Was There a Cambrian Explosion on Land?
A paper worth reading:
Tihelka, E.; Howard, R.J.; Cai, C.; Lozano-Fernandez, J.
Was There a Cambrian Explosion on Land? The Case of Arthropod Terrestrialization.
Biology 2022, 11, 1516.
https://doi.org/10.3390/biology11101516
This paper argues for far earlier life on land than the typical picture of the Cambrian period. That is, the geological record has land plants and animals appearing in the Silurian period 443-419 years ago.
DNA molecular clocks show a much earlier origin of terrestrial plants and animals, possibly at the end of the Cambrian/beginning of Ordovician.
This paper points out that there is some fossil evidence for terrestrial life even in the Cambrian, like a land (or at least beach) trackway left by an arthropod with 11 pairs of legs. There are very few terrestrial sedimentary rocks from before the Devonian period, so there may be a lot of evolution taking place on land that’s just missing from the fossil record.
That’s a pretty interesting idea, but the authors view it all through the “terrestrialization” of arthropods. It seems difficult to evolve the support structure (exoskeleton), means of absorbing oxygen from the air, and means of exuding carbon dioxide. It seems that at least 4 groups of arthropods managed it: millipedes and centipedes, insects, rolli-pollis (pill bugs or isopods, which are crustaceans), spiders and scorpions. There’s not much of a fossil record of how they managed this transition, as opposed to the record of vertebrates managing the same feat. So we know for a fact that some evolution took place on land and we don’t have a fossil record of it. We’ve only got phylogenomics to inform us of probably when it took place.