Imagine that a researcher has discovered a leaf fossil from the cambrian period that is 500 million years old. he notices that the leaf had very few stomata compared to present-day leaves. what does this tell the researcher about atmospheric co2 levels during the cambrian period?

It could mean that in the Cambrian Period the CO₂ levels present in the air could be much smaller than the CO₂ levels presents in present days.  

Explanation:

The stomata are porous presents in the leaves and stems of plants, that facilitates and control the gaseous flows from and into the plant. To make photosynthesis the plants needs to absorb CO₂ and water to convert this reactants in glucose, to produce their energy, and O₂ which is released into the atmosphere. This is represented in the following reaction:  

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The absorption of CO₂ and the release of O₂ are controlled by the stomata. An increase in the amount of CO₂ in the air will result in greater development of the stomata in the plants since this gas is used to produce glucose which is then transformed into energy.    

So, if the researcher had found a leaf fossil from the Cambrian Period with very few stomata, it could mean that in that period the CO₂ levels present in the air could be much smaller than the CO₂ levels present in present days.    

Have a nice day!