As the headlines reported,
(see below) these two late-surviving pre-mammals lived under the feet of Early Cretaceous dinosaurs and probably only came out after dark.
From the Mao et al. 2021 abstract:
“Mammaliamorpha comprises the last common ancestor of Tritylodontidae and Mammalia plus all its descendants. Tritylodontids are nonmammaliaform herbivorous cynodonts that originated in the Late Triassic epoch, diversified in the Jurassic period and survived into the Early Cretaceous epoch. Eutriconodontans have generally been considered to be an extinct mammalian group, although different views exist.”
“Here we report a newly discovered tritylodontid and eutriconodontan from the Early Cretaceous Jehol Biota of China. Eutriconodontans are common in this biota, but it was not previously known to contain tritylodontids.”
Confirmation on those points!
In the large reptile tree (LRT, 1825+ taxa; subset Fig. 4) Fossiomanus nests with Oligokyphus and the tritylodonts. The other new burrowing pre-mammal, Jueconodon nests with Liaocondon, and other eutriconodonts close to Gobiconodon and Repenomamus.
Mao et al. continue:
“These fossils shed light on the evolutionary development of the axial skeleton in mammaliamorphs, which has been the focus of numerous studies in vertebrate evolution and developmental biology. The phenotypes recorded by these fossils indicate that developmental plasticity in somitogenesis and HOX gene expression in the axial skeleton—similar to that observed in extant mammals—was already in place in stem mammaliamorphs. The interaction of these developmental mechanisms with natural selection may have underpinned the diverse phenotypes of body plan that evolved independently in various clades of mammaliamorph.”
Usually, ‘No hypotheses like this can proceed without first establishing a valid phylogeny.’ Parts of Mao et al. match the LRT. Unfortunately, Mao et al. follow invalid academic tradtion as they also include and therefore nest multituberculates with pre-mammals, rather than with rodents and plesiadapiformes in the gnawing clade, Glires. Just add pertinent taxa to resolve this problem. So far PhDs have been reluctant to do this and so the myth continues untested except here.
Mao et al. nest Jueconodon between Liaoconodon and Chaoyangodens (Fig. 3). In the LRT (Fig. 4) Jueconodon also nests with Liaoconodon, but Chaoyangodens nests as a monotreme mammal, basal to the echidna and platypus (Tachyglossus and Ornithorhynchus).
Mao et al. mention Liaoconodon often:
- “The triangular shape of the skull may have been exaggerated by the crush of
the specimen, but compared to those that have the similar preservation, such as Jeholodens, Liaoconodon, and Chaoyangodens, the triangular shape of Jueconodon is distinctive.
- The morphology of the mandible is similar to those of other eutriconodontans, such as Liaoconodon (Meng et al., 2011). Given that Liaoconodon was interpreted as a semiaquatic animal (Chen and Wilson, 2015), the similar mandible in both species indicate that the lower jaw and teeth of Jueconodon were not specialized for digging.
- The ossified Meckel’s cartilage on each side is preserved but displaced from its anatomical position. This suggests that the transitional mammalian middle ear, as best shown in Liaoconodon (Meng et al., 2011), was present in the fossorial eutriconodontans.“
Mao et al. report, “the Manda cynodont and mammaliaforms that are considered terrestrial.
Compared to extant mammals, Fossiomanus sinesis is superficially similar in body size and shape to the Cape dune mole-rat Bathyergus suillus, the largest subterranean scratch-digger species of the African mole-rats (Montoya-Sanhueza et al., 2019). However, they differ fundamentally in the axial skeleton in that mole-rat has the rodent body plan with the ancestral PV count of mammals.”
Mao F-Y, Zhang C, Liu C-Y and Meng J 2021. Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature (advance online publication)