Ptychadena in Mauritania and the first record of Ptychadena schillukorum

Abstract The study of specimens of the genus Ptychadena of the herpetological collection of the Museo Nacional de Ciencias Naturales de Madrid reveals the first record of Ptychadena schillukorum from Mauritania, extending the known distribution range of the species in West Africa more than 450 km northwards. A key is provided for the four Mauritanian species of Ptychadena to solve problems in identification encountered in previous studies.


Introduction
Ptychadena Boulenger, 1917 is a genus of medium-sized Afrotropical frogs that comprises 50 known living species (Frost 2017) which occur mainly in savannahs and wetlands. Several species of Ptychadena share a similar appearance and show interspecific overlap in morphological variation hampering the identification of diagnostic characters (Poynton 1970, Rödel 2000. Padial et al. (2013) show a map of Mauritania with the known localities of all amphibian species represented except for Ptychadena, where the locations are pooled by genus, due to the ambiguity of its specific identification in literature. However, Guibé and Lamotte (1957) revised the Ptychadena of West Africa providing a morphological key and Bwong et al. (2009) and Dehling and Sinsch (2013) suggested that morphologically similar species of Ptychadena may be identified using quantitative and qualitative morphological data (Zimkus et al. 2017).
Abrupt climate changes and hydrological fluctuations that took place in southern Mauritania during Pleistocene-Holocene, created a dynamic transitional zone promoting dispersal and contraction of distributional areas of Afrotropical species (Gasse 2000;Foley et al. 2003;Brito et al. 2014;Sow et al. 2015). Most amphibians in Mauritania are restricted to the southern part of the country where they inhabit the Sahel savanna ecoregion (including all known Ptychadena). Only Hoplobatrachus occipitalis, Sclerophrys xeros, and Tomopterna cryptotis, are present across the north in the Saharan realm (Padial et al. 2013).
In this note, a new species record of Ptychadena is reported from southern Mauritania based on material of the herpetological collection of the Museo Nacional de Ciencias Naturales of Madrid (MNCN) and on field data obtained by RM.
Morphological characters used in this study for comparison between the species of Ptychadena from Mauritania are: tarsal tubercle, inner metatarsal tubercle, outer metatarsal tubercle, lateral ridge, dorsal ridge, thigh pattern, supernumerary tubercles on toes and webbing extension in the fourth toe (Table 1).
The measure of the range extension of P. schikullorum was taken by airline distance using Google Earth tool, from the northern most locality in West Africa at Niokolo-Koba in Senegal to Aleg in Mauritania.

Results and discussion
The poor conditions in which we found the specimen of P. cf. mascareniensis collected in Mauritania by Padial and De la Riva (Padial and De la Riva 2004) makes its identification difficult due to the lack of morphological characters such as the dorsal and lateral ridges. The existence of three phalanges of the fourth toe free of web and the mottled pattern of thigh makes us question its identity (see Table 1). Unfortunately, the only existing picture provided by J. M. Padial (Fig. 5C) of the same specimen does not allow us to conclude its specific identification as Ptychadena of the mascareniensis group. Genetic samples of it will be analysed to solve this question (in prep.). In this work the criteria of Padial and De la Riva considering the presence of P. cf. mascareniensis in Mauritania are followed.
The morphology of the studied individuals of Ptychadena schillukorum from Mauritania fitted with the description of the species Howell 2006, Rödel 2000): dorsal and lateral ridges are not continuous; the legs are relatively short: tibiotarsal articulation reaches the level of the eye; lack of tarsal tubercles; outer metatarsal tubercles are inconspicuous; the pattern of thigh are speckled (fine vermiculation); the web bing is reduced: 2.5-three phalanges of the fourth toe free of web and no supernumerary tubercles are present in the toes. Tympanum small: its diameter reaches less than 0.7 diameter of the eye. The nearest locality to Mauritania where Ptychadena schillukorum has been reported is in southern Senegal at Niokolo-Koba (Lamotte 1969, Joger andLambert 2002). The record from Aleg in Mauritania extends 450 km northwards the distribution of the species (Fig. 1).
Ptychadena schillukorum ( Fig. 2A, B) inhabits the dry savannahs and grasslands from Senegal to the extreme southern Somalia and also in isolation from Egypt (Baha el Din 2006, Nago et al. 2006) and southwards to east Africa from Kenya to Mozambique (Channing and Howell 2006). The reproduction of P. schillukorum in West Africa takes place in shallow savannah ponds during the rainy season (Nago et al. 2009).    Aleg is located in the Sahel region, characterised by a subtropical dry arid climate with rainy season from July to October. Most individuals were collected in October 1998 by RM, a few days after a (sporadic) event of rainfall that caused the formation of shallow pools and the growth of herbaceous vegetation on the normally barren sand (Fig. 3). Other anuran species that were found concomitantly were Sclerophrys xeros and Tomopterna cryptotis (Fig. 4).
The four Mauritanian species of Ptychadena ( Figure 5) are easily distinguishable from each other based on morphological characters. Ptychadena schillukorum differs from P. trinodis by the lack of tarsal tubercle, smaller inner metatarsal tubercle and inconspicuous outer metatarsal tubercle. Also a fine yellow vertebral line is sometimes present (8 of 29 specimens of P. schillukorum examined) while in P. trinodis a fine white-yellow vertebral line and light colored lateral ridge is generally present. Ptychadena schillukorum differs from P. bibroni by its more compact body, shorter legs, lack of supernumerary tubercles on toe IV, always interrupted dorsal ridges, by its inconspicuous outer metatarsal tubercle and by thigh pattern, that is speckled in P. schillukorum and usually with yellow longitudinal and irregular line in P. bibroni. Finally, P. schillukorum is distinctive from P. mascareniensis by its compact body shape, dorsal and lateral ridges, always interrupted, and shorter legs.
Based on Rödel (2000) and personal data, a key for the Ptychadena species that occur in Mauritania is presented. Studies concerning amphibian fauna in Mauritania are scarce, suggesting that other populations of Ptychadena schillukorum may be discovered in the future. Molecular studies of P. schillukorum should be carried out to know how complex is the species across their distribution (expected to be high or a group of species as suggested by Nago et al. 2006) and also to determine what is the systematic position and taxonomy of the Mauritanian population.