Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
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Late Precambrian Bilaterians: Grades and Clades
Late Precambrian Bilaterians: Grades and Clades
Pages 87-108
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From page 87... ...
The relatively few body fossils known from the late Precambrian do not throw light on the sequence of evolutionary advances that led to the Cambrian taxa. The purpose of this paper is to characterize the evolution of metazoan body plans during the late Precambrian and Early Cambrian, with evidence drawn chiefly from Phanerozoic fossils and from living forms.
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From page 88... ...
Despite this expansion of the Cambrian, new absolute age estimates have caused the length of time believed to be available for the Cambrian explosion to be shortened (Bowring et al., 1993~. The relationship of the dates given in Figure 2 to the boundaries of the Stages of the Lower Cambrian remains a difficult stratigraphic problem, but it is likely that the most critical Stages, the Tommotian and Atdabanian, are probably only ~10 my in
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From page 89... ...
The earliest fossils that may be metazoans are preserved in two modes. One is as body fossils, chiefly as impressions in this case.
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From page 90... ...
It seems likely on present evidence that most of the late Precambrian forms do have cnidarian affinities. Even so, the late Precambrian body fossils do not represent direct ancestors of any of the higher metazoans and do not help to resolve the puzzle of the origin of the remaining phyla.
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From page 91... ...
Middle Cambrian Faunas. The rate of appearance of novel body plans slows greatly during the Middle Cambrian with the exception of the fauna of the Burgess Shale and its correlatives, which create a diversity "spike" (Whittington, 1985; Conway Morris, 1992~.
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From page 92... ...
= 100 50 o 700 A_ ~ ves .,,r~r Actinopterygi~ fry f r Cephalopoda Agnatha Arthropoda · Echinodermata, Annelida Haemocoelic Bilaterian Cnidaria · Porifera f |,~ Amphibia ~, - Diapsida Hominidae ..~ 600 500 400 300 200 100 0 Millions of Years Before Present FIGURE 3 Estimated somatic cell-type number (except nerve cells) required by the body plans of various taxa, plotted against the time of origin of those taxa indicated by the fossil record.
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From page 93... ...
Although the model is not meant to replicate the history of metazoan complexity, the behavior of the upper bound suggests that no forcing mechanism may be necessary to account for the empirical complexity increase in early metazoan body plans. The relatively rapid initial increase in complexity, created partly by a "floor" of two cell types and partly by the diversification of clades, may have been a feature of real clades even in the absence of forcing agents.
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From page 94... ...
If the unresolved rRNA branching events leading to various protostomes occurred within proto-mollusks, it may not be possible in principle to characterize them by synapomorphies that are germane to the derived features of living phyla. (Molecular data from various sources but chiefly Wainright e!
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From page 95... ...
Other interpretations are possible but they are less parsimonious. Late Precambrian Body Plans.
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From page 96... ...
A plausible scenario would begin with a seriated form, probably preferring hard grounds and supplementing a fundamentally peristaltic creeping locomotion by lateral body projections that served as accessory gripping mechanisms. Sclerotization may have begun as protection in such a habitat, since burrowing would not be possible, but flexibility of the body wall and peristaltic efficiency would have been sacrificed as it became heavier.
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From page 97... ...
or horizontal plowing. Undoubted annelidan body fossils have not been found in rocks older than Middle Cambrian; the earliest penetrating burrows may have been made by so-called pseudocoelomates such as priapulids or paleoscolecids, both of which are known in the Lower Cambrian.
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From page 98... ...
Late Vendian and Manykaian bilaterian body plans are thus visualized as consisting of an array of vermiform types, including flatworms and "round flatworms" with blood-vascular systems, many with hemocoels or "pseudocoels," and some with seriation of one organ system or another (Valentine, 1989, 1990; Bergstrom, 1989~. Organ coeloms were doubtless present in many lineages.
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From page 99... ...
These worms have left us an array of trace fossils and evidently little else, except their descendants, among whom we number. Tempo and Mode and Bodily Plans The more complex of the late Precambrian worms must have been higher invertebrates in every sense, with, by invertebrate standards, sophisticated organ systems consisting of appropriately specialized tissues and these in turn composed of differentiated cell phenotypes that, judging by the body plans of living organisms, probably numbered in the 40s.
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From page 100... ...
A form interpreted as an ancestral chelicerate (it lacks chelicerae) is described from the Middle Cambrian Burgess Shale (Briggs and Collins, 1988~; whether any cluster duplication had occurred by then is unknown.
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From page 101... ...
It is possible to speculate on aspects of the general course of evolution of metazoan body plans, assuming that the preceding picture of early metazoan evolution is approximately correct. Homeobox genes presumably arose within protistans, but took on a role in the specification of cell fates, movements, and patterns as cell differentiation accompanied the rise of multicellular organisms.
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From page 102... ...
within the Early Cambrian, an unequaled explosion of morphological novelty, the ancestral lineages represented chiefly or entirely by trace fossils. Evidence from the fossil record can be combined with that from molecular phylogenetic trees to suggest that the last common ancestor of (i)
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From page 103... ...
Living phyla appearing during the Cambrian explosion have a HoxlHOM gene cluster, implying its presence in the common ancestral trace makers. The explosion required a repatterning of gene expression that mediated the development of novel body plans but evidently did not require an important, abrupt increase in genomic or morphologic complexity.
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From page 104... ...
(1994) Vendian body fossils and trace fossils.
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From page 105... ...
(1991) A cluster of Antennapedia-class homeobox genes in a nonsegmented animal.
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From page 106... ...
(1991) Homeobox genes in mouse development.
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From page 107... ...
(1975) Genetic regulation and the fossil record.
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