The ‘spine-brush complex’ of Akmonistion: dorsal fin? or dorsal shield?

A strange taxonomic addition today.
Akmonistion (Figs. 1, 2) nests in the large reptile tree (LRT, 1634+ taxa) between Falcatus (Fig. 3) and Heterodontus (horn shark. Fig. 4), a taxon that nests basal to members of the Holocephali.

So Akmonistion, Falcatus and Iniopteryx are basal members of Chondrichthyes (1880). These are late-surviving, but basal to both Elasmobranchii (1838, sans Rhincodon and Manta) and Holocephali (1832). Clearly a more plesiomorphic, but currently unknown Silurian or Devonian taxon without all the novel secondary sexual traits awaits discovery.

Figure 1. Stethacanthus in situ, diagram and reconstruction from DGS methods.

Figure 1. Akmonistion in situ, diagram and reconstruction from DGS methods. The tan crest portion appears to be the postparietals or at least the dorsal shield.

Traditional paleontology
considers the famous, bizarre ‘spine-brush’ of Akmonistion (Figs. 1,2) an enlarged and specialized dorsal fin. I will present evidence that it is something else.

Figure 2. Akmonistan, a relative of Stethacanthus.

Figure 2. Akmonistion, a relative of Stethacanthus, from Coates and Sequeira 2001.

What is a spine-brush complex?
Phylogenetic bracketing and simple morphology indicate the spine-brush is convergent with the thoracic shield in placoderms or the postparietals in catfish. This is either a completely novel structure or the elaboration of a smaller older structure from ancestral taxa. In any case the spine-brush complex is not a co-opted dorsal fin.

Ordinary dorsal fins
are made of parallel rods with roots that have sharp ventral tips buried deeply into fish flesh.

By contrast, the brush-shield complex
has a one-piece solid root, like a dorsal shield. If a shield, then primitively a low dorsal shield could have been operational at a low height with a few bumps, improving and growing.generationally by sexual selection.

According to Maisey 2009,
in Falcatus the spine appears relatively late in ontogeny and only in males. Unfortunately, we don’t have an ontogenetic series for Akmonistion. I can only imagine a greatly reduced or absent spine-brush when in the egg. Thereafter, if it followed the pattern in male Falcatus, the spine would appear at puberty.

Weighing one spine-brush hypothesis against another,
the “late-appearing, strange-looking anterior dorsal fin with spine” hypothesis competes with “the late-appearing, strange-looking, well-anchored thoracic shield” hypothesis.

The anterior dorsal fin placement patterns
of the few tested taxa in the LRT produce relatively few strong placement patterns. (Here LRT clades are divided by spaces).

  • In Pseudoscaphorhychus, the sturgeon, a series of dorsal shields are present posterior to the skull, anterior to the pectoral fin.
  • In Rhincodon, the whale shark, the dorsal fins are posterior to the pectoral fins
  • In Falcatus (Fig. 2), the spine extends over the skull, rooted largely anterior to the pectoral fin, but directly over the pectoral girdle.
  • In Iniopteryx, one small posterior dorsal fin appeared dorsal to the pelvic fin, but the pectoral fins are rooted dorsally.
  • In Akmonistion (Fig. 2), the spine brush is rooted posterior to the skull, anterior to the pectoral fin. It could be the post parietal.
  • The following taxa are not in the ancestry of Akmonistion and Falcatus.
  • In Heterodontus, the horn shark, the dorsal fins are posterior to the pectoral fins.
  • In Chimaera, the anterior dorsal fin is dorsal to the pectoral fin.
  • In Belantsea, the anterior dorsal fin is dorsal to the pectoral fin.
  • In Cladoselache, the dorsal fins are posterior to the pectoral fin.
  • In Chlamydoselachus, the dorsal fins posterior to the pelvic fin.
  • In Isurus, the mako shark, the dorsal fins are posterior to the pectoral fins.
  • In Sphyrna, the mako shark, the dorsal fins are posterior to the pectoral fins.
  • In all rays, mantas, skates and angel sharks, both dorsal fins are posterior to the pelvic fins
  • In Polyodon, the paddlefish, the dorsal fin is posterior to the pelvic fins.
  • In Hybodus, the the dorsal fins are posterior to the pectoral fins.
Figure 3. Falcatus skull. This taxon is close to Polyodon in the LRT.

Figure 3. Falcatus skull. This taxon is close to Polyodon in the LRT. Note the anterior placement of the antler/spine/dorsal shield just behind the postparietals.

Does the LRT document any Akmonistion ancestors with dorsal shields?
In other words, is the dorsal shield of Akmonistion a reversal? a reappearance of something already in the gene pool? Or is it a novel trait?

FIgure 1. Ratfish (chimaera) and Heterodontus to scale.

FIgure 4. Ratfish (chimaera) and Heterodontus to scale.

We have three armored ancestors, according to the LRT.

  • Jawless Birkenia has dorsal hooks and ossifications
  • Jawless Hemicyclaspis (Fig. 5) is covered in armor.
  • Semi-jawless sturgeons, like Pseudoscaphorhychus (Fig. 5), retain a series of dorsal plates and other armor.
Figure 1. The osteostracan Cephalaspis (above) compared to the sturgeon Pseudoscaphorhynchus (below). The similarities of these armored morphologies have been overlooked previously. In both cases the jawless or tubular mouth is below the skull, the former towards the front, the latter below the eyes.

Figure 5. The osteostracan Cephalaspis (above) compared to the sturgeon Pseudoscaphorhynchus (below). Note the dorsal armor just behind the skull.

Akmonistion zangerli (HMV8246; Coates and Sequeira 2001; Early Carboniferous) had a larger spine-brush complex. Note the great distance between the skull and pectoral girdle along with the short rostrum and large orbit.

Figure F. Basal tetrapods 2020.

Figure 6. Basal vertebrates and tetrapods 2020. Akmonistion nests between Falcatus + Iniopteryx and Heterodontus + the rest of the Chondrichtheys.

Stethacanthus altonensis (St. John and Worthen 1875; Late Devonian to Early Carboniferous; 70cm) has a posterior-leanding brush-spine.


References
Bonaparte 1832. Iconografia della fauna italica, per le quatro classi degli animali vertebri. Rome, 78 pp.
Coates MI and Sequeira SEK 2001. A new stethacanthid chondrichthyan from the lower Carboniferous of Bearsden, Scotland. Journal of Vertebrate Paleontology. 21 (3): 438–459.
Maisey JG 2009. The spine-brush complex in Symmoriiform sharks (Chondrichthyes: Symmoriiformes), with comments on dorsal fin modularity. Journal of Vertebrate Paleontology, 29(1), 14-24.
St. John OH and Worthen AH 1875. Palaeontology of Illinois. Descriptions of fossil fishes. Geological Survey of Illinois, 6: 245–488.
Zangerl R 1984. On the microscopic anatomy and possible function of the spine-“brush” complex of Stethacanthus (Elasmobranchii: Symmoriida). Journal of Vertebrate Paleontology. 4 (3): 372–378.

wiki/Stethacanthus
wiki/Stethacanthidae
wiki/Akmonistion

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