[2] In 2016, a newly described fossil arachnid, Idmonarachne, was also included in the Tetrapulmonata; as of March 2016[update] it has not been assigned to an order.

In addition to the two pairs of book lungs, other synapomorphies of Tetrapulmonata include a large postcerebral pharynx (reduced in Uropygi), prosomal endosternite with four segmental components, subchelate chelicerae, a complex coxotrochanteral joint in the walking legs, a pretarsal depressor muscle arising in the patella (convergent with Dromopoda, lost in Amblypygi), a pedicel formed, in part, by ventral elements of the second opisthomal segment and a spermatozoon axoneme with a 9+3 microtubule arrangement.

[2][6] †Trigonotarbida †Haptopoda Uropygi s.s. Schizomida Amblypygi †Uraraneida Araneae In 2016, a fossil arachnid from the Late Carboniferous (Pennsylvanian) age was described in the genus Idmonarachne.

Based on its overall morphology, it was considered to belong to the Serikodiastida, although the presence of silk-producing spigots was not demonstrated.

A cladogram based on morphology placed Idmonarachne between uraraneids and spiders:[3] †Uraraneida †Idmonarachne Araneae The Late Carboniferous appears to be a time when there was a greater diversity of tetrapulmonate arachnids.