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Aguilera, O & D Rodrigues De Aguilera, 2004. Amphi-American Neogene sea catfishes (Siluriformes, Ariidae) from northern South America. Special Papers in Paleontology, 71: 29-48.

This shows that modern ariid species have been around for a fairly long time.

...and the abstract:

Fossil sea catfishes (Ariidae) collected from the Tropical American Neogene are essentially modern species that inhabited estuarine environments similar to those that characterize their living counterparts. Eight extant species of ariids. 'Arius' couma. 'A'. dowii, 'A'. herzbergii, 'A'. quadriscutis, 'A'. kessleri, 'A.' rugispinis, Bagre marinus and Sciadeops troscheli are described as fossils. These taxa are represented by neurocrania from the lower Miocene Cantaure. lower Miocene Castillo, upper Miocene Urumaco (middle and upper members) and upper Miocene-lower Pliocene Cubagua (Cerro Verde Member) formations of the Venezuelan marine basin. Additional indeterminate eroded or broken neurocrania assigned to cf. 'Arius'. cf. 'Cathorops' cf. 'Bagre', and Ariidae incertae sedis were also collected. Using a nonlinear regression such that total fish length (TL) in 'A'. couma, 'A'. dowii, and 'A'. herzbergii (TL = 3.4407 + 3.473 x total skull length), 'A'. quadriscutis (TL = 0.7082 + 5.9033 x distance between both post-temporal bones). 'A'. kessleri (TL = -0.2016 + 12.1467 x ventral length of the sustentaculum), and B. marinus (TL = 3.0482 + 4.9632 x distance between both lateral ethmoid processes), the total length of fossil ariids is inferred to be very close to the observed body-size range in the living counterpart. Pacific species reported here ('A'. dowii, 'A'. kessleri and S. troscheli) inhabited estuarine environments along the Caribbean coast of South America during the wide-open connection between the Pacific and the Atlantic Ocean, prior to the closure of the Panamanian isthmus in the early Pliocene.