Taxonomy & naming
Palaeoplex palimpsest was described in 2020 by Frederic D. B. Schedel, Viviane M. S. Kupriyanov, Cyprian Katongo and Ulrich K. Schliewen in Zootaxa (vol. 4718, issue 2, pp. 191-229), in a paper that erected two new monotypic cichlid genera from northern Zambia at once — Palaeoplex and its companion Lufubuchromis. Both are single-species genera, and Palaeoplex palimpsest is the type and only species of its genus. Eschmeyer's Catalog of Fishes and FishBase (species ID 69125) both list it as valid. The holotype, an adult male of 5.5 in standard length, is held in the Bavarian State Collection of Zoology in Munich as ZSM 47492 (formerly part of lot ZSM 44438); a substantial paratype series is split between Munich, the Royal Museum for Central Africa at Tervuren, the South African Institute for Aquatic Biodiversity, and Cornell University.
The fish had been collected and noticed for decades before it had a name. It appears in the older literature misidentified as Pseudocrenilabrus philander (Balon & Stewart, 1983, working on the Luongo) and as Chetia mola (Friedmann et al. 2013), and in molecular studies of the 2010s it circulated simply as the 'New Kalungwishi cichlid' or as Orthochromis sp. 'New Kalungwishi.' That history of shuffling is the whole point of the eventual name. Within the broad Pseudocrenilabrus group of small African haplochromines, the species sits in an unstable position — and not because the work was sloppy. Two careful genetic studies put it in two different places: an analysis based on nuclear DNA (Weiss et al. 2015) and one based on mitochondrial DNA (Schedel et al. 2019) returned contradictory phylogenetic placements for the same fish.
Morphology
This is a medium-sized, fairly deep-bodied haplochromine. The largest wild-caught male measured 5.5 in standard length (the holotype, about 7 in total length); the largest female was 4.5 in SL, so males clearly outgrow females. Body depth runs roughly 29-36% of standard length, deepest just behind the pelvic fins and tapering to the tail; the head is about a third of the body length, and there is no exaggerated forehead hump. The mouth is essentially terminal with lips of ordinary thickness, and the fish carries the two separate lateral lines typical of cichlids. The dorsal fin has 14-15 spines and 10-12 soft rays, the anal fin 3 spines and 7-9 rays, and there are usually 27-29 vertebrae.
Colour is strongly sex-linked. Breeding males are the striking sex: metallic greenish to turquoise along the flank and tail, the scale-edge sheen lining up into shimmering oblique bars, a whitish lower lip with a turquoise gleam, a golden-orange wash on the gill cover, jet-black pelvic fins, and black dorsal-fin lappets set off by a thin reddish line. On the soft anal fin sits an orange blotch — the egg-spot-like marking characteristic of the Pseudocrenilabrus group. There is no distinct spot on the tail. Females are far plainer: grey to olive with a silvery sheen, a yellowish unmarked anal fin, and no green or gold. Both sexes can show seven to nine faint vertical bars that are usually hard to see in adults but more obvious in juveniles. A useful, if subtle, field-and-lab character separates the two river populations: fish from the Luongo have 18-20 scale rows around the caudal peduncle, those from the Kalungwishi have 16-18.
Habitat
Palaeoplex palimpsest is a river fish, not a lake fish, and its known world is small: the Luongo River, a tributary of the Luapula, and several sites in the Kalungwishi River drainage, which empties into Lake Mweru. Both rivers belong to the Upper Congo system of northern Zambia (Luapula Province and Northern Province), and the species is, on present evidence, endemic to this corner of the Bangweulu-Mweru region. The collection sites cluster around named landmarks of the Zambian north — the Luongo near the Kawambwa-Mansa road and above Musonda Falls, and the Kalungwishi above Lumangwe Falls and around the Kabwelume and Kundabwika cataracts.
The describers' field notes give a clear picture of the type locality on the Luongo: a river about 82 ft wide and roughly 5 ft deep, with a rocky bed broken by patches of sand and mud, its banks fringed with dense reeds and small trees. Tellingly, the fish was found in the slow-moving stretches and was neither seen nor caught in the small rapids nearby, so it appears to favour the quieter, flowing reaches over fast water. No published water-chemistry figures (pH, hardness, temperature) exist for the species in situ; the honest position is that its preferred chemistry is simply not yet on record, beyond the general expectation for a clear, rocky Upper Congo river.
Feeding
There is no formal diet study for Palaeoplex palimpsest — no one has yet worked through a series of stomach contents — but the fish leaves good circumstantial evidence of how it makes a living. X-rays of the type series showed that about half the specimens had guts almost completely packed with a dense, fine-grained material, almost certainly sand, and one specimen contained recognisable fragments of snail shell. Paired with that is the anatomy of the throat: the lower pharyngeal jaw is broad and heavy, carrying stout molar-like (molariform) teeth in its central rows. That combination — a snail-crushing pharyngeal mill plus shell fragments and ingested sediment — points to a fish that takes at least part of its food by crushing hard-shelled molluscs gathered from the bottom, swallowing sand along with its prey as bottom-feeders routinely do. So the working description is a benthic, partly molluscivorous forager, stated as a well-supported inference from anatomy and gut contents rather than as a measured diet.
Mating
Courtship and the mating system of Palaeoplex palimpsest have not been observed — the species is known essentially from preserved specimens and a handful of field photographs, and nobody has reported watching it spawn in the wild or in a tank. What can be said is read off its body and its relatives. The pronounced colour difference between the sexes, with males in bright nuptial dress carrying an orange anal-fin blotch and females left plain, is the signature of the polygynous, male-displaying mating systems usual among haplochromine cichlids: a coloured male advertises and competes for females while females choose. The orange anal marking, set against the black pelvic fins, is the kind of signal these fishes use during courtship. Beyond that genus-level expectation, the specifics — whether males hold territories, dig pits, or form leks; how mates are chosen — are simply undocumented for this fish, and it would be invention to describe a courtship sequence as though it had been seen.
Breeding
Breeding biology is, likewise, essentially unknown for Palaeoplex palimpsest. No spawning has been described, no clutch counted, no incubation timed, and the describers did not report brooding behaviour. The strong sexual colour difference is consistent with the maternal mouthbrooding that dominates the haplochromine cichlids of Africa, and membership in the Pseudocrenilabrus group — small mouthbrooders whose females carry eggs and fry in the mouth — makes maternal mouthbrooding the most likely mode. But that is an expectation drawn from relatives, not an observation of this species, and the responsible thing is to flag it plainly: clutch size, brooding duration, fry size and parental roles for Palaeoplex palimpsest are all blanks waiting to be filled by the first person to keep or closely observe it.
In the aquarium
Palaeoplex palimpsest is, for practical purposes, not an aquarium fish. It was described in 2020 from a remote part of northern Zambia, it has no common name, and there is no evidence of it in the ornamental trade or in hobbyist collections — no import records, no spawning reports, no husbandry accounts. Anyone who did encounter it would be improvising. The sensible starting point would be to treat it as a medium-sized (to ~5.5 in), bottom-oriented Upper Congo river cichlid: a long tank with a sand substrate it can sift, rounded rockwork and current in the moderate range to echo the slow rocky reaches it favours, and robust tankmates rather than delicate ones. Given the molar-like throat teeth and shell fragments in wild guts, snails and other hard-shelled invertebrates would likely feature in its diet, alongside the usual prepared and frozen foods a haplochromine of this size accepts. All of this is reasoned extrapolation, not tested practice — the candid summary is that this is a wild-only, scientifically interesting species with no established place in the hobby, and the value in writing about it is documentary rather than a keeping guide.
Conservation
Palaeoplex palimpsest has not been assessed by the IUCN Red List; FishBase records it as Not Evaluated, as do the CITES and CMS listings. That is unsurprising for a fish named only in 2020, but the gap matters, because on present knowledge this is a narrow-range endemic — known from a single tributary system (the Luongo) and one further drainage (the Kalungwishi), both in the Upper Congo of northern Zambia. Range-restricted river fish of this kind are exactly the species for which the absence of an assessment can hide real exposure: the rivers it lives in are subject to the ordinary pressures of the region, including dam and reservoir development (the type series itself includes fish from the Luongo Reservoir above Musonda Falls), sedimentation from land clearing, and habitat alteration. None of this has been quantified for the species, and no threats have been formally documented. The honest conservation statement is therefore twofold: there is no current evidence that the fish is in trouble, and there is also no assessment confirming that it is safe — a recently described, small-range Congo-basin endemic that warrants a Red List evaluation it has not yet received.