The ubiquity of the Tarantula returns to the Cretaceous

Tarantulas are among the best known spiders, due in part to their size, vibrant colors and worldwide prevalence. But one thing most people don’t know is that tarantulas are household items. Females and their young rarely leave their burrows and only mature males will get lost in search of a mate. How then did such a sedentary spider come to inhabit six of the seven continents?

An international team of researchers, including Saoirse Foley of Carnegie Mellon University, went on a similar investigation to ancestry.com to find the answer to this question. They looked at transcriptomes, the sum of all the transcripts in mRNA, many tarantulas and other spiders from different time periods. Their findings were published online by PeerJ on April 6.

They used transcriptomes to build a genetic spider tree and then calibrated their tree with fossil data. Tarantula fossils are extremely rare, but the software used in the study was able to estimate the ages of older tarantulas compared to the ages of fossils from other spiders.

They discovered that tarantulas are ancient, first appearing in the piece of land now considered America about 120 million years ago during the Cretaceous. At that time, South America was allegedly attached to Africa, India and Australia as part of the Gondwana supercontinent. The spiders have finally reached their current destinations due to the continental drift, with some interesting departures.

For example, the nature of their entry into Asia suggests that tarantulas may also be surprisingly competent dispersers. The researchers were able to establish two separate offspring of tarantulas that diverged on the Indian subcontinent before it collapsed in Asia, with one offspring being predominantly ground dwelling and the other predominantly arboreal. They discovered that these lines colonized Asia about 20 million years ago. Surprisingly, the first group to reach Asia also managed to cross the Wallace Line, a border between Australia and the Asian islands, where many species are found in abundance on one side and rarely or not on the other.

“Previously, we did not consider tarantulas to be good dispersers. Although continental drift has certainly played its part in their history, the two Asian colonization events encourage us to reconsider this narrative. The differences in microhabitat between these two lines also suggest that tarantulas are experts in exploiting ecological niches, while simultaneously showing signs of niche conservation, “Foley said.