This is part five in a series by Keith Miller (earlier parts can be found on the sidebar). It is an updated extension of Miller and Campbell's 2003 essay “The ‘Cambrian explosion’: A challenge to evolutionary theory?” from the book Perspectives on an Evolving Creation: Grand Rapids, and it coincides with our Question, "Does the Cambrian Explosion pose a challenge to evolution?". A pdf version of Miller's full paper can be found here.
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Ground Zero: What were the Cambrian Animals Like?
One of the most important features of the Cambrian "explosion" was the very rapid diversification of organisms with shells, plates, and various other types of hard parts. A wide variety of soft-bodied organisms are also known from the Cambrian. Although some fossils can be assigned to living phyla, there are also specimens that appear to represent stem groups or intermediates between modern phyla, as well as specimens of unknown relationship. Representatives of several living classes and other lower taxonomic categories also appear in the Cambrian. A few deposits with exceptionally good preservation of fossils, such as the Burgess Shale in Canada (Image: Wikipedia), contribute to the wide range of taxa known from the Cambrian. Such deposits with exceptional preservation are known as Konservat-Lagerstätten (from the German "conservation deposits"). Similar deposits have since been found around the world in the Early to Middle Cambrian, notably the Early Cambrian Chengjiang fauna of China. Additionally, trace fossils become much more varied, complex, and abundant in the Cambrian, suggesting a newly widened range of animal activity.
Some of the very first fossils to appear near the base of the Cambrian are tiny skeletal plates, spines, tubes, and cap-shaped shells that have been called the “small shelly fossils.”1 Among these are the spicules of different groups of sponges, and the shells of the earliest known “crown group” mollusks and brachiopods (see part 2 of the series for more on steam and crown groups). However, the biological identities of many of these tiny skeletal elements were completely unknown until fairly recently. Well-preserved complete fossils in the Chengjiang, and other fossil lagerstätten around the world, have revealed that some of these small shelly fossils were actually the spines and “armoring” of larger metazoans. More detailed analysis of other fossils has revealed that they may represent the stem groups of living phyla, rather than evolutionary dead ends.
The discovery of complete specimens from later in the early Cambrian has revealed that a variety of scales, plates and spines found among the “small shelly fossils” actually fit together and overlapped to cover the bodies of slug-like organisms.2 These organisms are the halkieriids (Image: Wikipedia) and wiwaxiids (Image: Wikipedia). The halkieriids bore conical mollusk-like shells as well as calcareous structures similar to the chitinous bristles typical of polychaete annelid worms. The slightly younger Wiwaxia was covered in scale-like and spine-like structures even closer to those of the polychaetes, and also possessed a radula diagnostic of mollusks. These various unusual organisms bear resemblances to both mollusks and polychaete annelid worms, which are closely related phyla. Thus these organisms would appear to be positioned somewhere on the evolutionary tree near the branching point of the mollusks with the annelids.
Other cap-shaped fossils from the earliest Cambrian are the helcionelloids. These are interpreted as monoplacophoran-like crown group mollusks. As discussed earlier in the section on “Classifying Animals”, there is good fossil evidence of the transition from these primitive cap-shaped helcionelloids to the first bivalves. There are also likely fossil transitions from helcionelloids to the first gastropods.
Another important group of organisms represented by small plates in the early Cambrian are the lobopods. Lobopodians, until very recently an enigmatic group of strange fossils, were "caterpillar-like" organisms with fleshy lobed limbs and mineralized plates or spines running along their backs. They are similar to the living Onychophora(Image: Wikipedia), or velvet worms, but are considered a distinct group.3 The oldest known lobopodian bears certain similarities to a distinctive group of worms called the palaeoscolecid priapulids that also bore small plates or tubercles along their bodies.4 Lobopods may have been derived from these worms that also have an early Cambrian fossil record. Furthermore, the lobopods have become recognized as the critical link in reconstructing the assembly of the arthropod body plan. They have anatomical features in common with the arthropods, particularly with peculiar Cambrian stem arthropods such as Opabinia (Image: Wikipedia) and Anomalocaris (Image: Royal Ontario Museum, Toronto) that are preserved in the younger Chengjiang and Burgess fossil beds. These later organisms possessed lobopod limbs but also had gill flaps along their bodies and jointed feeding appendages. Intermediates between lobopodians and the early stem group arthropods have also been discovered that possessed gills.5 Of even greater interest is the evidence available from the extraordinary preservation of muscle tissue (this paper is available online here) in a few of these transitional organisms. These specimens suggest a progression of steps in the transformation of internal anatomy from lobopodians to true arthropods.6
The tommotiids (Image: Catalogue of Organisms), a group of roughly conical-shaped shells composed of calcium phosphate, have until recently been one of the most enigmatic of the small shelly fossils. However, new discoveries of articulated specimens have shown that pairs of symmetrical skeletal elements fit together to form an open cone that was attached to the seafloor at the base. An opening at the base indicates the presence of a muscular attachment structure likely similar to the pedicle of brachiopods. The paired shells also have features similar to the tiny paterinids (Image: Paleos), crown group brachipods with calcium phosphate shells that also appear in the early Cambrian.7 These fossils therefore appear to represent stem brachiopods that were themselves derived from armored tubular filter feeders attached to the seafloor.
Following the appearance of the small shelly fossils, the diverse metazoan fossil communities of the Chengjiang in China are dated at around 525-520 million years, 20 million years after the beginning of the Cambrian. The exceptional preservation in these fossil beds is similar to that of the Burgess Shale deposits that are dated around 515-505 million years. These extraordinary fossil sites give us our best views into the composition of marine biological communities from this time, preserving both soft-bodied organisms and those with mineralized skeletons.8These beds contain abundant and diverse sponges and cnidarians, as well as priapulid worms, annelid worms, lobopods, stem mollusks such as Wiwaxia, and brachiopods. However, probably the most dramatic characteristic of the Chengjiang and Burgess type deposits is the abundance and diversity of arthropods.
Arthropods comprise 50% or more of all of the fossil specimens collected from these beds. These fossils include stem arthropods such as the anomalocarids (Image: Wikipedia), trilobites (Image: Fossilmuseum.net) which came to dominate the Paleozoic, and some species that appear to be crustaceans and chelicerates (Image: Fossilmuseum.net). However, most of the fossils belong to primitive stem groups that likely represent evolutionary experimentations after the appearance of true arthropods but before the rise of most living arthropod groups. In the Burgess Shale one such primitive species alone (Marrella, Image: Wikipedia) comprises a third of all fossil specimens. These fossils show unusual arrangements, and types, of appendages.
The chordates (that include vertebrates), hemichordates (that include the living “acorn worms”, Image: Wikipedia), and echinoderms (that include the living starfish and echinoids) are all deuterostomes and have the same pattern of early embryo development. Although the modern representatives of these phyla appear extremely different, they are actually closely-related branches on the tree of life, and are understood to have evolved from a common ancestor. Some rare, but very significant, specimens in the Chengjiang seem to be stem chordates and stem echinoderms, as well as specimens that have been interpreted as organisms close to the common ancestors of chordates and echinoderms. These rather simple Cambrian organisms possess the anatomical characteristics that would be expected in organisms that had acquired some but not all of the distinctive features of chordates or echinoderms.
A very primitive stem group of deuterostomes, called ventulicolians has recently been described that might represent the anatomy of organisms near the base of the deuterostome evolutionary branch that were ancestral to both the chordates and echinoderms.9 These soft-bodied organisms possessed segmentation and oval structures interpreted as gill slits, and a terminal mouth. Significantly, another group of primitive deuterostomes, calledvetulocystids (Image: Nature), bears similarities to the ventulicolians as well as to some of the bizarre early echinoderms. These organisms were likely anchored to the sediment and possessed an echinoderm-like mouth and respiratory openings.10 They may in fact represent organisms ancestral to the first echinoderms that were characterized by peculiar globular and asymmetrical shapes.
The most primitive group of chordates are the urochordates, or tunicates, (Image: Wikipedia) that have a sack-like adult body that filters seawater through pharyngeal slits. In their tadpole-like larval form, they possess stiff notochords (a structure diagnostic of chordates) that is lost in the adult form. A likely tunicate (Image: Nature) has been described from the Chengjiang.11 Another group of primitive chordates are the cephalochordates(represented today by the lancelets, Image: Wikipedia) that possess a notochord as adults, pharyngeal slits, and muscles arranged in parallel bundles. Some fossils have been interpreted as stem cephalochordates.12 Lastly, and of particular interest, is a fossil that may be a stem vertebrate.13 Haikouichthys, (Image: Wikipedia)in addition to a notochord, gill pouches and muscle bundles, also appears to have had some structures characteristic of vertebrates. These vertebrate features include a cavity surrounding the heart, a dorsal fin, and cartilage around the head and as a series of elements along the notochord. The Chengjiang thus includes fossil specimens that occupy several significant transitional positions from primitive deuterostomes, to stem echinoderms and stem chordates.
The fossils of the Cambrian “explosion” were indeed diverse and included organisms that can be assigned to a number of living phyla. As we have seen, these fossil organisms were also largely representative of stem groups that possessed some, but not all, of the diagnostic features that define the major groups of living organisms. The body plans of phyla were assembled piecemeal. Furthermore, important transitional steps between living phyla and their common ancestors are also preserved. These include: the rise of mollusks from their common ancestor with the annelids, the evolution of arthropods from lobopods, the likely evolution of brachiopods from tommotids, and the rise of chordates and echinoderms from early deuterostomes. While the picture is far from complete, the spectacular fossil discoveries from the early and middle Cambrian strongly support the conclusion that the major branches of the animal tree of life are joined to a common metazoan (multi-cellular animal) trunk.