How do Osteoglossiformes differ from Hiodontiformes?

Osteoglossiformes and Hiodontiformes are the two orders that comprise the superorder Osteoglossomorpha, a group of bony fishes characterized by using their tongues as an opposing surface for teeth when feeding.1

They differ primarily in their geographic distribution and evolutionary status:

• Geographic Range: Osteoglossiformes are biologically and morphologically diverse, found in freshwater environments across Africa, Asia, South America, Australia, and the tropical Pacific islands, with some species occasionally entering brackish water. In contrast, Hiodontiformes are confined exclusively to North America.12
• Evolutionary Status: Hiodontiformes, represented by the family Hiodontidae (goldeyes and mooneyes), are considered the most primitive group of living osteoglossomorphs. While Osteoglossiformes also exhibit primitive traits, they include highly specialized groups such as the Mormyridae, which possess electric organs.12

Classification between these groups is determined by skeletal characteristics, including the caudal fin skeleton, gill arches, and inner ear anatomy.2