Published on January 25th, 2023 | by David Marshall
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Episode 151/152: Lissamphibian Origins
Caecilians, sometimes known as ‘blind worms’, are a lesser-known group of lissamphibians (all living amphibians). Most modern caecilians are all fossorial (burrowing) and are restricted to the moist soils and leaf litter of tropical forests. Adaptation to this specific ecology has led to radical modification of their bodies, from fusion of the bones in the head and the function of the jaw, to the loss of limbs and development of unique sensory organs.
The fossil record of caecilians is incredibly poor, with only 10 specimens available for researchers to piece together their evolutionary history with. This is further problematic because without a firm understanding of caecilian evolution, we can’t understand the origins of lissamphibians, which includes the ecologically significant groups of frogs and salamanders.
Published in Nature today, a new study led by Ben Kligman, Petrified Forest National Park and Virginia Tech, details numerous specimens of a new fossil caecilian. Discovered within a Triassic deposit, Funcusvermis gilmorei is not only the oldest known caecilian, but it also displays a unique combination of anatomical characteristics that helps illuminate the evolutionary origins of caecilians and all lissamphibians.
Whilst this discovery goes some way to answer some of the bigger questions, other problems that are raised, most notably why there is such an over-representation of Funcusvermis‘s lower right jaw.
Liassamphibia is the group of animals containing all modern amphibians, including frogs, toads, salamanders and caecilians (pictured). These ‘blind worms’ are mostly fossorial (burrowing) and predatory animals. They possess numerous adaptations to this ecology, leading them to look very similar to other fossorial groups such as amphisbaenians (worm lizards). Image: Caecilia pulchraserrana by Acosta-Galvis et. al. 2019 CC BY-SA 4.0.
Their morphology (physical form) isn’t the only adaptation to this ecology, strategies such as the young possessing teeth specifically for feeding off of their mother’s flesh is another, particularly gruesome but evidently beneficial, example.
The fossil record of caecilians is exceptionally poor, particularly for a group of animals that have been estimated to have originated even as far back as the Late Devonian! The earliest-known definitive caecilian fossil before now was Eocaecilia micropodia from the Early Jurassic of Arizona. This species, though possessing many of the burrowing adaptations of modern caecilians still possessed limbs, thus indicating that critical events in their evolution leading to the acquisition of their modern form happened before the Jurassic. Image: Eocaecilia micropodia Nobu Tamura CC BY 3.0.a: Palaeogeographic distribution of fossil caecilian (white) and other close relatives (orange) through time with modern caecilian distribution (solid yellow). b: Phylogenetic tree of lissamphibians including Batrachians (frogs and salamanders) and Gymnophionomorpha (caecilians). Coloured dots show molecular estimates for divergence of major groups against time, with averages shown as a vertical bar (blue/pink: Batrachia/Gymnophionomorpha ; yellow: Salientia (frogs)/Caudata (salamanders); Green: Rhinatrematidae/Stegokrotaphia (modern caecilians groups). This image shows just how far away the estimated origins of bratrachians/caecilians is from the first physical evidence of caecilians in the fossil record. There are a lot of caecilian fossils still to be found in earlier sediments! Image: Kligman et. al. 2023 CC BY 4.0.Following the evidence from coprolites (fossil droppings), Ben was part of a survey team in Petrified Forest National Park that discovered a new microvertebrate-bearing deposit. Image: Ben Kligman.The site, they named Thunderstorm Ridge, is part of the Late Triassic Chinle Formation (223-218 million years old). This site probably represents a pond or an oxbow lake. Image: William Reyes excavating fossils at the ‘Thunderstorm Ridge’ fossil site by Ben Kligman.Various tiny fossils have been recovered from this site by bulk processing of sediments. These are first sieved using various sized screens. Image: Study co-author Adam Marsh screen washing sediments by Ben Kligman.Any fossils are then hand-picked out of whatever is left from the screening. This is an extremely time-consuming process performed here by co-author Matthew Smith. Image: Ben Kligman.From this process, almost 100 new caecilian specimens were discovered, greatly increasing the number of caecilian fossils many times over. Interestingly, these specimens were predominantly (about 75%) of the lower right jaw. This allowed a minimum number of individuals to be estimated, but why so many of the same bone were found remains a mystery. Image: Ben Kligman.Despite this lack of diversity in the kinds of bones recovered, the lower jaw possessed enough anatomical characteristics to identify it as a new species which they named Funcusvermis gilmorei after the song Funky Worm by the Ohio Players. Image: Kligman et. al. 2023 CC BY 4.0.Anatomical data from this fossil was compared with other related species in a phylogenetic analysis, which produces a computer generated hypothesis of the most likely relationship between different species and groups. This added weight to a previous hypothesis that lissamphibians have origins within the dissorophoid temnospondyls. Image: Kligman et. al. 2023 CC BY 4.0.Working with this dissorophoid temnospondyl hypothesis, a tree featuring the step-wise acquisition of characteristics was produced. This helps show the physical evolution of caecilian jaws and teeth and gives further insights into the adaptation of this group to its specific fossorial (burrowing) ecology. Image: Kligman et. al. 2023 CC BY 4.0.Reconstruction of Funcusvermis gilmorei (foreground) and the crocodile relative Acaenasuchus geoffreyi (background) in the tropical forest of Petrified Forest National Park about 220 million years ago. Artwork by Andrey Atuchin. Courtesy of Andrey Atuchin, the National Park Service, and the Petrified Forest Museum Association.Whilst this research is incredibly significant for our understanding of caecilians and lissamphibians, there are numerous other fossils from other groups that have been discovered at Thunderstorm Ridge. We expect to hear a lot more from both Ben and Thunderstorm ridge in future. Image: Ben Kligman at Thunderstom Ridge by Adam Marsh.
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