Carbonemys cofrinii (Cadena et al. 2012; Late Paleocene, 60mya; carapace 1.7m) was one of the largest terrestrial turtles and it lived alongside the giant snake, Titanoboa. Originally considered a member of the Pleurodira (side-necked turtles), here Carbonemys nests with Macrochelys. The skull UF/IGM 41 scale bars indicate it was too small for the carapace UF/IGM 71. Note the small plastron, narrower rostrum and new identities for dorsal skull bones, as in snapping turtles.

Macrochelys temminckii (Gray 1856, originally Chelonura temminckii , Macroclemys temminckii, Troost in Harlan, 1835; extant) the alligator snapping turtle has powerful jaws, a long extensible neck and sharp ridges on the shell. This cryptodire turtle nests in the large reptile tree with Terrapene.

Archelon ischyros (Wieland 1896; Late Cretaceous; 4.6m or 15 feet in length) is the largest turtle ever documented. Along with Protostegus, Archelon is traditionally considered a member of the Protostegidae. In the LRT Archelon nests with Macrochelys, the snapping turtle (above), distinct from and parrallel to other sea turtles, all of which have a shorter, transverse premaxilla and different skull bone patterns. Previous workers had removed protostegids from other sea turtles, but their relationship to snapping turtles was not tested or known. A leathery carapace, like that of Dermochelys, covered the similarly open ribs, but the two tax are not related. Distinct from Macrochelys, the naris opens dorsally in Archelon.

Cadena EA, Ksepka DT, Jaramillo CA and Bloch JI 2012. New pelomedusoid turtles from the late Palaeocene Cerrejón Formation of Colombia and their implications for phylogeny and body size evolution. Journal of Systematic Palaeontology. 10 (2): 313.

Troost G 1835. In: R. Harlan. Medical and Physica Researches or Original Memoires in Medicine, Surgery, Physiology, Geology, Zoology, and Comparative Anatomy. L.R. Bailey, Philadelphia. xxxix + 653 pp.

Wieland GR 1896. Archelon ischyros: a new gigantic cryptodire testudinate from the Fort Pierre Cretaceous of South Dakota. American Journal of Science. 4th series. 2 (12): 399–412.

wiki/Carbonemys
wiki/Macrochelys
wiki/Archelon