Scorching climate drove lampreys apart during cretaceous period

"Lampreys are simultaneously creepy and special -- even though people tend to think of them as just tubes with a freaky circle of teeth," says Lily Hughes, corresponding author of the study. Hughes is a research assistant professor at North Carolina State University and the curator of ichthyology at the North Carolina Museum of Natural Sciences.

"They are an ancient and amazing species, and their larvae play an important role in filtering nutrients and maintaining water quality in stream beds," Hughes says. "But very little is known about their evolutionary relationships, or tree of life."

There are only 48 currently recognized species of lampreys, and they are divided into three families: one that lives in the Northern Hemisphere, and two in the Southern Hemisphere. No lamprey species are found in the tropics near the equator.

Hughes, study co-first author Devin Bloom, and the research team set out to determine when these groups diverged from their common ancestor. Bloom is an associate professor at Western Michigan University.

Using a combination of DNA samples from living lampreys and information from the fossil record, the team constructed phylogenomic trees that revealed a split between the Northern and Southern Hemisphere lampreys around 93 million years ago, during the Cretaceous period.

"We do know that during this time period, average temperatures globally were about 82 F (28.1 C), and the average tropical temperature was 93.5 F (34.2 C)," Hughes says. "Lamprey larvae like to be cooler, which is why they burrow into stream beds. The tropical temperatures at this point were much hotter than larval lampreys can survive.

"A couple of things could have happened as a result of these temperatures to explain the split: maybe tropical lampreys went extinct and the groups lost a common genetic link that way, or maybe the lampreys just moved away from the equator to more temperate climes. While we don't know how the lampreys were distributed at the time, we do know that this is when they split."

The findings represent a new hypothesis for lamprey distribution.

"One appealing explanation for the highly disjunct distribution of lampreys was existence of the ancient super continent Pangea, which could have physically separated the groups," Bloom says. "But our study, which combines fossil and genomic information, shows divergence between Northern and Southern Hemisphere lampreys happened long after the breakup of Pangea."

The research appears in Proceedings of the Royal Society B: Biological Sciences and is supported by the National Science Foundation under grant DEB-1754627. Kyle Piller of Southeastern Louisiana University, Nicholas Lang of Lane Tech College Prep High School and Richard Mayden of St. Louis University also contributed to the work.