Beneath Giants

Who you are

The mother is a small mammal of the lineage that led to placental mammals, set in a Hell Creek-style forest in what is now western North America, shortly before the asteroid struck. Which species, if any we know, was on our own line is unknown.

Mammals do not come from dinosaurs. Our line split from the reptile line, which includes dinosaurs and birds, more than 300 million years ago.

What the evidence shows

• Molecular clocks place the origin of placental mammals before the extinction, while definitive placental fossils before it are lacking. One 2023 model places their origin shortly before the impact.
• Fossil limb bones suggest mammals in western North America were becoming more ground-dwelling towards the end of the Cretaceous.

What is uncertain

• Whether true placentals already existed at this time is debated, so the game calls the mother “of the placental line” rather than a placental.

Sources

• Carlisle et al. 2023. A timescale for placental mammal diversification based on Bayesian modeling of the fossil record. Current Biology. (opens the publisher’s site)

  What it supports: A model placing the origin of placental mammals shortly before the asteroid impact, with most modern mammal orders appearing at or after it. Definitive Cretaceous placental fossils are still lacking.

• Janis et al. 2025. Down to earth: therian mammals became more terrestrial towards the end of the Cretaceous. Palaeontology. (opens the publisher’s site)

  What it supports: Fossil limb bones suggesting that mammals in western North America were becoming more ground-dwelling towards the end of the Cretaceous.

Summaries here are our own. Some publishers limit access to full papers; learning on this site is never paywalled.