TERMINAL SEGMENT
Converging on Convergence
JOHN ACORN
S
ome entomologists
mandible of a large ground beetle, Pasimachus elongatus. Of
progress toward their
course, Dr. Fox didn’t believe
chosen career in a
us, and really, who would? You
remarkably efficient fashion, but for others, including
have to be a pretty extreme beetle nerd to recognize a species
myself, the outcome was much
(there are hundreds in most
less predictable than perhaps
places) from one disarticulatit should have been. Before
ed mandible. But that is exactbeginning grad work in entomology, I spent more than a
ly what it was—not a fossil at
year on a paleontology M.Sc.,
all, but a fragment of a recently
studying Paleocene fossil mamdeceased beetle.
mal teeth from the foothills of
The reason this story struck
Alberta. My fieldwork was a bit
me as remarkable is that multituberculate jaws are themlike mining, and for a variety of
selves remarkable. In this
reasons, I realized (eventually)
extinct group of mammals,
that I would be happier with
the incisor teeth stick out forinsects.
ward like forceps, reminiscent
I would hike to my site, fill
of rodent teeth, but more like
sacks with as much soft, crumbly sandstone as I could carry,
chopsticks than chisels. Behind
and then heave these heavy The “plagiaulacoid” ground beetle, Pasimachus elongatus, with a) dorsal the incisors, there is a gap, and
sacks back to the vehicle for view of right mandible, and b) mandible of multituberculate mammal then a huge, semicircular prefor comparison.
molar blade, with a somewhat
sorting in the lab. There, I
serrated cutting edge and rearwashed the “matrix” under a
ward-sweeping ridges on its sides, making
hose, sieved out the resulting small pebmultituberculate mammal, but instead
bles, dried them, and sorted them from
the lower jaw look a bit like a table saw. At
of teeth set in bone, this was a singular,
the (very rare) fossil teeth under a disthe back of the jaw lie rectangular grinding
fused structure, shining black like the rest
secting microscope. It was very, let’s say,
molars, with two parallel rows of cusps.
of the fossil teeth from that site. When
meditative work. My supervisor, Richard
One of the multituberculates that exhibits
Asta left for the day, Dr. Fox looked over
Fox, employed an elderly woman, Asta, to
this type of dentition is Plagiaulax, and the
her discoveries, and was grumbling loudly
do his sorting, and Asta was vastly more
great evolutionary biologist George Gaylord
to himself when David Maddison stopped
patient than I was. In a day’s work, she
Simpson adopted the term “plagiaulacoid
by. David is now a well-known entomologist working at Oregon State University.
would find a few teeth, and every once in
dentition” for its teeth, in a classic paper
“What on earth is this?” Dr. Fox wona while, a jaw or part of a jaw, with teeth
(Simpson 1933) in which he argued that this
dered, and invited us to look at it, adding,
intact. These jaws were the true prizes.
oddball arrangement had evolved in at least
“It looks like a multituberculate jaw, but
One day in 1982, Asta set aside an
four separate branches of the mammalian
it isn’t!” I looked first, and thought that
extraordinary object, sorted from matrix
evolutionary tree. I say “oddball” since the
perhaps I recognized it. David took a turn,
collected in the Alberta badlands. It
(continued on page 126)
and we both agreed that it was the right
looked just like a tiny lower jaw of a
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American Entomologist • Summer 2014
Terminal Segment, from page 128
only living mammals with plagiaulacoid
dentitions are some of the rat kangaroos
of Australia (not to be confused with the
kangaroo rats of North America).
In any event, here before us was a “plagiaulacoid” beetle jaw, and one convincing
enough to perplex a prominent specialist in Paleocene mammals. At the time,
I thought that we should write a paper
about it, but I quickly realized that a few
elements were missing in order for it to be
a “remarkable convergence” story. For one
thing, the beetle does not have anything
like grinding molars on its mandibles,
possibly because it eats other invertebrates
(and possesses a grinding proventriculus
in its foregut), while rat-kangaroos eat
tough plant material. So I put the idea on
the shelf, even though it still struck me
as deeply puzzling that the same unlikely-looking jaw arrangement could evolve
not only four times among the mammals,
but in a beetle as well. It seemed like evidence that some sort of mysterious shaping force of evolution was at play. Convergence is usually explained as the result
of similar selection pressures in similar
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environments, and I just couldn’t fathom
what those forces might be for plagiaulacoid jaw owners. Some things just look
more accidental than others.
We now generally believe that evolution results in the particular organisms
we see around us, and not others, largely
because of what Stephen J. Gould called
“historical contingency.” In this view, some
of what happens during the evolutionary
process involves quirky happenstance,
ranging from fortuitous mutations to asteroid impacts. Gould argued, in Wonderful
Life, that if the metaphorical “tape of life”
was “rewound” and allowed to play again,
it would no doubt come out differently.
Others, however, were not so sure, and
even though paleontologist Simon Conway Morris was one of the heroes of Wonderful Life (he studies the Burgess Shale
fossils, on which the book was based),
he countered forcefully that evolutionary
convergence suggested a different interpretation, in which certain general types
of organisms will inevitably arise, given
enough time and evolutionary diversity,
such that the tape of life would, in truth,
come out similarly each time, although
not identically. The reason it “matters” is
that we humans either were, or were not,
inevitable here on Earth.
Convergence involves distantly related and dissimilar organisms coming to
resemble one another, in whole or in part,
through evolution. Vertebrate convergences get a lot of attention, and everyone has
heard of whales converging with fish, ichthyosaurs converging with porpoises, and
bats converging with birds. Insects too have
also produced some amazing convergences, and my favorites are probably praying
mantids and mantispids (neuropterans
that I think of as “kung fu lacewings”).
Ranatra water scorpions should qualify
here as well, since they look very little like
scorpions, and a lot like slender underwater mantids. Another great example of
convergence in insects involves swallowtail
butterflies and a variety of large, tropical,
day-flying tropical moths in the family
Uraniidae, such as Urania itself.
Partial convergence is more common
than whole-body convergence, and thus
it is the phenomenon we encounter most
often. Plagiaulacoid Pasimachus jaws are
one example, but the most spectacular
case of arthropod partial convergence
is probably the long-horned beetle that
American Entomologist • Summer 2014
possesses a bulbous, venomous sting on
the end of its antenna that looks for all the
world like the sting of a scorpion (Berkov
et al. 2007). Apparently, this is an extremely rare beetle, so this fantastic adaptation
doesn’t seem to have given it any special advantage in its Peruvian forest habitats. One could put together quite a list
of partial convergences, including short
wing covers (in various beetles and earwigs), raptorial legs (not only in mantids,
mantispids, and water scorpions), oar-like
swimming legs (in various aquatic bugs
and beetles), and halteres (in strepsipterans and flies). Convergence can also be
physiological, behavioral, and molecular.
It is amazingly common once you start
looking for it.
Evolutionary biologists go to some
lengths to distinguish convergence from
parallel evolution, by pointing out that parallel evolution involves the same, homologous trait evolving to become the same
novel trait in two or more related lineages.
For those who reconstruct evolutionary
history, however, any type of convergence
or parallelism is unwelcome “homoplastic noise” that obscures the “true signal”
of evolution. In a more general sense,
American Entomologist • Volume 60, Number 2
however, particular instances of convergence are extremely interesting, since they
relate to how and why evolution actually
happens.
More than most aspects of evolution,
convergence seems to inspire philosophical speculation. Whether something like
a human being is an inevitable product
of evolution leads naturally toward discussions of whether there are humanoid creatures on other planets. You rarely
hear anyone ask whether there are insects
on other planets, but why not? Are there
plagiaulacoid jaws on other planets? Earwig-like creatures? “Mantoids”? And if
there are, do these organisms really represent the “same thing” somewhere else,
or is the similarity partly a matter of how
we, as humans, categorize things (the philosophical “problem of universals”)? Since
convergence is based on non-relatedness,
does it really matter that two convergent
lineages share the same planet? This sort
of thinking may seem like quite a stretch
given our starting point—a single beetle
jaw that was mistaken for a mammal fossil—but I still can’t shake the thought that
these sorts of questions represent more
than just background noise. At some level,
every time we think about insects, we think
about life in general, and we speculate on
what it all really means. Or not. Sometimes,
we just look at bugs.
References
Simpson, G. G. 1933. The “plagiaulacoid” type
of mammalian dentition: a study of convergence. Journal of Mammalogy. 14: 97-107.
Gould, S. J. 1989. Wonderful Life: The Burgess Shale and the Nature of History. W.
W. Norton and Co. 347 pp.
Morris, S. C. 1998. The Crucible of Creation:
The Burgess Shale and the Rise of Animals.
Oxford University Press. 242 pp.
Berkov, A., N. Rodríguez, and P. Centeno.
2007. Convergent evolution in the antenna of a cerambycid beetle, Onychocerus
albitarsis, and the sting of a scorpion.
Naturwissenshaften DOI 10.1007/s00114007-0316-1.
John Acorn lectures at
the University of
Alberta. He is an entomologist, broadcaster,
and writer, and is the
author of fifteen
books, as well as the
host of two television
series.
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