Kimberella and Controversial Relationships — A Chronological Synopsis
Editor’s note: We are delighted to present a series of posts by paleontologist Günter Bechly on the Ediacaran organism Kimberella. If identified as an animal, it would “predate the Cambrian explosion of bilaterian animal phyla as a kind of ‘advance guard.’” The question is of interest for debates about evolution and arguments about intelligent design raised by Stephen Meyer, among others. Find the full series about Kimberella here.
Glaessner & Daily (1959) figured a specimen of Kimberella as a “problematic fossil, possibly belonging to the Siphonophora.”
Glaessner & Wade (1966) compared Kimberella with Leptomedusae, Trachymedusina, and Carybdeida (Cubozoa or box jellies). They agreed that “the existence of most of its characters in one or more of these groups indicates its medusoid affinities.” Their main argument was an assumed homology of the apparent “segmented zones” with the gonads in some jellyfish groups.
Wade (1972) discussed the assumed medusoid affinities, agreed with the interpretation of the segmented zones as medusoid gonads and gastric pouches, and even believed themselves to see possible tentacles. Wade suggested that Kimberella is “probably a Scyphozoan” (true jelly).
Jenkins (1984, 1992) thoroughly reviewed and reevaluated the morphology of Kimberella and came to somewhat different interpretations (e.g., concerning the puckered gonads) but also concluded that a medusoid affinity is obvious. He described in detail for Kimberella the presence of medusoid organs like the bell, 2-3 puckered gonads, subumbrella, rhopalia, and pedalia supporting the tentacles. He suggested that Kimberella is a cubozoan box jelly and even is the direct ancestor of the modern chirodropid sea wasps, which are responsible for many severe accidents for swimmers on the Australian coast. Such a cubomedusoid affinity was also endorsed by Glaessner (1984). It is remarkable, indeed, how detailed a medusoid morphology was projected onto Kimberella fossils and later recognized as pure fantasy.
After the new and better preserved White Sea material was discovered in 1994 and described by Fedonkin & Waggoner (1997), Fedonkin (1998), and Waggoner (1998), these authors totally rejected the previous medusoid interpretation of Kimberella and found that this fossil organism “had no tentacles, no tetraradiate symmetry, and no bell.” They also rejected a possible affinity with the more typical Ediacaran organisms. Instead they interpreted Kimberella as “a bilaterian, more complex than a platyhelminth [flatworm], and more like a mollusk than anything else living.” Nevertheless, Fedonkin & Waggoner offered the qualification, “We cannot prove a molluscan affinity conclusively; some definitive molluscan synapomorphies, like the radula, cannot be seen, and others are open to other interpretations.”
Seilacher (1997, 1999) thought that the trace fossils “record a bilateral radula and its usage in pendulum grazing, which is definitely a molluscan feature … as is the impression of a broad muscular foot.” Seilacher (1999) speculated that “the Kimberella animal could have been a halkieriid,” which is a mollusk-like Cambrian organism. Independent of its affiliation, however, he thought that it could be properly modeled after modern “soft limpets.”
Erwin (1999) was very skeptical of previous attributions of Ediacaran organisms to animals. He said that “those who would assign Ediacaran fossils to protostomes and deuterostomes have generally failed to identify any diagnostic characters which unambiguously support such assignments. At present then, claims of a higher metazoan affinity must be regarded as unproven.” Nevertheless, he characterized Kimberella as “triploblastic, benthic organism with a broad, well-muscled foot and a broad visceral cavity. Such an organism would have required at least a hemaeocoel. No appendages are present (as is true of all Ediacaran forms) nor is a mouth apparent.” He said that “although this is not clearly a mollusc (Fedonkin and Waggoner’s preferred interpretation) or even a protostome, it is more complex than a cnidarian” and “Kimberella could also represent a stem-group bilaterian” before the protostome-deuterostome ancestor.
Budd & Jensen (2000) commented as follows: “Perhaps the strongest case for an Ediacaran bilaterian body fossil has been made by Fedonkin & Wagonner [sic] (1997) for Kimberella, which they describe as mollusk-like. It is a relatively large, bilaterally symmetrical organism, which they reconstruct as possessing both a dorsal shell and a ‘foot’. Nevertheless, 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 (cf. Erwin, 1999).”
Martin et al. (2000) called Kimberella the “oldest well-documented triploblastic bilaterian,” but still admitted that “whether Kimberella at 555 Ma indicates that the protostome-deuterostome split is even older is not yet resolvable.”
Erwin & Davidson (2002) considered Kimberella as “the oldest generally accepted bilaterian fossil” and “oldest convincing remains of bilaterians are fossils of an already well-developed animal” and “demonstrable adult bilaterian form.” Furthermore, they said that Kimberella “is interpreted as a benthic animal, most likely of protostome affinity, and possibly a mollusk.”
An Artifact of Fossilization
Dzik (2003) did not consider the Kimberella fossils as ventral casts but rather as “external molds of the collapsed upper surface of the body.” Therefore, he claimed that the similarity to limpet soles is quite superficial and rather an artifact of the fossilization process. He also said that “the main argument for the molluscan nature of Kimberella remains thus its association with putative radular marks proposed by Seilacher (1999). However, it is unclear what these structures truly represent.” Instead, Dzik thought that the stable width between supposed gut and the body surface was filled with a kind of mesoglea and mesogleal muscles, which “would point to ctenophoran affinities of Kimberella.” He therefore interpreted Kimberella as a freely swimming animal and possible comb jelly.
Fedonkin (2003) of course affirmed the “new interpretation of Kimberella as a mollusk-like triploblastic animal.”
Seilacher et al. (2003, 2005) called Kimberella a “soft limpet” and “primitive mollusk,” and said that “the molluscan nature of Kimberella is expressed by impressions of the flat foot and the margin of a soft dorsal shield seen in ventral deathmasks of the bodies,” which he obviously considered as putative synapomorphies.
Valentine (2004) listed Kimberella as “a possible eutrochozoan” and “a mollusc-like bilaterian organism.” However, this author also commented that “although Kimberella is interpreted as having a cap-shaped shell, there is evidence only of a relatively rigid dorsal region, and assignment to Mollusca is based on general similarity of form.”
Mainly based on Fedonkin & Waggoner (1997), Caron et al. (2006) considered Kimberella as an earliest stem group mollusk, succeeded by the Lower Cambrian stem group mollusks Odontogriphus, Wiwaxia, and Halkieria. These authors especially mentioned the following commonalities between Kimberella and the soft-bodied organism Odontogriphus: dorso-ventrally flattened ovoid shape; large size; a cuticular dorsal exoskeleton shown by the integrity of the dorsum relative to soft anatomy; a non-cuticularized ventral sole; and iterated structures; as well as the (inferred) presence of an anterior radula.
Butterfield (2006) disagreed with a molluscan affinity of Odontogriphus and Wiwaxia because a “more detailed analysis of these fossil structures, however, reveals pronounced anatomical and histological discrepancies with molluscan analogues, such that they are more reliably interpreted as primitive features of the superphylum Lophotrochozoa.” This author also remarked that contrary to the Kimberichnus traces attributed to Kimberella, “no such traces are found in association with Odontogriphus or Wiwaxia, and, in any event, it is questionable whether a two-rowed (or even a four-rowed) feeding apparatus could work in any way analogous to a belt-like radula.” Butterfield concluded that “The dilemma of interpreting fossil ‘worms’ as molluscs or annelids — or for that matter brachiopods, chaetognaths, arthropods or chordates — is hardly new. In some cases, the problem is simply a dearth of diagnostic features (e.g. Kimberella, which on current evidence can only be reliably identified as a probable bilaterian).”
Two Alternative Positions
Conway Morris & Caron (2007) considered two alternative positions for Kimberella and Odontogriphus, either as successive stem lophotrochozoans or as successive stem mollusks, but they preferred the former alternative as better supported by their cladistic analysis. However, they admitted that the “bootstrap values are generally very low and the most parsimonious tree is far from robust,” which is just another way to say that the results are not well supported and thus not reliable.
Fedonkin et al. (2007a) simply listed Kimberella as a member of the phylum Mollusca, which “may be related to living chitons (amphineurans) and monoplacophorans.” Fedonkin et al. (2007b) closed their abstract with the statement that “the structural complexity of Kimberella poses questions about the time of origin of the triploblastic metazoans.” They also compared many structures with those of mollusks and concluded that Kimberella had a “fundamentally mollusc-like architecture.”
Budd (2008) said that the claim for a bilaterian affinity of Kimberella “has been revitalized by the discovery of the molluscan affinities of the rather similar Odontogriphus from the Burgess Shale” (Butterfield 2006, Caron et al. 2006).
Butterfield (2008) described isolated microscopic teeth from the early Cambrian Mahto Formation in Alberta (Canada), which he identified as components of a molluscan radula and the oldest fossil record of this structure. He showed that similarities with the feeding apparatus of the Cambrian organisms Odontogriphus and Wiwaxia are only superficial. Concerning Kimberella he said: “Tellingly, Caron et al. (2006) extend their identification of radula-bearing fossils to Late Ediacaran Kimberella based solely on the presence of co-occurring sedimentary scratch marks, whereas Conway Morris and Caron (2007) argue that both Kimberella and Odontogriphus are just as likely to represent stem-group lophotrochozoans (despite coding both forms as having a ‘muscular foot’ and ‘distichous radula’). Whether Kimberella, Orthrozanclus, or Halkieria … had a radula remains to be seen ….”
Parkhaev (2008) mentioned that Kimberella “was interpreted as a primitive mollusc-like organism” and that “the recent finds of Kimberella with traces of crawling (Figure 3.1B) prove the animal nature of this organism and favor its molluscan affinity,” but cautioned, “however, to determine whether Kimberella is really a mollusc, specimens are needed that show a defined head with a ventral mouth.”
Peterson et al. (2008) noted that “one taxon in particular, Kimberella, has generated much discussion as a possible triploblastic metazoan.” Based on several similarities with mollusks (i.e. external form, inferred radula-like organ, association with scratch marks) as well as their molecular clock estimate of a divergence between annelids and mollusks about 570 million years ago, the authors concluded that “it is possible, if not probable, that Kimberella is allied with modern molluscs.”
Davidson & Erwin (2009) discussed the usual characters (body size and shape, apparently muscular foot, associated scratch marks), but cautioned that “Although many of these features are similar to those of a mollusk …, it is probably premature to assign Kimberella to the Mollusca.” They concluded that among the Ediacaran fossil so far only Kimberella “shows convincing bilaterian characters” and “can reasonably be considered as representing a protostome.” More specifically they claimed that Kimberella “marks the first appearance of a lophotrochozoan lineage” and represents the only Ediacaran organism that “clearly lies above the protostome–deuterostome divergence,” which therefore must predate 555 million years. They also thought that “the appearance of Kimberella and the oldest metazoan traces at this time is unlikely to be a coincidence.”
Freeman (2009) commented that “another bilaterian, Kimberella, is a putative stem group mollusc.”
Ivantsov (2009) concluded that “most likely, Kimberella was related to some very early stage in the evolution of trochophore animals, may be even to the proto-molluscan stage.”
Xiao & Laflamme (2009) said that Kimberella “has the best phylogenetic resolution so far” as a bilaterian animal “and is interpreted as possible mollusc.”
Ivantsov (2010b, 2011) considered Kimberella as a “trochophore animal of a pre-molluscan stage.”
Rozhnov (2010) mentioned that “Some Vendian organisms, primarily Kimberella probably had a higher level of organization, judging from its more complex structure and traces of feeding … However, even this quite exceptional case cannot be unequivocally interpreted. Anatomically they could be more simply organized than would be required by their supposed molluskan level of organization.”
Seilacher & Hagadorn (2010) discussed Kimberella as “early bilaterian” and “earliest mollusks.” They thought that the “fan-shaped scratches — perhaps more than the inferred morphology of Kimberella or the early Cambrian appearance of radular teeth … indicate that such key crown-group mollusk features as a radula and muscular foot had already evolved before the Cambrian. This also implies that Lophotrochozoa, in which Mollusca are nested … had already originated by this time.”
A Possible Relationship
Dzik (2011) wrote that even though Kimberella is believed to be a proto-mollusk, “the anatomy of Kimberella, as currently understood, is rather distant from typical mollusks and this introduces some uncertainty regarding its widely accepted identity as a pre-Cambrian member of the Spiralia.” He also discussed a possible relationship with the mollusk-like Cambrian genera Halkieria, Wiwaxia, and Odonatogriphus.
Erwin et al. (2011) considered Kimberella as “confidently assigned to the crown of Metazoa” and as “bilaterians, and possible molluscs.” They erected a new clade Kimberellomorpha to accommodate Kimberella and the somewhat similar genus Solza.
Next, “Kimberella and Controversial Relationships — A Chronological Synopsis, Continued.”