Nyctibates corrugatus, Boulenger, 1904

Schweiger, Susan, Rödel, Mark-Oliver, Hammel, Jörg. U. & Müller, Hendrik, 2024, Don’t go with the flow: cranial adaptations of stream tadpoles in the Afrobatrachian family Arthroleptidae, Zoological Journal of the Linnean Society 202, pp. 1-17 : 7-9

Nyctibates corrugatus and Trichobatrachus robustus

Both taxa had more muscular tails than the previously described two taxa but were otherwise similar to them in having low dorsal and ventral fins (Fig. 1). The oral disc of N. corrugatus consisted of semicircular, wide, and flattened lobes that were surrounded by marginal papillae (Fig. 3C, also see Griesbaum et al. (2019) for a detailed description). The oral apparatus of T. robustus was highly modified into a large oral sucker with multiple rows of small papillae (Fig. 3D).

In N. corrugatus , the cornua trabeculae were fused together and formed a broad, trapezoidal plate, which was very different from the other three tadpoles examined (Fig. 4C). Laterally, the cornua trabeculae articulated with the cartilago labialis superior. In T. robustus , the cornua trabeculae were also fused, but each trabecular horn curved ventrad at its anterior end (Fig. 4D). The processus muscularis of the palatoquadrate was much wider in T. robustus compared to N. corrugatus (Fig. 5C, D). In N. corrugatus , the commissura quadratoorbitalis formed a curved, cartilaginous strip that connected the processus muscularis of the palatoquadrate to the trabecula cranii, whereas it was broad and massive in T. robustus (Fig. 5C, D). In N. corrugatus , a long processus hyoquadratis extended from the medial part of the palatoquadrate (Fig. 5C). The processus hyoquadratis articulated widely with the hypobranchial apparatus via the lateral part of the ceratohyale. There was a wide gap between this process and the processus muscularis. The ceratohyal fitted into this gap, articulating with the palatoquadrate. This structure was not seen in any of the three other tadpoles examined in this study. In N. corrugatus and T. robustus , the palatoquadrate was connected anteroventrally to the neurocranium by a broad commissura quadratocranialis (Fig. 6C, D). Laterally, the palatoquadrate of T. robustus bore an extremely elongated processus ventralis, which was much shorter in N. corrugatus (Fig. 5C, D). In both tadpoles, the cartilago labialis superior formed a massive, single element of the upper jaw (Fig. 6C, D). Compared to N. corrugatus , it appeared much shorter and more compressed in T. robustus . The cartilago labialis inferior consisted of two separated elements; it was of a more triangular shape in T. robustus (Fig. 6D). The cartilago meckeli was transversally oriented and L-shaped in frontal view in T. robustus , while it appeared thicker in N. corrugatus (Fig. 6C, D). For a detailed description of the complete cranial skeleton see Supporting Information (Fig. S3 for N. corrugatus , Fig. S4 for T. robustus ; Table S2 ).

The hyobranchial skeleton in T. robustus was transversally orientated.Incontrastitwasmoreobliquein N.corrugatus .Theceratohyale of N. corrugatus and T. robustus were even more massive compared to A. occidentalis (Fig. 7). The ceratohyale of T. robustus was more stout and transversely oriented in comparison to N. corrugatus (Fig. 7C, D). A basihyale was only present in N. corrugatus (Fig. 7C). Nyctibates corrugatus had four separated ceratobranchialia, whereas ceratobranchialia I and II were fused in T. robustus (Fig. 7C, D).

The m. orbitohyoideus in N. corrugatus was more elongated in comparison to T. robustus . In N. corrugatus , it originated at the lateral part of the processus muscularis and the processus ventralis on the palatoquadrate and inserted along the lateral part of the ceratohyale and on the processus hyoquadratis (Fig. 8C). For a complete description of the cranial musculature of N. corrugatus see Supporting Information (Fig. S7; Table 2 ). In T. robustus , the m. orbitohyoideus was very thick and consisted of three layers of muscle, a feature which we did not observe in any of the three other tadpoles ( Figs 8D , 9 ). The fibres of the deepest layer originated dorsolaterally on the processus muscularis and the inner side of the processus ventralis and inserted at the anterior region of the ceratohyale (Fig. 9). The middle layer originated anterolaterally on the processus muscularis and along the processus ventralis and inserted medioventrally and mediodorsally on the ceratohyale. The outer layer originated dorsolaterally on the processus muscularis and also on along the processus ventralis, and inserted distally on the ceratohyale (Fig. 9). For a complete description of the cranial musculature of T. robustus see Supporting Information (File S1, Fig. S8; Table 2 ).

Figures

Figure 8. 3D reconstruction of the chondrocranium and the m. orbitohyoideus, lateral view. A, slow-flowing stream tadpole of Leptopelis parkeri. B, moderate-flowing stream tadpole of Astylosternus occidentalis. C, fast-flowing stream tadpole of Nyctibates corrugatus. D, fast-flowing stream tadpole of Trichobatrachus robustus. The processus ventralis functions as an extended origin and therefore expands the surface of the large m. orbitohyoideus in A. occidentalis, N.corrugatus, and T.robustus. Note that the m. orbitohyoideus has separate deep and superficial muscle layers in T. robustus.

Figure 9. 3D reconstruction of the chondrocranium and associated musculature of Trichobatrachus robustus. Left, dorsal view. Right, lateral view. Shown separately are the three layers of the m. orbitohyoideus: (1) superficial layer, (2) medial layer, (3) deeper layer.