Earlier the horned turtles, Meiolania and Niolamia, were nested in the large reptile tree (LRT) as basalmost hardshell turtles, closely related to the toothed horned stem turtle/pareiasaur, Elginia. This was heresy when introduced.
newly discovered turtle eggs (Lawver and Jackson 2016) add evidence to the basal status of Meiolania.
From the Lawver and Jackson 2016 abstract:
“A fossil egg clutch from the Pleistocene of Lord Howe Island, Australia that we assign to Testudoolithus lordhowensis, oosp. nov. belongs to the stem turtle Meiolania platyceps. Thin sections and scanning electron microscopy demonstrate that these eggs are composed of radiating acicular aragonite crystals. This mineral composition first evolved either before the split between Meiolaniformes and crown Testudines or prior to Proterochersis robusta, the earliest known stem turtle. Meiolania platyceps deposited its eggs inside an excavated hole nest. This nesting strategy likely evolved no later than the Early to Middle Jurassic.”
All known meiolanids
are from later, higher Late Cretaceous and Tertiary strata.
soft-shell and hard-shell turtles have a dual origin from separate small Late Permian and Middle Triassic pareiasaur ancestors, Elginia and Sclerosaurus. Both were also horned. The traditional earliest known turtles, Proganochelys and Odontochelys are both known from later, Late Triassic, strata.
Not on topic, but worth watching on YouTube:
Here’s a video about the origin of oil in the Jurassic. It runs for 90 minutes and is fascinating throughout. The video reminds us what a recent Golden Age we currently live in based on a limited supply of petroleum products. The video concludes we have long passed the tipping point for climate change based on the flood of cheap energy. And the end of the oil age is something our children will see. Ironically, climate change in the ice-free Jurassic was one factor in the Earth producing the oil we now use.
Lawver DR and Jackson FD 2016. A fossil egg clutch from the stem turtle Meiolania platyceps: implications for the evolution of turtle reproductive biology. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1223685.