Saurichthys: a Triassic ‘tuna’ close to the last of the lobefins

Saurichthys (Fig. 1) looks like a Triassic barracuda, but here in the large reptile tree (LRT, 1535 taxa, subset Fig. 5) nests between the last of the Devonian putative lobefins, Strunius (Fig. 2)  and the extant tuna (Thunnus, Figs. 3,4).

Figure 1. Saurichthys, shaped like a barracuda, sister to the tuna.

Figure 1. Saurichthys, shaped like a barracuda, sister to the tuna. Note: the diagram, from Gregory 1938, lacks teeth. More complete specimens have teeth.

Saurichthys sp. (Agassiz 1834; up to 1m in length; Early Triassic to Mid Jurassic) is a predatory tuna sister with a long pointed snout and a long, barracuda-like body. Traditionally considered a member of the Saurichthyformes, that clade now appears to be a junior synonym for the previously named Scombridae. Only one dorsal fin appears here. More than 30 species are known. Several taxa are junior synonyms.

Figure 2. Strunius skull enlarged to show detail. Inset shows the second origin of the dual external naris as the original apparently splits by the addition of a skin bridge creating two openings. Compare to figure 1.

Figure 2. Strunius skull enlarged to show detail. Inset shows the second origin of the dual external naris as the original apparently splits by the addition of a skin bridge creating two openings. Compare to figure 1.

Strunius walteri (Jessen 1966; originally Glyptomus rolandi Gross 1936; 10 cm in length; Late Devonian) was considered a lobe-fin fish with ray fins. Here it nests with Cheirolepis, a traditional and transitional ray fin fish. The origin of the double naris in this lineage appears here as a split dividing the original single in two. The palate and possible choana are not known.

Figure 3. Thunnus, the tuna, skeleton and skin.

Figure 3. Thunnus, the tuna, skeleton and skin. More primitive than traditional cladograms recover it.

This appears to be a novel hypothesis of interrelationships
that links previously unlinked taxa. If I missed a citation that predates this one that supports this hypothesis of interrelationships, please send me the citation.

Figure 2. Thunnus, the tuna, nests with some of the most basal bony fish, like Strunius and Pholidophorus.

Figure 4. Thunnus, the tuna, nests with some of the most basal bony fish, like Strunius and Pholidophorus.

Thunnus thyrnnus (Linneaus 1758; 4.6m long) is the extant Atlantic tuna. Traditionally it is considered a member of the perch family. Here it nests with Triassic Pholidopterus. The jugal is retained. The squamosal is a vestige. The intertemporal, supratemporal and tabular are disconnected from one another. The maxilla is toothless. Note the lacrimal contacts the ventral jugal, creating an orbit not confluent with a lateral temporal fenestra. The tip of the premaxilla rises to produce procumbent teeth, but the rest extends posteriorly beyond the maxilla.

Figure 4. Subset of the LRT focusing on basal vertebrates (fish) and the tuna clade.

Figure 5. Subset of the LRT focusing on basal vertebrates (fish) and the tuna clade.

The LRT continues to bring diverse clades of fish together,
reducing the number of clades and illuminating interrelationships.

Figure 1. Pholidophorus in situ + two skull drawings relabeled with tetrapod names.

Figure 6. Pholidophorus in situ + two skull drawings relabeled with tetrapod names, looks more like a tuna than the other clade sisters.

Pholidophorus sp. (Agassiz 1832; Middle-Late Triassic; 40cm long) was a herring-like fish with primitive ganoid scales and poorly ossified spine. Traditionally considered an early teleost, with large eyes, here it nests with Late Devonian Strunius, but lacks the central process of the tail. Here the skull bones are re-identified with tetrapod labels. The pectoral and pelvic fins were similar in size. Earlier we looked at the connection between Pholidophorus and Strunius.


References
Agassiz L 1832. Untersuchungen über die fossilen Fische der Lias-Formation. Jahrbuch für Mineralogie, Geognosie, Geologie und Petrefaktenkunde, 3, 139–149.
Agassiz JLR 1835. Recherches sur les Poissons fossiles, 5 volumes. Imprimerie de Petitpierre et Prince, Neuchaatel, 1420 pp.
Agassiz JLR 1835. On the fossil fishes of Scotland. Report of the British Association for the Advancement of Science, British Association for the Advancement of Science, Edinburgh.
Gross W 1933 1936 Die Fische des baltischen Devons, Palcteontographica A 79:1-74.
Jessen 1966. in Piveteau (Ed.). Traite de paleontologie. Tome 4. L’origine des Vertebres, leur expansion dans les eaux douces et le milieu marin. Vol. 3. Actinopterygiens, Crossopterygiens, Dipneustes. Masson & Cie, Paris
Linnaeus C von 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Wu F-X, Sun Y-L and Fang G-Y 2018. A new species of Saurichthys from the Middle Triassic (Anisian) of Southwestern China. Vertebrata PalAsiatica 56(4):287–294. pdf

wiki/Strunius
wiki/Pholidophorus
wiki/Thunnus
wiki/Saurichthys

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.