Illustrations of Xenacanthus
typically show a skull with very few sutures. Usually you just see the palatoquadrate sutured to the chondrocranium (e.g. Fig. 1).
in a traced fossil (Fig. 2) sutures clearly define individual bones… and they tell us what bone makes the cranial spine: a central extension of the postparietal.
According to Wikipedia,
Xenacanthus decheni (Beyrich 1848; originally Pleuracanthus Agassiz 1837; Devonian to Triassic, 1m up to 4m in length) is a genus of prehistoric sharks. At least 21 species are known. Traits include:
- Two knife-like cusps on teeth
- Serrated spine arises from back of skull
- Two (= split) anal fins
- The short broad pectoral and pelvic fins are the same size
Such two-pronged teeth
are also found in a more primitive relative from the Early Devonian, Doliodus (Fig. 3), originally considered the ‘oldest articulated chondrichthyan’ and a transitional taxa between acanthodians and sharks.
The head spine
(Figs. 1, 2) is distinct from the dorsal fin spines found in Hybodus and acanthodians (= spiny sharks). Phylogenetically it is also homologous with the anterior spine-brush complex of Akmonistion.
clade members are quite diverse and among the first taxa in the lineage leading toward bony fish in the large reptile tree (LRT, 1655+ taxa; subset Fig. x below).
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72.
Beyrich E 1841. Über Xenacanthus decheni und Holacanthus gracilis, zwei Fische aus der Formation des Rothliegenden in Nord Deutschland. Bericht Verhandle. k. preuss. Akad. Wissens. zu Berlin 1848: 24–33.
Long JA 1995. The Rise of Fishes. Johns Hopkins University Press. Baltimore and London.