Experts unearthed a fossilized frog that used to exist 100 million years ago. According to the study, the female frog was killed during mating with a male species through gripping.
Scientists noted that mesozoic fossils of frogs are rare in the palaeontological record, particularly those exhibiting soft tissues that offer limited insights into early life-history characteristics.
Mesozoic Frogs
In a recent study, they reported 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.
The new fossil, which could have represented a younger individual than the holotype of Gansubatrachus, showed that sexual maturation occurred before full adulthood in this frog and provides evidence of death linked to mating behavior.
The experts also discussed other potential sources of variation and life-history traits of Gansubatrachus. The new finding represented the oldest Early Cretaceous frog preserving in situ eggs and provided a glimpse into ancient anuran development during Mesozoic times.
The analysis suggested that the frog was still skeletally immature, meaning the species was able to sexually reproduce before being physically fully developed.
Frogs are unique among living land vertebrates, being extremely diverse in their development, reproductive modes and other life-history traits. They have been conserved in their skeletal body plan since Jurassic times.
Most frogs have a biphasic life cycle characterized by different larval (tadpole) and adult (frog) body forms, but some frogs have evolved direct development, lacking a free-living larva.
In all cases, though, they develop from soft, 'jelly'-coated eggs and exhibit a high diversity in the size and number of eggs produced by female frogs.
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Fossil Record
In contrast with the extraordinary current diversity of frogs, which is more than 7600 species worldwide, their fossil record is relatively sparse, yet fossils of both tadpoles and adults have been described.
Most of these fossils typically preserve skeletal data only and, in a few cases, part of the body outline, being extremely rare in the preservation of soft body parts, including eggs.
Fossil eggs of frogs have been reported from a few localities with ages ranging from the Mid-Cretaceous to the Pliocene. These include a single isolated egg preserved in Cretaceous amber, eggs associated with Early Oligocene adult palaeobatrachids of Bechlejovice, Czech Republic, and an isolated egg containing an emerging tadpole in Miocene amber from the Dominican Republic.
The reproductive characteristics are critical components of the life history of frogs, with size at sexual maturity and sexual dimorphism being important traits for understanding the population dynamics of species.
The stage at which sexual maturity occurs greatly influences fitness and is a critical transition in the life history of organisms.
In extant species, this stage is most often determined by gonadal characteristics or external secondary sexual traits, although skeletochronology has also been applied to determine the relative age of anurans and other vertebrates.
However, in the case of fossils most of these approaches are typically hindered by incomplete preservation of body parts.
Skeletochronology has been used to determine the age with relative success in a few studies, although it does not allow for direct inference of sexual characteristics.
In exceptional cases, sexual maturity of extinct vertebrates such as dinosaurs has been established directly from closely associated eggs, but these findings are very rare in the fossil records.
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