Title from https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2132288

Abstract

A review of the Southern Hemisphere Mesozoic tribosphenic mammal fossil record supports the hypothesis that Tribosphenida arose in the Southern Hemisphere during the Early Jurassic, around 50 million years prior to the clade’s reliably dated first appearance in the Northern Hemisphere. Mesozoic Southern Hemisphere tribosphenic mammals are known from Australia, Madagascar, South America and the Indian subcontinent, and are classified into three families: Bishopidae (fam. nov.), Ausktribosphenidae and Henosferidae. These are stem therians, and considerable morphological evolution occurred within the lineage between the Jurassic and late Early Cretaceous. Important dental modifications include a graduated transition between premolars and molars, development of molar wear facets V and VI, loss of facets for postdentary bones, reduction in the Meckelian groove and development of a true dentary angle. Previous classifications of Southern Hemisphere tribosphenic mammals are ambiguous because information from the upper dentition has been lacking. Upper molars attributed to the late Early Cretaceous (Albian) Southern Hemisphere group Bishopidae fam. nov. are now known to possess a prominent protocone and stylar cusp C. We thus consider bishopids to be the sister group to Theria.

This strongly suggests the origin of Tribosphenida was in the Jurassic, and
in the southern hemisphere. (The prevailing understanding is that the group was more or less synonymous with Boreosphenida (see https://en.wikipedia.org/wiki/Tribosphenida)

On Sun, 25 Dec 2022 09:11:58 -0800 (PST), erik simpson
<eastside.erik@gmail.com> wrote:

>Title from https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2132288
>
>Abstract
>
>A review of the Southern Hemisphere Mesozoic tribosphenic mammal fossil record supports the hypothesis that Tribosphenida arose in the Southern Hemisphere during the Early Jurassic, around 50 million years prior to the clade’s reliably dated first appearance in the Northern Hemisphere. Mesozoic Southern Hemisphere tribosphenic mammals are known from Australia, Madagascar, South America and the Indian subcontinent, and are classified into three families: Bishopidae (fam. nov.), Ausktribosphenidae and Henosferidae. These are stem therians, and considerable morphological evolution occurred within the lineage between the Jurassic and late Early Cretaceous. Important dental modifications include a graduated transition between premolars and molars, development of molar wear facets V and VI, loss of facets for postdentary bones, reduction in the Meckelian groove and development of a true dentary angle. Previous classifications of Southern Hemisphere tribosphenic mammals are ambiguous
>because information from the upper dentition has been lacking. Upper molars attributed to the late Early Cretaceous (Albian) Southern Hemisphere group Bishopidae fam. nov. are now known to possess a prominent protocone and stylar cusp C. We thus consider bishopids to be the sister group to Theria.
>
>This strongly suggests the origin of Tribosphenida was in the Jurassic, and
>in the southern hemisphere. (The prevailing understanding is that the group was more or less synonymous with Boreosphenida (see https://en.wikipedia.org/wiki/Tribosphenida)

This is interesting to me. I understood mammal dentition to be a
defining characteristic of mammalian clades. But IIUC your cited
Wikipedia article, tribosphenic molars are found in several
paraphyletic groups. Are there other molar types which were once
considered diagnostic of a clade but are no longer?

Is it me or does it seem like you have to be absolutely obsessed with dentistry to study post-Triassic mammals?

On Sunday, December 25, 2022 at 11:54:24 PM UTC-8, 69jp...@gmail.com wrote:
> On Sun, 25 Dec 2022 09:11:58 -0800 (PST), erik simpson
> <eastsi...@gmail.com> wrote:
>
> >Title from https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2132288
> >
> >Abstract
> >
> >A review of the Southern Hemisphere Mesozoic tribosphenic mammal fossil record supports the hypothesis that Tribosphenida arose in the Southern Hemisphere during the Early Jurassic, around 50 million years prior to the clade’s reliably dated first appearance in the Northern Hemisphere. Mesozoic Southern Hemisphere tribosphenic mammals are known from Australia, Madagascar, South America and the Indian subcontinent, and are classified into three families: Bishopidae (fam. nov.), Ausktribosphenidae and Henosferidae. These are stem therians, and considerable morphological evolution occurred within the lineage between the Jurassic and late Early Cretaceous. Important dental modifications include a graduated transition between premolars and molars, development of molar wear facets V and VI, loss of facets for postdentary bones, reduction in the Meckelian groove and development of a true dentary angle. Previous classifications of Southern Hemisphere tribosphenic mammals are ambiguous
> >because information from the upper dentition has been lacking. Upper molars attributed to the late Early Cretaceous (Albian) Southern Hemisphere group Bishopidae fam. nov. are now known to possess a prominent protocone and stylar cusp C. We thus consider bishopids to be the sister group to Theria..
> >
> >This strongly suggests the origin of Tribosphenida was in the Jurassic, and
> >in the southern hemisphere. (The prevailing understanding is that the group was more or less synonymous with Boreosphenida (see https://en.wikipedia.org/wiki/Tribosphenida)
> This is interesting to me. I understood mammal dentition to be a
> defining characteristic of mammalian clades. But IIUC your cited
> Wikipedia article, tribosphenic molars are found in several
> paraphyletic groups. Are there other molar types which were once
> considered diagnostic of a clade but are no longer?

I'm not that well-versed in the mammal fossil record of the Mesozoic. For a long time teeth
were the major part of that record, but my impression is that more recent finds have opened
things up some. Just glancing at phylogentic tree found by googling shows lots of polytomies
and short-branched clades. The overall view is pretty consistent, but there are lots of clades
that are less well-supported than you'd like.

On Monday, December 26, 2022 at 7:19:20 AM UTC-8, thesigh...@gmail.com wrote:
> Is it me or does it seem like you have to be absolutely obsessed with dentistry to study post-Triassic mammals?

I don't know if it's absolutely necessary, but an awful lot of the post-Triasssic mammal fossils are little jaws and
teeth. Mammals made a run for it in the Triassic, but following the end-Triassic extinction the dinosaurs got
the upper hand, and mammals hunkered down until the end-Cretaceous event gave them another chance.