The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotlan

Morphology and histology of acanthodian fin spines from the late Silurian Ramsåsa E locality, Skåne, Swede

“…Scan slices of morphotype E show a thicker ring of mineralized tissue around the central cavity ( Figure 6.1), and data from morphotypes D and F show that the hard tissue around the central cavity is not very vascularized in these morphotypes (Figures 5 and 6.2). This suggests that these spines could have a tripartite structure similar to several other acanthodian spines (Denison, 1979;Burrow and Turner, 2010), including Ischnacanthus (Denison, 1979), Diplacanthus (Burrow et al, 2016), and Par-12 exus . Denison (1979) also described a fourth layer of tissue, composed of either a thick inner "osteon" or "denteon" (cb) located between the basal layer (b) and the central cavity in the closed portions of acanthodian fin spines (also figured by Gross, 1971), and can be observed from the scan data for morphotype D (Figure 5).…”

“…The median canals extend for almost the entire length of the closed portion of the spine (Figures 4 and 5), but there is no data at the tip to suggest they extend the spine's full length. Many other acanthodian dorsal fin spines also show evidence for longitudinal growth by the presence of accessory canals that run the entire length of the fin spines (Burrow et al, 2016). Callorhinchus also has a median canal that runs the length of the closed portion of the spine (Jerve et al 2014), which has also been identified in fossil sharks, such as Sphenacanthus (Maisey, 1982).…”

“…Abbreviations: cc, central cavity; k, keel; med.c, median canal; r, ribbing; rib.c, ribbing canals; s, sulcus. squamation in early gnathostomes (e.g., Trinajstic, 2001;Brazeau, 2012;Burrow, 2013;Burrow et al, 2013Burrow et al, , 2015Burrow et al, , 2016Chevrinais et al, 2017) will undoubtedly influence future designations of isolated material. On the other hand, the taxonomical framework based on isolated material that Vergoossen (2004) described from Ramsåsa site E may enable an assignment of the spines treated herein on a higher taxonomic level.…”

“…These discoveries, along with cranial data, have led to the rejection of a monophyletic Acanthodii (Brazeau, 2009;Davis et al, 2012) and to new ideas about how these animals relate to osteichthyans and chondrichthyans. The working hypothesis of vertebrate phylogeny has now shifted to accept acanthodians as a paraphyletic assemblage within the chondrichthyan total group (Zhu et al, 2013;Giles et al, 2015;Burrow et al, 2016;Qiao et al, 2016).…”

“…With the aid of the micro-vertebrate framework presented by Vergoossen (2004), and by comparing them to described articulated material (see later sections), we aim to give these vertebrate remains taxonomical and morphological assignments. Given the current close phylogenetic position of acanthodians to chondrichthyans based on recent hypotheses (Zhu et al, 2013;Giles et al, 2015;Burrow et al, 2016;Qiao et al, 2016) it is necessary to understand how similar their fin spines are in terms of morphology and growth. The morphology and histology are investigated (where possible) from synchrotronmicrotomography data collected from the European Synchrotron Radiation Facility in Grenoble, France.…”