A cretaceous frog with eggs from northwestern China provides fossil evidence for sexual maturity preceding skeletal maturity in anurans

Abstract

Mesozoic fossils of frogs are rare in the palaeontological record, particularly those exhibiting soft tissues that offer limited insights into early life-history characteristics. Here we report on a skeletally immature frog from the Lower Cretaceous of northwest China, with egg masses in the body and eggs in the oviduct, indicative of a gravid female. CT reconstruction of the specimen allows referral to Gansubatrachus qilianensis and we assign it as a paratype complementing the diagnosis of the type species. The new fossil, which might represent a younger individual than the holotype of Gansubatrachus, shows that sexual maturation occurred before full adulthood in this frog and provides evidence of death linked to mating behaviour. We also discuss other potential sources of variation and life-history traits of Gansubatrachus. The new finding represents the oldest Early Cretaceous frog preserving in situ eggs and provides a glimpse into ancient anuran development during Mesozoic times.

Keywords: Anura; Early Cretaceous; Gansubatrachus; northwest China; sexual maturation.

Figure 1.

Holotype and paratype locality (star) for Gansubatrachus qiliansis (a) and stratigraphic details for the Zhonggou Formation (b) [34].

Figure 2.

Paratype of Gansubatrachus qilianensis. (a,b) Photography of part and counterpart (JQ-HX-QW-02A and JQ-HX-QW-02B), showing the skeleton and soft-body parts, with the red box in dorsal view boxes indicating eggs preserved in the body cavity; (c) CT reconstruction of dorsal view, showing the intact right hind limb covered in rock. The parasphenoid bone was excluded from reconstruction because it was obscured by the frontalparietal. Scale bar: 5 mm.

Figure 3.

Anuran eggs within body cavities of the fossil frog Gansubatrachus qilianensis (paratype JQ-HX-QW-02) and extant ranid frog. (a) G. qilianensis, enlarged view of left red box in figure 2a, showing fossil eggs arranged in columns and likely enclosed within oviduct; (b) G. qilianensis, enlarged view of right red box in figure 2a showing cluster of eggs, possibly within ovary; (c) passage of eggs through the paired oviducts in the extant frog Rana pipiens (Image revised from the Biodiversity Heritage Library. Contributed by MBLWHOI Libraries. (www.biodiversitylibrary.org) [64]). Scale bars in a and b: 1 mm.

Figure 4.

Energy dispersive X-ray spectroscopic (EDS) elemental maps illustrating preservation of eggs ascarbonized fossil. (a) EDS analysis of the field of view shown rotated 90° clockwise by the red box in figure 3a; (b–d) C, Al and Si maps are the same as the field of view of figure 4a.

Figure 5.

Close-ups of holotype (JQ-HX-QW-01) and paratype (JQ-HX-QW-02A) of Gansubatrachus qilianensis showing their different developmental stages: a₁, b₁, skulls (fp, frontoparietal; psh, parasphenoid; sph, sphenethmoid); a₂, b₂, Forelimbs (different carpal ossification states were shown in the two specimens). Scale bar: 2 mm.

Figure 6.

Reconstructed Early Cretaceous living environment of the Gansubatrachus qilianensis.