[2] This species ranges from Tomales Bay to the Gulf of California, generally in inshore waters no deeper than 6 m (20 ft).

The most distinctive traits of this plain-colored ray are the three parallel rows of large, hooked thorns that start from the middle of the back and run onto the tail.

Encountered singly or in groups, the thornback guitarfish feeds on small, benthic invertebrates and bony fishes.

[1] Based on morphology, John McEachran and Neil Aschliman concluded in a 2004 phylogenetic study that Platyrhinoidis and Platyrhina together form the most basal clade of the order Myliobatiformes, and are thus the sister group to all other members of the order (encompassing stingrays and their relatives), rather than being closely related to the true guitarfishes of the family Rhinobatidae, a possibility that had long been considered by taxonomists.

[6] Molecular phylogenetics, by contrast, consistently recovers Platyrhinidae as being a close relative of neither guitarfish nor stringrays, but rather as the sister-group to Torpediniformes, the electric rays.

[2] The pectoral fin disc of the thornback guitarfish is heart-shaped, slightly longer than it is wide, and thick towards the front.

It inhabits coastal habitats with muddy or sandy bottoms, including bays, sloughs, beaches, and lagoons, and can also be found in kelp beds and adjacent areas.

[10] Thornback guitarfish mate in late summer, and females give birth the following year at around the same time, peaking in August.

[4][7] Off the United States, this ray is common and faces no substantial threats: it is only occasionally caught incidentally by commercial and recreational fishers, and has no economic value.

However, in Mexican waters the thornback guitarfish population is small and fragmented, and the degree to which it is affected by fishing is uncertain.

There, the IUCN has listed it locally under Data Deficient while noting its susceptibility to inshore lagoon fisheries and shrimp trawlers, and the urgent need for additional information to ensure its long-term regional survival.