3.1 | Reconstruction of behavioral niche
Ancestral state reconstructions of retreat type and entrance type resulted in largely consistent and complementary evolutionary patterns (Fig. 1), and there are clear associations between the two: web-building taxa are almost all opportunists, taxa that modify their burrow entrance with a purse, turret, collar, or trapdoor are almost always burrowers or nest-builders, and nest-builders always have a trapdoor.
In the MP analyses, the ancestral mygalomorph and the ancestors of both the Atypoidea and the Avicularioidea were recovered as opportunists with web entrances (funnel, sheet, or space webs). The ML analyses contrasted with this in recovering the most likely state for the ancestral mygalomorph as a burrower, and the ancestral atypoid as a burrower with a purse-web entrance. However, these differences are likely due to the absence of several opportunist, web-building atypoid taxa from the ML analysis (Hexurella , Mecicobothrium , Megahexura ) and we therefore prefer the hypothesis of the more taxon-rich MP analysis.
Assuming an opportunist ancestor, obligate burrowing has arisen at least four times independently in the Mygalomorphae: in the Atypoidea (Atypidae and Antrodiaetidae), the Euagridae (some Cethegus ), the Hexathelidae (Mediothele , Plesiothele , and someScotinoecus and Hexathele ), and in the ancestor of the Bipectina (-Paratropididae). Most of the early branching avicularioid families have opportunistic, web-building ancestors, however the ancestral hexathelid was recovered as ambiguous in the MP analysis (which has several additional hexathelid taxa) being either an opportunist with a web entrance, or a burrower with an open entrance.
We recovered the ancestor of the Bipectina (-Paratropididae) as a burrower with a trapdoor entrance, and this behavior was retained in the ancestor of three of the four major bipectine clades: the Venom Clade+, the Domiothelina, and the Theraphosoidina. The ancestor of the Nemesioidina, however, was recovered as a burrower with an open entrance. In the Venom Clade+ , burrowing and trapdoor-building have both been lost in the Atracidae, most of which are opportunists with web-entrances (Atrax and many Hadronyche ). In the Domiothelina, the burrowing and trapdoor-building combination is largely conserved, but the trapdoor has been lost several times independently in favor of an open entrance or another type of entrance modification (collar or turret). Nest-building has also evolved at least three times independently in the Domiothelina (in the Idiopidae, Halonoproctidae and Migidae), always from burrowing, trapdoor-building ancestors, and all nest-builders retain the trapdoor. This nest-building + trapdoor niche evolved in the same way in the Theraphosoidina, in the Barychelidae. Although our analysis includes only a fraction of theraphosid diversity, we recovered the ancestral tarantula as a burrower with an open hole. Finally, in the Nemesioidina almost the full spectrum of behaviors has evolved from the burrowing + open-entrance ancestor: trapdoors and other entrance modifications have evolved several times, as has opportunism, and the ancestral mygalomorph niche of opportunism + web-construction has evolved in the Dipluridae.
Overall, behavioral niche-space in the Mygalomorphae can be described in terms of two extremes: at one end are opportunists that build webs at the entrance to the burrow, and at the other are burrowers and nest-builders that structurally modify their burrow entrance. Intermediate taxa usually burrow, but neither construct a web nor structurally modify their entrance. Shifts across this niche space in both directions have been common in mygalomorph evolution, with almost all major clades including representatives of several/most behavioral niches, despite disparate evolutionary histories (Fig. 1).