Is the prefrontal bone in Alpine newt (Triturus alpestris Laurenti, 1768) of dual origin?

Abstract

According to current knowledge, only the prefrontal bone (os prefrontale) of the circumorbital series is preserved in the family Salamandridae. However, the exact origin and number of ossification centres creating this bone is unknown. Detailed examination of the prefrontal bone during ontogeny of juvenile and adult specimens of the Alpine Newt (Triturus alpestris) indicates its dual origin (prefrontal and lacrimal). We found that the prefrontal bone originates from four ossification centres, i.e. three prefrontal centres and one posterior lacrimal centre. The anterior lacrimal centre participates in the maxillar ossification. The development of these ossification centres occurs very late in ontogeny (at stage 54), and starts after differentiation of the nasal capsules. The total fusion of the lacrimal ossification centre with the prefrontal bone of T. alpestris is distinct from the fully differentiated lacrimal bone attached to the prefrontal bone of the fossil family Branchiosauridae (Temnospondyly). We propose that heterochrony, observed in the recent species, is a delayed development followed by accelerated ossification that resulted in the fusion of the anterior lacrimal centre with the maxilla and the posterior lacrimal centre with the prefrontal bone.

Fig. 1

Daily water temperatures during early development of the Alpine newt (T. alpestris). The white girdle, within the embryonic period, denotes the date of hatching of embryos.

Fig. 2

The skull (exocranium) of the T. alpestris adult female with its bone configuration. State after bone and cartilage staining, dorsal view, scale bar = 1 mm.

Fig. 3

The left paired os praefrontale in the adult specimen of T. alpestris. Dorsal (A) and ventral (B) aspect; ma.fr., margo frontalis; cr.prf., crista praefrontalis; ma.na., margo nasalis; inc.nl., incisura nasolacrimalis; pr.rnl., processus retronasolacrimalis; ma.orb., margo orbitalis; pr.fr., processus frontalis; rec.fr., recessus frontalis; pr.mx., processus maxillaris; ma.mx., margo maxillaris; pr.na.i., processus nasalis inferioris; pr.na.s., processus nasalis superioris. Scale bar = 0.5 mm.

Fig. 4

Different levels of ossified centres of prefrontal bone in three larvae of T. alpestris at stage 54. (A) Arrows represent the direction of ossification of the maxilla, nasale and prefrontale; lacrimal centre is a small triangle below the prefrontal centre. (B) Two ossified centres – lacrimale and prefrontale (?) – anterior ossified centre of the lacrimal bone, which is probably attached to pars facialis ossis maxillaris. (C) Advanced ossification of both centres of prefrontale (lacrimal and prefrontal) and facial part of maxilla. Anterior facial part of maxilla, in the nasal opening, where the ossified centre is observed (*) – this is probably associated with septomaxilla or nariodal bone (Jarvik, 1980). Abbreviations: lacr., ossification centre of os lacrimale; pfron., ossification centre of os praefrontale; imax., intermaxilla; max., maxilla; cob., cartilago obliqua of the nasal capsula; nas., os nasale; fron., os frontale. Scale bar = 1 mm.

Fig. 5

Time difference in the development of four bones between fossil branchiosaurid Apateon caducus (after Boy, 1972; Schoch, 1992; Boy & Sues, 2000) and recent newt Triturus alpestris (after author).