Abstract
Acanthodians may represent a paraphyletic assemblage of stem chondrichthyans, stem osteichthyans, stem gnathostomes, or some combination of the three. One of the difficulties in determining the phylogenetic affinities of this group of mostly small, spiny fishes is that several subgroups of acanthodians are represented by relatively little information in the fossil record. It is becoming increasingly apparent that to understand the evolution of gnathostomes, we must understand more about acanthodians. This study uses micro-computed tomography to test hypotheses about acanthodian jaw function, and in doing so provides insight into the form, function, and ecological role of ischnacanthiform acanthodian jaws and teeth from an extraordinary Early Devonian fossil locality in the Northwest Territories of Canada. The results of this study suggest that ischnacanthiform acanthodians may have coexisted by trophic niche differentiation, employing specialized feeding strategies during the Silurian and Early Devonian.
Highlights
Acanthodians were generally small, spiny fishes that are represented in the fossil record from the Early Silurian (Hanke et al 2001b; Karatajute-Talimaa and Smith 2002; Burrow 2003) to the Permian (Denison 1979) by mostly disarticulated scales, fin spines, and teeth
The occlusion of the teeth was restricted by the anterior and posterior lateral flanges associated with each lateral tooth cusp in E. rugosus and by the flanges and posterior teeth in T. gannitus
The occlusion in T. gannitus is closer to the cog-like vertical tooth closure suggested by Burrow (2004) than is the occlusion in E. rugosus, neither species exactly fits the model proposed for Silurian-Middle Devonian ischnacanthiforms
Summary
Acanthodians were generally small, spiny fishes that are represented in the fossil record from the Early Silurian (Hanke et al 2001b; Karatajute-Talimaa and Smith 2002; Burrow 2003) to the Permian (Denison 1979) by mostly disarticulated scales, fin spines, and teeth. The vast majority of ischnacanthiform acanthodian specimens are disarticulated dermal tooth-bearing elements referred to as dentigerous jaw bones (Long 1986; Hanke et al 2001b; Burrow 2004, 2007; Hairapetian et al 2006; Voichyshyn and Szaniawski 2012).
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