Amber fossils are a rich source of invertebrate fossils, providing insights into new families, genera, and species. For an organism to become an amber fossil, it needs to be trapped in tree resin that oozes from injured trees, which then hardens and becomes buried beneath sediment before fossilization occurs at high pressure and temperature. Amber has been discovered at several localities worldwide, dating to various time periods. For instance, Dominican amber dates back to around 20-15 million years ago (MYA), Baltic amber dates back to around 45 MYA, New Jersey amber is dated around 94 MYA, and Burmese amber is one of the oldest amber deposits, dating back to around 100 MYA.
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The origin of Burmese amber is significant because of the interesting geological history of the region in which it is found. Myanmar and other south Asian countries were formed through the accretion of various landmasses that moved northwards until they collided with Asia and amalgamated into the existing landmass south of the Himalaya mountains.
Myanmar is squeezed between India and Thailand and is formed from two landmasses that came together, the Sibumasu and West Burma terranes. The Hukawng basin, where Burmese amber is found, was postulated to be part of the West Burma terrane. Deciphering the origin of each of the landmasses that make up southern Asia is one of the geological challenges from this region. These landmasses started to accrete around 40 MYA.
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It is of interest that Burmese amber was formed around 100 MYA, long before the landmasses had accredited, implying that the Burmese amber fossils were trapped in amber far from their current locality. The West Burma terrane is believed to have originated from northern Australia, where it rifted around 150 MYA to drift across the Indian Ocean before colliding with Asia. At 100 MYA, this landmass was located in the tropical equatorial regions of the world, which is an expected locality for the trees that produce amber resin.
Tick fossils were discovered in Burmese amber that could be placed in extant genera found only in Australasia, which was of considerable interest. These genera include Bothriocroton (7 extant species) and Archaeocroton (1 species found in New Zealand). The occurrence of these ticks with Australasian affinities is consistent with the current geological hypothesis that the Burma terrane rifted from Australia and moved across the ocean before accreting to Asia. It also implies that while the amber is dated at around 100 MYA, the Burma terrane rifted from Australia around 150 MYA. This would suggest that Bothriocroton and Archaeocroton, and by extension all lineages postulated to have diversified before the origin of these genera, would be older than 150 million years. These include the prostriates (Ixodes), basal metastriates, the soft ticks, and the Nuttalliellidae, which raises the minimum age for these lineages to 150 MYA.
This finding is crucial for the origin of ticks, as studies that suggest an origin for ticks younger than 150 MYA would be inconsistent with this hypothesis. The data also support previous suggestions that the origin of ticks in general lies in Gondwanaland. This latest study is part of an unfolding saga of tick biodiversity found in amber, with far-reaching implications for the study of tick evolution.
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