

Spriggina floundersi

"Soft-bodied trilobite"

Archaeaspinus fedonkini Archaeaspinus fedonkini

Precambrian "arthropods"

Although many of the fossils of the Precambrian seem incomparable to living forms today, quite a few have been interpreted as metameric bilaterians (segmented creatures with a "head" and "tail" end, and mirrored side structures). Much has been made, for example, of the Ediacaran fossil Spriggina floundersi from Australia (top left), which most workers agree shows a cephalic shield with genal spines, and numerous similar segments running backward in a rather worm-like manner (Glaessner 1979). Recent arguments that Spriggina is a trilobitoid ecdysozoan have been made (McMenamin 2003), but the segments of the body alternate along the midline in a decidedly non-arthropod manner, arguing strongly against arthropodan nature (e.g. Ivantsov 2001). Another very rare Precambrian species cited as an early arthropod, Bomakellia kelleri (top right), bears a semicircular cephalon and a tri-lobed body bearing what might be filamentous gills typical of arachnomorph arthropods. It should be noted, however, that none of the Precambrian species clearly shows antennae or jointed limbs (but then again, such features are rarely preserved even among bona fide trilobites). Assessments of Precambrian taxa such as these and their relationships to arthropods and other groups have often generated as many questions as they sought to answer (e.g., Waggoner 1996). An un-named "soft-bodied trilobite" from the Flinders site in Australia (middle left) also might seem a reasonable Precambrian candidate antecedent to true trilobites (Gehling 1991). At first glance, it resembles the many-segmented Redlichiida, but it also resembles small specimens of Archaeaspinus (bottom left), Dickinsonia, or Vendia, which most workers do not consider to be of arthropod affinity. Nonetheless, this taxon, as well as Parvancorina minchami (Glaessner 1980) both suggest that arthropods did not miraculously appear in the Lower Cambrian, but took form during the Precambrian. The specimen of Parvancorina to the right even bears fine lines that some workers have interpreted as evidence of paired limbs. But how could such a simple body plan be thought of as arthropodan?

