Another clade no longer extinct: Acanthodii (spiny sharks)

The LRT has invalidated several traditional clades
like Parareptilia, Ornithodira and Cetacea. The LRT has also resurrected and supported a few long forgotten clades like Enaliosauria and Volitantia. The LRT also recovered extant members for traditionally extinct clades like Placodermi (catfish), Desmostylia (mysticete whales) and Pareiasauria (turtles).

Key to phylogenetic analysis
is the idea that it does not matter if certain clade members lack one trait or another, or have traits shared by taxa that are not clade members. It only matters that a clade is determined by a unique suite of hundreds of traits compared to all other tested clades. That makes it important to test as many taxa as possible to minimize the possibility that any pertinent taxa are excluded and no inappropriate taxa are included. Monophyletic clade members include all descendants of a last common ancestor.

Some of the earliest known fish fossils
belong to spiny sharks (e.g. Mesacanthus, Fig. 1; clade: Acanthodii) in the Early Devonian. Other disarticulated specimens attributed to spiny sharks (scales and spines) are found in Early Silurian strata.

Instead of fin rays or lobe fins,
spiny sharks have sharp dentine spines trailing unreinforced membranes. Like sharks, acanthodian skeletons were made of cartilage because they do not preserve well. Unlike sharks, the scales were made of bone-like material. Due to taxon exclusion and the reliance on just a few traits (= Pulling a Larry Martin) acanthodians have been traditionally difficult to nest in prior phylogenetic analyses.

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

With the addition of the spiny shark Mesacanthus
(Early Devonian, Fig. 1) the large reptile tree (LRT, 1654+ taxa; subset Fig. 2) nests spiny sharks between anchovies, like the extant Engraulis, and palaeoniscids like Pteronisculus (Early Triassic).

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Spiny sharks are also basal
to Perleidus (Triassic) and Notopterus (extant; Fig. 3) among tested taxa in the LRT.

Figure 3. The extant knife fish or featherback, Notopterus, is an extant descendant of spiny sharks in the LRT.

Figure 3. The extant knife fish or featherback, Notopterus, is an extant descendant of spiny sharks in the LRT. Note the spiny pectoral fins and similar skull morphology.

According to Wikipedia:
“Burrow et al. 2016 provides vindication by finding chondrichthyans to be nested among Acanthodii, most closely related to Doliodus and Tamiobatis. A 2017 study of Doliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a transitional fossil and further reinforcing this idea.”

That’s only true due to taxon exclusion,
according to the LRT (Fig. 2) where Doliodus (Fig. 4) is closer to Akmonistion and Xenacanthus.

Figure 1. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Figure 4. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Does a spiny shark have to have spiny fins?
No. Several taxa unrelated to spiny sharks, like catfish, have more or less spiny fins and sometimes these give rise to ray fins, as they do in Notopterus (Fig. 3) for the pectoral fins, but not the pelvic fins.

Taxa in the LRT are nested at nodes based on hundreds of traits
more closely shared with sister and cousin taxa than with more distantly related taxa in toto.

Key to scoring included taxa
is the creation of a reconstruction (Fig. 1) that moves disarticulated bones back to their in vivo positions. That simply must be done to understand the data.

Mesacanthus mitchelli (renamed with Traquair 1888; Early Devonian, 410 mya; 3cm) is a basal acanthodian transitional to Notopterus and Perleidus. So spiny sharks are not extinct. Distinct from other tested spiny sharks, Mesacanthus has open cheeks exposing the quadrate and hyomandibular. Soft tissue preserves membranes posterior to all the spines.


References
Baron MG 2015. An investigation of the genus Mesacanthus (Chordata: Acanthodii) from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland using traditional morphometrics. PeerJ. 3: e1331. doi:10.7717/peerj.1331
Burrow C; den Blaauwen J; Newman M and Davidson R 2016. “The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotland”. Palaeontologia Electronica 19 (1): Article number 19.1.10A.
Traquair RH 1888. Notes on the nomenclature of the Fishes of the Old Red Sandstone of Great Britain. Geol. Magazine (3)5:507–517.

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/osteoglossiformes

 

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