Cockroaches: the earliest known cave-dwellers

Cockroaches are the first and only creatures to be discovered living in caves before the dinosaurs were wiped out 66 million years ago. 

Two new species of the critter have been identified preserved in amber from around 99 million years ago in the mid-Cretaceous period when dinosaurs roamed the Earth. 

All other cave-dwelling animals originate from the current Cenozoic era, beginning 66 million years ago when three quarters of plant and animal species, including the dinosaurs, went extinct after a huge asteroid smashed into Earth.

“It is a small miracle,” says lead investigator Peter Vršanský at the Slovak Academy of Sciences in Bratislava, Slovakia. “But one can expect that in 110 tons of amber you will find something peculiar.”

The specimens were originally discovered in amber deposits from a mine located in Hukawng Valley, Myanmar, which were provided to the researchers for analyses. 

200226 crenocticola

Mulleriblattina bowangi 

Credit: Peter Vršanský, Sendi et al. Gondwana Res 2020 (Copyright Elsevier 2020)

 “It was clear from the first look that these were fantastic, but a lot of time is needed to determine exactly what you see,” explains Vršanský. “It is a time abyss. You must imagine that these are in dino caves, with pterosaurs in the air, and nearly no flowers. It is another world.”

Being from one of the world’s most important fossil sites, the amber’s age was already known. Crystals in the mine’s volcanic rocks had been dated to 98.8 million years ago – give or take half a million years – which means that trees oozed resin and trapped the unsuspecting cockroaches in amber around a similar time. 

The researchers analysed the specimens using microscope photography to determine the appearance and anatomical features of the ancient amber-locked arthropods. The team reports its findings in the journal Gondwana Research.

“It’s a new genus and new species, the full determination and description took several months,” explains Vršanský. “Definitely they are the earliest known unequivocal cave-dwellers.” 

The two species were named Mulleriblattina bowangi and Crenocticola svadba and placed in the Nocticolidae family, which comprises other species thriving today. Their cave-dwelling life was evident based on lost colouration, reduced wings and eyes, elongated antennae for sensing, and reduced leg spines for passive defence.

But if they were cave-dwellers, how did they become trapped in tree resin?

“Most probably the source tree was growing directly at the cave entrance,” says Vršanský. “Alternatively, the resin was produced from roots which grew down inside.”

Other cave animals undoubtedly existed in the Mesozoic “dinosaur” era spanning 253 to 66 million years ago. But caves likely collapsed or didn’t allow fossil preservation over such vast timescales. Plus some fossilised animals may lack vital clues that betray cave-dwelling tendencies.

Nevertheless, all cave animals living today have a late-Cenozoic origin, meaning that all other Mesozoic-origin cave-dwellers went extinct. It’s a mystery that Vršanský can’t explain, especially as cave environments should protect organisms from extreme environmental fluctuations that often caused extinctions out in the open.

While it’s not certain if the new cockroach species survived the mass extinction event that killed the dinosaurs, Vršanský says it’s plausible because cave environments remained in the Myanmar region afterwards. Indeed, the species may have never died out.

“It cannot be fully excluded that they still survive today. Not all species in this group are known and not all living Australian nocticolid species are described,” explains Vršanský. “The question is if survivors will be still the same species, which is doubtful, or if they transferred into other taxon. But this lineage survives today.”

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