Pimoidae

Hormiga, Gustavo, Kulkarni, Siddharth, Moreira, Thiago Da Silva & Dimitrov, Dimitar, 2021, Molecular phylogeny of pimoid spiders and the limits of Linyphiidae, with a reassessment of male palpal homologies (Araneae, Pimoidae), Zootaxa 5026 (1), pp. 71-101 : 93

Biogeography of Pimoidae

Our biogeographic reconstructions suggest that ancestrally pimoids and Pimoa were widely distributed across the Palearctic, the Nearctic and the Sino-Japanese regions (see Holt et al. 2013 for definitions of zoogeographic regions). It has been suggested that during that time (80 – 48 Ma) a large continuous boreotropical forest was present at higher latitudes in the northern hemisphere ( Tiffney 1985a, b; Lavin & Luckow 1993). Pimoa fossils from Baltic amber suggest pimoids have been associated with this forest. Gradual cooling, particularly pronounced after the late-Eocene, led to the retreat and fragmentation of the boreotropical forest and this has likely led to the vicariance events leading to the Nearctic, the Spanish Massif and the Italian peninsula lineages.

The biogeographical history of the Asian Clade, which is also the most speciose group of Pimoa , is more complex and dynamic. The dating of the common ancestor of this clade (with varying combinations of fossil placements) ranges between 47.8 Ma (mid-Eocene) and 23.07 Ma (Late Oligocene). Our biogeographic analyses indicate a widespread ancestral area for the Asian pimoids, which spans from the Western Himalayas to Hunan mostly within the Oriental region and with limited presence in the Sino-Japanese region .

The collision of the Indian raft with Eurasia is hypothesized to have begun during the early Eocene at about 50 Ma, with the Indian plate and mainland Asia contacting between 45-35 Ma and leading to the rise of the Himalayas by 23 Ma ( Ali & Aitchison 2008, Clift et al. 2008, Metcalfe 2013). Some studies on rosefinches ( Tietze et al. 2013), geckos ( Agarwal et al. 2014), spiders ( Zhao et al. 2020) and crabs ( Klaus et al. 2010) have shown that the India-Asia collision along with the formation of the Himalayas has profoundly affected the biogeography of several organisms.

The timing of origin of the Asian Pimoa Clade and its known distribution (see Fig. 4 distribution map) coincides with the India-Eurasia collision. It can be conjectured that the connectivity of two biotas (of insular India and mainland Asia) and the rise of mountain barriers together with global cooling and increase of aridity in large parts of central Asia (to a large extent also due to the rise of the Himalayas) may be a driving force behind the diversification of this most speciose clade in the family. Our biogeographic analyses suggest multiple dispersal and vicariance events between the different Oriental and Sino-Japanese subregions that we considered here. However interesting, these biogeographical hypotheses cannot be tested here due to the limitations of our data set, since the majority of Asian Pimoa have only COI sequence data. It is possible that the missing data may have influenced the branch lengths and relationships of taxa within this clade. In addition, many Asian Pimoa species are known only from the type material (providing a single point for the species distribution). It is therefore necessary to further study whether the known narrow ranges represent the actual distribution or are an undersampling artifact. Given these limitations we abstain from discussing in further details the inferred biogeographical history of the Asian Clade of Pimoa .

Figures

FIGURE 4. A, Biogeographic hypothesis obtained from the Dispersal-Extinction-Cladogenesis (DEC) model of RASP analysis on a dated phylogeny reconstructed with fossil calibrations using treePL. B, Biogeographic areas used as input in RASP (see Table 4 for details). Note that this is not a distribution map of pimoids, the range of any given pimoid species does not occupy all of the shaded biogeographic area (e.g., in North America pimoids are exclusively found in the west). C, Dispersal and vicariance rates optimized by the DEC model of RASP.