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The origin of the Mammalia and Marsupialia in the LRT

Sunday, July 21, 2024 14:59

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Here
(Fig 1) is a four-frame comparison of Megazostrodon, the basalmost mammal in the constantly updated large reptile tree (LRT, 2322 taxa) with Ukhaatherium, the basalmost marsupial in the LRT. Monotremes (Fig 3) are derived from Megzostrodon.

The similarities in size and shape indicate a minimal phylogenetic difference despite their great chronological difference (Early Jurassic vs Late Cretaceous).

Figure 1. The basal monotreme in the LRT, Megazostrodon, compared to the basal marsupial, Ukhaatherium to scale. ” data-image-caption=”

Figure 1. The basal monotreme in the LRT, Megazostrodon, compared to the basal marsupial, Ukhaatherium to scale.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/megazostrodon-ukhaatherium-skull588.gif?w=186″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/megazostrodon-ukhaatherium-skull588.gif?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87682″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/megazostrodon-ukhaatherium-skull588.gif” alt=”Figure 1. The basal monotreme in the LRT, Megazostrodon, compared to the basal marsupial, Ukhaatherium to scale.” width=”584″ height=”943″ />

Figure 1. The basal monotreme in the LRT, Megazostrodon, compared to the basal marsupial, Ukhaatherium to scale.

Megazostrodon rudnerae
(Crompton & Jenkins, 1968; Gow 1986) early Jurassic ~200 mya, ~12 cm in length, was a basal mammal and basal monotreme. In all other reptiles, including mammal predecessors, the articular forms a jaw joint with the quadrate. In mammals the dentary forms a jaw joint with the squamosal. The tiny articular and quadrate become miniaturized ear bones.

Figure 2. Megazostrodon and Ukhaatherium at full scale @ 72 dpi. ” data-image-caption=”

Figure 2. Megazostrodon and Ukhaatherium at full scale @ 72 dpi.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/ukhaatherium_megazostrodon588.jpg?w=265″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/ukhaatherium_megazostrodon588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87688″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/ukhaatherium_megazostrodon588.jpg” alt=”Figure 2. Megazostrodon and Ukhaatherium at full scale @ 72 dpi.” width=”584″ height=”661″ />

Figure 2. Megazostrodon and Ukhaatherium at full scale @ 72 dpi. Ukhaatherium is a Late Cretaceous late survivor of that Late Triassic radiation and split.

Ukhaatherium nessovi
(Novacek et al. 1997; Campanian, Late Cretaceous, 80 mya) is known from eight partial skeletons. Wikipedia reports, 4 premolars and 3 molars, but the break in dental shape happens between premolar 3, typically the largest and last one, and the next tooth, molar 1 of 4. The lumbar region is curved. Traditionally considered an Asioryctidae, close to placentals, but with epipubic bones, here it nests at the base of the Metatheria = Marsupialia as a late survivor of that Late Triassic radiation.

According to Wikipedia – Ukhaatherium,
“Ukhaatherium is a genus from the family of the Asioryctitheria, which lived in the late Cretaceous. Ukhaatherium and Asioryctes form the subfamily Asioryctinae, while Kennalestes stands somewhat outside this group.”

That hypothesis is not confirmed by the LRT. Asioryctes nests apart from Ukhaatherium in the LRT.

Figure 3. Tiny basal monotremes in the LRT include Sinodelphys, Juramaia, Chaoyangodens, Brasilitherium and Lactodens. Note the trend toward a longer, lower snout culminating in extant platypus and echidna. ” data-image-caption=”

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/monotreme_skulls588.jpg?w=120″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/monotreme_skulls588.jpg?w=408″ tabindex=”0″ role=”button” class=”size-full wp-image-87698″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/monotreme_skulls588.jpg” alt=”Figure 3. Tiny basal monotremes in the LRT include Sinodelphys, Juramaia, Chaoyangodens, Brasilitherium and Lactodens. Note the trend toward a longer, lower snout culminating in extant platypus and echidna.” width=”584″ height=”1466″ />

This remains a novel hypothesis of interrelationships since 2018.
We looked at Ukhaatherium earlier here. With present data, marsupials had a Late Triassic origin alongside very similar montremes. Placentals evolved just a little later based on the presence of placental Glires multituberculates in the Jurassic.

References
Crompton AW and Jenkins FA Jr 1968. Molar occlusion in late Triassic mammals, Biological Review, 43 1968:427-458.
Novacek MJ, Rogier GW, Wible JR, McKenna MC, Dashzev g D and Horovitz I 1997. Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia Nature 389: 483-486.

wiki/Megazostrodon
wiki/Ukhaatherium

Source: https://pterosaurheresies.wordpress.com/2024/07/22/the-origin-of-the-mammalia-and-marsupialia-in-the-lrt/

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