OSTEOLOGY OF BIBARBA

The cranial skeleton of Bibarba is typical of cobitids sensu Sawada (1982), with an evident metapterygoid– quadrate fenestra in the suspensorium and an interorbital fenestra in the neurocranium (Figs 4, 5).

In cave-dwelling B. parvoculus, the pterosphenoid forms the posterior rim of the interorbital fenestra, dorsoventrally separating the frontal and the parasphenoid, and posteriorly contacting only the sphenotic (Fig. 4C). The bifurcate pharyngeal process of the basioccipital is short and tapers to an end at a vertical approximately anterior to centrum 2 + 3 (Fig. 4C, F). The entopterygoid is a slim bone with a blade-like posterior part and, anteriorly, articulates with the palatine and, posteriorly, with the metapterygoid (Fig. 4D). The posteroventral process of ceratobranchial 5 (pharyngeal bone) extends ventrally, forming a right angle with the ventral limb (Fig. 4E). A single fontanelle enclosed by paired frontals, paired parietals and the supraoccipital is on the roof of the skull (Fig. 6C). The anterior part of the frontal is narrow at the orbital region and about 36% as wide as the posterior part (Fig. 6C, F). The suborbital spine is relatively stout. A weakly ossified supraorbital with a grey value similar to that of the sclera ring in the 3D rendering (Fig. 6C) is at a vertical approximately anterior to the laterocaudal process of the suborbital spine. In the caudal skeleton, there are five hypurals, with hypurals 4 and 5 fused (in all specimens except for one female with hypurals 4 and 5 separate) (Fig. 4H). The vertebral count (including the first four centra forming the Weberian apparatus and the last pleural centrum) is 45 (N = 2) or 46 (N = 2).

In surface-dwelling B. bibarba, the skeleton is similar but has the following differences. The bifurcate pharyngeal process of the basioccipital is short and stout, ending at a vertical approximately anterior to centrum 1 (Fig. 5C, F). The posterior part of the entopterygoid is shallow (Fig. 5D). The posteroventral process of the pharyngeal extends posteriorly, forming an obtuse angle with its ventral limb (Fig. 5E). The anterior frontal is narrower, about 20% as wide as the posterior part (Fig. 6I, L). The vertebral count is 39 (N = 2).

Sexual dimorphism is demonstrated in the pectoral fin and in the pectoral girdle, nasal bones and anterior intermuscular bones based on the two males and four females (Figs 1C, D, 5C, F, 6). In the pectoral fin, a duplication of the lamina circularis on the second and third pectoral rays occurs in males (Fig. 1C, D). In the pectoral girdle, the coracoids, mesocoracoid and scapula are much stouter in the male (Fig. 6D, E, J, K) and all are fused with the cleithrum in surfacedwelling B. bibarba (Fig. 6J, K), but autogenous in cave-dwelling B. parvoculus (Fig. 6D, E). The lateral lamina at the ventral limb of the cleithrum is smaller in males than in females of B. bibarba (Fig. 6H, K). In addition, a vertically positioned lamina derived from the second pectoral radial contacting the mesocoracoid (Fig. 6A, D, G, J), is much stronger in males (Fig. 6D, J). In the neurocranium, a slim laterally convex nasal is lateral to the supraethmoid–ethmoid complex, which is stouter in males than in females of B. bibarba (Fig. 6I, L), but similar in male and female B. parvoculus (Fig. 6C, F). The anterior intermuscular bone associated with vertebrae 4 and 5 is ramified and hatchet-like and stouter in male than in female B. bibarba (Fig. 5C, F), but similar in male and female B. parvoculus (Fig. 4C, F).