What is the origin of plantigrade?
The adjective plantigrade comes from the Latin noun planta “sole (of the foot)” and the verb gradī “to take steps, step, walk.” The Proto-Indo-European root ghredh- “to step, stride” is not very common, and all current English words are borrowings from Latin, e.g., gradual, grade, and verbs ending in -gress, e.g., ingress, regress, transgress. Planta, however, is another story: it shows the infix n, but its Proto-Indo-European root is the very common plat-, plet-, plot- “flat, broad.” Plat- is the source of the Lithuanian adjective platùs “wide, broad,” the all but identical Greek adjective platýs “flat, wide” (as in platypus “flatfoot”), the English adjective and noun flat, the noun flet (also flett) “dwelling, hall,” familiar to readers of Beowulf and J.R.R. Tolkien’s Lord of the Rings (probably the same crowd), and flan (the Spanish custard). Plantigrade entered English in the 19th century.