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Crustaceans and the "Cambrian Explosion"

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Science  07 Dec 2001:
Vol. 294, Issue 5549, pp. 2047
DOI: 10.1126/science.294.5549.2047a

We noted with interest the report by Siveter et al. (1) of new phosphatocopid specimens showing soft-part preservation from the Lower Cambrian Comley Limestones, but we question some of the interpretations both in the report and in the accompanying Perspective by Fortey (2). First, Siveteret al. incorrectly equate the sister group of the phosphatocopids (their “Eucrustacea”) with the crown group of the Crustacea, thereby inferring their coexistence. Although the phosphatocopids necessarily coexisted with their sister group, the crown group crustaceans constitute only a subset—and quite possibly a post–Early Cambrian subset—of this group. Between the divergence of the phosphatocopids and that of the last common ancestor of all living crustaceans remains an as-yet-unresolved interval that may have been occupied only by members of the stem group (interval X onFig. 1). Although Cynognathus bears a close resemblance to mammals, we would hesitate to infer from its discovery the existence of Early Triassic shrews.

Figure 1

Fossil record of crustaceans in the Cambrian. The phosphatocopids of (1) fall in their expected sequence, as derived stem group crustaceans, consistent with the earliest arthropodan trace and body fossils in the middle and lower Lower Cambrian. The Mount Cap fossils show several crown group features and are placed in a trichotomy with the Comley fossils and crown group crustaceans. Interval X is the period between the appearance of phosphatocopids and crown group crustaceans in the fossil record, during which the sister group of phosphatocopids existed, but crown group crustaceans need not have existed. Dating from (6,7).

Second, with an age estimate of around 511 million years ago (Ma), these Toyonian, or terminal Lower Cambrian, fossils postdate the base of the Cambrian by some 32 million years, nearly two-thirds of the duration of the Cambrian Period and well beyond the range appropriate for testing Cambrian Explosion hypotheses. Moreover, the Comley fossils postdate crustaceans from Botoman-age shales of NW Canada (3) and precede the oldest undoubted crown group crustaceans [from the lower Upper Cambrian Orsten Formation of Sweden (4)] by no more than perhaps 10 million years, timing relationships that somewhat limit the evolutionary implications that can be drawn from the Comley fossils.

More broadly, both Siveter et al. (1) and Fortey (2) suggest that these new fossils erode, if not explode, the concept of a Cambrian Explosion. Unfortunately, they provide no formal definition of this term, and their suggestion that it is synonymous with the origin and earliest cladogenesis of the Metazoa is nothing more than a straw man; we are aware of no recent hypotheses proposing post-Ediacaran roots for the metazoan clade. In our view, the Cambrian Explosion is better defined as the Early Cambrian adaptive radiation [in the sense of (5)] of several eukaryotic clades, including the Metazoa, documented by the truly rapid expansion in the fossil record of characteristics such as large size, morphological complexity, skeletalization, and infaunal activity. If metazoan roots extend deep into the Proterozoic, then early metazoan cladogenesis was clearly decoupled from the Cambrian Explosion; if not, then we are simply looking at a case of accelerated cladogenesis initiated no later than around 565 Ma.


Crustaceans and the "Cambrian Explosion"

Response: Budd et al. contend that Phosphatocopida are stem-line Crustacea set off from “crown group Crustacea” of their terminology. Our paper [(1); see also (2)] detailed the many apomorphies shared by the Phosphatocopida and the monophylum Eucrustacea (3). These apomorphies (Fig. 1) define a complete reorganization of the cephalic locomotion and feeding structures and are absent from the stem-line derivatives of the Crustacea or, more precisely, the stem-line derivatives of an as-yet-unnamed taxon (N.N. of Fig. 1) that embraces Eucrustacea and its monophyletic sister taxon, the Phosphatocopida (4).

Figure 1-1

Proposed relationships within Crustacea. Sets of autapomorphies are numbered consecutively in square boxes; the question mark points to the unresolved situation in the stem line of the as-yet-unnamed taxon N.N. All taxa herein coexisted in the Late Cambrian Orsten assemblages, whereas the Phosphatocopida record extends down to the Early Cambrian.

Coeval occurrence of Eucrustacea and Phosphatocopida follows the logic of phylogenetic systematics (5), which demands coexistence of sister taxa irrespective of whether these embrace only fossil forms. Even if this approach is disputed, the late Cambrian existence of several precursors of N.N., in the form of derivatives of the early stem line (Fig. 1) that share the first autapomorphies of Crustacea (6), implies that branches preceding the node between Phosphatocopida and Eucrustacea must have originated earlier than the first record of Phosphatocopida—which is our new find. Consequently, branches within Arthropoda, Gastroneuralia, Bilateria, and Metazoa must have originated still earlier. The record of “remarkably advanced … Crustacea” from the lower Cambrian of Canada (7), albeit very fragmentary and showing no assignable body details, supports that observation.

Budd et al. state that our material is not pertinent to the Cambrian Explosion because it is “terminal Lower Cambrian.” Our conservative age estimate is based on a 511 Ma isotopic date for the boundary between the Lower and Middle Cambrian (8). The phosphatocopid specimens with soft anatomy are from a condensed limestone sequence (9) and lie about 0.5 m above the Red Callavia Sandstone of Olenellus Biozone age and 0.6 m below the local top of the Lower Cambrian. The specimens come from an intra–Protolenus Biozone horizon, equivalent to the Siberian late Botomian or Toyonian Stage and within the age range 511 to 517 Ma (10). The Cambrian evolutionary radiation achieved its “explosive” character in an interval correlative with the Atdabanian to Botomian (10). Our crustacean fossil is at most a few million years younger than this, and no more than a few million years younger than the earliest Avalonian trilobites (8). Moreover, fossil assemblages of Atdabanian age contain the head shields of phosphatocopids (11), which hints at an even earlier origin for the arthropod evolutionary line.



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