Tiny Hapalodectes IVPP V5235 and descendant marsupial sabertooths now nests with a much larger slightly restored Megistotherium

Not readily apparent
until taphonomically repaired and rescored in a phylogenetic matrix, giant Early Miocene Megistotherium (Fig 1) now nests with smaller sabertooth marsupials (Patagosmilus and Thylacosmilus, Fig 1) and their ancestor, tiny Hapalodectes IVPP V5235 (Fig 1) in the large reptile tree (LRT, 2337 taxa).  Apparently all had a postorbital bar and lacked premaxillary teeth. A pseudo-mastoid process was created by a lateral process of the exoccipital (dark green).

Here (Fig 1) a postorbital bar is restored attaching to the broken areas of the 66cm long Megistotherium skull.

According to Wikipedia – Megistotherium,
a long list of closer relatives to Megistotherium have been identified, but none have been tested in the LRT – other than Hyainailourous (aka Pterodon, Fig 2), which nests in a sister clade to these four taxa (Fig 1). Note the large canines and narrow premaxilla in all.

Megistotherium osteothlastes
(Savage 1973; Miocene, 23mya; 66cm skull length) was originally considered a giant hyaenodontid creodont. Here it nests with the much smaller ancestor to marsupial sabertooths, Hapalodectes hetangensis (below). The jaw muscles were enormous. The large diameter canines were housed in large, laterally expanded maxillae. The braincase was narrow. The fragile postorbital bars appear to have broken off. The premaxillae each include room for only one tiny tooth, if not absent altogether, as in sabertooth marsupials.

Hapalodectes hetangensis
(Ting and Li 1987; 4.5cm skull length; Paleocene, 55 mya; IVPP V 5235) This skull was originally considered a mesonychid, but here nests with the marsupial clade leading to sabertooth taxa with a shrinking premaxilla. Note the encircled orbits rotated anteriorly as in Thylacosmilus, convergent with primates.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

References
Rich TH, Flannery TF, Trusler P, Kool L, van Klaveren NA and Vickers-Rich P 2001. A second tribosphenic mammal from the Mesozoic of Australia. Records of the Queen Victoria Museum 110: 1-9.
Savage RJ 1973. Megistotherium, gigantic hyaeonodont from Miocene of Gebel Zelten, Libya. Bulletin of the British Museum (Natural History) Geology 22(7):483–511.
Ting S and Li C 1987. The skull of Hapalodectes (?Acreodi, Mammalia), with notes on some Chinese Paleocene mesonychids. Vertebrata PalAsiatica. 25: 161–186.

wiki/Megistotherium

On a side note:
A few Reddit users responded to “What is this community opinion about David Peters(reptileevolution.com)?” here about four years ago. I just found it.

Par for the course.

Some quotes from comment 1:
“Peters willfully misconstrues digital image artifacts”
“hare-brained radical reconstructions of phylogenetic relationships.”
“Not only is he absolutely batshit insane, he’s also simultaneously charismatic”
“ReptileEvolution and PterosaurHeresies are basically the Qanon of paleontology.”

These comments echo those of Darren Naish in 2012 and 2020. You can read that editorial in its entirety (with footnotes) here.

Some quotes from comment 2:
“David Peters isn’t doing weird speculation stuff. He’s just misinterpreting fossils, and a lot of his conclusions are NOT scientifically possible. For example, last I checked he still thinks pterosaurs gave live birth and the babies clung to the parents like opossums.”
“Peters published a paper saying the baby pterosaurs are actually adult pterosaurs that crawled into turtle eggs.”

Mea culpa. These last two comments arise from fanzine articles I sent in around 2003, just prior to and just following the description of the IVPP pterosaur egg (Figs 3, 4). I was wrong. Completely wrong. Unfortunately that bit of freshman naiveté now overshadows and follows me around like an albatross hanging from my neck – despite all the publications in academic journals and subsequent work (including corrections) over the next two decades.

On the one hand, consider this a cautionary tale.
On the other hand, acknowledge your mistakes and keep pressing forward.

Explanation:
the IVPP pterosaur embryo (Fig 3) was almost as large in the egg as all but one adult anurognathid, and twice as tall as that relatively large outlier. That’s why it appeared to me, at the time, to be an adult anurognathid inside a leathery egg shell. Logical, but wrong.

Interesting side note:
To date, no one else over the past twenty-two years has attempted to trace the IVPP embryo in detail and reconstruct it. Embryos like the IVPP specimen are complete, articulated, perfect. Packaged that way. And adult pterosaurs are identical to late term embryos, only 8x larger, which means you can use them as taxa in phylogenetic analysis. Which no one else does. See those tracings here (Fig 4).

That’s what I mean by ‘keep pressing forward.’ And learn something while doing it. Every scientist who makes a discovery or two gets the same treatment from experts and the public, even Yale professor John Ostrom.

Source: https://pterosaurheresies.wordpress.com/2025/10/29/tiny-hapalodectes-ivpp-v5235-and-descendant-marsupial-sabertooths-now-nests-with-a-much-larger-slightly-restored-megistotherium/