Remembering the time periods... 

Passing forward a memory trick from a fossil sage,

Pregnant camels often sit down carefully;
perhaps their joints creak too quietly!

• Pregnant: Pre Cambrian period
• Camels: Cambrian period
• Often: Ordovician period
• Sit: Silurian period
• Down: Devonian period
• Carefully: Carboniferous period
• Perhaps: Permian period
• Their: Triassic period
• Joints: Jurassic period
• Creak: Cretaceous period
• Too: Tertiary period
• Quietly: Quaternary period