In 1914, Barnum Brown and Peter Kaisen, working for the American Museum of Natural History, in Alberta at the Sand Creek near the Red Deer River, eight miles southeast of Steveville, excavated an ankylosaur skeleton, specimen AMNH 5337.

The generic name is derived from Greek platys, "wide", and plate, "small shield", in reference to the broad osteoderms.

The specific name honors Walter P. Coombs Jr., who pioneered the study of ankylosaurs in the late twentieth century.

[3] The holotype, AMNH 5337, was found in a layer of the Dinosaur Park Formation at an altitude of 667 metres, which indicates an age of about 76,8 million years.

Some of the armour plates on its rump are large, exceeding twenty-five centimetres in length, and have a rough top surface with a sharp keel continuing into a pointy beak-shape.

The squamosal horns, at the rear corners of the skull roof, are low and rough with a broad base and a pitted point extending to behind.

On the first cervical halfring, the transverse armour band protecting the top front of the neck, the paired middle osteoderms are elongated and partly keeled with a conical projection.

More precisely, it was a member of the clade Euoplocephalini, established by Penkalski, which is an approximate synonym of Ankylosaurini in several studies.