Small taxa at the origin of sharks and kin (= Chondrichthyes)

Housekeeping continues on the LRT
untangling interrelationships in the fish subset. Here (Figs 1,2) are taxa that now nest at the separation of sharks from sturgeons, placoderms and bony fishs.

Figure 1. Tiny Early Silurian Shenacanthus compared to Carboniferous Gregorius, Hybodus fraasi and Ozarcus. ” data-image-caption=”

Figure 1. Tiny Early Silurian Shenacanthus compared to Carboniferous Gregorius, Hybodus fraasi and Ozarcus.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/gregorius_hybodus588.jpg?w=161″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/gregorius_hybodus588.jpg?w=550″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/gregorius_hybodus588.jpg” alt=”Figure 1. Tiny Early Silurian Shenacanthus compared to Carboniferous Gregorius, Hybodus fraasi and Ozarcus. ” class=”wp-image-84172″ />

Two factors affect the present hypothesis of interrelationships:
1) the rarity of fossils and 2) the lack of valid data using tetrapod homologs (standard colors used here). The second problem improves a little every day.

Figure 2. Another clade split of early, the iniopterygidae, known only from derived Carboniferous taxa. Here’s Iniopteryx and Gregorius. They both compare well with tiny Early Silurian Shenacanthus, lookng more like a tiny arthrodire placoderm just beginning to develop its own version of jaws. ” data-image-caption=”

Figure 2. Another clade split of early, the iniopterygidae, known only from derived Carboniferous taxa. Here’s Iniopteryx and Gregorius. They both compare well with tiny Early Silurian Shenacanthus, lookng more like a tiny arthrodire placoderm just beginning to develop its own version of jaws.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/gregorius_iniopteryx588.jpg?w=191″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/gregorius_iniopteryx588.jpg?w=584″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/gregorius_iniopteryx588.jpg” alt=”Figure 2. Another clade split of early, the iniopterygidae, known only from derived Carboniferous taxa. Here’s Iniopteryx and Gregorius. They both compare well with tiny Early Silurian Shenacanthus, lookng more like a tiny arthrodire placoderm just beginning to develop its own version of jaws.” class=”wp-image-84175″ />

Which taxon was the outgroup?
A jawless, spiny taxon with high-set pectoral fins and low-set finfolds, Lasanius (Fig 3).

Figure 2. The jawless holotype of Early Silurian Lasanius also has pectoral fins anchored higher than the gill slits, like an iniopterygian by convergence. ” data-image-caption=”

Figure 2. The jawless holotype of Early Silurian Lasanius also has pectoral fins anchored higher than the gill slits, like an iniopterygian by convergence.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/lasanius-holotype-skull588.jpg?w=163″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/lasanius-holotype-skull588.jpg?w=558″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/lasanius-holotype-skull588.jpg?w=558″ alt=”Figure 2. The jawless holotype of Early Silurian Lasanius also has pectoral fins anchored higher than the gill slits, like an iniopterygian by convergence. ” class=”wp-image-84179″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/02/lasanius-holotype-skull588.jpg?w=558 558w, https://pterosaurheresies.files.wordpress.com/2024/02/lasanius-holotype-skull588.jpg?w=82 82w, https://pterosaurheresies.files.wordpress.com/2024/02/lasanius-holotype-skull588.jpg?w=163 163w, https://pterosaurheresies.files.wordpress.com/2024/02/lasanius-holotype-skull588.jpg 588w” sizes=”(max-width: 558px) 100vw, 558px” />

Lasanius problematicus
(Traquair 1899; Middle Silurian; 5cm) is a birkeniid with longer pectoral fins and a distinct set of dorsal spines.

Shenacanthus vermiformis
(Zhu et al 2022, Early Silurian, est 3cm long) is the earliest known of the jawed placoderms and the basalmost member of the spiny sharks (Osteichthys). Ozarcus is a much larger and much later sister from the Late Mississippian. Claspers are present.

References
Traquair R 1899. XXXII Report on Fossil Fishes collected by the Geological Survey of Scotland in the Silurian Rocks of the South of Scotland. Transactions of the Royal Society of Edinburgh, 39(3): 827–864.

wiki/Lasanius