Mygalomorphae

Sphodros rufipes, an atypical mygyalomorph

Megarachne servinei was thought to be a giant mygalomorph from the Upper Carboniferous (about 350 million years ago), but was later found to be a eurypterid.[7] The oldest known mygalomorph is Rosamygale grauvogeli (Hexathelidae) from the Triassic of northeastern France. No mygalomorphs from the Jurassic have yet been found.[8]

The number of families and their relationships have both been undergoing substantial changes since a cladogram showing family relationships was published in 2005,[9] with two significant studies in 2018.[10][11] The division of Mygalomorphae into two superfamilies, Atypoidea and Avicularioidea, has been established in many studies. The Atypoidea retain some vestiges of abdominal segmentation in the form of dorsal tergites; the Avicularioidea lack these. Molecular phylogenetic studies undertaken between 2012 and 2017 have found somewhat different relationships within the Avicularioidea. Some families appear not to be monophyletic and further changes are possible in the future.[4] Rosamygale belongs Avicularioidea, based on the absence of an abdominal scutum and well-separated posterior lateral spinnerets.[12]

Mygalomorphae tend to be very morphologically conserved, which makes it difficult to find reliable morphological features to use in taxonomy. It has been hypothesized that because Mygalomorphae tend to be fossorial and live in tubular webs, they are subjected to similar selective pressures, so most species should evolve in similar ways. Additionally, this may also mean that homoplasies are more likely to occur, further complicating taxonomy based on morphology.[13]

The relationships of taxa in the Mygalomorphae were restructured based on a comprehensive phylogenetic study by Opatova et al. (2020)[14] The generic composition of the families Ctenizidae, Cyrtaucheniidae, Dipluridae, and Nemesiidae were relimited. Five subfamilies were raised to the rank of family: Anamidae, Euagridae, Ischnothelidae, Pycnothelidae, and Bemmeridae. Three new families were created: Entypesidae, Microhexuridae, and Stasimopidae. Lastly, a new subfamily, Australothelinae, was generated and placed in the family Euagridae.

The preferred cladogram from Optova et al. (2020) is:[14]

In 1985, Robert Raven published a monograph of the Mygalomorphae in which he proposed an internal classification for the Mygalomorphae, based on morphological features. In 2020, Opatova et. al. commented "In short, much of today’s classification scheme dates back to Raven (1985)".[14] Raven used various compound ranks, such as "gigapicoorder" and "hyperpicoorder". Ignoring these unusual rank names, his classification can be shown diagrammatically:[16]

Subsequent research, largely based on molecular phylogenetic studies, has not upheld some of Raven's groupings. In particular his primary division between Tuberculotae and Fornicephalae has been replaced by a very different division between Atypoidea (expanded from Raven's Atypoidina) and Avicularioidea, which has no counterpart in his system. As another example, the families Mecicobothriidae and Microstigmatidae, which Raven placed in Mecicobothrioidina (a "gigapicoorder"), are now placed very far apart in the Atypoidea and Avicularioidea respectively. Other groups, such as Crassitarsae and Domiothelina, are more recognizable, allowing for some changes in family circumscriptions.[14]