Santanachelys gaffneyi (Hirayama 1998, Early Cretaceous, 110 mya, 20 cm long, carapace: 14.5 cm, Fig. 1) was once considered the oldest sea turtle, Note the small flippers. The skull is specialized in the manner of later chelonioids, with large interorbital foramina that are indicative of huge lachrymal salt glands surrounding the eyes.
Thalassochelys caretta (Loggerhead turtle) is a living sea turtle. The flippers (hands) are much larger in this much larger extant sea turtle.
You might wonder if the Santanachelys holotype is a hatchling.
You might be right because in 10 million years older sediments we find Desmatochelys padillai (Cadena and Parham 2015, Fig. 2). and it is bigger than some living sea turtles. So we’re not yet seeing any phylogenetic miniaturization at the base of this clade. We may never.
Desmatochelys has larger forelimb paddles than Santanachelys and a more open carapace and plastron architecture. So, given the extra bone in the shell and smaller paddles,. Santanachelys may be more primitive, despite its later appearance in the fossil record.
The minimally invasive posterior margin of the skull indicates that sea turtles split from other turtles at an early stage in turtle evolution.
Cadena EA and Parham JF 2015. Oldest Known Marine Turtle? A New protostegid from the Lower Cretaceous of Colombia. PaleoBios. 32(1).
Hirayama R 1998. Oldest known sea turtle. Nature 392 (6677): 705–708.