Later medieval periods always depict spears or longswords
About the 2nd century AD, the gladius Hispaniensis began to be phased out by the spatha, a longer Celtic-style sword originally used by cavalry auxiliaries; the spatha had more reach than the gladius, and by the 3rd century it had effectively replaced it for both infantry and cavalry. For a more detailed history, I'd suggest The Gladius: The Roman Short Sword.
The spatha in turn influenced subsequent sword design, which was traced in the sword typologies of Jan Petersen, R.E.M. Wheeler, and Ewart Oakeshott - that takes you right up to the "knightly sword" of the Middle Ages.
Funnily enough, the story doesn't end there. The gladius found a second life in the 19th century as the inspiration for a number of artillery swords - the result of a Neoclassical aesthetic in France. Examples include the French Model 1816 artillery sword, which was revised in 1831 and served to inspire the United States Model 1832 foot artillery sword. Military swords as combat weapons generally fell out of favor in early contemporary warfare, with most of them relegated to ceremonial items by the end of World War I.
I asked a similar question here
Top comment is by /u/Ambarenya
Later Roman armies spent more time on horseback, had taller soldiers, and relied less on tight combat formations. All these factors made longer swords more attractive