A redescription of the three longest-known species of the acanthodian Cheiracanthus from the Middle Devonian of Scotland

Abstract

The cheiracanthid acanthodiforms were widespread during the Middle Devonian, often being the most abundant acanthodians in northern European vertebrate macroand microfaunal assemblages. Three species of cheiracanthids, Cheiracanthus murchisoni, C. grandispinus, and C. latus, have been known from the Middle Devonian (Eifelian-Givetian) of northern Scotland since the nineteeth century. Here we review the anatomy of these species and show that the main distinguishing features for the three species are the scale ornamentation, general body shape, and relative robustness of their scapulocoracoids and fin spines. They also show different stratigraphic and geographic distributions in the Orcadian Basin. All three species appear at the base of the Coccosteus cuspidatus + Pterichthyodes milleri placoderm biostratigraphic zone; C. latus disappears towards the upper limit of this zone, C. murchisoni extends into the base of the overlying Dickosteus threiplandi zone, and C. grandispinus reaches up to the middle of this zone. Some cheiracanthid taxa based on isolated scales from the Baltic countries, Belarus, and Russia are considered junior synonyms of the Scottish species. The new data we provide should prove helpful in further elucidating the taxonomic position of cheiracanthids.

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

“…In diplacanthids [75,76] they are better interpreted as occlusal plates [25]. Although a mandibular splint has been identified in the putative cheiracanthid Protogonacanthus [50], the taxon in question is likely not a cheiracanthid but an acanthodid [60]. Finally, as we show, descriptions of mandibular splints in ischnacanthids [32,49] instead represent a reinforced ventral margin of the endoskeletal mandible.…”

“…'Acanthodian' stem-group chondrichthyans display a diverse array of oral and dental morphologies, including an apparently independent origin of teeth deep within an edentulous clade. However, interpreting patterns of tooth evolution is complicated by conflicting and unresolved phylogenetic hypotheses, both for the chondrichthyan (figure 7; figures S1,2) and gnathostome stem [6,58,[82][83][84]. CT-and synchrotron-based investigations seem likely to provide the anatomical information necessary to resolve these instabilities, and likelihood-based methods provide another potential way of overcoming uncertainties [7].…”

“…Our phylogenetic analysis supports the view of Long [43] and Burrow [23] that Acanthodopsis is closely related to Acanthodes , with the presence of teeth representing the only difference between the genera. A possible morphological comparison to these teeth lies in the branchial and hyoid rakers found in acanthodiform fishes like Acanthodes , Cheiracanthus , and Homalacanthus [29, 58]. As with the teeth of Acanthodopsis , these rakers are triangular in shape, sometimes striated ([59] fig.…”

“…Promesacanthus [35]) and cheiracanthids (e.g. Cheiracanthus [58]). In this latter group, tooth- like hyoid rakers have sometimes been mistaken for teeth [60].…”

“…Doliodus [17]). They have also been described in the acanthodiform-like Latviacanthus [67], although as this is based on x-ray plates these may be mischaracterised hyoid rakers as in Homalacanthus [58]. Tooth whorls are also present in some stem-group holocephalans such as iniopterygians [68], although these are likely to be secondarily derived given their phylogenetic remoteness from the chondrichthyan stem-group.…”