Within the extant centipedes (Chilopoda), two thirds of the species (nearly two thousand) are in the clade Epimorpha, all of which form all their trunk segments and appendages before or very shortly after hatching (Minelli and Sombke, 2011; Brena, 2014). Nevertheless, different species of Epimorpha grow up to very different body sizes and their maximum length spans two orders of magnitude, from millimetres to decimetres. Such huge differences are found within each of the two major clades within the Epimorpha, i.e. the Scolopendromorpha (~700 species) and the Geophilomorpha (>1200 species). Among scolopendromorphs, at least two species of Scolopendra Linnaeus, 1758 have been reported to overreach 30 cm in length, i.e. S. gigantea Linnaeus, 1758 and S. galapagoensis Bollman, 1889 (Shear and Peck, 1992; Shelley and Kiser, 2000; Kronmüller, 2013). Among geophilomorphs, individuals surpassing 20 cm have been reported in a few species of Himantarium Koch, 1847, Orya Meinert, 1870 and Titanophilus Chamberlin, 1915, all in the superfamily Himantarioidea (Attems, 1929; Bonato et al., 2011; C. Kronmüller, pers. comm.). At the opposite extreme, body miniaturization evolved repeatedly in the history of the Epimorpha (Lewis, 2002; Foddai et al., 2003; Pereira, 2013a) but the smallest species are hard to single out, mainly because many candidate species are known from single or very few specimens only, preventing a confident estimate of the maximum length that they may reach at full growth (Appendix 1). The available data point to the species of Dinogeophilus Silvestri, 1909 as the smallest species in the Epimorpha, but this has been rarely highlighted in the literature.
Up to now no more than half a dozen specimens of Dinogeophilus have been reported, all from a narrow region in South America, partly in Argentina and partly in Uruguay. The first specimen was collected in the early 20th century (Silvestri, 1909a, 1909b) and described as D. pauropus Silvestri, 1909, as the first representative of a distinct genus. Another five specimens were collected much later and described (Pereira, 1984) as belonging to a second species D. oligopodus Pereira, 1984. All these specimens are less than 5.5 mm long.
Since its discovery and over more than a century, Dinogeophilus has been almost invariantly considered a member of the family Geophilidae. This taxonomic position has been explicitly maintained also in recent taxonomic synopses (Foddai et al. 2000; Bonato et al., 2011), in evolutionary analyses of anatomical features (Minelli and Bortoletto, 1988; Turcato et al., 1995) and in biogeographical overviews (e.g., Pereira et al., 1997; Bonato and Zapparoli, 2011). However, new specimens collected by one of us (L.A. Pereira) and a recent molecular phylogenetic analysis of the Geophilomorpha (Bonato et al., 2014) have provided evidence that Dinogeophilus does not belong to the family Geophilidae but to the family Schendylidae, which is morphologically very different and only distantly related to the former (Edgecombe and Giribet, 2007; Bonato et al., 2014). Geophilidae and Schendylidae have been recently classified in different superfamilies (Geophiloidea and Himantarioidea, respectively; Bonato et al., 2014) and their separation has been estimated to date back from the Mesozoic (Murienne et al., 2010).
Such preliminary evidence prompted us to reassess the phylogenetic position of Dinogeophilus by means of both morphological and molecular evidence. In particular, we tested the two competing hypotheses (Geophilidae vs. Schendylidae) by (i) examining newly available specimens, also applying scanning electronic microscopy for the first time to this taxon, (ii) performing similarity analyses and phylogenetic analyses on the molecular data recently obtained, and (iii) revisiting critically all previously published data and opinions on Dinogeophilus. This allowed us to (iv) reinterpret the morphology of these peculiarly miniaturized centipedes in a more solidly established evolutionary context, especially exploring morphological correlates of miniaturization, including putative paedomorphic traits and novelties.