Teinolophos

Teinolophos is a prehistoric species of monotreme, or egg-laying mammal. It is known from four specimens, each consisting of a partial lower jawbone collected from the Wonthaggi Formation at Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous.
The species name honours the artist Peter Trusler. The genus name, "Teinolophos", means "extended ridge", a reference to its tooth structure.
Originally, "Teinolophos" was thought to be a eupantothere. Further research revealed similarities to "Steropodon", except in size: the animal was around 10 cm long. It is usually listed as a steropodontid, though it may be more basal.
The holotype is a partial left dentary known as NMV P208231. An age of approximately 123 million years makes this the earliest known monotreme. The lower molar is broadly similar in morphology to the m2 of "Steropodon". The trigonid is compressed and the talonid has no basin. The dentary is about one sixth the size of "Steropodon", and wear facets indicate an "orthal" occlusion with the upper molars.
Description.
Jaw.
The construction of the lower jaw differs from existing monotremes. Among the contrasts are the condyle, which is well above the tooth row (instead of at about the same height); and the ascending ramus, which is also higher. Also different is that "Teinolophos" probably had a strong bite. A unique feature for known toothed monotremes is that the trigonid is tall, while the talonid is set much lower. This is more like the general mammalian arrangement. The molar is double-rooted, which is plesiomorphic when compared to ornithorhynchids, but is a shared characteristic with "Steropodon" and "Kollikodon". Subsequent monotreme molars are multi-rooted.
Unlike modern monotremes, "Teinolophos" lacked a beak.
Ears.
Unlike modern monotremes, which have suspended ear bones much like placentals and marsupials, "Teinolophos" still had them connected to the jaw via the Meckel's cartilage. This reinforces the idea that the modern ear condition evolved independently among monotremes and therians.