Brachycephalus nanicus Nunes, Lyra, Machado, Carrasco-Medina, Andrade, Haga, Botelho, Pedrozo, Velasco, Jacinavicius, Gray, Blackburn, Kohlsdorf, Muscat & Toledo, 2025, sp. nov.

Brachycephalus nanicus sp. nov. https://zoobank.org/ B7A9AA10-0F6A-41FE-AFE3-AAF78FB87B4D Figure 2 Chresonymy Brachycephalus sp. Toledo et al. 2024 Holotype. Adult male (ZUEC-AMP 25581; field number LaHNAB 251) collected in Trilha do Ouro, Estação Ecológica de Bananal (22S 48’ 14”, 44W 22’ 17”, ~ 1.220m a.s.l.; WGS84 Datum; Figure 3), municipality of Bananal, state of São Paulo, Brazil by L.M. Botelho, L.A.O.S. Ferreira, E. Muscat, M. Moroti, T.M. Machado, and J.P. Bovolon on 28 March 2022. Paratopotypes. ZUEC-AMP 25577–25579, 25610 and 25617 (Figure 4) adult males, and 25580 indetermined sex, all of them collected with the holotype by the same collectors; and ZUEC-AMP 25259, an adult female, also collected in the type-locality by J. Prado on 10 December 2021. Etymology. The specific epithet nanicus an adjective in Latin (meaning those with reduced size) in allusion to the extremely small body size of the adult individuals of this Brachycephalus species (less than 1 cm). Diagnosis. Brachycephalus nanicus can be assigned to the genus Brachycephalus by observed phalangeal reduction, and phylogenetic affinities. Member of the Brachycephalus puri species group (new group proposal), with the content of B. puri, B. nanicus and Brachycephalus sp. 8 (sensu Condez et al. 2020). The new species can be diagnosed from its congeners by the following combination of characters: leptodactyliform body; adult SVL 6.9–9.0 mm for males and 9.2–9.6 mm for females; smooth skin lacking hyperossification; snout rounded in dorsal view; finger II with pointed tips (arrow-shaped); toe I externally absent (lacking phalanges), and toes II, III, IV and V distinct; toe II with rounded tips; brown background with few golden spots scattered aleatory in the general coloration of dorsal surfaces and cloacal region; presence of a dark brown stripe along the flanks and the thigh; yellow blotches on the ventral surfaces; presence of dark brown strips on the legs. The new species is also genetic distinct. Holotype description. Body slender and leptodactyliform (Figure 2); head as wide as long (HL/HW = 1.02%), narrower than the body; head length approximately 34% of SVL; snout shorter than the eye diameter, rounded in dorsal and lateral views; nostrils oval and protuberant, directed anterolaterally; canthus rostralis distinct; loreal region slightly concave; mouth nearly sigmoid; eye slightly protruding in dorsal and lateral views, 38.5% (ED/HW) and 37.5% (ED/HL); tympanum indistinct; vocal sac not externally expanded; vocal slights present; tongue longer than wide, posterior half not adherent to the floor of mouth; choanae relatively small and round; vomerine teeth absent. Arm and forearm slender; arm slightly shorter than forearm (UAL 15.4% and FAL 30.5% of SVL); hands shorter than the forearm and longer than the upper arm; externally, finger I and IV small, vestigial; fingers II and III robust and distinct; relative lengths of fingers I ≤ IV <II <III; inner and outer metacarpal tubercles absent; subarticular tubercles undefined in hand (Figure 2C). Leg relatively long, total leg length 81% of SVL; shank slightly shorter than thigh (SHL/SVL = 35% and THL/SVL = 45.8%); foot longer than tarsus and the shank; toe I externally not visible; toes II, III, IV and V distinct; relative toe length V <II <III <IV; metatarsal tubercles present, both oval, outer slightly larger than inner; subarticular tubercles undefined in foot (Figure 2D). Skin smooth without dermal ossifications. Measurements of the holotype (and the type series) are available in Table 1. Comparison with other species. The leptodactyliform body of Brachycephalus nanicus distinguishes it from B. clarissae and the species belonging to the groups of B. ephippium, B. pernix, and B. vertebralis which have bufoniform phenotype (Almeida-Silva et al. 2021; Condez et al. 2021; Folly et al. 2021, 2024; Lyra et al. 2021; Nunes et al. 2021; Mângia et al. 2023). Brachycephalus nanicus adult female size (SVL 9.55 mm) is smaller than that of B. clarissae (female SVL 10.1–10.9 mm; Folly et al. 2022), B. didactylus (female SVL 10.2 mm; Izecksohn 1971), B. tabuleiro (female SVL 10.9–12.7 mm; Mângia et al. 2023), and B. puri (female SVL 10.2 mm; Almeida-Silva et al. 2021). Brachycephalus nanicus can also be distinguished from B. clarissae by the smooth dorsal skin [dorsum with a median row of small light-colored (yellow in life, cream in preservative) tubercles; Folly et al. 2022], HL/SVL = 34%, SHL/SVL = 35%, and abdominal muscles transparent also can distinguish Brachycephalus nanicus from B. dacnis (HL/SVL = 23.6%, SHL/SVL = 40.6%, and muscles with a slight dark-brown coloration – melanocytes; Toledo et al. 2024). Brachycephalus nanicus has toe I externally not visible, and toes II, III, IV and V distinct. This condition distinguishes the new species from B. didactylus (toe I not visible, toes II and III distinct and functional, and toe IV and V vestigial; Izecksohn, 1971; Napoli et al. 2011), B. hermogenesi (toes IV [externally not visible] and V reduced; Napoli et al. 2011), B. pulex (toes I, II, and V not visible, and toes III and IV distinct and functional; Napoli et al. 2011), and B. puri (toe I not visible, II, III, and IV functional, and V vestigial; Almeida-Silva et al. 2021). The new species can be diagnosed from B. dacnis, B. didactylus, B. hermogenesi, B. pulex, B. puri, and B. sulfuratus by having brown background with few golden spots scattered randomly in the general coloration of dorsal surfaces and cloacal region [dark ornamentations (some with x-shaped mark) in these species; Izecksohn 1971; Giaretta & Sawaya 1998; Napoli et al. 2011; Condez et al. 2016; Almeida-Silva et al. 2021; Toledo et al. 2024)]. Brachycephalus nanicus can also be distinguished from B. didactylus, B. hermogenesi, B. puri, and B. pulex by having yellow blotches on the ventral surfaces (absent in these species; Izecksohn 1971; Giaretta & Sawaya 1998; Almeida-Silva et al. 2021). The new species can also be diagnosed from B. puri by the rounded snout in dorsal view, finger II with pointed tips (arrow-shaped), and the presence of a dark brown stripe along the flanks and of the thigh (mucronate snout, finger II with rounded tips, and dark brown stripe absent in B. puri; Pie & Ribeiro 2015; Almeida-Silva et al. 2021; Folly 2022). The new species can also be diagnosed from B. didactylus by toe II with pointed tips (toe II with rounded tips in B. didactylus; Izecksohn 1971). The new species can also be diagnosed from B. pulex by the dark brown strips on the legs (reddish brown stripes in B. pulex; Napoli et al. 2011). The smooth skin lacking discernible osteoderms can diagnose Brachycephalus nanicus from B. alipioi, B. bufonoides, B. crispus, B. ephippium, B. garbeanus, B. guarani, B. herculeus, B. margaritatus, B. nodoterga, B. pitanga, B. toby, and B. vertebralis (which present hyperossification: Clemente-Carvalho et al. 2009; Haddad et al. 2010; Pombal 2010; Clemente-Carvalho et al. 2012; Condez et al. 2010, 2014; Folly et al. 2021, 2024). Though many flea-toads are often externally similar looking, there is substantial variation in the extent of reductions and patterns of co-ossification in their skeletons. In contrast to the extensively synostosed cranium of B. nanicus, the cranial bones are not coossified in B. dacnis or B. sulfuratus, but is similarly synostosed in B. clarissae, B. didactylus, and B. hermogenesi, (for comparisons, see Figure 7 of Toledo et al., 2024); in B. hermogenesi, the nasals are not synostosed to the composite cranium. Like B. nanicus, B. clarissae and B. didactylus are edentulous, whereas B. dacnis and B. sulfuratus have a handful of small teeth or odontoids on the maxillae and premaxillae, and teeth are well developed and more numerous in B. hermogenesi. The squamosal of B. nanicus has an anterior flange that incorporates a small zygomatic ramus and is like that of B. dacnis, B. didactylus, B. hermogenesi, and B. sulfuratus, whereas B. clarissae has only a small anterior flange along the body of the squamosal. The quadratojugal of B. nanicus is like that of B. didactylus in being small, restricted to the distal end of the squamosal, and lacking an anterior process, whereas there is a thin, rodlike anterior process in B. clarissae, B. dacnis, B. hermogenesi, and B. sulfuratus. The pterygoid of B. nanicus is less robust than in B. dacnis, and B. hermogenesi, but more robust than the thin rod-like pterygoid of B. clarissae and B. didactylus. In contrast to the vertebral fusions observed in a skeletally mature B. nanicus (ZUEC-AMP 25610; none observed in the smaller ZUEC-AMP 25580), there are no vertebral fusions in B. clarissae, B. dacnis, B. hermogenesi, or B. sulfuratus; in B. didactylus, presacral vertebrae VI, VII, and VIII are co-ossified. Among the flea-toads examined, the urostyle of B. didactylus is unique in having laterally expanded flanges extending along its length. In B. nanicus, the single phalanx of fingers I and IV are tiny elements (similar to B. clarissae), whereas these are slightly larger in B. dacnis and B. sulfuratus, even more robust in B. hermogenesi, and lacking completely in B. didactylus. Osteology. Based on ZUEC-AMP 25610 (sex unknown). The skeleton of B. nanicus is other small species in the genus, including by lacking osteoderms and having unornamented dermal skull bones that are not co-ossified to the skin. This individual (Fig. 5) seems to be osteologically mature with ossified mesopodials and complete, ossified distal long bones. The skull is compact, slightly longer than wide, and lacking ornamented dermal roofing bones.The frontoparietals, sphenethmoids, prootics, exoccipitals, parasphenoid, nasals, and ossified nasal cartilages are completely synostosed. There are well-defined and bony margins to the optic fenestrae and prootic foramina as well as within the sphenethmoid and nasal cartilages, including for the olfactory nerve. The premaxillae are broad, clearl