Lyson et al. 2019 bring us a peek
into a selection of mammal skulls (Fig. 1) preserved in concretions buried in the first few hundreds of thousands of years (up to 1million years) of sediment following the K-T extinction event (= KPgE) 66mya.
Figure 1. From Lyson et al. 2019 showing skulls of increasing size following the KPgE. These come from a variety of clades, not a single one. Tiny taxa were omitted from every stratum.
The illustration of skull through time from Lyson et al. 2019
from one locality (Fig.1 ) suggests that mammals were small immediately following the KPgE and thereafter increased and diversified over time. No doubt that happened in a general sense. However…
Placed into a phylogenetic context
(using the large reptile tree (LRT, 1590 taxa) indicates the skulls are from a wide variety of mammals, not a single clade. Lyson et al. omitted tiny taxa from every stratum, evidently to make them tell this tale.
- Didelphodon is a highly derived creodont marsupial in the LRT (Fig. 2).
- Baioconodon is a related marsupial not yet in the LRT
- Loxolophus A is a third creodont in the LRT
- Loxolophus B is a fourth creodont in the LRT
- Ectoconus is a basal terrestrial herbivorous placental (= basal condylarth) in the LRT
- Carsioptychus (originally Plagioptychus) nests with Sinonyx in the Anagale / tenrec / odontocete clade in the LRT. It was a mesonychid mimic.
- Taeniolabis is a highly derived multituberculate member of Glires
- Eoconodon nests with Mesonyx or Sinonyx, a mesonychid mimic (Fig. 3). I need more data than just a mandible.
In other words,
these taxa come from a variety of marsupial and placental clades, all with origins deep in the Mesozoic. The increases in skull size in the graphic (Fig. 1) following the extinction event was done by cherry-picking these skulls and omitting small taxa. We know that tiny rodents, primates and tree shrews were present in the earliest Paleocene because we have them today and we have them in the Jurassic. The authors told the story they wanted to tell and my hat is off to them. The publicity rush (see links below) and PBS NOVA special (see YouTube video below) that attend the publication of their paper attests to the industry they tapped into that exists to promote stories that otherwise would not have risen to this level of interest. After all, other fossils found in concretions don’t get this sort of press.
Even so,
it’s always good to see paleontology told so well on the screen. And discoveries are always worthwhile. Some of these taxa (see list above) had to be added to the LRT to figure out just what they were in a phylogenetic sense, and that’s always interesting as well.
Figure 2. Didelphodon from Wilson et al. 2016 had a 9 cm long skull.
Figure 3. Eoconodon was either a mesonychid like Mesonyx, or a pre-tenrec mesonychid-mimic like Sinonyx. You can see how similar the mandibles are to each other. Even the teeth are similar.
References
Lyson TR et al. (15 co-authors) 2019. Exceptional continental record of biotic recovery after the CretaceousâPaleogene mass extinction. Science: eaay2268 (advance online publication) DOI: 10.1126/science.aay2268
Wilson GP, Eddale EG, Hoganson JW, Calede JJ and Vander Linden A 2016. A large carnivorous mammal from the Late Cretaceous and the North American origin of marsupials. Nature Communications 7:13734 PDF
https://science.sciencemag.org/content/early/2019/10/23/science.aay2268
https://www.sciencemag.org/news/2019/10/how-life-blossomed-after-dinosaurs-died
https://www.pbs.org/wgbh/nova/article/fossils-million-years-after-dinosaurs-died/
https://phys.org/news/2019-10-fossil-trove-life-fast-recovery.html
The Pterosaur Heresies 