the work of Nelson, Grande and Wilson 2016 and van der Laan 2016. Superclass †Thelodontomorphi Jaekel 1911 Class †Thelodonti Kiaer 1932 Family †Oeseliidae Märss 2005 Order †Archipelepidiformes Wilson & Märss 2009 Family †Boothialepididae Märss 1999 Family †Archipelepididae Märss ex Soehn et al. 2001 Order †Furcacaudiformes Wilson & Caldwell 1998 (Fork-tailed thelodonts) Family †Nikoliviidae Karatajūtė-Talimaa 1978 Family †Lanarkiidae Obručhev 1949 Family †Pezopallichthyidae Wilson & Caldwell 1998 Family †Drepanolepididae Wilson & Marss 2009 Family †Barlowodidae Märss, Wilson & Thorsteinsson 2002 Family †Apalolepididae Turner 1976 Family †Furcacaudidae Wilson & Caldwell 1998 Clade Thelodontida Stensiö 1958 non Kiaer 1932 s.l. Family †Talivaliidae Marss, Wilson & Thorsteinsson 2002 Family †Longodidae Märss 2006b Family †Helenolepididae Wilson & Märss 2009 Order †Sandiviiformes Karatajūtė-Talimaa & Märss 2004 Family †Angaralepididae Karatajūtė-Talimaa & Märss 2004 Family †Stroinolepididae Karatajūtė-Talimaa & Märss 2002 Family †Sandiviidae Karatajūtė-Talimaa & Märss 2004 Order †Turiniida Stensiö 1958 Family †Turiniidae Obručhev 1964 Order †Thelodontiformes Family †Thelodontidae Jordan 1905 Order †Loganelliiformes Turner 1991 Family †Nunavutiidae Marss, Wilson & Thorsteinsson 2002 Family †Loganelliidae Märss, Wilson & Thorsteinsson 2002 Order †Phlebolepidiformes Berg 1937 s.s. Family †Phlebolepididae Berg 1937 corrig. Family †Shieliidae Märss, Wilson & Thorsteinsson 2002 Family †Katoporodidae Soehn et al. 2001 ex Märss, Wilson & Thorsteinsson 2002 Scales The bony scales of the thelodont group, as the most abundant form of fossil, are also the best understood – and thus most useful. The scales were formed and shed throughout the organisms' lifetimes, and quickly separated after their death. Bone – being one of the most resistant materials to the process of fossilisation – often preserves internal detail, which allows the histology and growth of the scales to be studied in detail. The scales consist of a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid upper surface and an aspidine (acellular bony tissue) base. Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. Beyond that, there appear to be five types of bone-growth, which may represent five natural groupings within the thelodonts – or a spectrum ranging between the end members, meta- (or ortho-) dentine and mesodentine tissues. Each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish. Scale morphology, alone, has limited value for distinguishing thelodont species. Within each organism, scale shape varies greatly