Tiny Santanichthys is a bonefish

Updated April 28, 2021
with a closer look at Santanichthys now nesting with bonefish like Albula and Opisthoproctus.

This was a paragraph from the earlier post:
Two very closely related taxa, one 20x times larger,
enter the LRT today. Santanichthys (Silva Santos 1995; Figs. 1, 5) is only 3m in length. Notelops (Woodward 1901; Figs. 2–4) reaches 60cm in length. Both are from the Santana Formation, Early Cretaceous.

Figure 1. Tiny Santanichthys is a phylogenetically miniaturized taxon at the base of the Ostariophysi clade.

Santanichthys diasii 
(Silva Santos 1958; Filleleul and Maisey 2004; Early Cretaceous; 3cm; DGM-DNPM 647P) was a tiny Santana Formation fish considered the oldest characiform and otophysan. Here Santanichthys nests with Albula the extant bonefish (Fig. 2). According to Filleleul and Maisey, this is the earliest appearance of a Weberian apparatus, a sound amplifier that connects the swim bladder to the auditory system.

Figure 2. The skull and diagram of tiny Santanichthys from Filleul and Maisey 2004, colors added.

These taxa are considered members of the Characiformes,
a clade that traditionally includes piranha. Likewise the large reptile tree (LRT, 1801 taxa then, 1839 taxa now; subset Fig. x) nests them together, derived from the piranha clade. Traditionally Characiformes also includes knife fish and catfish. These clades are not related to piranha in the LRT, nor are they related to bonefish.

Figure 3. Albula vulpes skull with highly derived facial bones reidentified here. Note the lateral premaxillary processes and ‘floating’ cheek bones. Green vertebrae are caudals.

Santanichthys and Albula
share a long list of traits. Tiny Opisthoproctus (Fig 4) has fewer vertebrae, like Santanitchthys.

Figure 4. Opisthoproctus nests with Santanichthys in the LRT.

Figure x. Rayfin fish cladogram

If your studies dive deep into fish science  
you’ll come across the traditional clade Ostariophysi, in which member taxa all have a Weberian apparatus (see above). The LRT indicates that some fish with this trait evolved it independently, while others later lost it by convergence. Be careful. Lumping taxa together using one trait or a dozen is called “Pulling a Larry Martin.” Try to always determine clades with a phylogenetic analysis that tests hundreds of traits and then determine your clades based on a last common ancestor and all of its descendants. Convergence is rampant.

Membership within the clade
Ostariophysi (Lord 1922) includes

Gonorynchiformes — milkfish, untested in the LRT

Cypriniformes — perch, a clade distally derived from Santanichthys in the LRT.

Characiformes — piranha, a clade that proximally precedes Santanicthys in the LRT

Siluriformes — catfish, a clade unrelated to Santanichthys in the LRT

Gymnotiformes — knife fish, a clade that distally precedes Santanichthys in the LRT

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References
Filleul A and Maisey JG 2004. Redescription of Santanichthys diasii (Otophysi,
Characiformes) from the Albian of the Santana Formation and Comments on Its Implications for Otophysan Relationships. American Museum Novitates 3455:21pp.
Forey PL 1977. The osteology of Notelops Woodward, Rhacolepis Agassiz Pachyrhizodus Dixon (Pisces: Teleostei). Bulletin of the British Museum (Natural History) 28(2):123–204.
Silva Santos R 1995. Santanichthys, novo epiteto generico para Leptolepis diasii Silva Santos, 1958 (Pisces, Teleostei) da Formacao Santana (Aptiano), Bacia do Araripe, NE do Brasil. Anais da Academia Brasileira de Ciencias 67:249–258.
Woodward AS 1901. Catalogue of the Fossil Fishes in the British Museum (Natural History), 4. xxxviii + 636 pp., 19 pis, 22 figs. Brit. Mus. (Nat. Hist.), London.

wiki/Santanichthys
wiki/Notelops
wiki/Characiformes
wiki/Ostariophysi