A new species of ischnacanthiform acanthodian from the Givetian of Mimerdalen, Svalbard

Abstract

A new ischnacanthiform acanthodian Serradentus armstrongi nov. gen. et sp. has been collected from the Middle Devonian (Givetian) Fiskekløfta Member, the upper member of the Tordalen Formation in the Mimerdalen Subgroup of Spitsbergen. The specimen comprises both pairs of upper and lower jaw bones, jaw cartilages, tooth or denticle whorls, dentition cones and spiky denticles. The latter two features have previously only been identified in Early Devonian ischnacanthiforms. The geology is consistent with a depositional environment of a brackish backwater lagoon with an anoxic bottom. The fish probably entered the lagoon during storm action and died due to the storm churning up bottom anoxic waters, or from being trapped and killed by hypersalinity. The fish possibly partially decomposed on the surface before the head detached and sank to the lagoon floor. The lack of scavengers due to the anoxic conditions prevented scattering of the individual elements, with denticles preserved along the labial surface of the dentigerous jaw bone, and jaw cartilage under the jaw bone.

Diverse stem-chondrichthyan oral structures and evidence for an independently acquired acanthodid dentition

“…Dentition cones — tooth-like cones with smaller denticles attached — are present in the mouths of some stem-group chondrichthyans. They are only known in partially-articulated ischnacanthids with dentigerous jaw bones: Zemylacanthus ( Poracanthodes ), Acritolepis , and Serradentus [34,69,70]. However, they are absent in the better characterised Ischnacanthus and relatives [19,23].…”

“…However, they are absent in the better characterised Ischnacanthus and relatives [19,23]. The lack of fully articulated fossils bearing dentition cones leaves open the possibility that they represent a displaced part of the branchial apparatus rather than oral structures [34]. Their presence may unite a subset of acanthodians with dentigerous jaw bones.…”

“…Dentigerous jaw bones are tooth-bearing dermal jaw bones present in the upper and lower jaws of a number of stem-chondrichthyan taxa [21]. Although isolated dentigerous jaw bones are comparatively common in the rock record, articulated fossils are relatively few and far between: they include Zemlyacanthus, Nerepisacanthus , and Serradentus [30,34,72]. By far the best anatomically characterised taxa with dentigerous jaw bones are Ischnacanthus and similar taxa [23,73].…”

“…An articulated specimen of Atopacanthus sp. from the Upper Devonian is also known [32], but its attribution to this genus is questionable [33], and it has subsequently been referred to Serradentus [34]. The specimen described here, NHMUK PV P.10978 (figure 2), is a dentigerous jaw bone collected from Elgin, Scotland, and is Middle Devonian in age.…”

“…The type species, Atopacanthus dentatus , is known from several dentigerous jaw bones from near Hamburg, New York and is presumed to be an ischnacanthid [23, 47]. The sole articulated specimen attributed to Atopacanthus sp., from the Upper Devonian of the Rhineland [48, 49], has since been referred to Serradentus [50] and so the genus is known only from disarticulated remains. The specimen described here, NHMUK PV P.10978 (figure 3), is a dentigerous jaw bone collected from Elgin, Scotland, and is Middle Devonian in age.…”