Euharamyidans include the squirrel-like Jurassic gliders
Shenshou (Figs. 1,2 ), Vilevolodon and Maiopatagium in the large reptile tree (LRT, 1265 taxa). These are sisters to the squirrel, Ratufa, the squirrel-like Paramys and two living rodents, Rattus and Mus (rat and mouse).
Mao et al. 2018 report, “The new evidence suggests presence of diphyodonty in euharamiyidans. While it will take time to amass data to resolve the discrepancy between competing phylogenetic hypotheses about ‘haramiyidans’, multituberculates, and/or allotherians, it is helpful to continue deepening our knowledge about the morphology of euharamiyidans. Our finding of potential diphyodonty in euharamiyidans provides an additional piece of evidence for mammalness of the peculiar group.”
The skull of Shenshou (Fig. 2), close to living squirrels. Evidently the molar cusps are convergent with those of Haramiyavia, but there are few other similarities.
Haramiyavia (Fig. 3), a pre-mammal cynodont with a small canine and large incisors not related to Shenshou. Note the dual articular/dentary jaw joint in Haramiyavia, missing (actually evolved into ear bones) in Shenshou. Such a jaw joint marks this taxon as a pre-mammal synapsid.
In the LRT, Haramyavia, a basal member of the Haramiyida
nests with other pre-mammals like Brasiliodon and Sinoconodon, hence: not related to euharamiyidans. Determining the clade based on traits (no matter what these traits may be) is the cause of the phylogenetic confusion based on tooth shape and replacement patterns, which can converge. Only a taxon’s placement on a cladogram can tell you what an animal really is. Sadly, that’s a current heresy, not widely appreciated.
According to Wikipedia
(ref below): “Haramiyidans are a long lived lineage of mammaliaform cynodonts. Their teeth, which are by far the most common remains, resemble those of the multituberculates. However, based on Haramiyavia, the jaw is less derived; and at the level of evolution of earlier basal mammals like Morganucodon and Kuehneotherium, with a groove for ear ossicles on the dentary. They are the longest lived mammalian clade of all time.”
As the LRT showed several years ago
the rodent-like Euharamiyidans (Fig. 1) nest with placental rodents in the clade Glires, not with the much more primitive pre-mammals like Haramiyavia (Fig. 3).
Mao et al. 2018 report, “presence of the diphyodont dentition alone is not diagnostic for mammals. This is because a diphyodont dentition exists not only in mammals but also in stem mammaliaforms, such as Morganucodon and docodonts, although there may be more than one replacement for the upper canine of Haldanodon (Martin et al., 2010b).”
By contrast, in the LRT
Morganucodon is a basal metatherian, not a stem mammaliaform. Which is one more reason why it has diphyodont dentition (milk teeth + permanent teeth). The late-surviving docodonts, Haldanodon and Castorocauda nest between the synapsids, Probainognathus and Pachygenelus in the LRT. Those four should be replacing all their teeth all the time. All four had a dual jaw joint that was not quite mammalian, but getting there!
Diphyodont dentition alone is diagnostic for mammals
because it implies toothless, milk-lapping/sucking hatchlings, (but be careful not to pull a Larry Martin here, because the LRT uses 231 traits and diphyodont dentition is not among them).
Mao et al. 2018 report, “tooth replacement is also complex among mammals. For instance, the molariform teeth of eutriconodonts show replacement and some species have the entire dentition replaced and show at least three tooth generations. Cheek tooth replacement is uncertain in “symmetrodontans”. In North American spalacotheriids deciduous canine and premolars were retained late in life and may never have been replaced; thus, their dentitions perhaps were monophyodont. This has been supported by the spalacolestine Lactodon from the Early Cretaceous Jehol Biota, in which there is no sign of cheek tooth replacement even though this taxon possesses deciduous-like antemolars. New CT scan data (unpublished) further confirmed that there is no tooth germ at any tooth locus, including incisors and canines, of Lactodon [= Lactodens”?]. Thus, presence of the diphyodonty in euharamiyidans, does not constitute a sufficient evidence for the group’s mammalian affinity.”
Let’s examine those arguments
in new light shed by the LRT.
- Eutriconodonts (Spinolestes, Gobiconodon and kin): These taxa do not nest within Mammalia in the LRT (contra Martin et al. 2015).
- Symmetrodontans (Zhangheotherium and kin): Zhangheotherium is a basal pangolin, hence the atavistic teeth, as in another placental clade, the archaeocete ‘whales’.
- Spalacotherids (Lactodon = Lactodens): Taxa like Lactodens nest within the prototheria in the LRT.
It always comes back down to phylogenetic analysis.
And the LRT answesr all such problems within its ken. The radiation of placental mammals was in the Early Jurassic based on the appearance of derived placental mammals in the Late Jurassic. Non-mammalian synapsids survived into the Middle Jurassic, so there was plenty of overlap.
PS. If you’re wondering about
Lactodens (= Lactodon; Fig. 4; Han and Meng 2018; Early Cretaceous) here it nests at the base of the echidna + platypus clade, two toothless (as adults) taxa. Perhaps that’s why the diphyodont dental rules start breaking down with this taxon, as described by Mao et al.
Mao F-Y et al. (5 co-authors) 2018. Evidence of diphyodonty and heterochrony for dental development in euharamiyidan mammals from Jurassic Yanliao Biota. Vertebrata PalAsiatica DOI: 10.19615/j.cnki.1000-3118.180803