Mississippian taxonomic orders

200px|thumb|left|Teeth of Petalodus|Petalodus ohioensis Petalodontiformes ("thin-plate teeth") is an extinct order of marine cartilaginous fish related to modern day chimaera found in what is now the United States of America and Europe.

right|thumb|Helicoprion bessonovi, teeth at the front of the lower jaw (reversed for more natural position) 225px|thumb|Restoration of Romerodus ([[Caseodontidae)]]

Xenacanthiformes (or Xenacanthida) is an order or superorder of extinct shark-like chondrichthyans (cartilaginous fish) known from the Carboniferous to the Late Triassic. They were native to freshwater, marginal marine, and shallow marine habitats. Some xenacanths may have grown to lengths of . Most xenacanths died out at the end of the Permian in the End-Permian Mass Extinction, with only a few forms surviving into the Triassic.

Iniopterygiformes (Originally spelled Iniopterygia and sometimes informally abbreviated as "iniops") is an extinct order of cartilaginous fish known only from the Carboniferous period of the United States. Iniopterygians are characterized by large, superficially wing-like pectoral fins positioned upwards behind the head, from which the name of the group (translated as "nape fin") is derived. Iniopterygians are also noted to possess proportionally large skulls and eyes, armor plates composed of dentin, and "tooth-whorls" of fused teeth. Their elongated pectoral fins bore large, denticle-covered