Acanthodii Owen 1846

Class ACANTHODII Owen, 1846

REMARKS

The order Climatiiformes is thought to contain the most primitive acanthodian species, including Lupopsyrus pygmaeus. The diagnosis of the Climatiiformes provided by Denison (1979) included acanthodians with enlarged cranial tesserae and scales, a dermal shoulder girdle including pinnal and lorical plates, and in some cases, prepectoral spines, plus all climatiiforms possessed two dorsal fins. Not surprisingly, the diagnosis of the order has changed with each new publication as new taxa are shoe-horned into the classification scheme. Gagnier & Wilson (1996a) revised this diagnosis and limited the climatiiform character list to include: scales with, or derived from Nostolepis Pander, 1856, type histological structure, the presence of two dorsal fins, and the presence of fairly large head scales to accommodate Kathemacanthus rosulentus Gagnier & Wilson, 1996a, and Brochoadmones milesi Bernacsek & Dineley, 1977; note that K. rosulentus has been recently reclassified as a putative chondrichthyan based primarily on scale growth (Hanke & Wilson 2010). Gagnier & Wilson (1996a) excluded pinnal and lorical plate armour and prepelvic spine presence from their climatiiform character list, to incorporate their new taxa and because mesacanthids also possess prepelvic spines (see: Egerton 1861; Miles 1966, 1973; Denison 1979; Gagnier 1996; Upenice 1996; Cumbaa & Schultze 2002; Hanke 2008). Some mesacanthids also have fairly large head scales, so we think that this feature too is not unique to climatiiforms. Support for Gagnier & Wilson’s decision to eliminate prepectoral and prepelvic spines as a climatiiform characteristic follows the discovery of several new taxa from MOTH which possess these spines but lack characteristic scales and perichondral bone of acanthodians (Hanke & Wilson 1998, 2004, 2010; Wilson& Hanke 1998). This leaves us with: 1) scales derived from a Nostolepis - type of histology; and 2) two dorsal fins, as potential features defining the order Climatiiformes.Two dorsal fins are present in non climatiid acanthodians such as: diplacanthids, ischnacanthids and also gyracanthids plus early chondrichthyans, osteichthyans and sarcopterygians (Janvier 1996). Furthermore, most “ Nostolepis ” species are known only from isolated microremains, with the exception of a few taxa (Valiukevičius 2003a; Burrow & Turner 2010), and acritolepid ischnacanthiforms have Nostolepis - type scale histology (Valiukevičius & Burrow 2005). As a result we cannot support the definition of climatiiform acanthodians using histological features now known to exist outside the group. This historical perspective shows there are no synapomorphies to unite the climatiiforms as historically defined (Janvier 1996; Hanke 2001; Davis 2002; Hanke & Wilson 2004; Burrow & Turner 2010), and as a result, the higher classification of L. pygmaeus is left open pending detailed reexamination of climatiiform fishes.