Caecilians aren’t exactly your run-of-the-mill amphibian. With no legs, teeth, and maggots, they spend their lives burrowing in the ground, viewing the world with tentacles that protrude between their eyes.
Little is known about these stealthy creatures, or how they evolved. Only 11 species of ancestral caecilians have been found, so our understanding of their interactions with other aquatic species, such as frogs and salamanders, is fragmented.
Now, a team of scientists from the US has uncovered a group of small jaws that represent a new species that closes a huge gap in the past records, revealing more about the unique features of modern caecilians and the beginning of the evolution of wet amphibians.
Virginia Tech paleontologist Ben Kligman and his colleagues describe finding fossils — some jaws less than a quarter of an inch long — in Arizona at a site called Thunderstorm Ridge where the remains of more than 70 caecilians have been unearthed. They named the new creation Funcus vermis gilmoreimeaning ‘worm funky’.
“Seeing the first jaw under the microscope, with two distinct rows of teeth, shocked me,” says Kligman, who worked on the excavation with Xavier Jenkins, now a graduate student at Idaho State University.
Given the known age of the fossil and its prominent teeth, “we immediately knew it was a caecilian, [and] the oldest caecilian fossil ever found,” says Kligman.
These fossils date back to about 220 million years ago, when the ancient supercontinent Pangea had not yet split and Arizona hung close to the equator. This means that the discovery pushes the ancient history of caecilians back 35 million years.
Before this jump back in time, scientists are struggling to connect the dots between four-legged batrachians (frogs, newts, and salamanders) and caecilians, which together are representatives of the group of tetrapods called Lissamphibia.
But the role of the caecilian within the group was uncertain. Where on the line did the caecilians lose their legs? Or from whom did the frogs come? Did their last ancestor have two sets of identical teeth?
There are three current and widely debated theories about how modern amphibians evolved – the evolutionary steps from when lobe-finned fish first crawled on land, that is.
One theory suggests that they descended from an extinct group of four-legged amphibians called the dissorophoid temnospondyls that lived in the Paleozoic era, about 250 million years ago. One suggests that their origins lie in the lepospondyls, a group of primitive tetrapods that resemble eels and snakes with stilts.
A third but less popular theory is that caecilians evolved from lepospondyls and frogs and salamanders evolved from dissorophoids.
In 2008, a creature called the ‘frogamander’ was discovered in Texas, with a large frog-like head and a salamander-like tail. The discovery strengthened the evolutionary link between frogs and salamanders, to keep them four-legged disorophoids, different from caecilians.
But the whole story of the origin of Lissamphibia still hinged on the relationship of caecilians to their aquatic relatives, the batrachians, and the ancient Palaeozoic tetrapods. There was a gap of more than 70 million years between the earliest known caecilians and their distant relatives in the Palaeozoic.
According to Kligman and colleagues, Fungusworm gaps up in the differences that with their new study provides “strong evidence to support the single origin of amphibians living within the dissorophoid temnospondyls.”
Fungusworm they share a skeleton with fossil frogs and salamanders, as well as dissorophoid temnospondyls. Like all modern mammals, it has a row of pedicellate teeth, the smaller ones look like little things on its jaws. But Fungusworm lacks some features of modern caecilians, viz a nerve organ that their early relatives developed, probably to help their noses pierce and smell.
“Unlike living caecilians, Fungusworm “It doesn’t have many changes related to burrowing, indicating a gradual acquisition of material related to underground life in the early years of caecilian evolution,” says Kligman.
Although fossils can certainly help confirm the history of the evolution of caecilians, because now archaeologists are enjoying the specimens they found.
“These findings show that fossils you can’t see can dramatically change our understanding of all the groups you see today,” says paleontologist and study author Sterling Nesbitt, also of Virginia Tech University.
This study was published in Nature.
