Inferring age and growth from remains of fossil and predated Recent anurans: a test case using skeletochronology

Large amounts of vertebrate remains regurgitated by owls are potentially available for continuous monitoring of predated populations, without human interference or the need to sacrifice any specimens. Presumably, Pleistocene and Holocene fossil microvertebrate assemblages found in caves or shelters are also principally the result of predation by nocturnal birds. Our study tests, for the first time, the standard skeletochronological technique on such prey remains. We analyse two Spanish samples from the genus Discoglossus (Amphibia, Anura, Discoglossidae), a living population of D. galganoi (n = 65 individuals) predated by the European Barn Owl, Tyto alba, and a Pliocene-Pleistocene sample of an extinct species (n = 22) presumably predated by Tyto balearica, a closely related ancestor of T. alba. The histological peculiarities and alterations found in bones of both samples are described, but the results indicate that, in general, this type of material is suitable for skeletochronological applications. In the living population, both size and longevity were greater than previously recorded for the species. For both samples, size and age are positively, though weakly, correlated. Comparisons by age-class show that the extinct species is larger than the living one, particularly for 4- and 5-year-old individuals. Growth rates in juveniles (or larvae) of the extinct species should have been slightly higher than in the living species, with similar growth rates after the third year. Recorded longevity is higher in the extinct species. All these life-history traits support a distinct taxonomic status for the extinct Balearic Discoglossus species.