Friday, May 31, 2019

That Little Rock Tells A Huge Story

This post’s title quotes Caltech entomologist Joseph Parker’s description of the tiny bit of amber, 5.5 mm long and 3.5 mm wide, pictured below.

(Images in this post were clipped from a Caltech video titled Time Capsule from the Cretaceous, available on the Caltech News website.  They are reproduced here with the generous permission of Caltech.)

What flows from this speck of amber is truly an amazing story, one built from a cascading series of logical and highly probable hypotheses and conclusions.  To my mind, it’s a brilliant example of how scientists can identify and make sense of the broader contextual implications of a discovery.  No finding truly exists in a vacuum.

We begin with this Burmese amber that came from a mine in Kachin state, Myanmar.  It is dated at approximately 99 million years old (Early Cretaceous period).  In and of itself, nothing surprising here.  There is an ongoing trade in Burmese amber from this area and the general consensus has been that all such amber is of that age.  Where things become interesting, indeed, the heart of the story, is that this amber contains a minute insect fossil, a beetle, and not just any beetle.

Before I get ahead of myself, I should stress that I have no special knowledge of fossil beetles or Burmese amber.  My entree to all of this was an interesting Caltech News piece titled These Beetles Have Successfully Freeloaded for 100 Million Years, written by Lori Dajose.  From that article, I turned to the peer-reviewed research paper behind it that was written by entomologist Yu-Lingzi Zhou and coauthors, including Joseph Parker.  Titled A Mesozoic Clown Beetle Myrmecophile (Coleoptera:  Histeridae), the paper was recently published in  eLife (April 16, 2019).

Zhou et al. identified the beetle discovered in this amber as a new, extinct species belonging to the Histeridae family of beetles and, more specifically and perhaps more importantly, to the Haeteriinae subfamily.  Histerids are commonly known as clown beetles of which there are over 4,600 extant species described.  The new species has been named Promyrmister kistneri.  Zhou et al marshal a long list of detailed morphological characteristics of this specimen that have direct counterparts in modern haeteriine beetles leading them to place this beetle in this family and subfamily with a great deal of certainty.

So, at this stage, what have we learned?  Some 99 million years ago a species of histerid lived here.  But this is a conclusion with profound ramifications because today’s histerids, particularly including the Haeteriinae subfamily of clown beetles, are myrmecophiles, that is, they live with ants.  The term myrmecophile is derived the Greek root words myrmeco, meaning “ant,” and phil meaning “love” – myrmecophiles are “ant lovers.”

Haeteriine clown beetles are one of two histerid subfamilies that have predominantly evolved the physical characteristics and behaviors necessary to spend much of their lives in ant nests; indeed some extant haeteriine beetles may live out their entire lifecycle in or near an ant colony.  Most are neotropical and live in army ant colonies in ways that differ by genus.  As Parker notes, some are “highly integrated guests” being fed by ant workers.  “The beetles have been observed feeding on the ants’ brood and harvested food, and to run with or phoretically attach to workers during emigrations.”  (A phoretical attachment is one between two different species in which one carries the other.)  This discussion of the haeteriine beetles and myrmecophily draws from Joseph Parker’s fascinating research paper Myrmecophily in Beetles (Coleoptera):  Evolutionary Patterns and Biological Mechanisms (Myrmecological News, February 2016).

Why would these beetles or other myrmecophiles (myrmecophily is an attribute of some 10,000 arthropod species) make the evolutionary transition from free-living to living in ant colonies?  Such colonies offer at least two strong attractions:  they offer an environment rich in nutritional resources and, if their robust lines of defense can be successfully penetrated, they offer protection to the interloper.  But this intimate relationship comes with a potential major cost:  the guest species’ fates are tied to the fates of the ant species they join.  (As will be discussed, histerid beetle species may be particularly able to circumvent this negative consequence.)

Pictured below is a modern clown beetle among its ant hosts.

And from these conclusions about the identity of this specimen, these scientists have taken the logical next step to posit that this new species also lived in an ant colony.  If that is so (and it most likely is), then this clown beetle was probably a symbiont in an ant nest composed of early (stem group) ants which are the kind found so far in Burmese amber.  (Zhou et al. acknowledge that the absence of crown group ants in Burmese amber does not prove that they were not there at this time in the Cretaceous and hosting this clown beetle species.)

The ramifications of the deduction that this beetle species had evolved for life in an ant colony are quite striking.  Entomologists believe that some 99 million years ago, ant social life was first taking shape, which means that, almost from the outset of the evolution of ant social life, clown beetles had evolved to integrate themselves into these urcolonies.  Parker has posited that the Histeridae have “a family-wide predisposition to evolving myrmecophily.”  “Perhaps the majority of histerid genera may be capable to making the evolutionary shift to this lifestyle quite readily, should ecological opportunity permit.”  (Myrmecophily in Beetles.)  That that far back, in the Early Cretaceous, a histerid had already made this evolutionary shift is certainly persuasive evidence of this predisposition.  Further convincing evidence of this predilection comes from the conclusion that this new species of clown beetle lived among early stem ants which have long since gone extinction, yet haeteriines continue to live among the modern counterparts of those early ants.  As Zhou et al. write
Whether haeteriines evolved in stem- or crown-group ant colonies, their original hosts are presumably long-extinct.  The present-day host associations of haeteriines imply that these myrmecophiles have host-switched between many modern ant lineages. . . .  We suggest that it is this capacity for host switching that may explain the great longevity of the clown beetle-ant symbiosis.  Through host switching, the clade as a whole has circumvented potential coextinction with host ant lineages that disappeared from the Cretaceous to the present.  (p. 8)
Indeed, as Parker put it, “That little rock tells a huge story.”

Additional Thoughts

I would note that any Burmese amber with resident fossils may have a dark side, one that is told by science writer Joshua Sokol in Troubled Treasure:  Fossils in Burmese Amber Offer an Exquisite View of Dinosaur Times – and an Ethical Minefield (Science, May 4, 2019).  I certainly am not suggesting that the disturbing aspects of Burmese amber described below necessarily apply to the piece considered in this post, only that Burmese amber in general raises such concerns.

Proceeds from the Burmese amber market are fueling an ongoing war between the Myanmar military and insurgents in the country’s Kachin state.  The recent spate of fossil discoveries from this amber apparently are mostly products of this amber market.  Further, as Sokol describes, because these fossils come to light through a commercial transaction, many important fossils are left in private hands, a situation that professional paleontologists find at least troubling.  That these amber mines are in a conflict area means that the detailed work needed to properly date these pieces of amber (and their fossils) is decidedly difficult to undertake.  The age of the Burmese, set at 99 million years old in a 2012 study, is now under some question with suggestions that it comes from a period of a few million years around that age.  Finally, the amber with fossils makes its way out of Myanmar despite the country’s legal prohibition against exporting fossils because amber is treated as an exportable gemstone.  Paleoentomologist Michael Engel has bemoaned this situation, stating, “It’s like Myanmar’s cultural heritage, paleontological heritage, is just being wholesale ripped out of the ground and distributed around the world.”  (As quoted by Sokol, p. 729.)

Where to draw the line on Burmese amber and paleontology may be clear to some, but not to me.  This amber has provided science with many, truly marvelous finds in recent years, yet it clearly comes at a cost.

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