Gladii are known from a number of extinct cephalopod groups, including teudopseids (e.g.Actinosepia, Glyphiteuthis, Muensterella, Palaeololigo, Teudopsinia, Teudopsis, and Trachyteuthis), loligosepiids (e.g.Geopeltis, Jeletzkyteuthis, and Loligosepia), and prototeuthids (e.g.Dorateuthis, Paraplesioteuthis, and Plesioteuthis).
^Hunt, S. & M. Nixon (1981). A comparative study of protein composition in the chitin-protein complexes of the beak, pen, sucker disc, radula and oesophageal cuticle of cephalopods. Comparative Biochemistry and Physiology Part B: Comparative Biochemistry68(4): 535–546. doi:10.1016/0305-0491(81)90071-7
(Russian) Bizikov, V.A. (1991). Squid gladius: its use for the study of growth, age, intraspecies structure and evolution (on the example of the family Ommastrephidae). Ph.D. Thesis. Institute of Oceanology, SSSR Academy of Sciences, Moscow. 513 pp. (English abstract)
Toll, R.B. (1982). The comparative morphology of the gladius in the order Teuthoidea (Mollusca: Cephalopoda) in relation to systematics and phylogeny. Ph.D. Thesis. University of Miami, Coral Gables, Florida. 390 pp.
Toll, R.B. (1998). The gladius in teuthoid systematics. Smithsonian Contributions to Zoology586(1): 55–67.