Enigmatic humerus is too late to bridge the ‘anatomical gap’ between pelycosaurs and therapsids. The LRT already did that years ago.

Readers, don’t create an ‘anatomical gap’ where none exists,
just to put a little splash on a paleo-headline.

Bishops et al 2023 report,
“The origin and radiation of therapsids from non-therapsid synapsid ancestors (“pelycosaurus”, for brevity) was a key event in the evolution of the mammalian lineage. Distinct from the problem of the biogeographic and temporal circumstances surrounding this event is the question of how the therapsid body plan was assembled. The incompleteness of the fossil record shrouds both questions in uncertainty.”

By contrast, the large reptile tree (LRT, 2222 taxa, subset Fig 1) and the therapsid skull tree (TST, 77 taxa, Fig 2) both documented this ‘key event’ several years ago. And those did not come as surprises. We’ve known for many years how the therapsid body plan was assembled. This is not a mystery. The fossil record is complete enough to present all the pertinent details so long as pertinent taxa are not omitted.

BTW,
the Pelycosauria is a valid clade (Figs 1, 2) arising out of Pantelosaurus and Haptodus. Therapsid ancestors went around this clade, arising directly from a sister to Ophiacodon. The last common ancestor of all therapsids in the LRT is Cutleria, known from good above-the-waist material (Fig 4). Hipposaurus (Fig 2), another basal theraposid preserves more elements of the post-crania than just a humerus.

The authors do note the presence of
Hipposaurus (Fig 3) and Biarmosuchus post-cranial material. Unfortunately the authors consider these better-represented specimens “as a partial proxy for the hypothetical ancestral therapsid.” They assert their single humerus“helps close the void between “pelycosaurus” and therapids.”

This is risky. A last common ancestor in a phylogenetic analysis of well-known specimens is the most important thing one needs to hypothesize interrelationships. What the authors are doing, putting their reputation on a single humerus, is akin to “Pulling a Larry Martin.”

References
Bishop PJ et al (5 co-authors) 2023. Enigmatic humerus from the Mid-Permian of South Africa brdiges the anatomical gap between “pelycosaurs” and therapsids. Journal of Vertebrate Paleontology Article: e2170805. https://doi.org/10.1080/02724634.2023.2170805

http://reptileevolution.com/cutleria.htm