Did the Seleucids Invent Heavy Cavalry?

[Alanus]

I would say the Seleucid Empire was a tad late in employing heavily-armored  "cataphracts." Early documentation is void of descriptive particulars, but all indications point to the East as influenced by Siberian chariot cultures. The oldest complete suit of armor was found in Omsk, dated to the early Bronze Age, 1900 to 1800BC.

The Siberian intrusion into late Shang and early Zhou gives us the earliest effective bronze helmets and lamellar, as used in chariot warfare. By the late Western Zhou period, the earliest cavalry units were so equipped. As soon as lamellar construction switched from bone to bronze or iron, it's weight increased to a point where it could not be practically used by infantry. This the important key, and it defines what we might call "cataphractism." In Siberia, the Sargatskya switched from bone to iron in the 4th century BC, most likely from contact with the Massagetae to their south. We have an "implied" use of heavily armored cataphacts in the two accounts of the Tomyris vs. Cyrus war of 550BC. First the Massagetae and Persians expended their arrows (from Herodotus), then the Massagetae "retreated" into a canyon followed by Cyrus' forces. The Massagetae then turned about and decimated the trapped Persians (from Julius Frontinus). This tactic could only be accomplished by a heavy cavalry armed with a contus.

In essence, heavy armor was developed for chariot warfare, then worn by cavalry beginning in the 10th to 9th centuries BC. During the late Spring & Autumn period, cavalry warfare increased, and it bloomed during the Warring States Period, ending in 221BC with Shihuangdi's consolidation of the First Chinese Empire. Basically, heavy armor did not change until the general use of chainmail. Here are pictorial examples of early heavy armor:

   
The bone lamellar armor found in Omsk, northwest of the Altai, dated to roughly 1900BC.

   
Typical bronze helmet, early Zhou, from Liulihe, Hebei, dated to first generation 1045-1015BC. It is identical to the helmet found in a woman general's grave at Baifu. dated to 950BC. These have also been found in Inner Mongolia and the Kuban above the Black Sea, mistakenly identified as "Scythian."

   
Terracotta representation of cavalry armor from Shihuangdi's Mausoleum. Originals were bronze and extremely heavy, and date back to Eastern Zhou, 771-400BC. We can rule out the use of heavy lamellar by foot soldiers-- it was chariot armor, then used by heavy cavalry.

   
Sargatskya armor, 4th century BC., likely based on Massagetae armor (the culture just south of the Sargatskya).

   
The Orlat Battle Plaque, found in Sogdiana, depicting a battle between the Yuezhi's heavy cavalry against the Sacarauli, c. 160BC. The helmets, lamellar, and swords are all based on Chinese prototypes.

   
Reconstruction of Roxolani heavy armor. You can see its provenance extends back to China and (before that) to Siberia. As far as I know, I'm the only person foolish enough to wear anything this crazy. It kills you, absolutely kills you, but it offered the best protection for its time. Personally, in view of its extreme north-eastern origin, I would say the Seleucids were preceded by early Siberians, the Zhou Chinese, and then the Massagetae and Sargatskya in chronological order.