Amniotes are a clade of vertebrates comprising the reptiles, birds and mammals that lay their eggs on land or retain the fertilized egg within the mother. They are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water.
Recently, endogenized sequences of Hepatitis B viruses (eHBV's) have been discovered in bird and snake genomes where they constitute direct evidence for the coexistence of these viruses and their hosts from the late Mesozoic until present.
Recently a report was published that showed the first eHBV's from crocodilian, snake and turtle genomes, including a turtle eHBV that endogenized >207 million years ago. This genomic 'fossil' is >125 million years older than the oldest avian eHBV and provides the first direct evidence that Hepattitis B viruses already existed during the Early Mesozoic.
The study reveals an unforeseen host range of prehistoric Hepatitis B viruses and provides novel insights into the genome evolution of Hepatitis B viruses throughout their long-lasting association with amniote hosts.
 Suh et al: Early Mesozoic Coexistence of Amniotes and Hepadnaviridae in Plos Genetics – 2016