That a flattened placoderm, Dickosteus, gave rise to a flattened shark, Gladbachus, in the Middle Devonian should come as no surprise

This image
of Gladbachus (Fig 1) was found in the SuppData for Coates et al 2018. Comparisons to the smaller, slightly earlier placoderm, Dickosteus, are warranted based on a long list of similar traits, apparently overlooked until now.

With the present results the traditional boundaries separating placoderms from sharks are getting thinner and blurrier.

Figure 1. Middle Devonian basal shark, Gladbachus, compared to Early Middle Devonian placoderm near the origin of arched jaws, Dickosteus, Note the breakup of the thoracic armor into branchial strips. ” data-image-caption=”

Figure 1. Middle Devonian basal shark, Gladbachus, compared to Early Middle Devonian placoderm near the origin of arched jaws, Dickosteus, Note the breakup of the thoracic armor into branchial strips.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/01/gladbachus.dickosteus.fossil588.jpg?w=203″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/01/gladbachus.dickosteus.fossil588.jpg?w=584″ src=”https://pterosaurheresies.files.wordpress.com/2024/01/gladbachus.dickosteus.fossil588.jpg?w=588″ alt=”Figure 1. Middle Devonian basal shark, Gladbachus, compared to Early Middle Devonian placoderm near the origin of arched jaws, Dickosteus, Note the breakup of the thoracic armor into branchial strips.” class=”wp-image-83918″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/01/gladbachus.dickosteus.fossil588.jpg 588w, https://pterosaurheresies.files.wordpress.com/2024/01/gladbachus.dickosteus.fossil588.jpg?w=101 101w, https://pterosaurheresies.files.wordpress.com/2024/01/gladbachus.dickosteus.fossil588.jpg?w=203 203w” sizes=”(max-width: 588px) 100vw, 588px” />

Earlier we looked at the evolutionary breakup
of the placoderm thoracic armor to create the branchials in basal sharks like Gladbachus. These two taxa (Fig 1) provide the latest example of that transition. Maybe it happened several times.

Gladbachus adentatus
(Heidtke & Krätschmer 2001; Burrow and Turner 2013; Coates et al. 2018; Middle Devonian, est. 54cm) was originally considered an ‘unfamiliar’ basal chondrichthyan close to acanthodians (spiny sharks). Here it nests with Doliodus near the base of the shark clade. This toothless or tiny-toothed taxon with a wide gape and flat skullwas apparently a filter feeder with enormous gill bars. The unique holotype preserves the pelvic area. The tiny Early Devonian placoderm > shark transitional taxon, Doliodus, provides clues to the evolution of Gladbachus as applied colors here indicate.

Coates et al 2018 reported,
“Despite many features resembling placoderm or osteichthyan conditions, phylogenetic analysis confirms Gladbachus as a stem chondrichthyan and corroborates hypotheses
that all acanthodians are stem chondrichthyans.”

Dickosteus (Fig 1) was not mentioned in the text. Overlooked.

“Gladbachus has never been included among acanthodians, but, like Doliodus, recent analyses and discussion suggest that it might illuminate conditions bridging the acanthodian–chondrichthyan transition. Thus, a primary aim of the present work is to test the assumed chondrichthyan affinity of Gladbachus in light of the current acanthodians-as-stem-chondrichthyans hypothesis.”

Try not to assume your results before you have them. Let your wide gamut cladogram tell you how clades are related to one another. Don’t omit pertinent taxa along the way.
You may be surprised by an outlier. Happens here all the time.

References
Burrow C and Turner S 2013. Scale structure of putative chondrichthyan Gladbachus adentatus Heidtke & Krätschmer, 2001 from the Middle Devonian Rheinisches Schiefergebirge, Germany. Historical Biology 25(3):385–390.
Coates MI, et al (7 co-authors) 2018. An early chondrichthyan and the evolutionary assembly of a shark body plan. Proceedings of the Royal Society B 285(1870):20172418. Heidtke UHJ and Krätschmer K 2001. Gladbachus adentatus nov. gen. et sp., ein primitiver Hai aus dem Oberen Givetium (Oberes Mitteldevon) der Bergisch Gladbach – Paffrath-Mulde (Rheinisches Schiefergebirge). Mainzer geowiss. Mitt. 30, 105–122.

wki/Gladbachus
wiki/Dickosteus