Krusatodon kirtlingtonensis (Sigogneau-Russell 2003, Paciroli et al 2024; Middle Jurassic) was considered a long-lived docodont. In the LRT it is an ordinary monotreme basal to the long-snouted montremes. Ornithorhynchus has a similar life span and maturation rate.

Juramaia sinensis (Luo et al. 2010; Late Jurassic. 160 mya; BMNH PM1343) was described as a tiny shrew-like eutherian mammal, perhaps the earliest known eutherian (placental) mammal. The large reptile tree nests Juramaia at the base of the Prototheria (= Monotremata or egg-laying mammals). The long rostrum is a basal trait shared by clade members and a basal therian, Ukhaatherium.

The long, slender forelimbs were not like the robust sprawling forelimbs of monotreme mammals and cynodonts. Thus this is the earliest known mammal to have a fully parasagittaly configuration.

Henosferus molus Rougier et al. 2007; Middle Jurassic) is known from broken specimens restored to a single mandible. The dentary formed the jaw joint, but a trough remained for the reduced post-dentary bones. This, plus the simple teeth and tooth formula nests Henosferos very close to the base of the Mammalia. Juramaia (above) is similar, but with more derived molars and incisors.

Lactodens sheni (Han and Meng 2016; Early Cretaceous) nests as a tiny mouse-like prototherian mammal, the last common ancestor of echidnas and platypuses, neither of which is mentioned in the original paper. Originally described as a spalacolestine and a symmetrodont, here it nests with prototheres (= monotremes).

‘Symmetrodontans’ are extinct mammals characterized by having a reversed-triangle molar pattern in which three main cusps define a triangular molar crown. This dental morpholgy has been regarded as being intermediate between the ‘triconodont’ tooth and the tribosphenic pattern characterizing therians.

Lactodens has a dental formula of three upper incisors, one canine, three premolars, and six molars/three lower incisors, one canine, five premolars and six lower molars, double-rooted canines, extremely low-crowned and transversely thin premolars, and acute angled molars. Deciduous canines and premolars in the slim and extremely long dentary imply a faunivorous diet.

Han G and Meng J 2016. A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian ‘symmetrodontans’. Zoological Journal of the Linnean Society 178:343–380 .PDF

Luo Z-X, Yuan C-X, Men Q-J and JiQ 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445. doi:10.1038/nature10291.

Panciroli E et al (7 co-authors) 2024. Jurassic fossil juvenile reveals prolonged life history in early mammals. Nature https://doi.org/10.1038/s41586-024-07733-1

Rougier, GW, Martinelli AG, Forasiepi AM and Novacek M J 2007. New Jurassic mammals from Patagonia, Argentina : a reappraisal of australosphenidan morphology and interrelationships. American Museum novitates, no. 3566. online here.

Sigogneau-Russell D 2003. Docodonts from the British Mesozoic. Acta Palaeontologica Polonica, 48(3)

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