Remains from a 95-million-year-old marine creature with nubs for legs is clarifying how some lizards shed their limbs as they crept through evolutionary time and morphed into slinky snakes.
Described in the current issue of the Journal of Vertebrate Paleontology, the snake-like lizard had a small head and willowy body. Extending 10 to 12 inches from snout to tail, the aquatic creature also sported a lengthy neck and relatively large rear limbs. Missing were all the bones of its forearms, including the hands and digits found in modern lizards.
The oddball creature, Adriosaurus microbrachis, is a member of a lineage of lizards thought to be snakes' closest relatives.
“It adds to the picture we have of what was happening 100 million years ago,” said lead researcher Michael Caldwell, a paleontologist at the University of Alberta, in Canada. “We now know that losing limbs isn't a new thing and that lizards were doing it much earlier than we originally thought.”
The new fossil reveals the earliest record of this limb-shedding in a lizard and gives scientists a rare glimpse back to the time when terrestrial lizards evolved to be limbless and returned to their watery origins. In fact, the ancestors of all animals lived in aquatic and marine environments.
Steps to limb loss
Body parts once used in an animal’s evolutionary past but tossed aside or morphed via natural selection to provide another function are called vestigial limbs.
“It has been clear for centuries that snakes are tetrapods (four-legged vertebrates) that lost their limbs,” Caldwell told LiveScience. “The process and pattern of this limb-loss has remained a mystery for a long time.”
Fossils of lizards in transitional states—as the four-legged critters begin to evolve into snakes—have been rare.
“What we have not had to this point is a fossil record of vestigial limbs in lizards,” Caldwell said. “This is the first.”
Scientists initially collected the fossil during the 19th century from a limestone quarry in Slovenia. For nearly 100 years, the little lizard remained in a collection bin at the Natural History Museum in Trieste, Italy, before Caldwell and a colleague found it in 1996 during a visit to Europe.
The scientists were surprised to find the lizard’s forelimbs were too small to be useful for walking, while its hind limbs appeared to be functional.
"For some oddball reason, the forelimbs were lost before the rear limbs, when you would think it would be the opposite," Caldwell said. "The front limbs would be useful for holding onto dinner or digging a hole, but it must be developmentally easier to get rid of the forelimbs."
Though the lizard find does not make for a “missing link,” Caldwell suggests it suffices as a critical data point for helping scientists understand the aquatic process of limb loss.