensissp. nov. seems to have been influenced by the formation of the sand seas of Arabia (Edgell 2006), it is not possible to draw any firm conclusions about the biogeographic history of the species, especially given the unsupported phylogenetic relationships and the complex and dynamic geological history of the area.

Thanks to the extensive DNA barcoding efforts across Arabia, we have fairly good knowledge on the distribution limits of the individual Pseudotrapelus species across the peninsula. Nonetheless, there are still regions that would require more attention both in terms of fieldwork and genotyping. One such region is eastern Yemen where P. dhofarensis and P. neumanni probably connect. Other such areas are the geographically intermediate, and somewhat isolated, populations in Hail Province in northwestern central Saudi Arabia and near Wadi ad-Dawasir close to the ‘Uruq Bani Ma’arid protected area in the southwest of the country. Both are part of the Arabian Shield rocky desert that stretches from the western coast inland to the Arabian interior and is typical for rugged, rocky outcrops and isolated inselbergs (Edgell 2006), all of which represent an ideal habitat for Pseudotrapelus. It therefore seems likely that they belong to some of the western mountain species, probably P. aqabensis. However, the area around Wadi ad-Dawasir lies at the southern edge of the Tuwaiq Escarpment and it cannot be ruled out that P. tuwaiqensissp. nov. occurs this far south along the ridge of the escarpment. These are, however, mere speculations that will need to be verified by means of DNA barcoding.

Interestingly, in the phylogenetic study of Pseudotrapelus by Melnikova et al. (2015), the authors hinted at the potential existence of an unidentified population in northern and central Saudi Arabia, that should be further taxonomically investigated. Their hypothetical population from Saudi Arabia around Riyadh most likely represents P. tuwaiqensissp. nov. They also implied a possible hybridization between populations of P. sinaitus with that of the central Arabian species in the area of Al Mudawwara in southern Jordan. Our findings indicate no evidence of previous hybridization events between the two species, and propose complete reproductive isolation between P. tuwaiqensissp. nov. and its geographically neighboring species. However, we must bear in mind that both our and Melnikova et al. (2015) results were based on a rather limited number of specimens, and thus more data and denser sampling of specimens and loci is needed to properly assess the distribution ranges and phylogenetic relationships within clade I of Pseudotrapelus.

Regarding conservation implications, considering the probable endemicity of the new species to central Saudi Arabia, its conservation status should be evaluated at national and worldwide levels, taking into account its limited distribution range, population density and trends, potential hybridization with other species, and future urbanization that is planned in the area of its occurrence. Interestingly, our field observations indicate that population densities of P. tuwaiqensissp. nov. are much lower than those of other Pseudotrapelus species (e.g., P. dhofarensis in southern Oman, P. sinaitus in Jordan). In March 2023, a comprehensive field survey was conducted for three weeks by a team of four herpetologists actively searching for reptiles. The survey took place in habitats suitable for Pseudotrapelus within the known range of P. tuwaiqensissp. nov. in the area north of Riyadh. Despite the efforts, the survey yielded the observation of only two individuals.