A tiny, ancient, African river fish, Cromeria, enters the LRT alongside an ancient Amazon river fish, Anableps (four-eyed fish)

Contra tradition and textbooks
the tiny fish pictured here (Fig. 1) are all basal taxa in the clade of bony ray fin fish. Their presumed origin and radiation in the Silurian is based on the presence of known, more derived taxa in the Late Silurian. Perhaps the small size of these taxa has helped them avoid detection in Silurain strata. Prohalecites, the basalmost taxon, is known from Middle Triassic strata. The rest are late-surviving extant taxa.

Figure 1. Small taxa from the base of the ray-fin bony fish clade shown at full scale. Cromeria is the sixth taxon down and the third illustration down. Taxa not shown are generally much larger in size.

The physical and phylogenetic separation of Anableps and Cromeria,
both freshwater taxa, occurred before South America separated from Africa in the Jurassic Period 180 mya.

Cromeria nilotica
(Boulenger 1901; 4.5cm; Figs 1-3) is an extant tiny African naked shellear previously not associated with Anableps (Fig. 4). They must have split apart prior to the South America – Africa rift. Two species are shown below (Fig. 3). Note the subterminal mouth, as in Anableps. The scaleless body lacks a lateral line. Like other clade members, the caudal fin extends anteriorly both dorsally and ventrally.

Figure 2. From Gregory 1933, diagram of Cromeria. Colors added here. Note the odd caudal fin extending anteriorly to the dorsal and anal fins.
Figure 3. Two species of Cromeria, the African naked shellear shown several times life size.
Figure 2. Anableps skull CT scan images from Michel et al. 2015.
Figure 4. The four-eyed fish Anableps skull CT scan images from Michel et al. 2015. Compare to Figs 1–3.

Anableps tetrophthalmus
(originaly Cobitis anableps Linnaeus 1758, Scopolis 1777; Michel et al. 2015; 32 cm) is the extant four-eyed fish (aka: cuatro ojos), a surface predator of insects falling into Amazonian fresh waters or shallow shores where Anableps beach themselves to eat them. The underslung jaw enables this behavior.

Females are much larger than males.
Internal fertilization (with a modified tubular anal fin) leads to live birth (viviparity) of up to 14 young. The vertebral number is higher than typical for most ray-fin fish.

Traditionally Anableps is a member
of the pupfish (guppy, killifish, topminnow, Danio) family. Here, in the LRT, Anableps nests with basalmost ray fin fish like Lepidogalaxias (Fig. 1) and tiny Cromeria (Fig. 1) from Africa, close to the origin of ray fin bony fish and far from the more highly derived guppies and pupfish.

References
Boulenger GA 1901. Diagnoses of new fishes discovered by Mr. W. L. S. Loat in the Nile. Annals and Magazine of Natural History, Including Zoology, Botany and Geology, Being a Continuation of the ‘Magazine of Botany and Zoology’, and of Louden and Charlesworth’s ‘Magazine of Natural History’, Series 7 8: 444-446.

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