One of the most exciting things about the Royal Tyrrell Museum is that it’s more than a Museum, it’s an active research facility, so there are exciting things happening all the time that further our understanding of ancient life.
Dr. James Gardner, our Curator of Palaeoherpetology (the study of prehistoric reptiles and amphibians), has just published a paper on fossil tadpoles in a special issue of the journal Fossil Imprint, available for free here.
Scientific research papers often owe their origins to chance events and may take years to come to fruition. Such is the case for “The Fossil Record of Tadpoles.” Dr. Gardner’s interest in fossil tadpoles was sparked in the early 1990s, when he recognized a 45 million year old tadpole fossil in the University of Florida Museum of Natural History’s fossil plant collection. He began working on a manuscript reviewing the fossil record of all tadpoles but it was set aside for decades, as he became busy with other projects – his PhD dissertation, for example. Fast forward a quarter century later, when Dr. Gardner was invited to contribute a paper to a volume commemorating the Czech palaeontologist Zdeněk Špinar (1916–1995). Knowing that Professor Špinar had published many papers on fossil tadpoles, Dr. Gardner decided to resurrect and update his dusty and almost forgotten manuscript.
One of the most characteristic features of anurans (frogs and toads) is their two-stage or biphasic life cycle consisting of a free swimming, vaguely fish-like, and typically herbivorous larval form (tadpole) that undergoes profound and rapid structural remodeling (metamorphosis) to become a four-legged, hopping, and exclusively carnivorous adult.
Metamorphosis of tadpole into frog. Source.
Metamorphosed or adult frogs are moderately well represented in the fossil record; but many people are surprised that tadpoles also appear. At first glance, tadpoles seem poor candidates for fossilization: they are small, have only a rudimentary skeleton, and the tadpole phase of a frog’s life cycle generally lasts only a few months.
However, lakes provide an ideal environment for preserving those kinds of fossils, thanks to the fine-grained sediments in lake bottoms that can quickly cover and preserve carcasses. They are also ideal habitats for tadpoles.
Figure 1: An indeterminate tadpole body fossil from the middle Eocene (approximately 45 million years ago) of Utah, USA. Head points towards top of figure and scale bar along left side is in 1 mm increments. Note the characteristic tadpole body form, consisting of a globular head + body and an elongate, narrow tail. This specimen lacks any indication of a skeleton; either the individual died at a young stage before the bones began ossifying or bones were present but did not preserve. Note the preserved eye pigments, brain, and digestive tract. This fossil is in the collections of the University of Florida Museum of Natural History, Gainesville, Florida.
Dr. Gardner’s review showed that the fossil record of tadpoles is better than most people realize. Tadpole body fossils have been known since the early 1800s, but in an interesting historical quirk, the first examples reported in 1828 were not recognized as tadpoles. It took another three years before tadpole fossils which were identified as tadpoles were published. Since then, fossil tadpoles have been reported from over 40 localities on most continents, except Antarctica and Australia. Those localities are in lake-style deposits and range in age from the Early Cretaceous to late Miocene (140–10 million years ago). A number of those localities have yielded multiple examples of different-sized tadpole body fossils, some so well preserved that their body outlines and details of internal structures like nerves and blood vessels can be seen.
Figure 2. Examples of fossil tadpoles preserving bones and at different growth stages, both with their head pointing towards top of figure. Left: Pre-metamorphic tadpole of Shomronella jordanica (basal Pipimorpha) from the Early Cretaceous (about 130 million years ago) of northern Israel or Palestinian West Bank. This locality has yielded over 250 tadpoles of Shomronella. Right: Later stage tadpole undergoing metamorphosis of Palaeobatrachus vicentinus (Palaeobatrachidae) from the middle or late Oligocene (about 28 million years ago) of northeastern Italy. This is the only fossil tadpole known from the locality. The Shomronella fossil is in the collections of Hebrew University, Jerusalem, Israel and the Palaeobatrachus fossil is in the collections of Paläontologisches Museum, Humboldt University, Berlin, Germany; both photographs are courtesy of Prof. Zbyněk Roček (Prague, Czech Republic).
Size series of tadpole fossils from the same species are especially valuable because they provide large enough samples for palaeontologists to trace patterns of growth and metamorphosis in fossil frogs (the human equivalent would be having examples of everything from a baby to a teenager). The tadpole fossil record demonstrates that the distinctive tadpole body form and lifestyle and its astounding metamorphosis into a four-legged and hopping adult are ancient attributes of frogs.
This fish-like larval form can appear similar to other animals, such as fish and insects. In addition to reviewing the fossil record of tadpole body fossils, Dr. Gardner also examined reports of other fossils that have been interpreted as tadpoles. Those include 385 million year old larval fish fossils from the Middle Devonian of Scotland, a recently metamorphosed and 250 million year old skeleton of a proto-frog from the Early Triassic of Madagascar, and a piece of 20 million year old amber from the Dominican Republic containing what may be a tadpole hatching from an egg. Dr. Gardner provided the evidence against interpreting most of these fossils as tadpoles, although some, such as the Dominican amber, are ambiguous.
This paper and a second paper by Dr. Gardner on North American frogs from the Campanian titled “The Hopping Dead” (proof that scientists have a sense of humour) are available here. Fossil Imprint is an open access journal.