Strange-spined Xenacanthus enters the LRT

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).

Figure 1. Xenacanthus diagram with dorsal vertebrae colorized. Note the difference between the skull of this species and the one in figure 2.

Figure 1. Xenacanthus diagram with dorsal vertebrae colorized. Note the difference between the skull of this species and the one in figure 2.

By contrast
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.

Figure 2. Xenacanthus skull with DGS colors added to show the unfused mandible bones.

Figure 2. Xenacanthus skull with DGS colors added to show the unfused mandible bones. Image from Long 1995.

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:

  1. Freshwater
  2. Two knife-like cusps on teeth
  3. Serrated spine arises from back of skull
  4. Two (= split) anal fins
  5. 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.

Figure 1. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Figure 1. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

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.

Figure 2. Akmonistan, a relative of Stethacanthus.

Figure 2. Akmonistion, a relative of Stethacanthus, Doliodus and Xenacanthus.

Note,
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).

Figure x. Subset of the LRT focusing on basal vertebrates. This represents the latest hypothesis of interrelationships and includes several changes from prior versions of this section.

Figure x. Subset of the LRT focusing on basal vertebrates. This represents the latest hypothesis of interrelationships and includes several changes from prior versions of this section.

References
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.

wiki/Xenacanthus

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