Traditionally, Spinosauridae is divided into two subfamilies: Spinosaurinae, which contains the genera "Icthyovenator", "Irritator", "Oxalaia", "Sigilmassasaurus" and "Spinosaurus", is marked by unserrated, straight teeth, and external nares which are further back on the skull than in Baryonychinae. And Baryonychinae, which contains the genera "Baryonyx" and "Suchomimus", is marked by serrated, slightly curved teeth, smaller size, and more teeth in the lower jaw behind the terminal rosette than in spinosaurines. Others, such as "Siamosaurus", may belong to either Baryonychinae or Spinosaurinae, but are too incompletely known to be assigned with confidence. "Siamosaurus" was classified as a spinosaurine in 2018, but the results are provisional and not entirely conclusive.