Basal Mammalia to scale: Megazostrodon, Ukhaatherium and Morganucodon

Short one today…
but interesting. Three basal mammals representing the basal dichotomy splitting monotremes from therians are shown here (Fig 1).

Figure 1. Basal mammals include Megazostrodon, Ukhaatherium and Morganucodon. ” data-image-caption=”

Figure 1. Basal mammals include Megazostrodon, Ukhaatherium and Morganucodon.

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Figure 1. Basal mammals include Megazostrodon, Ukhaatherium and Morganucodon.

Megazostrodon is basal to monotremes.
Ukhaatherium and Morganucodon are basal therians = marsupials. Based on only these three taxa (admittedly a small sample, and Ukhaatherium was a Late Cretaceous late survivor of that Triassic split) phylogenetic miniaturization was present at the transition from egg-laying to live birth.

Note the gracile jugal in all three taxa,
the long rostrum and the large number of molars in these basal mammals.

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.

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.

Morganucodon watsoni
(Kühne 1949; Late Triassic, 235–200mya; 2–3cm skull length) is known from fissure fills (abundant, 3D and completely disarticulated) fossils from Wales and China. This insectivore was like a tree shrew. The anterior teeth had a single replacement. Traditionally considered a mammaliaform, here Morganucodon nests with Ukhaatherium at the base of the Theria, close to the base of the Mammalia (= Prototheria). The jaw retains some tiny rear jaw elements, so the hearing was not perfected yet.

References
Crompton AW and Jenkins FA Jr 1968. Molar occlusion in late Triassic mammals, Biological Review, 43 1968:427-458.
Gow CE 1986. A new skull of Megazostrodon ( Mammalia, Triconodonta) from the Elliot Formation (Lower Jurassic) of Southern Africa. Palaeontologia Africana 26(2):13–22.
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.

wiki/Megazostrodon
wiki/Ukhaatherium
wiki/Morganucodon