Abstract
BackgroundThe relationships between early jawed vertebrates have been much debated, with cladistic analyses yielding little consensus on the position (or positions) of acanthodians with respect to other groups. Whereas one recent analysis showed various acanthodians (classically known as ‘spiny sharks’) as stem osteichthyans (bony fishes) and others as stem chondrichthyans, another shows the acanthodians as a paraphyletic group of stem chondrichthyans, and the latest analysis shows acanthodians as the monophyletic sister group of the Chondrichthyes.Methodology/Principal FindingsA small specimen of the ischnacanthiform acanthodian Nerepisacanthus denisoni is the first vertebrate fossil collected from the Late Silurian Bertie Formation Konservat-Lagerstätte of southern Ontario, Canada, a deposit well-known for its spectacular eurypterid fossils. The fish is the only near complete acanthodian from pre-Devonian strata worldwide, and confirms that Nerepisacanthus has dentigerous jaw bones, body scales with superposed crown growth zones formed of ondontocytic mesodentine, and a patch of chondrichthyan-like scales posterior to the jaw joint.Conclusions/SignificanceThe combination of features found in Nerepisacanthus supports the hypothesis that acanthodians could be a group, or even a clade, on the chondrichthyan stem. Cladistic analyses of early jawed vertebrates incorporating Nerepisacanthus, and updated data on other acanthodians based on publications in press, should help clarify their relationships.
Highlights
The group of early jawed fishes traditionally referred to the Acanthodii is emerging as pivotal in our understanding of relationships between extinct and extant gnathostomes, i.e. jawed fishes [1], [2]
All specimens of one such rare species, the ischnacanthiform Onchus graptolitarum [5] from the Late Silurian (Pridoli) of the Czech Republic – to date, the only Silurian acanthodian represented by several partial articulated fish – have been long lost
We describe a single, almost complete representative of Nerepisacanthus denisoni that confirms the assignment of this species to the ischnacanthiform family Acritolepidae, based on a combination of features including dentigerous jaw bones, shallowly inserted laterally flattened fin spines ornamented with smooth longitudinal ridges, absence of prepelvic fin spines, body scales with superposed growth crowns formed of odontocytic mesodentine, and scales in the head/branchial region with polyodontode, appositional growth crowns
Summary
The group of early jawed fishes traditionally referred to the Acanthodii is emerging as pivotal in our understanding of relationships between extinct and extant gnathostomes, i.e. jawed fishes [1], [2]. The holotype specimen of the only other undisputed Silurian acanthodian taxon based on an articulated fossil, Nerepisacanthus denisoni [4] from the Ludlow or Pridoli of New Brunswick, Canada, lacks most of the head and the posterior half of the body, with the anterior dorsal spine being the only fin spine preserved. The Fiddlers Green and Williamsville members of the Bertie Formation in Ontario represent the northwestern edge of a much broader outcrop belt of long-famous ‘‘waterlime’’ deposits that extends into east-central New York State. These massive cement-like to argillaceous dolostones originated in subtidal and intertidal lagoonal settings, under alternating hypersaline and brackish estuarine conditions. Whereas one recent analysis showed various acanthodians (classically known as ‘spiny sharks’) as stem osteichthyans (bony fishes) and others as stem chondrichthyans, another shows the acanthodians as a paraphyletic group of stem chondrichthyans, and the latest analysis shows acanthodians as the monophyletic sister group of the Chondrichthyes
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