Ukhaatherium: a late-surviving Morganucodon in the LRT

Ukhaatherium nessovi (Late Cretaceous, Novacek et al. 1997) has traditionally been nested with the proto-placental, Asioryctes. Here in the large reptile tree (LRT, 1243 taxa), Ukhaatherium nests as a basal therian in the large reptile tree. Morganucodon (Latest Triassic/Earliest Jurassic, Küehne 1949) nests with Ukhaatherium, with virtually identical scores and are similar in size.

Figure 1. Morganucodon skull in several views. Compare to Ukhaatherium in figure 2.

Figure 1. Morganucodon skull in several views. Compare to Ukhaatherium in figure 2.

When you see them together,
(Figs. 1,2) the similarities between Morganucodon and Ukhaatherium are strikingly obvious.

Figure 2. Skull and dentition of Ukhaatherium. Compare to Morganucodon in figure 1.

Figure 2. Skull and dentition of Ukhaatherium. Compare to Morganucodon in figure 1.

The long snout of prototheres
is retained in basal therians like Morganucodon and Ukhaatherium. The epipubic bones found in Ukhaatherium are no surprise in the LRT, where it nests in the middle of other taxa that share this trait.

Figure 3. Ukhaatherium in situ.

Figure 3. Ukhaatherium in situ.

Rowe 1988 defined Mammaliaformes
as Morganucondontidae + Mammalia. Here Morganucodontidae nest within the Mammalia, so Mammaliaformes is a junior synonym of Mammalia.

Figure 3. Subset of the LRT with the addition of Perameles and Morganucodon.

Figure 4. Subset of the LRT with the addition of Perameles and Morganucodon. Sharp-eyed readers with good memories will not subtle changes in this tree near protorotheres.

And, by the way,
Ukhaatherium + Morganucodon is still close to Asioryctes in the LRT. If we let a little time go by from the Latest Triassic to the Latest Cretaceous, then that’s enough time for the tiny posterior jaw bones in Morganucodon to evolve into the tiny ear bones of Ukhaatherium, convergent and parallel to those of other therian and prototherian mammals (which already show distinct patterns in this regard).

References
Kuehne WG 1949. On a triconodont tooth of a new pattern from a fissure-filling in South Glamorgan. Proceedings of the Zoological Society of London 119:345-350
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.
Rowe TS 1988. Definition, diagnosis and origin of Mammalia. Journal of Vertebrate Paleontology 8(3):241–264.

wiki/Ukhaatherium
wiki/Morganucodon

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