Critics of Huldtgren’s proposal instead think they may well be tiny metazoan embryos, though of unknown affiliation. [Xiao et al., “Comment on ‘Fossilized Nuclei and Germination Structures Identify Ediacaran ‘Animal Embryos’ as Encysting Protists’,” 1169. Huldtgren and colleagues have defended their interpretation here: Huldtgren et al., “Response to Comment on ‘Fossilized Nuclei and Germination Structures Identify Ediacaran “Animal Embryos” as Encysting Protists’,” 1169.]
Another interpretation is that the fossils represent giant sulphur bacteria, since “sulphur bacteria of the genus Thiomargarita have sizes and morphologies similar to those of many Doushantuo microfossils, including symmetrical cell clusters that result from multiple stages of reductive division in three planes.” [Bailey et al., “Evidence of giant sulphur bacteria in Neoproterozoic phosphorites,” 198–201.] Critics of this hypothesis doubt that sulphur bacteria could be fossilized because they “collapse easily and have only patchy biofilms that are limited to the multi-layered envelope.” [Cunningham et al., “Experimental taphonomy of giant sulphur bacteria: implications for the interpretation of the embryo-like Ediacaran Doushantuo fossils,” 1857–64.]
The debate over whether the Doushantuo microfossils should be interpreted as metazoan embryos, protozoans, or giant sulphur bacteria will doubtless continue. Whatever the outcome, however, the fact remains: small, fragile, and soft-bodied organisms of some kind have been found fossilized in this Precambrian strata, raising the question of why the same layers of rock were unable to preserve the immediate precursors to the numerous metazoan phyla that emerge so abruptly in the Cambrian layers above them.
52. Similarly, paleontologists rarely find the remains of parasites that live in the soft tissues of other organisms (indeed, parasitic organisms represent several of the phyla that have no fossil record). As noted, the geological record does preserve soft tissues, but only infrequently. When it does, researchers fortunate enough to make such finds will rarely want to destroy important specimens (of soft-tissue organs) in order to examine them for traces of parasitic infection or habitation. Not surprisingly, therefore, paleontologists have not found the remains of many parasitic organisms in the fossil record.
53. The Emergence of Animals, 91. This point is also based on personal conversation with Professor (Mark) McMenamin.
54. Erwin and Valentine, The Cambrian Explosion, 8.
55. Foote, “Sampling, Taxonomic Description, and Our Evolving Knowledge of Morphological Diversity,” 181. Another statistical paleontologist, Michael J. Benton, and his colleagues have reached a similar conclusion. They note that “if scaled to the … taxonomic level of the family [and above], the past 540 million years of the fossil record provide uniformly good documentation of the life of the past” (Benton, Wills, and Hitchin, “Quality of the Fossil Record Through Time,” 534). In another article Benton also writes: “It could be argued that there are fossils out there waiting to be found. It is easy to dismiss the fossil record as seriously, and unpredictably, incomplete. For example, certain groups of organisms are almost unknown as fossils… . This kind of argument cannot be answered conclusively. However, an argument based on effort can be made. Paleontologists have been searching for fossils for years and, remarkably, very little has changed since 1859, when Darwin proposed that the fossil record would show us the pattern of the history of life” (“Early Origins of Modern Birds and Mammals,” 1046).
56. Foote, “Sampling Taxonomic Description, and Our Evolving Knowledge of Morphological Diversity,” 181. I should note that there is one way in which my analogy to colored marbles in a barrel fails to capture the nature of the challenge of Cambrian fossil discontinuity. If after pulling samples from a barrel for a while you finally came up with a green and orange ball to go along with the piles of red, blue, and yellow balls, you still wouldn’t have much confidence that the barrel had a rainbow of ball colors finely grading from one to another. Yet you could at least say that the orange ball stands between the yellow and red ball, and the green ball stands between the blue and yellow balls (like the hybrid produced from two plants). But many of the new Cambrian animal forms that have been discovered since Darwin’s time aren’t seen as intermediates between the previously known animal forms representing known phyla. They aren’t evolutionary intermediates between one existing phylum and another. Instead, scientists consider them as existing out in morphological space all their own, standing not as intermediates but as phyla that themselves are in need of intermediate forms—almost as if, by stretching my analogy, some new primary color had been discovered.
57. See Erwin et al., “The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals,” 1091–97.
58. Bowring et al., “Calibrating Rates of Early Cambrian Evolution.”
59. Bowring et al., “Calibrating Rates of Early Cambrian Evolution,” 1297.
60. Lili, “Traditional Theory of Evolution Challenged,” 10.
61. There are a few putative survivors of the Ediacaran fauna. For example, the enigmatic and frond-like Thaumaptilon found in the Burgess Shale might be a descendant of Ediacaran fronds, though this is contested. Jensen et al., “Ediacara-Type Fossils Cambrian Sediment,” 567–69; Conway Morris, “Ediacaran-like Fossils in Cambrian Burgess Shale-Type Faunas of North America,” 593–635.
62. Bowring et al., “Calibrating Rates of Early Cambrian Evolution,” 1297. See also McMenamin, The Emergence of Animals.
63. Erwin et al., “The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals,” 1091–97.
64. Shu et al., “Lower Cambrian Vertebrates from South China.”
65. Chen et al., “A Possible Early Cambrian Chordate”; Chen and Li, “Early Cambrian Chordate from Chengjiang, China”; Dzik, “Yunnanozoon and the Ancestry of Chordates.” Note, however, that the assertion that Yunnanozoon is a chordate has been challenged. See Shu, Zhang, and Chen, “Reinterpretation of Yunnanozoon as the Earliest Known Hemichordate”; Shu, Morris, and Zhang, “A Pikaia-like Chordate from the Lower Cambrian of China.” Paleontologists also have found a single specimen of a possible cephalchordate, Cathaymyrus, from the lower Cambrian Qiongzhusi Formation near Chengjiang. The status of Cathaymyrus as a valid taxon has also been challenged; some paleontologists argue that the single specimen of Cathaymyrus may actually be a dorsoventrally compressed Yunnanozoon; see Chen and Li, “Early Cambrian Chordate from Chengjiang, China.”
66. Chen, Huang, and Li, “An Early Cambrian Craniate-like Chordate,” 518.
67. Shu et al., “Lower Cambrian Vertebrates from South China.”
68. Shu et al., “An Early Cambrian Tunicate from China.”
69. The chordates discovered in the Cambrian strata near Chengjiang represent just one of many new animal body plans found there, some of them designated as new phyla, others as new subphyla, classes, or families within existing phyla. For example, paleontologists classify Occacaris oviformis, a round, egg-shaped animal with large forcep-like structures, as a member of the well-known phylum Arthropoda. [Hou et al., The Cambrian Fossils of Chengjiang, China, 130.] Nevertheless, Occacaris, which has been found only in the Maotianshan shale, clearly exemplifies a unique way of arranging organs and tissue—one unlike any previously known arthropod outside the Chengjiang biota. Arguably, it represents a unique body plan. In other cases, Chinese paleontologists have discovered animals with such unusual morphologies that they have been unable to classify them within any known phylum. These so-called problematica, such as the mysterious Batofasciculus ramificans, a hot-air-balloon-shaped cactus-like animal, has been given a species and genus name, but as yet, no specific phyletic designation, though it clearly exemplifies a unique body plan. [Hou et al., The Cambrian Fossils of Chengjiang, China, 196.] While such difficult-to-classify organisms do not increase the official count of novel phyla that first arose during the Cambrian, they do frequently display novel body plans.
Chapter 4: The N
ot Missing Fossils?
1. Gradstein, Ogg, Schmitz, and Ogg, The Geological Time Scale 2012.
2. Grotzinger et al., “Biostratigraphic and Geochronologic Constraints on Early Animal Evolution.” A few Ediacarans may have survived until the middle Cambrian. See Conway Morris, “Ediacaran-like Fossils in Cambrian Burgess Shale-Type Faunas of North America.”
3. Monastersky, “Ancient Animal Sheds False Identity.”
4. Monastersky, “Ancient Animal Sheds False Identity.”
5. Monastersky, “Ancient Animal Sheds False Identity.”
6. Fedonkin and Waggoner, “The Late Precambrian Fossil Kimberella is a Mollusc-like Bilaterian Organism,” 868.
7. For example, Graham Budd, a Swedish paleontologist and Cambrian expert, has expressed skepticism about this classification. He acknowledges that “the strongest case for an Ediacaran bilaterian body fossil has been made by Fedonkin and Waggoner (1997) for Kimberella,” but nevertheless disputes the classification of Kimberella as a true mollusk. He argues that “Kimberella does not possess any unequivocal derived molluscan features, and its assignment to the Mollusca or even the Bilateria must be considered to be unproven” (Budd and Jensen, “A Critical Reappraisal of the Fossil Record of the Bilaterian Phyla,” 270).
8. Another reason the Ediacaran body fossils cannot be assigned to the animal phyla in a decisive manner is because of the coarse grain size of the beds in which they occur. Details of body form are too vague to allow a clear decision, and until better means of analysis or new beds with finer grain texture are found, these fossils will remain as intriguing “problematica,” problematic forms about which it is not possible to come to a decision. See Miklos, “Emergence of Organizational Complexities During Metazoan Evolution.” See also Bergström, “Metazoan Evolution Around the Precambrian–Cambrian Transition.”
9. Retallack, “Growth, Decay and Burial Compaction of Dickinsonia,” 215. As Retallack goes on to explain, “Like fungi and lichens, Dickinsonia was firmly attached to its substrate, ground-hugging, moderately flexible, and very resistant to burial compaction” (236).
10. Glaessner, The Dawn of Animal Life, 122.
11. Birket-Smith, “A Reconstruction of the Pre-Cambrian Spriggina,” 237–58.
12. Glaessner, The Dawn of Animal Life, 122.
13. Glaessner, The Dawn of Animal Life, 122.
14. McMenamin, “Spriggina Is a Trilobitoid Ecdysozoan,” 105.
15. Glaessner, The Dawn of Animal Life, 122–23. McMenamin, The Emergence of Animals, 20, 24, 118.
16. In addition, a recent depiction of an Ediacaran organism closely related to Spriggina, called Yorgia, by Ediacaran expert Andrey Ivantsov, shows it exhibiting a smoother, less jagged edge than the bodies of trilobites with no protruding spines. Ivantsov’s reconstruction seems to underscore a shift in opinion away from regarding Spriggina–like organisms as basal arthropods in possession of distinctive arthropod characteristics (such as genal spines). Ivantsov, “Giant Traces of Vendian Animals.” See also Ivantsov, “Vendia and Other Precambrian ‘Arthropods.’ ”
17. Brasier and Antcliffe, “Dickinsonia from Ediacara,” 312.
18. Brasier and Antcliffe, “Dickinsonia from Ediacara,” 312. Ivantsov, “Vendia and other Precambrian ‘Arthropods.’ ”
19. “Asymmetry in the Fossil Record,” 137.
20. Erwin, Valentine, and Jablonski, “The Origin of Animal Body Plans,” 132.
21. Erwin, Valentine, and Jablonski, “The Origin of Animal Body Plans,” 132.
22. Erwin, Valentine, and Jablonski, “The Origin of Animal Body Plans,” 132.
23. Cooper and Fortey, “Evolutionary Explosions and the Phylogenetic Fuse,” 151–56.
24. Fortey, “Cambrian Explosion Exploded,” 438.
25. Knoll and Carroll, “Early Animal Evolution,” 2129.
26. Ward, On Methuselah’s Trail, 36.
27. “Life on Land,” 153–54. Recently, Gregory Retallack has published a controversial hypothesis about the Ediacaran fauna. Retallack has studied the depositional environments of key Ediacaran fossils such as Dickinsonia. He has concluded that these organisms should not be classified as marine animals, because they were deposited on land. According to Retallack, the rocks that bore these Ediacaran fossils “have a variety of features that are more like the biological soil crusts of desert and tundra than the parallel wrinkled, and undulose hydrated microbial mats of intertidal flats and shallow seas.” [Retallack, “Ediacaran Life on Land,” 89.] Retallack’s thesis has received a cool reception from other Ediacaran experts, however. They have not only questioned his analysis of ancient sediments but pointed out that the Ediacaran forms that he analyzed from Australia are also preserved in clearly marine sediments (from Newfoundland, for example) and that it is unlikely that the same organisms would live both on land and in the sea. [Callow, Brasier, Mcilroy, “Discussion: ‘Were the Ediacaran siliciclastics of South Australia coastal or deep marine?’’’ 1–3.]
28. Erwin et al., “The Cambrian Conundrum.” Others believe that these Precambrian “worm” trails could have been created by giant protists. See Matz et al., “Giant Deep-Sea Protist Produces Bilaterian-like Traces,” 1849–54.
29. Valentine, Erwin, and Jablonski, “Developmental Evolution of Metazoan Body Plans”; Runnegar, “Evolution of the Earliest Animals.”
30. Runnegar, “Proterozoic Eukaryotes”; Gehling, “The Case for Ediacaran Fossil Roots to the Metazoan Tree.”
31. Budd and Jensen, “A Critical Reappraisal of the Fossil Record of the Bilaterian Phyla,” 270.
32. See Matz et al., “Giant Deep-Sea Protist Produces Bilaterian-like Traces.”
33. Erwin et al., “The Cambrian Conundrum.”
34. Sperling, Pisani, and Peterson, “Poriferan paraphyly and Its Implications for Precambrian Palaeobiology”; Erwin and Valentine, The Cambrian Explosion, 80.
35. Conway Morris, “Evolution: Bringing Molecules into the Fold,” 5.
36. McMenamin and McMenamin, The Emergence of Animals, 167–68.
37. Peterson et al., “The Ediacaran Emergence of Bilaterians.”
38. Shen et al., “The Avalon Explosion,” 81.
39. See, e.g., Cooper and Fortey, “Evolutionary Explosions and the Phylogenetic Fuse.” The Cambrian period 543 mya is marked by the appearance of small shelly fossils consisting of tubes, cones, and possibly spines and scales of larger animals. These fossils, together with trace fossils, gradually become more abundant and diverse as one moves upward in the earliest Cambrian strata (the Manykaian Stage, 543–530 mya).
40. Bowring et al., “Calibrating Rates of Early Cambrian Evolution,” 1293–98; Erwin et al., “The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals,” 1091–97.
41. Meyer et al., “The Cambrian Explosion: Biology’s Big Bang,” 323–402.
42. Valentine, “Prelude to the Cambrian Explosion,” 289.
43. Animals with fivefold symmetry extending from a central body cavity are described technically as radially symmetric “pentamerous” animals.
44. Budd and Jensen, “A Critical Reappraisal of the Fossil Record of the Bilaterian Phyla,” 261. As Budd and Jensen explain in more detail: “although this fossil possesses pentaradial symmetry, its small size, combined with preservation in relatively coarse sand means that other echinoderm-specific features are not readily visible. Its assignment to the Echinodermata thus largely rests on this single character, and must be at present regarded as an open question” (261).
45. Valentine, On the Origin of Phyla, 287, 397.
46. Bottjer, “The Early Evolution of Animals,” 47.
47. Cnidarians and ctenophores, for example, are radially symmetric. (One might think that echinoderms, which as adults have pentaradial [fivefold] symmetry, are not bilaterians, but “in early development, echinoderms are bilateral” and thus classed among the Bilateria.) For more discussion, see Valentine, On the Origin of Phyla, 391.
48.
Bengtson and Budd, “Comment on ‘Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian,’ ” 1291a.
49. As Bengtson and Budd explain, “The specimens presented by Chen et al. represent a common mode of preservation of microfossils in phosphatic sediments, including those of the Doushantuo.” In particular, they note that “the layers have a regular banding of color and thickness that is different between the specimens but consistent within the individual specimens.” They argue that “this pattern defies biological explanation but is easily explained as representing two to three generations of diagenetic overgrowth.” They also note that “rather than being sinuously folded, as would be expected from deformed tissue layers,” the layers in the imprint display features typical of (inorganic) diagenetic crusts. They conclude that although the imprint may have encased the remains of eukaryotic microfossils, “their reconstructed morphology as bilaterians is an artifact generated by cavities being lined by diagenetic crusts. The appearance of the fossils now has little resemblance to that of the living organisms that generated them” (“Comment on ‘Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian,’ ” 1291a).
50. Bengtson et al., “A Merciful Death for the ‘Earliest Bilaterian,’ Vernanimalcula,” 421.
51. Bottjer, “The Early Evolution of Animals,” 47.
52. Bengtson et al., “A Merciful Death for the ‘Earliest Bilaterian,’ Vernanimalcula,” 426.
53. Bengtson et al., “A Merciful Death for the ‘Earliest Bilaterian,’ Vernanimalcula,” 426.
54. Marshall and Valentine, “The Importance of Preadapted Genomes in the Origin of the Animal Bodyplans and the Cambrian Explosion,” 1190, emphasis added.
55. Budd and Jensen, “The Limitations of the Fossil Record and the Dating of the Origin of the Bilateria,” 183.
56. Budd and Jensen, “The Limitations of the Fossil Record and the Dating of the Origin of the Bilateria,” 168.
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