SVP abstracts 25: Acanthodians misunderstood

Schnitz et al. 2020 attempt to bring
a new understanding to the spiny sharks, the acanthodians (Fig. 1, left column). In the large reptile tree (LRT, 1751+ taxa; subset Fig. 2) acanthodians are basal bony fish transitional taxa leading to several clades of other basal bony fish.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

From the abstract:
“Acanthodians are a poorly understood paraphyletic group of extinct fishes from the Paleozoic.”

They are better understood now after nesting them in the LRT, which minimizes taxon exclusion. I hate to keep repeating this, but all I add to do was add taxa.

“While they show comparatively little diversity in lifestyle and range of body shape, they play a prominent part in our understanding of vertebrate evolution as part of the chondrichthyan stem-group.”

This is false. Spiny sharks are osteichthyans in the LRT, not basal to sharks. Taxon exclusion has been a continuing problem with fish workers.

Figure 2. Subset of the LRT focusing on basal vertebrates (= fish) highlighting acanthodians (=spiny sharks).

Figure 2. Subset of the LRT focusing on basal vertebrates (= fish) highlighting acanthodians (=spiny sharks).

“Their evolutionary history, however, is poorly understood, largely due to the limited preservation of their mostly cartilaginous skeleton that results in a bias towards isolated remains such as fin spines and scales.”

This is false. The LRT provides a complete evolutionary history back to headless Cambrian chordates.

“Thus, considerable uncertainties remain in how the completeness of acanthodian fossils impact on the phylogenetic narrative of both chondrichthyans and other vertebrates.”

This is false. The LRT provides the certainties that come from minimizing taxon exclusion.

“Here, we address these issues by using a variation of the previously defined Skeletal Completeness Metric (SCM), an approach that calculates how complete the skeletons of individuals are compared to their theoretical complete skeleton, to quantify the quality of the acanthodian fossil record.”

This might be the next step after first recovering a valid phylogeny from the more complete acanthodian fossils.

“Acanthodians show a significantly lower completeness distribution than many tetrapods, including theropods, plesiosaurs, sauropodomorphs, ichthyosaurs, pelycosaurs and parareptiles, but a similarly low distribution to bats. Analysis of completeness distribution between acanthodian orders reveals significant differences, with the Acanthodiformes and Diplacanthiformes showing highest overall completeness. Our assessment of completeness reveals only weak spatial biases influencing the acanthodian fossil record while temporal biases are much higher.”

In other words, no phylogenetic conclusions and sort of a waste of time because no valid phylogeny was created. Don’t follow authorities, textbooks or invalid traditions. When you minimize taxon exclusion you’ll understand acanthodian phylogeny. THEN proceed with more detailed studies.


References
Schnitz L, Butler RJ, Coates MI and Sansom IJ 2020.
Skeletal and soft tissue completeness of the acanthodian fossil record through time. SVP abstracts 2020.

wiki/Acanthodii
wiki/Mesacanthus
wiki/Climatius

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