Erpetoichthys

Meunier, François J. & Gayet, Mireille, 2020, Comparative morphology of the finlet spines of the extant Polypteridae (Osteichthyes; Cladistia). Systematic interest, Cybium 44 (1), pp. 19-37 : 28

Morphology of the finlet spines in Erpetoichthys calabaricus ( Fig. 12 )

The spine height of Erpetoichthys is approximately constant from the first to the last, with a slight decrease for the two most posterior spines. They are stockier than the spines of Polypterus species and their articular head distinctly differs in shape and organization of the processes ( Fig. 12 ).

The ganoine completely covers the anterior face of the spines with no superior transversal pad at its lower margin. Thus, the upper anterior processes are missing, and the lower anterior processes are scarcely visible, bounding an inferior transversal pad narrower than the articular head. In addition, the median processes are weakly individualized ( Fig. 12B ). The median groove never shows any constrictions and is widely open at its base ( Fig 12A ). Because of the constant size of the spine for the whole series, its lower margin is always localized at the same distance from the articular head. The only position-related difference observed is the basal foramen shape: oval only for the first spine and rounder for the last. In contrast to Polypterus , the lepidotrichium is never dichotomised and remains unibranched whatever the individual age in Erpetoichthys .

The lateral processes do not have the same bulky shape ( Fig 12A ) as those of Polypterus ( Fig. 4D ). They appear flat and free from the rest of the articular head. This articular head shows weakly differentiated posterior processes, which merge with the two basal processes to form a rounded surface extended to the whole pinnule base ( Fig. 12A ). This organization is very similar to the one described for P. polli in which the lateral processes are marginal to the genus Polypterus .

In basal view, the articular head of the Erpetoichthys spine is oval, slightly higher than wide ( Fig. 12B ), and not rectangular or square as observed for the Polypterus species ( Fig. 4F ). A perfectly round glenoid cavity is located in its centre. Finally, the two lateral processes are more distant from the base than they are in the Polypterus species.

Figures

Figure 4. – A-B: Polypterus bichir katangae; A: The anterior face of the spine is covered by an irregular ganoine layer, and shows the basal articular head (AH) and the shaft (SH); B: The posterior face of the spine shows, the median groove and the basal foramen on the shaft; the two arrow-heads indicate the two lateral pads. C-F: P. senegalus; detail of the anterior (C), the posterior (D), the left lateral (E), and the basal face (F) of the articular head, respectively, to show the various significant processes; the arrowhead points to the median groove (F). Scale bars:A, B = 1 mm; C-F = 0.5 mm.

Figure 12. – Erpetoichthys calabaricus. A: The posterior face of 6th finlet spine. B: Basal view of the articular head of the same finlet spine showing the glenoid cavity and the processes. Scale bars = 0.5 mm.