‘One of the earliest known stem elasmobranchs’ is another shark-like placoderm
Coates et al 2019 wrote,
“The jaws of Tristychius (Fig. 2, A to C, G, and I) differ in numerous respects from the standard jaw morphology found in most Paleozoic chondrichthyans.”
The large reptile tree (LRT, 2312 taxa) nests Middle Mississippian Tristychius (Figs 14) apart from traditional chondrichthyans and elasmobranchs, originating closer to the tiny unnamed Devonian placoderm, ANU V244 (Fig 1). Other similar taxa nest alongside.
After analysis, resemblance to traditional sharks is the result of convergence.
Figure 1. Tristychius compared to the larger Maghriboselache and the smaller unnamed specimen, ANU V244.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-maghriboselache588.jpg?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-maghriboselache588.jpg?w=584″ class=”size-full wp-image-84552″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-maghriboselache588.jpg?w=584&h=502″ alt=”Figure 1. Tristychius compared to the larger Maghriboselache and the smaller unnamed specimen, ANU V244.” width=”584″ height=”502″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-maghriboselache588.jpg?w=584&h=502 584w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-maghriboselache588.jpg?w=150&h=129 150w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-maghriboselache588.jpg?w=300&h=258 300w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-maghriboselache588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Tristychius compared to the larger Maghriboselache and the smaller unnamed specimen, ANU V244. The labial cartilages (red) on Maghriboselache are traced from displaced elements found in the throat area of the fossil. Note the evolution and migration of the plastron in ANU V244 anteriorly in Maghriboselache and Tristychius.
According to Coates et al
Tristychius was a suction feeder, using throat bones (here considered former plastron bones) to rapidly expand the volume of the mouth cavity.
Figure 1. Tristychius, a basal shark from the Early Carboniferous,
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius_overall588.jpg?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius_overall588.jpg?w=584″ class=”size-full wp-image-84559″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius_overall588.jpg?w=584&h=226″ alt=”Figure 1. Tristychius, a basal shark from the Early Carboniferous, ” width=”584″ height=”226″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius_overall588.jpg?w=584&h=226 584w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius_overall588.jpg?w=150&h=58 150w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius_overall588.jpg?w=300&h=116 300w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius_overall588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Tristychius, a basal shark from the Early Carboniferous, The gill bars and long skull in the decades-old drawing of another local and coeval specimen do not match the µCT scan here, but at least similar gill bars are known from the scanned specimen. See figure 4.
Tristychius arcuatus
(Agassiz 1837; Dick 1978; Coates et al. 2019; Early Carboniferous; 60cm est.) was considered a Hybodus relative, but here nests with Maghriboselache and Devonian ANU V244. The nares point anteriorly. Teeth are nearly absent with only a few in the anterior dentary and premaxilla homologs. Note the large labial cartilages (red) that restrict jaw depression and create lateral walls for the jaws to create suction when snapped open.
Figure 3. The skull of Tristychius updated from prior interpretations, based on a placoderm Bauplan.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-skull588.jpg?w=272″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-skull588.jpg?w=584″ class=”size-full wp-image-84561″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-skull588.jpg?w=584&h=644″ alt=”Figure 3. The skull of Tristychius updated from prior interpretations, based on a placoderm Bauplan.” width=”584″ height=”644″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-skull588.jpg?w=584&h=644 584w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-skull588.jpg?w=136&h=150 136w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-skull588.jpg?w=272&h=300 272w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius-skull588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 3. The skull of Tristychius updated from prior interpretations, based on a placoderm Bauplan.
A new Bauplan
for the skull of Tristychius is shown here (Fig 3) in which several skull bones are re-identified. Tristychius has an antorbital fenestra confluent with the orbit, and a dorsal fenestra convergent with many extant sharks. Note the anterior orientation and placement of the nares in Tristychius, as in the unnamed ANU V244 specimen, which has similar circumorbital bones and other structures.
This is what can happen when you compare taxa together that have not been tested together, which is the value adding taxa to your own LRT.
Figure 4. Nodule and µCT scans in dorsal and ventral views of Tristychius.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius.nodual588.gif?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius.nodual588.gif?w=584″ class=”size-full wp-image-84564″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius.nodual588.gif?w=584&h=328″ alt=”Figure 4. Nodule and µCT scans in dorsal and ventral views of Tristychius.” width=”584″ height=”328″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/02/tristychius.nodual588.gif?w=584&h=328 584w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius.nodual588.gif?w=150&h=84 150w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius.nodual588.gif?w=300&h=168 300w, https://pterosaurheresies.files.wordpress.com/2024/02/tristychius.nodual588.gif 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 4. Nodule and µCT scans in dorsal and ventral views of Tristychius. Note the shark-like gill bars, evolved from former tabulars and the plastron of placoderm ancestors like ANU V244.
This appears to be a novel hypothesis of interrelationships.
If not, please provide a citatioin so I can promote it here. Other similar sharks nesting with placoderms were recovered earlier. See links below.
References
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72.
Coates MI, Tietjen K, Olsen AM and Finarelli JA 2019. High-performance suction feeding in an arly elasmobranch. Sci. Adv. 2019; 5 : eaax2742
Dick JRF 1978. On the carboniferous shark Tristychius arcuatus Agassiz from Scotland.
Earth and Environmental Sciences Transactions of the Royal Society, Edinburgh. 70: 63–108.
Millerosteus and Amazichthys link to a clade of basal sharks, including Maghriboselache
Source: https://pterosaurheresies.wordpress.com/2024/02/27/one-of-the-earliest-known-stem-elasmobranchs-is-another-shark-like-placoderm/
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