You heard it here in 2011: diadectids are amniotes

Co-author, David S. Berman,
has been saying diadectids are amniotes since the 1990s, but not with a comprehensive taxon list, and, apparently nobody listened. The consensus apparently prefers their diadectids with tadpoles.

Here’s what Wikipedia reports
“Diadectes (meaning crosswise-biter) is an extinct genus of large, very reptile-like amphibians that lived during the early Permian period (Artinskian–Kungurian stages of the Cisuralian epoch, between 290 and 272 million years ago^[1]). Diadectes was one of the very first herbivorous tetrapods, and also one of the first fully terrestrial animals to attain large size.”

Klembara et al. 2019 report
on the inner ear morphology of diadectids and seymouriamorphs. From the abstract:
“Two pivotal clades of early tetrapods, the diadectomorphs and the seymouriamorphs, have played an unsurpassed role in debates about the ancestry of amniotes for over a century, but their skeletal morphology has provided conflicting evidence for their affinities. Both maximum parsimony and Bayesian inference analyses retrieve seymouriamorphs as derived non‐crown amniotes and diadectomorphs as sister group to synapsids.”

Dr. David Marjanovic wrote in the DML:
“Amniota is a crown-group; there’s technically no such thing as a “stem-amniote”, because if it’s on the stem, it’s not an amniote.”

Unfortunately, Klembara et al. don’t have enough taxa
to understand that Amniota is a junior synonym for Reptilia. So Repitilomorpha works well for pre-reptiles. More importantly, for the subject at hand, Diadectes (Fig. 1) and kin have been deeply nested within the large reptile tree (LRT, 1583 taxa) since 2011. This is an online resource you can use to double check your taxon list, just to make sure it is up to date. The Klembara et al. taxon list (Fig. 2) is so inadequate it nests several reptiles apart from one another and omits dozens of others pertinent to this issue.

Bottom line, when you add enough taxa
diadectomorphs are not close to synapsids, but arise from millerettids.

At least the Klembara team
moved diadectomorphs inside the Amniota. That’s a minor victory. Add the above taxa to your cladogram (Fig. 2) and see where Diadectes nests. That’s what the LRT is here for… to help workers avoid taxon exclusion.

References
Klembara J, Hain M, Ruta M, Berman DS,  SEPierce and Henrici AC 2019. Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by highâresolution xâray microcomputed tomography, and the origin of the amniote crown group. Palaeontology (advance online publication) Future publication date: August 5, 2020
doi: https://doi.org/10.1111/pala.12448
https://onlinelibrary.wiley.com/doi/full/10.1111/pala.12448

wiki/Diadectes