Polypterus

Schultze, Hans-Peter, 2018, Hard tissues in fish evolution: history and current issues, Cybium 42 (1), pp. 29-39 : 32

Polypterus and elasmodine

Sire (1989) discovered elasmodine in the scales of the extant Polypterus between the dentine and the basal isopedine. He constructed an additional palaeoniscoid type scale, the polypteroid type ganoid scale, with elasmodine lying between dentine and the bony basal plate. Sire et al. (2009: fig. 16) considered the polypteroid-type ganoid scale as the basal osteoichthyan scale type and argued that elasmoid scales are paedomorphic structures in actinopterygians and sarcopterygians.

Polypterus is considered the most primitive living actinopterygian, but it is not the most primitive fossil actinopterygian. The genus was placed close to scanilepiforms by Aldinger (1937) and Schultze (1968) on histological grounds (canal system and arrangement of bone cells around the canals, respectively) and by Jakovlev (1973), Selezneva (1985) and Sytchevskaya (1999) on morphological similarities of Evenkia with Polypterus . Cloutier and Arratia (2004) cladogram shows Polypterus together with Acipenser and neopterygians as the sister group of Tarrasius and Guildaichthys, separate from palaeoniscimorphs. A similar placement appears in a new phylogenetic analysis of actinopterygians that includes scanilepiforms ( Giles et al., 2017). The two polypterid genera Polypterus and Erpetoichthys and the sister group Evenkia are hypothesized to be the most advanced scanilepiforms ( Fig. 1 ), and the Chinese Middle Triassic Fukangichthys appears as the most basal scanilepiform in this analysis. Scanilepis occurs in the Late Triassic (Rhaetian), the earliest fossil polypterid in the Late Cretaceous (Cenomanian). Thus, there is still a time gap of about 110 million years between the two groups, which is not comparable to a ghost line of about 320 million years if the polypterids are considered to be the most primitive actinopterygians. The youngest palaeoniscimorph with rhomboid palaeoniscoid scales can be found in Pteroniscus from the Late Jurassic, which is not related to scanilepiforms.

The relationship of polypterids as the most advanced scanilepiforms suggests that the histological structure of their scales represents an advanced form of the palaeoniscoid scale type within palaeonisciforms, and the occurrence of elasmodine between dentine and isopedine is an advanced character within palaeonisciforms and not a basal character in actinopterygians or even of osteichthyans as proposed by Sire et al. (2009). Scanilepis shows no indication of elasmodine. Thus, it could be said that the polypterids are another group converging on elasmoid scales within scanilepiforms.

Figures

Figure 1. – Appearance of elasmoid scales within osteichthyans shown on an interrelationship scheme of Osteichthyes after Long et al. (2015), Schultze (2015), and King et al. (2017) for the lower part and after Giles et al. (2017) for the actinopterygians above Ligulalepis. Node A = Osteichthyes: de, dentine, i, isopedine, r, rhomboid scale; node B = de, dentine, en, enamel, i, isopedine, r, rhomboid; node C = Sarcopterygii: el, elasmoid; node C’ = r, rhomboid, co, cosmine (dipnoi are represented by Diabolepis and their elasmoid scale by that of Neoceratodus); node D = Actinopterygii: ga?, questionable for basal taxa, r, rhomboid; canal system of Andreolepis typical for lower actinopterygians and osteichthyans; node D’ = a, acrodine in Ligulalepis (after Schultze, 2015: fig.13) and higher actinopterygians, ga, ganoine, r, rhomboid (only branches with elasmoid scale are named in lower actinopterygians). Scales of Lophosteus superbus after Gross (1968, fig. 1A), of Actinistia after Smith (1940: fig. 70, Latimeria chalumnae), Onychodontida after Andrews et al. (2006: fig. 60b, Onychodus jandemarrai), of Dipnoi after Kerr (1955: fig. 4B, Neoceratodus), of Rhizodontida after Long (1989: fig. 13, Barameda decipiens), of Tristichopterida after Jarvik (1980: fig. 138 B3, Eusthenopteron foordi), of Holoptychiidae after Ørvig (1957: fig. 13A, Holoptychius sp.), of Sphaerolepis after Fritsch (1893: pl. 109, fig. 5), of Polypteridae after Rauther (1929: fig. 136, Erpetoichthys calabricus), of Coc- colepis combination after Agassiz (1843: vol. 2, pl. 26, fig. 7) and Schultze (1996: fig. 4), of Eurycormus (original), of Leptolepis cory- phaenoides (original), and of an amiid after Grande and Bemis (1998: fig. 201, Cyclurus macrocephalus).