SVP abstracts – 14: on the evolution of the clade Placentalia

Shelley 2022 reported,
“Resolving the phylogenetic relationships among Paleocene mammals has been a longstanding goal in paleontology.

The large reptile tree (LRT, 2171 taxa) achieved that goal a while back, based on traits of genus-based fossil and extant taxa, not genes, which too often recover untenable results (e.g. horses and bats as sister taxa) and do not test fossil taxa.

“Constructing an accurate and comprehensive phylogeny for Paleocene mammals is a worthwhile objective in itself, but it also provides a framework on which we can better understand the origin of placental mammals and the evolutionary processes underlying the diversification of mammals before, during, and after the end-Cretaceous mass extinction.

A laudable goal that should have been achieved by the year 2000 in academia.

“More recently, a robust Palaeocene mammal phylogeny has become a much-coveted tool for reconciling discrepancies between morphological and molecular evidence for the phylogeny and diversification of Placentalia.

Dr. Shelley: are you saying you know of a third robust phylogenic method that will reconcile traits and genes? Why not just drop genes, which too often recover untenable results? And isn’t a phylogeny a result? Not a tool? And is this tool really coveted? Or is that hyperbole?

“Here, we present a novel phylogenetic dataset to test hypotheses regarding Paleocene mammal phylogeny and the origin and diversification of Placentalia.

Which hypotheses regarding Paleocene mammal phylogeny are not novel? The suprageneric results (see below) are suspect. Why not test taxa, like the LRT does?

“To date, our matrix combines phenomic data for 36 extant mammal species and 107 fossil species scored for 2540 morphological characters alongside 26 genes sequenced for 47 species.

36 and 107 are not big numbers for taxa. This is called ‘cherry picking’.

Why go overboard with 2540 traits when a tenth of that number is sufficient in the LRT? More traits might be useful when dealing with scraps or just teeth, but such studies should follow wider gamut studies of the entire skeleton to determine tree topologies. Then add the scrappy taxa.

Stay away from genes. Why would you go there knowing you’ll have “discrepencies”?

“Within Placentalia, our phylogenies provide support for the divergence of Atlantogenata (Afrotheria and Xenarthra) from Boreoeutheria (Euarchontoglires and Laurasiatheria).

Wait. Wait. Wait. Which genera of pre-placentals did you cherry-pick as your outgroup? Or did this study lack a suitable outgroup (like the pouchless marsupial, Monodelphis, Fig 1), recovered by testing a wide gamut of taxa?

Shelley’s genomic results recover vague and untenable suprageneric taxa. so they are suspect from the start. We want to see genera in the results, not these hopeful, nebulous, invalid genomic clades. Be specific. These results are no better than those published in Novacek 1992 thirty years ago.

“Among the Paleocene taxa, ‘condylarths’ are distributed along the base of Laurasiatheria with members of ‘Arctocyonidae’ recovered as sister taxa to Artiodactyla; enigmatic groups such as Pantodonta and Taeniodonta are recovered as crown placentals whereas Leptictida is not.

Meanwhile, in the LRT traditional members of the Pantodonta nest in a variety of nodes making this clade a phylogenetic wastebasket. The same goes for Taeniodonta with some marsupials and placentals included. Other than Leptictis and Leptictidium, the traditional clade Leptictida seems to be based on dental traits. Odontocetes, tenrecs and sengis are extant members of this clade. Adding taxa reveals this.

“Our Paleocene mammal phylogeny is a critical step toward better understanding placental mammal evolution. Ultimately, this work will facilitate the investigation of fundamental questions previously encumbered by the lack of a well-resolved phylogeny.”

The longer workers waste their time in deep time genomic tests, the longer we’ll all have to wait for someone or some group to confirm, refute or modify the results recovered in the LRT, which tests traits in fossil and extant taxa.

References
Naish D 2015. tetrapod-zoology/refined-fine-tuned-placental-mammal-family-tree/
Novacek M 1992. Mammalian phylogeny: shaking the tree. Nature 356, 121-125.
Shelley SL 2022. The phylogeny of paleocene mammals and the evolution of Placentalia. SVP abstracts 2022: 318.

Epigenetics
wiki/Pantodonta
wiki/Taeniodonta
wiki/Leptictida