Over the last few days, I’ve been doing some research into Precambrian animals. This is a direct effort to refute the claims of Meyer that all these forms appeared in the Cambrian explosion. This is the status of my efforts to date.
| Age (mya) | Reference | Event | Notes |
| 3,500 | Schopf 1993 | microfossils resembling cyanobacteria (Australia) | |
| 2,100 | Han & Runnegar 1992 | Algal cysts with cells largers than any known in modern prokaryotes | |
| 1,500 | Javaux et al 2001 | complex specrical and spinose forms of eukaryotes | |
| 1,100 | red and green algae appeared | ||
| 1,000 | Wray 1996 | Time of divergence of inverts and verts; molecular data | A literal reading of the fossil record suggests that the animal phyla diverged in an ‘explosion’ near the beginning of the Cambrian period. Calibrated rates of molecular sequence divergence were used to test this hypothesis. Seven independent data sets suggest that invertebrates diverged from chordates about a billion years ago, about twice as long ago as the Cambrian. Protosomes apparently diverged from chordates well before echinoderms, which suggests a prolonged radiation of animal phyla. These conclusions apply specifically to divergence times among phyla; the morphological features that characterize modern animal pbody plans, such as skeletons and coeloms, may have evolved later. |
| 635-580 | Budd, 2007 | Doushanto Fm. | The significance of these dates is that all of the Doushantuo fossils would predate the oldest of the famous Ediacaran fossils such as Dickinsonia, and thus would provide an independent record of the animal life during a period of time that no large body or trace fossils are known. Indeed, the overlying Dengying Fm does yield Ediacaran-type fossils, which could be said to support this contention. |
| 630 | Budd, 2007 | Stem Group Animals Present | |
| 600 | no multicellular prior to this | ||
| 590 | Xiao et al 1998, 2002 | brown algae; highly diverse multicellular algae | |
| 590 | Xiao et al 1998, 2002 | probably microscopic embryos in stages of cleavage with 2, 4, or 8 cells | Ediacaran |
| 590 | Li et al 1998 | sponges with monaxonal spicules and preserved soft tissue | |
| 590 | Chen et al 2008 | Bilaterian fossils appear | Furthermore, the organization of these fossils, taken together with their provenance, indicates that the genetic tool kit and pattern formation mechanisms required for bilaterian development had already evolved by Doushantuo times, long before the Cambrian. Therefore, the diversification of body plans in the Early Cambrian followed from the varied deplument of these mechanisms once conditions permitted, not from their sudden appearance at od just before the Cambrian boundary. |
| 575 | Narbonne & Gehling 2002 | Mackenzie Mountains, Canada: organisms in disc, trianle, and fronds | |
| late Precam | Fedonkin 1997 | Kimbrella – a bilateral mollusc-like organism | http://www.nature.com/nature/journal/v388/n6645/full/388868a0.html |
| 541 | Cambrian Period Begins | ||
| 505 | Caron 2006 | Odontogriphus omalus was originally described as a problematic non-biomineralized lophophorate organism. Here we reinterpret Odontogriphus based on 189 new specimens including numerous exceptionally well-preserved individuals from the Burgess Shale collections of the Royal Ontario Museum. This additional material provides compelling evidence that the feeding apparatus in Odontogriphus is a radula of molluscan architecture comprising two primary bipartite tooth rows attached to a radular membrane and showing replacement by posterior addition. Further characters supporting molluscan affinity include a broad foot bordered by numerous ctenidia located in a mantle groove and a stiffened cuticular dorsum. Odontogriphus has a radula similar to Wiwaxia corrugata but lacks a scleritome. We interpret these animals to be members of an early stem-group mollusc lineage that likely originated in the Neoproterozoic Ediacaran Period, providing support for the retention of a biomat-based grazing community from the late Precambrian until at least the Middle Cambrian. |
Notice that last entry “Cambrian period begins”. All of the stuff on this list is Pre-Cambrian. We have the presence of bilateral organisms. Indeed according to the Wray paper, we have the majority of the animal forms already present. That being said, specific features may have appeared later, but the genetic plan for those phyla are already present.
The Wray paper is particularly interesting as it also opens with a brief discussion of Darwin and how the appearance of the sudden development of animal fossils posed a problem for natural selection. The paper describes the use of molecular divergence rates for a large number of taxa. They used seven different genes for this process and the results are pretty interesting.
The existence of an extended but cryptic Precambrian history of metazoans also has some interesting implications for understanding the origin and diversification of animal body architecture. In particular, the rapid appearance of diverse skeletonized taxa in the fossil record during the middle Early Cambrian may reflect an exceptional period of simultaneous morphological innovation within distinct lineages rather than a rapid branching of phyla. It has long seemed likely, for example, that mineralized skeletons evolved independently in several phyla at this time (39). It is unlikely, however,that all “body plan” features evolved during the Cambrian. A cephalized, bilaterally symmetrical body composed of three germ layers predates the protostome-deuterostome split (28, 40, 41) and thus probably evolved much earlier than is generally recognized. Coeloms are shared by the two deuterostome phyla we examined and may predate the Cambrian by several hundred million years (the coeloms of protostomes may have an independent origin, and dating their appearance will require more
information about the divergence times of the various protostome phyla) .
Four of the major ‘innovations’ in body plans are in place well before the Cambrian. Radial anatomy, bilateral anatomy (with a specific head and rear), three germ-layers (we have three germ layers, it’s basically a difference in cell types that make up the outside layer, the inside tube (intestines, stomach , etc) and everything between those two layers (in humans things like the heart, skeleton, etc), and coeloms (a fluid-filled body cavity between the intestines and the body wall of some higher metazoans) were present long before the Cambrian.
We also have molluscs (Caron 2006) and cnidarians (Chen 2002) well before the Cambrian. We also have segmented organisms (Valentine 1989) and worms.
So, what’s Meyer’s issue here? Aside from the fact that there’s only four of these articles listed as references in his book (they are bolded below) and he draws completely opposite conclusions from them than the authors do.
Meyer’s book, even the Kindle edition, isn’t crosslinked from references back to text, so we’ll just have to see what he says when we find them.
A final note is that this isn’t even close to all the material on precambrian organisms. This is just a sample… i.e. what was available in PDF for free or via Science.
We’ll talk about Meyer’s “diagrams” in the next installment. Suffice to say that he must not have actually read the papers he referenced in his book, because his diagrams are nothing like what is actually in the papers.
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References:
Chen, Jun-Yuan et al. “Precambrian animal life: probable developmental and adult cnidarian forms from Southwest China.” Developmental biology, 31 Jul. 2002. Available: http://www.ncbi.nlm.nih.gov/pubmed/12142030.
