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cephalopod beak
All extant cephalopods have a two-part beak, or rostrum, situated in the buccal mass and surrounded by the muscular head appendages. The dorsal (upper) mandible fits into the ventral (lower) mandible and together they function in a scissor-like fashion.〔Young, R.E., M. Vecchione & K.M. Mangold (1999). (Cephalopoda Glossary ). Tree of Life Web Project.〕〔Young, R.E., M. Vecchione & K.M. Mangold (2000). (Cephalopod Beak Terminology ). Tree of Life Web Project.〕 Composed primarily of chitin and cross-linked proteins,〔Saunders, W.B., C. Spinosa, C. Teichert & R.C. Banks (1978). ''Palaeontology'' 21(1): 129–141.〕〔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 Biochemistry'' 68(4): 535–546. 〕〔Miserez, A., Y. Li, J.H. Waite & F. Zok (2007). ''Acta Biomaterialia'' 3(1): 139–149. 〕〔(Organic composite is exceptionally robust: jumbo squid ). Ask Nature.〕 beaks are more-or-less indigestible and are often the only identifiable cephalopod remains found in the stomachs of predatory species such as sperm whales.〔Clarke, M.R. (1986). ''A Handbook for the Identification of Cephalopod Beaks''. Oxford University Press, Oxford.〕 They can be used to estimate the mantle length and total body weight of the original animal as well as the total ingested biomass of the species.〔Clarke, M.R. (1962). (The identification of cephalopod "beaks" and the relationship between beak size and total body weight ). ''Bulletin of the British Museum (Natural History), Zoology'' 8(10): 419–480.〕〔Wolff, G.A. (1981). ''Fishery Bulletin'' 80(2): 357–370.〕〔Wolff, G.A. (1984). NOAA Technical Report NMFS 17, NOAA/National Marine Fisheries Service.〕〔Jackson, G.D. (1995). The use of beaks as tools for biomass estimation in the deepwater squid ''Moroteuthis ingens'' (Cephalopoda: Onychoteuthidae) in New Zealand waters. ''Polar Biology'' 15(1): 9–14. 〕〔Jackson, G.D. & J.F. McKinnon (1996). Beak length analysis of arrow squid ''Nototodarus sloanii'' (Cephalopoda: Ommastrephidae) in southern New Zealand waters. ''Polar Biology'' 16(3): 227–230. 〕〔Jackson, G.D., N.G. Buxton & M.J.A. George (1997). Beak length analysis of ''Moroteuthis ingens'' (Cephalopoda: Onychoteuthidae) from the Falkland Islands region of the Patagonian Shelf. ''Journal of the Marine Biological Association of the United Kingdom'' 77(4): 1235–1238. 〕〔Gröger, J., U. Piatkowski & H. Heinemann (2000). ''Polar Biology'' 23(1): 70–74. 〕 Cephalopod beaks gradually become less stiff as one moves from the tip to the base, a gradient that results from differing chemical composition. In hydrated beaks of the Humboldt squid (''Dosidicus gigas'') this stiffness gradient spans two orders of magnitude.〔Miserez, A., T. Schneberk, C. Sun, F.W. Zok & J.H. Waite (2008). The transition from stiff to compliant materials in squid beaks. ''Science'' 319(5871): 1816–1819. 〕
Fossilised remains of beaks are known from a number of cephalopod groups, both extant and extinct, including squids, octopuses, belemnites, and vampyromorphs.〔Zakharov, Y.D. & T.A. Lominadze (1983). New data on the jaw apparatus of fossil cephalopods. ''Lethaia'' 16(1): 67–78. 〕〔Kanie, Y. (1998). New vampyromorph (Coleoidea: Cephalopoda) jaw apparatuses from the Late Cretaceous of Japan. ''Bulletin of Gumma Museum of Natural History'' 2: 23–34.〕〔Tanabe, K. & N.H. Landman (2002). Morphological diversity of the jaws of Cretaceous Ammonoidea. ''Abhandlungen der Geologischen Bundesanstalt, Wien'' 57: 157–165.〕〔〔Tanabe, K., P. Trask, R. Ross & Y. Hikida (2008). Late Cretaceous octobrachiate coleoid lower jaws from the north Pacific regions. ''Journal of Paleontology'' 82(2): 398–408. 〕〔Klug, C., G. Schweigert, D. Fuchs & G. Dietl (2010). First record of a belemnite preserved with beaks, arms and ink sac from the Nusplingen Lithographic Limestone (Kimmeridgian, SW Germany). ''Lethaia'' 43(4): 445–456. 〕〔Tanabe, K. (2012). Comparative morphology of modern and fossil coleoid jaw apparatuses. ''Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen'' 266(1): 9–18. 〕 Aptychi – paired plate-like structures found in ammonites – may also have been jaw elements.〔Morton, N. (1981). Aptychi: the myth of the ammonite operculum. ''Lethaia'' 14(1): 57–61. 〕〔Morton, N. & M. Nixon (1987). Size and function of ammonite aptychi in comparison with buccal masses of modem cephalopods. ''Lethaia'' 20(3): 231–238. 〕〔Lehmann, U. & C. Kulicki (1990). Double function of aptychi (Ammonoidea) as jaw elements and opercula. ''Lethaia'' 23: 325–331. 〕〔Seilacher, A. (1993). Ammonite aptychi; how to transform a jaw into an operculum? ''American Journal of Science'' 293: 20–32. 〕
The beak may also be referred to as the mandibles or jaws.〔Tanabe, K., Y. Hikida & Y. Iba (2006). Two coleoid jaws from the Upper Cretaceous of Hokkaido, Japan. ''Journal of Paleontology'' 80(1): 138–145. 〕
==Measurements==
The abbreviations LRL and URL are commonly used in teuthology to refer to ''lower rostral length'' and ''upper rostral length'', respectively. This is the standard measure of beak size in Decapodiformes; ''hood length'' is preferred for Octopodiformes.〔
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