POST-METAMORPHIC DEVELOPMENT OF EARLY CRETACEOUS FROGS AS A TOOL FOR TAXONOMIC COMPARISONS

Specimens of a single but currently undetermined species of the anuran genus Liaobatrachus from the Lower Cretaceous of northeast China, including four articulated skeletons preserved on a single block of sediment plus two skulls and several additional adult skeletons on other slabs, illustrate post-metamorphic developmental stages from early juvenile to fully grown adult. The relative ages of the individuals were estimated based on body size combined with degree of ossification. Post-metamorphic developmental data not only provide a basis for comparisons with other fossil and recent anuran taxa, but also help to identify both age-independent (i.e., size-independent) characters that can be used for taxonomic comparisons among individuals of various sizes (i.e., ages) and age-dependent (i.e., size-dependent, ontogenetically variable) characters that can only be used for comparisons across corresponding developmental stages. Our study revealed that the following characters are independent of size in Liaobatrachus: frontoparietals with a fontanelle between their frontal portions; quadratojugal present; vertebral centra amphicoelous and pierced by a notochordal canal; and free (not ankylosed) ribs attached to diapophyses of second to fourth vertebrae. The tibiofibula/femur and presacral vertebral column/snout-vent length ratios were also found to be size independent. In contrast, ossification of the carpus, distal tarsals, and pubis appears to be ontogenetically variable (i.e., they vary with age), and this is presumably also true of fusion between the tibiale and fibulare. These characters should only be used for comparisons among individuals at a common developmental stage.