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A clade of Devonian spiny sharks with mosaic skull bones

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Today’s blogpost is fraught with guesswork
because the skull bones of this clade of spiny sharks have dissolved into a mosaic of small scale-like patterns (Fig 1). Determining homologies for these skull bones depends on comparative anatomy, a dollop of hope and a willingness to make changes over and over again.

This skull bone dissolution (= cracking up) is not scored as a trait in the LRT. So the nebulous shapes that chains of these bones is used instead. It’s a challenge that others have avoided. That’s the nature of research, going off to examine dark corners to shed light there.

Figure 1. A clade of spiny sharks with mosaic skull bones. These include Triazeugacanthus, Brachyacanthus, Climatius, Ptomacanthus and Brochoadmones. ” data-image-caption=”

Figure 1. A clade of spiny sharks with mosaic skull bones. These include Triazeugacanthus, Brachyacanthus, Climatius, Ptomacanthus and Brochoadmones.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2023/12/brachyacanthus-ptomacanthus-brochoadmones-skull588.jpg?w=83″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2023/12/brachyacanthus-ptomacanthus-brochoadmones-skull588.jpg?w=283″ src=”https://pterosaurheresies.files.wordpress.com/2023/12/brachyacanthus-ptomacanthus-brochoadmones-skull588.jpg” alt=”Figure 1. A clade of spiny sharks with mosaic skull bones. These include Triazeugacanthus, Brachyacanthus, Climatius, Ptomacanthus and Brochoadmones. ” class=”wp-image-83066″ />

Figure 1. A clade of spiny sharks with mosaic skull bones. These include Triazeugacanthus, Brachyacanthus, Climatius, Ptomacanthus and Brochoadmones. These taxa have many more skull bones than usual, each one subdivided in its own pattern of scales.

A 2017 paper on Triazeugacanthus ontogeny
by Chevrinais, Sire and Cloutier reported, “Ontogenetically, scales form first in the region below the dorsal fin spine, then squamation spreads anteriorly and posteriorly, and on fin webs.”

“Phylogenetically, Triazeugacanthus scales show similarities with acanthodians (e.g. “box-in-box” growth), chondrichthyans (e.g. squamation pattern), and actinopterygians (e.g. ganoine). Scale histology and growth are interpreted in the light of a new phylogenetic analysis of gnathostomes supporting acanthodians as stem chondrichthyans.”

Acanthodians are among the last of the 483 taxa in the fish subset of the LRT to find a firm node, which currently changes weekly. There will come a time (perhaps today) when interpretations no longer waver. There is value in putting taxa together in one graphic (Fig 1), like a police line-up, to make similarities and differences more apparent.

This is yet another tool colleagues should employ in their own studies.

References
Chevrinais M, Sire, J-Y and Cloutier R 2017. From body scale ontogeny to species ontogeny: Histological and morphological assessment of the Late Devonian acanthodian Triazeugacanthus affinis from Miguasha, Canada. PLOS ONE. 12 (4): e0174655. Bibcode:2017PLoSO..1274655C. doi:10.1371/journal.pone.0174655
Miles RS 1966. The acanthodian fishes of the Devonian Plattenkalk of the Paffrath. Trough in the Rhineland. Arkiv för Zoologica (Stockholm)18: 147–194.

wiki/Triazeugacanthus


Source: https://pterosaurheresies.wordpress.com/2023/12/20/a-clade-of-devonian-spiny-sharks-with-mosaic-skull-bones/


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