points. Thus, ichthyosaur
paddles may be dead ringers for fish fins in external form, but they are built
of finger bones from a terrestrial past; and the eyes of squid and vertebrates,
although so similar in final form, follow markedly different embryological routes
in their construction.We may be confident that Walossek and Müller's
specimens are true pentastomes because (as for their tardigrades) the similarities
are so numerous, so detailed, and so pervasive. These features include the basic
body plan of a globular head with two pairs of limbs suited for attachment to
a host, and a thin, tapering, wormlike body behind. Fossils and moderns also share
the basic embryological design of "segment constancy," with growth in
size through successive molts, but no addition of further segments (as insects
and most arthropods generally do) in growth. (The Orsten fossils include both
larvae and adults, so even these details of growth may be inferred.)Beyond
this identity of basic form and growth, fossils and moderns also match part for
apparently trivial part. Both have distinctive pores on the inner edges of the
limbs; both can withdraw the limbs partially into their sockets; both grow a pair
of papillae, or nodes, at the rear end surrounding the anus. Such a suite of distinct
and apparently minor features would not evolve twice in such detailed similarity
of form and position.Moreover, in one prominent feature, the fossils
teach us something important by revealing a structure unknown in modern forms.
The body of modern pentastomes (behind the head) seems to consist of four segments.
But these divisions are not marked by clearly repeating structures in each zone--the
usual sign of true segmentation in several invertebrate phyla. Nerve ganglia are
separated and repeated, but since pentastomes are so morphologically degenerate
(with no respiratory, circulatory, or excretory organs), few other possibilities
exist for crucial evidence of true segmentation. In particular, the most indicative
of all features--limbs on each segment--do not exist in any modern pentastome.
But several of the fossils contain small, paired limbs on the second and third
body segments! In fact, one might say that the fossils are entirely comparable
to the moderns, with this one added (and highly informative) feature.These
fossils clearly disprove the favored hypothesis of later derivation for pentastomes
after the evolution of terrestrial vertebrates. Of the pentastomes' profound stability
for more than 500 million years, Walossek and Muller conclude: "The long
history of the group and its remarkable morphological stasis invalidates any hypothesis
of their evolution from terrestrial arthropods" (for arthropods did not invade
the land until long after the Cambrian).The existence of Cambrian pentastomes
raises the obvious question of their original hosts, since terrestrial vertebrates
had not yet evolved. Switching of hosts, even from one phylum to another, is a
common phenomenon in the evolution of parasites, so the need to postulate such
a transition raises no theoretical problems, but we still want to know the potential
candidates. The original Cambrian hosts need not have been closely related to
vertebrates, but one prominent fossil group, the conodonts, has been enigmatic
throughout the history of paleontology (for their soft bodies provide little opportunity
for fossilization and only their microscopic "tooth elements" are generally
preserved). But soft-bodied remains of entire animals have been found in the last
decade and latest evidence indicates that conodonts belonged to the vertebrates
after all, as Walossek and Müller suggest. Conodont fossils are common in
all Cambrian localities that have yielded pentastomes.But what of the
biochemical claims for crustacean affinities and origin much later than the Cambrian?
Molecular data have won such prestige over the last few years that such a contention
might seem indisputable--yet the hard evidence of Cambrian pentastomes seems
even less subject to refutation. But close reading of Abele's 1989 paper provides
a lovely resolution.I have often pointed out in these essays how theories
strongly constrain (often unconsciously) our interpretation of data. (For this
reason we must be particularly vigilant and probing as we explicitly consider
the consequences of our theoretical preferences.) The solution lies in the last
sentence of the paper by Abele and colleagues--but they didn't see it, presumably
because later origin from crustaceans fit the usual assumptions of evolutionary
theory and its preference for continuous origin of major groups. The last line
reads (I will quote verbatim and then translate):Thus, over a period of
time very roughly estimated to be 287 . . . million years, the 18S rRNA of these
two groups has diverged about 10.8 percent, or about 1.9 percent per fifty million
years, a higher rate than the one percent per fifty million years previously reported
for eucaryote 18S rRNA. Given the potential errors in making such estimates the
significance of this difference remains unknown. In other words, assuming
that pentastomes arose only 287 million years ago from crustaceans, the rate of
evolution for their RNA is nearly twice as fast as average rates calculated for
other multicellular organisms. This doesn't trouble them too much (as the very
last sentence states) because RNA rates are subject to so much error. But
they never even mention the obvious alternative hypothesis--which now turns
out to be almost certainly correct. If the pentastomes really diverged from crustaceans
(or from some other group) during the Cambrian, some 530 rather than 287 million
years ago, then the total measured difference does not translate to an unusually
high rate of change, but to an average rate after all--for the 10.8 percent
difference, spread over 530 rather than 287 million years, works out to just about
the average of 1 percent over 50 million years! In other words, the molecular
data and the fossil evidence coincide and remove an anomaly in the molecular data
considered alone under the conventional (and false) assumption of later origin
for the minor phyla.The Cambrian explosion is the key event in the
history of multicellular animal life. The more we study this episode, the more
we are impressed by its uniqueness and of its determining effect on the subsequent
pattern of life's history. The basic anatomies that arose during the Cambrian
explosion have dominated life ever since, with no major additions--and with
subtractions imposed for reasons that may more resemble the luck of the draw than
the predictable survival of superior lines. The pattern of life's history has
followed from the origins and successes of this great initiating episode. I can,
therefore, only end with another well-remembered line from Gettysburg, when Lincoln
so misspoke in the first phrase (a cruel irony for schoolchildren forced to memorize
against their will), but stated so truly and eloquently in the second--and
which applies so well to the extraordinary influence of successful Cambrian groups:
"The world will little note nor long remember what we say here, but it can
never forget what they did here."Stephen Jay Gould teaches biology,
geology, and the history of science at Harvard University.
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