By Scott A. Rowan
The plastron is the bottom (ventral) half of the shell for turtles, tortoises and terrapins. Like the upper (dorsal) half of the shell, plastrons are composed of several hardened scutes that allow for complete protection of the rounded bottom of the shell.
While some species, such as the horseshoe crab, are known to possess a carapace but no plastron, no living animal is known to possess a plastron but no carapace. The oldest known turtle dinosaur fossil, Odontochelys semitestacea (“half-shelled turtle with teeth”), possessed a plastron, but no carapace, leading many scientists to believe that turtles developed their shell from the bottom up, needing to protect their organs from attack from beneath more so than above.
The Vertebrate Integument Volume 1, Theagarten Lingham-Soliar, (Springer, Verlag Berlin Heidelberg).
Homology, Genes, and Evolutionary Innovation, Günter P. Wagner (Princeton, N.J.: Princeton University Press, 2014).