The earliest direct evidence of frogs in wet tropical forests from Cretaceous Burmese amber

Abstract

Frogs are a familiar and diverse component of tropical forests around the world. Yet there is little direct evidence from the fossil record for the antiquity of this association. We describe four fossil frog specimens from mid-Cretaceous (~99 mya) amber deposits from Kachin State, Myanmar for which the associated fauna provides rich paleoenvironmental context. Microcomputed tomographic analysis provides detailed three-dimensional anatomy for these small frogs, which is generally unavailable for articulated anurans in the Mesozoic. These crown-group anuran specimens provide the earliest direct evidence for anurans in a wet tropical forest. Based on a distinct combination of skeletal characters, at least one specimen has clear similarities to living alytoid frogs as well as several Mesozoic taxa known from the Jehol Biota in China. Whereas many Mesozoic frogs are from seasonal and mesic paleoenvironments, these fossils provide the earliest direct evidence of anurans in wet tropical forests.

Figure 1

Photograph images of four fossil frog specimens referred to Electrorana, including the holotype (A; DIP-L-0826) and three additional specimens (B: DIP-V-16119; C: DIP-V-16127; D: DIP-V-16121). Specimens in (B) and (D) are presented with two views of the amber specimen and the oval in (D) indicates the presence of the anuran specimen. Scale bars equal 5 mm.

Figure 2

Holotype (a) of Electrorana limoae (DIP-L-0826) as visualized via microCT scanning. (b) Unidentified coleopteran embedded in amber with holotype. Skull in dorsal (c), left lateral (d), and ventral (e) views. Ventral views of pectoral girdle (f), left hand (g), vertebral column (h), and left foot (i). Abbreviations of anatomical terms are as follows: clav – clavicle; cor – coracoid; dent – dentary; exoc – exoccipital; fpar – frontoparietal; max – maxilla; phyd – parahyoid; pmax – premaxilla; postmed proc – posteromedial process of hyoid; pro – prootic; prsph – parasphenoid; pter – pterygoid; qj – quadratojugal; raduln – radioulna; scap – scapula; sept – septomaxilla; sq – squamosal; supscap – suprascapula; unc – uncinate process. The number of each digit is indicated with roman numerals. Scale bar for (c–e), 2 mm; scale bar for (b,f–i) 1 mm.

Figure 3

Three fossil frog specimens referred to Electrorana visualized via microCT scanning. (A) DIP-V-16119, (B) DIP-V-16127, (C) DIP-V-16121. Scale bars equal 2 mm.

Figure 4

Majority-rule consensus trees representing phylogenetic analyses using parsimony of matrices from (a) Henrici et al., (b) Báez, and (c) Gao & Chen including Electrorana from the Cretaceous of Myanmar. Numbers adjacent to nodes represent the percentage of equally parsimonious trees containing that node. The phylogeny based on Henrici et al. is rooted using the extant alytoid clade, that based on Báez is rooted with Ascaphus, and that based on Gao & Chen is rooted with Triadobatrachus. Extinct taxa are indicated with a dagger, and Electrorana is indicated in boldface.