Woodlice

145 - 66 mya Cretaceous

Animals Insects Springtails Mites Worms

Isopods

Living isopods of the suborder Oniscidea, which we call woodlice, are the only group of Crustacea almost entirely composed of terrestrial forms. I suspect many people would think that isopods evolved on land earlier than this period. They look ancient, and the idea of an isopod walking out of the sea on to land to become a woodlouse is a powerful image. But it did not happen until this period.

The best fossil evidence prior to this period is 'thin on the ground' to coin an apt phrase. The most widely quoted isopod fossil find (Ferrier & Charbonnier 2014) was in well preserved Carboniferous (360-300mya) habitats in France. But these have since been reclassified as insect nymphs (Garwood et al 2012). Some try to explain the lack of fossils on the nature of the exoskeleton. But it may be because there were no/few isopods about!

Fossil paucity

It is often said that the paucity of fossils between these periods. It may be they were dying in places which do not enable fossilisation. That would help explain the gap until when there is amber for better fossilisation, which doesn’t appear until this Cretaceous period. However, I don’t buy this. The ‘lateness’ of this first fossil woodlice is intriguing, as the rigid exoskeleton appears to look strong. Perhaps it is more obvious explanation - they weren't there.

In this period, there were more developed soils and diverse terrestrial ecosystems that could support the woodlice lifestyle. That involves feeding on decaying organic matter and living in environments with high humidity. Were they the first true 'terrestrialisers'?
"Living isopods of the suborder Oniscidea (commonly called woodlice) are the only group of Crustacea almost entirely composed of terrestrial forms. Furthermore, woodlice are completely independent from the aquatic environment from which they originally arose." (Broly et al 2013)

Better habitat

The diversification of the flowering plants would also have led to increased leaf litter and organic debris, which woodlice would feed on. The Cretaceous terrestrial ecosystems had a broader range of habitats, with microhabitats suitable for the first woodlice to establish themselves.

One march.

It now seems that "phylogenetic analyses support monophyly of terrestrial isopods..and... Dating analyses suggest..origin of isopod terrestriality, much more recently than other terrestrial arthropods" (Thorpe 2024). Although she confirms earliest fossil evidence is ca. 105mya, she believes molecular analysis points to 350-250mya for their great march. But, we have often seen molecular analyses predict emergence earlier than morphological evidence.

Single march into land

"Dating the divergence of terrestrial isopods in Oniscidea also dates the origin of terrestriality in Isopoda. The earliest oniscid fossils date to the mid-Cretaceous, ~105 mya . However, the earliest oniscid divergence in this analysis, between the littoral groups Diplocheta and Tylida and the remaining Oniscidea, dates to the Carboniferous-Permian boundary, ~298 (249-348) mya. These dates suggest isopods made the transition to land considerably later than other terrestrial arthropods; molecular estimates for hexapods, myriapods and arachnids date to between the Ordovician up to the Silurian or Cambrian, alongside the emergence of terrestrial plants. " (Thorpe 2024)

But “to date", the first attested occurrences of Oniscidea are recorded from the Early Cretaceous. At this time, woodlice were already widely distributed (from Western Europe to Eastern Asia) with several species" (Broly et al 2013) This would make sense. While their ancestors may have been around for a couple of hundred million years, the new leaf litter would have provided them with an ideal environment to run around - just as we know them today. Whenever there is a bit of litter, there are woodlice.

Cuticles

The cuticles of terrestrial and marine isopods consist of calcite, calcium carbonate , and calcium phosphate (ACP), in an organic matrix. Marine species have a higher concentration of magnesium within the cuticle. Hard cuticles have a low percentage of organic material, and, in terrestrial species, a higher percentage of Calcium Carbonate indicates that the mineral composition of the crustacean cuticle depends on its function and the habitat of the animal (Neues et al 2007) Many fossils are formed from calcium carbonate which petrifies soft bodies, so it is hard to see why woodlice fossils would be so elusive.

Walk from water?

While there is evidence of their ancestors 200 million years before, we do not know where these ancestors lived. They were probably marine. For their march, their cuticles do not contain wax, hence they depend on damp conditions. The period to walk would be this period, as there is now somewhere dark and damp with much larger living spaces, under litter, for them to walk into. They could not fit the pores nor burrow into the ground.

There are around 3,500 species all across the world, and have 14 legs and moult a couple of times. They can live 3-4 years but would have been preyed upon then by centipedes… and spiders They have an exoskeleton and can roll up – hence its common name ‘pill bug’. They eat rotting plants, fungi, bacteria, so help disperse bacteria and fungal spores large distance, in terms of spore measurements. They also eat their own faeces. But they don’t pee! They get rid of their waste by producing ammonia which passes out through their shells as a gas.

These arthropods are much bigger than springtails so would need looser environment - as dead leaves would provide

The woodlice do a great job at turning over and digesting leaf litter. But they do not carry leaves into the soil.

Woodlice need moisture because they rapidly lose water by excretion and through their cuticle, and so are usually found in damp, dark places, such as under rocks and logs, They have a close relationship with soil organisms and we are going to see later that these humble creatures may be playing an important role in relation to global warming, by eating bacteria, that completely belies their lowly status.

Dont forget to link to global warming..woodlice save the day!

Is the reason why there are no woodlice fossils from soil before this period because they were not there? Despite their ancient appearance, they may have only arrived in litter in this period, because only now was there enough to house them?

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