Megistotherium rescored in the LRT as a giant didelphid
According to Wikipedia – Megistotherium
“Megistotherium is an extinct genus of hyaenodont belonging to the family Hyainailouridae that lived in Africa and possibly Asia as well. It first appeared in Early Miocene to late Middle Miocene from 22.5 to 12.0 million years ago, existing for 10.5 million years.”
The Virginia opossum (Didelphis, Figs 1–3) has a much more ancient ancestry. Chronological bracketing pushes that genesis back to the Early Cretaceous or Jurassic.
I finally noticed the resemblance.
Then I rescored that present line drawing data for Megistotherium (Fig 1) alongside that of Didelphis, the Virginia opossum in the large reptile tree (LRT, 2339 taxa). The available data for Megistotherium from a British Museum publication (Savage 1973) was vague enough to manipulate DGS colors toward the Didelphis Bauplan.
Differences include the expansion of the maxilla over the canines and a relatively smaller braincase and nuchal crest.
If valid, this is what happens when an opossum phylogenetically grows
until it has a 66cm long skull.
Figure 1. Lion-sized Megistotherium compared to scale and to size with extant Didelphis, the Virginia opossum. Now they nest together in the LRT.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/megistotherium-didelphis588.jpg?w=149″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/megistotherium-didelphis588.jpg?w=510″ class=”size-full wp-image-93858″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/megistotherium-didelphis588.jpg” alt=”Figure 1. Lion-sized Megistotherium compared to scale and to size with extant Didelphis, the Virginia opossum. Now they nest together in the LRT.” width=”584″ height=”1172″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/megistotherium-didelphis588.jpg?w=584&h=1172 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/megistotherium-didelphis588.jpg?w=75&h=150 75w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/megistotherium-didelphis588.jpg?w=149&h=300 149w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/megistotherium-didelphis588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Lion-sized Megistotherium compared to scale and to size with extant Didelphis, the Virginia opossum. Now they nest together in the LRT.
That rare pinch atop the frontal,
was a trait I had not seen in any of the other 600+ synapsid taxa in the LRT. The rest of the traits were similar enough to nest the two together in the LRT and apart from competing relatives.
Figure 2. Didelphis, the Virginia opossum, with several young riding bareback.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis_young.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis_young.jpg?w=584″ class=”size-full wp-image-93862″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis_young.jpg” alt=”Figure 2. Didelphis, the Virginia opossum, with several young riding bareback. ” width=”584″ height=”338″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis_young.jpg?w=584&h=338 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis_young.jpg?w=150&h=87 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis_young.jpg?w=300&h=173 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis_young.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Didelphis, the Virginia opossum, with several young riding bareback.
Figure 3. Didelphis blind and helpless neonates.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis.neonates588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis.neonates588.jpg?w=584″ class=”size-full wp-image-93864″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis.neonates588.jpg” alt=”Figure 3. Didelphis blind and helpless neonates.” width=”584″ height=”292″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis.neonates588.jpg?w=584&h=292 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis.neonates588.jpg?w=150&h=75 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis.neonates588.jpg?w=300&h=150 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/07/didelphis.neonates588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 3. Didelphis blind and helpless neonates.
Did Megistotherium continue
carrying young on its back (Fig 2) after giving birth to premature neonates (Fig 3)? Or did giant Megistotherium adopt yet another origin of the placentalia? Good question.
This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.
References
Savage RJ 1973. Megistotherium, gigantic hyaeonodont from Miocene of Gebel Zelten, Libya. Bulletin of the British Museum (Natural History) Geology 22(7):483–511.
Source: https://pterosaurheresies.wordpress.com/2025/07/25/megistotherium-rescored-in-the-lrt-as-a-giant-didelphid/
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