Inside San Diego’s Natural History Museum, Tom Deméré pulled out drawers that line cabinets in the paleontology department. They hold ancient shells and bones that come to San Diego from all over the world.
The fossils can be tucked away in the drawers for years after their initial examination. But paleontology curators like Deméré never know what hidden treasures lie among the museum's 1.6 million artifacts.
“It's possible that there are fossils in here that are new to science but they haven’t been studied yet. And that’s why we have collections,” Deméré said.
In 1985, he remembered handling a 324 million-year-old fossil. It arrived at the museum from west Texas. At the time, the fossil was classified as an ancient crustacean, or a shelled creature such as a crab, that typically lives in aquatic environments.
The fossil was studied once or twice over the years. But it spent the majority of its four decades at the museum safely nestled in its drawer.
Until a researcher from across the world had a hunch it could be something special.
“To him he saw an insect, not a crustacean," Deméré said.
Forty years after its arrival, the relic has been identified as one of the oldest insect fossils on record. Genetics have shown that insects evolved from crustaceans. But an 80 million year gap in the fossil record smudges the evidence of how insects went from living in water to their modern form. This fossil, however, reveals there may have been an in-between phase.
“They (the researcher and his team) had this 'ah-ha' moment that it was, in their interpretation, a really early form of an insect coming out of the water,” Deméré said. “It’s kind of a missing link.”
The discovery was made by Erik Tihelka, a doctoral student at the University of Cambridge. He first encountered the specimen when flipping through pages of a second hand paleontology textbook. There, he spotted a lined drawing of the creature. But something about its crustacean label seemed off. It was enough to prompt a visit to the museum to take a closer look.
“He contacted our museum requesting research access. And so he traveled to the U.S. I think it was his first trip,” Deméré said.
Tihelka used polarized light and a microscope to examine the fossil in finer detail. He found the fossil had surprisingly insectlike features. Its head was adorned with two slender antennas. Its body was broken into three distinct sections. And located in the middle of its body, a pair of three legs designed for walking.
“If you see something with six legs, and it's on land, it's probably an insect,” said Shahan Derkarabetian, the curator of invertebrate zoology at the museum.
But the fossil also had 18 additional appendages that did not fit the criteria of modern insects. The extra legs were wide and flat — features that are better suited for slicing through water. Derkarabetian suggests the combination of the fossil’s terrestrial and aquatic adaptations may indicate it is an evolutionary bridge.
“They have these little paddled appendages they used to swim around. And so that kind of characteristic is what bridges that gap between things that swim and things that walk purely on land,” Derkarabetian said.
Tihelka and his team named the fossil Chosha praecursor, which is the Navajo or Dine name for "insect predecessor." The fossil sits inside the gap in the fossil record known as the hexapod gap. Evidence of six-legged insects is missing between 385 and 322 million years ago. But Chosha praecursor is helping to fill in the blanks.
It also reveals clues about how insects transitioned from water to land. Invertebrates must endure many morphological changes in order to survive above the surface, such as swapping gills for organs that can inhale oxygen.
“Fossils like this give us that insight into the order of events and the full morphological changes that led to the full terrestrial organism,” Derkarabetian said.
Chosha praecursor is currently on display in the Amazement in the Basement exhibit.
