these exponents of a crustacean link to the reasonable hypothesis that
pentastomes originated long after the Cambrian explosion, thus affirming
the conventional view that fundamentally new body plans may originate throughout
geological time, and that the Cambrian explosion is a great episode of intensification,
but not an exclusivity. In fact, Abele and colleagues proposed a divergence
date of 350 to 225 million years ago for pentastomes and crustaceans.
But Walossek and Muller have now described-indisputable pentastomes--a
whole fauna of several species, not just a single example--from upper
Cambrian strata (see Dieter Walossek and Klaus J. Muller, "Pentastomid
Parasites from the Lower Paleozoic of Sweden," Transactions of
the Royal Society of Edinburgh, Earth Sciences, vol. 85, 1994). These
fossils come from the upper Cambrian "Orsten" beds of Sweden,
an extraordinary deposit that has, over the last two decades, yielded
a vitally important fauna of tiny, exquisitely beautiful fossils from
this early time in the history of multicellular animal life. (The fossils
are phosphatized and preserved in tiny calcareous nodules. The nodules
can be dissolved away in acid, leaving perfectly formed and fully three-dimensional
fossil, hollow on the inside but recording all elaborate and intimate
detail of the surface architecture. Unfortunately, since the nodules are
so tiny, no fossil of more conventional and larger marine invertebrates
can be preserved, and the Orsten fauna consists primarily of arthropod
larvae and other tiny adult creatures including these newly described
pentastomes.)
In all such cases of proposed linkage between ancient fossils and living
forms (especially in the face of such a temporal gap, for no later pentastome
fossils are known between these ancient Orsten specimens and the living
species), one must consider the obvious alternative that the fossils are
merely convergent upon modern pentastomes and represent an entirely independent
genealogical line. The history of life is replete with examples of astonishing
similarity separately evolved--fish and ichthyosaurs, marsupial and
placental moles, eyes of squid and vertebrates. But convergence, however
stunning in general adaptive features of basic form and function, can
never be intricately precise in hundreds of detailed and highly particular
parts--because converging lines begin from such different antecedents
and must craft similarities from disparate starting
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