The Cambrian evolutionary ‘explosion’: decoupling cladogenesis from morphological disparity

May combine two claims.

Larvae of crustaceans, trilobites and primitive arachnomorphs are all about 1 mm long with single dorsal shields, matching the small sister groups in size.

“Larvae of crustaceans (nauplius), trilobites (protaspis) and primitive arachnomorphs (so-called trilobite larva' of Limulus) are all about 1 mm long with single dorsal shields, and thus conform in size to the small sister groups mentioned above.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

The topology and derivation of advanced animal phyla in Willmer's tree show obvious similarity to previously described trees.

“The topology of the tree and the derivation of the advanced animal phyla shows an obvious similarity to those described previously.”

May combine two claims.

If any or all of the Ediacaran identifications were correct, the major steps in the radiation of arthropods must have taken place in the Precambrian.

“The Trilobita occupy a high position on the tree; hence if any or all of these Ediacaran identifications were correct, the major steps in the radiation of the arthropods must have taken place in the Precambrian.”

May combine two claims.

The lack of late Vendian fossils attributable to known metazoan groups is fact, but the complete absence of ancestors of modern groups at that time is inference.

“The lack of late Vendian fossils easily attributed to known groups of Metazoa is fact, but the complete absence of ancestors of 'modern'groups at the same time is inference.”

May combine two claims.

If the Vendian Arkarua is correctly interpreted as an echinoderm, the important events in echinoderm phylogenesis must have occurred in the Precambrian, regardless of the lack of preserved fossils.

“If the Vendian Arkarua is correctly interpreted as an echinoderm (Gehling, 1987), the important events in their phylogenesis must have occurred in the Precambrian, regardless of the lack of preserved fossils.”

• cites Gehling, 1987 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Marrella falls among the most primitive arthropods on most analyses, yet its sole and earliest representative is Middle Cambrian.

“The lace crab' Marrella, for example, falls among the most primitive arthropods on most analyses, yet its sole and earliest representative is Middle Cambrian.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

Echinodermata and Chordata have a comparatively poor Cambrian record, particularly in the early part.

“Echinodermata and Chordata have a comparatively poor Cambrian record, particularly in the early part.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

In either case, the record of soft-bodied pre-brachiopods is lacking.

“In either case, the record of soft-bodied, pre-brachiopods' is lacking.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

The capacity to secrete a shell is a brachiopod character, which provides evidence for an early history unrepresented by fossils, like that of Mollusca or Trilobita.

“This suggests that the capacity to secrete a shell is a brachiopod character, which provides evidence for an early history unrepresented by fossils, like that of the Mollusca or Trilobita.”

Probably not a substantial claim: may not pass S3 (load-bearing).

A recent cladistic analysis of brachiopods using a phoronid outgroup concluded that the monophyletic Brachiopoda can be divided into two broad groups, Lingulata and Calciata.

“A recent cladistic analysis of the brachiopods (Popov et al., 1993; Bassett et al., 1994), however, using a phoronid outgroup, concluded that the monophyletic clade Brachiopoda can be divided into two broad groups, Lingulata and Calciata (Fig. 3B).”

• cites Popov et al., 1993Not checked yet

  Not checked in the free run.

• cites Bassett et al., 1994Not checked yet

  Not checked in the free run.

Classified as probably not a substantial claim.

A trilobite-like soft animal has been reported from the Ediacara Member of the Rawnsley Quartzite.

“A trilobite-like soft animal has recently been reported from the Ediacara Member of the Rawnsley Quartzite (Jenkins, 1992).”

• cites Jenkins, 1992Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

A hypothetical early Cambrian echinoderm stem group left no record and would still need to be related satisfactorily to the chordates.

“However, such a hypothetical stem group left no record, and would still need to be related satisfactorily to the chordates.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

The kind of arguments used for arthropods can be repeated for other phyla, but robust cladistic analyses of their relationships are more difficult to construct.

“The kind of arguments rehearsed for the arthropods can be repeated for other phyla, but robust cladistic analyses of relationships are arguably more difficult to construct (Runnegar, 1994).”

• cites Runnegar, 1994Not checked yet

  Not checked in the free run.

May combine two claims.

Earlier molecular studies did not support arthropod monophyly, but later investigations re-established the group as monophyletic.

“The coherence of the Arthropoda as a clade was not supported by some earlier molecular studies, although subsequent investigations have re-established the monophyly of the group”

• cites Turbeville et al., 1991Not checked yet

  Not checked in the free run.

• cites Wheeler, Cartwright & Hayashi, 1993 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

If the low insertion of Uniramia below most Burgess Shale arthropods is correct, this reveals a stratigraphic mismatch, because all uniramians are post-Ordovician apart from one disputed example.

“More strikingly, if the low insertion of Uniramia below most of the Burgess Shale arthropods in our cladograms is correct, this reveals a more fundamental stratigraphic mismatch; apart from one disputed example (Robison, 1990), all the uniramians are post-Ordovician.”

• cites Robison, 1990Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S2 (checkable).

If this model is correct, no special dispensation waiving evolutionary rules need be invoked.

“If this model is correct, no special dispensation waiving evolutionary rules need be invoked.”

Probably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing). May combine two claims.

If the different shell types evolved independently, their appearance in the Tommotian could be interpreted as the moment when shells were acquired, implying a more complete record than that of the molluscs.

“In this case, their appearance in the Tommotian could be interpreted as the moment when shells were acquired, implying a more complete record than that of the molluscs.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Trilobita consistently form a clade in cladistic analyses of Arthropoda.

“The Trilobita consistently form a clade in cladistic analyses of the Arthropoda (see Wills et al., 1994, for review).”

• cites Wills et al., 1994 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

The preferred model resembles mammalian history, with a long Mesozoic prehistory of phylogenetic groundwork before rapid deployment into varied terrestrial habitats in the Palaeocene, accompanied by increases in size and disparity.

“The preferred model becomes similar to the history of the mammals, with a long Mesozoic prehistory that witnessed much phylogenetic groundwork before the rapid deployment into varied terrestrial habitats in the Palaeocene, with an accompanying increase in size and disparity.”

Probably not a substantial claim: may not pass S2 (checkable). May combine two claims.

It remains an important question why changes in skeletization and expression of clades are evident at the base of the Cambrian, but this question has no necessary connection to cladogenesis.

“It remains an interesting and important question why the changes in skeletization and expression of clades are evident at the base of the Cambrian, but it is a question with no necessary connection to the cladogenesis.”

May combine two claims.

Because Pikaia is usually regarded as a chordate and true echinoderms are known from the Tommotian, early Cambrian chordates must have existed even though they are not preserved in the fossil record.

“The Burgess Shale Pikaia is usually regarded as a chordate, while true echinoderms are known from the Tommotian, and hence, as in the case of Marrella discussed above, early Cambrian chordates must have existed even though they are not preserved in the fossil record.”

Probably not a substantial claim: may not pass S3 (load-bearing).

The distribution of Naraoia is consistent with a phylogenetic argument based on vicariance biogeography.

“Such a distribution is consistent with a phylogenetic argument based upon vicariance biogeography.”

• AssumptionRests on an assumption: A late Precambrian supercontinent existed before the breakup into separate palaeocontinents near the beginning of the Phanerozoic.

Probably not a substantial claim: may not pass S2 (checkable), S4 (based).

The possibility of accommodating all the facts within an actualistic framework has not been eliminated.

“However, the possibility of accommodating all the facts within an actualistic framework has yet to be eliminated.”

May combine two claims.

Disparity among Cambrian arthropods is close to that of the Recent, and the morphospace occupied by Recent Arthropoda was already filled by Cambrian time.

“Disparity among Cambrian arthropods is close to that of the Recent (taking a representative selection of morphologies from the Recent fauna for comparison), ot mpds mpnt t , t t t at occupied by Recent Arthropoda was already filled by Cambrian time.”

• cites Briggs & Fortey, 1989 sourceNot checked yet

  Not checked in the free run.

• cites Briggs, Fortey & Wills, 1992a,bNot checked yet

  Not checked in the free run.

• cites Wills, Briggs & Fortey, 1994 sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

Only a strictly stratophenetic approach can deny the existence of substantial unpreserved ranges.

“Only a strictly stratophenetic approach, however, can deny the existence of substantial unpreserved ranges.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Most authors consider that the shell-bearing molluscs constitute a clade, the Conchifera.

“most authors consider that the shell-bearing forms constitute a clade, the Conchifera (which includes clams, gastropods, scaphopods, chitons, cephalopods, monoplacophorans and rostroconchs, as well as sundry early Cambrian shells, such as Pelagiellida) (Stasek, 1972; see Runnegar, 1996, for a recent review).”

• cites Stasek, 1972Not checked yet

  Not checked in the free run.

• cites Runnegar, 1996Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Like calcified trilobites, shelled molluscs appear instantly in the fossil record.

“Thus, like the calcified trilobites, the shelled molluscs appear instantly' (but see Conway Morris and Peel 1995).”

• cites Conway Morris and Peel 1995 sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S4 (based).

Previous analyses of Cambrian arthropods have revealed shortcomings in the explosion hypothesis.

“Previous analyses of the Cambrian arthropods have revealed shortcomings in the explosion' hypothesis.”

May combine two claims.

Inferred relationships between lophophorate phyla suggest that phoronids must have pre-dated the Tommotian, which yields the earliest brachiopods, even though the first phoronid record is doubtfully Pennsylvanian.

“Similarly, inferred relationships between lophophorate phyla (Nielsen, 1994) suggest that phoronids must have pre-dated the Tommotian, which yields the earliest brachiopods, even though the first phoronid record is doubtfully Pennsylvanian (Benton, 1993).”

• cites Nielsen, 1994Not checked yet

  Not checked in the free run.

• cites Benton, 1993Not checked yet

  Not checked in the free run.

May combine two claims.

The morphological range of Cambrian molluscs may be as great as that of trilobites, although the time available for their explosion is shorter because the earliest shelled records are older than those of trilobites.

“Their morphological compass may be as great as that in trilobites although the time available to 'explode' is even shorter since the earliest records of shelled forms are older than those of trilobites — Tommotian as opposed to Atdabanian.”

Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).

Similar evidence for incompleteness can be found in other major groups.

“similar evidence can be accrued for other major groups.”

May combine two claims.

Early trilobites were vagrant benthos, not plankton, and no special distributional factor seems to have been at work.

“The functional morphology of early trilobites indicates that they were vagrant benthos, not plankton, and there does not seem to have been any special distributional factor at work with these animals (Bergström, 1973).”

• cites Bergström, 1973Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

Cephalodiscus is likely to be close to the basal deuterostome morphology.

“Among these, Cephalodiscus, which forms loosely attached aggregations of small zooids rather than true colonies in the manner of Rhabdopleura (Bulman, 1970), is likely to be close to the basal morphology.”

• cites Bulman, 1970Not checked yet

  Not checked in the free run.

• AssumptionThe authors state this as an assumption: Pterobranchs are primitive among deuterostomes.

Probably not a substantial claim: may not pass S3 (load-bearing).

It is generally accepted that hemichordates insert at a basal level in deuterostome phylogeny.

“it is generally accepted that hemichordates insert at a basal level in the phylogeny of the group (Neilsen, 1994).”

• cites Neilsen, 1994Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S4 (based).

This size increase made manifest in the fossil record what had previously been achieved by normal evolutionary mechanisms over a much longer period.

“This made manifest in the record what had previously been achieved by normal evolutionary mechanisms over a much longer period.”

May combine two claims.

Phylactolaemata, interpreted as the most primitive bryozoans, include species with few zooids and organic walls that are not candidates for fossilization under normal conditions.

“Phylactolaemata, which have been interpreted as the most primitive bryozoans (Jebram, 1986), includes species with few zooids and organic walls, which are not candidates for fossilization under normal conditions.”

• cites Jebram, 1986Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

It is not simple to demonstrate heterochronic change without an ancestor-descendant series, a condition unlikely at the Precambrian-Cambrian interface.

“It is not a simple matter to demonstrate heterochronic change without an ancestor-descendant series of species, a condition unlikely to be fulfilled at the Precambrian-Cambrian interface.”

May combine two claims.

Whether or not late Precambrian large-animal niches were occupied by Ediacaran vendobionts, the basal Cambrian appears to have triggered release from small-size constraints.

“Whether or not the niches for large animals' in the late Precambrian were occupied by Ediacaran vendobionts it does seem that the basal Cambrian triggered a release from the constraints of small size.”

• AssumptionRests on an assumption: Niches for large animals in the late Precambrian were available to all parallel lineages.

Probably not a substantial claim: may not pass S2 (checkable), S4 (based).

Adherence to an explosion scenario and the formalisation of uncertainty in these diagrams have no logical relationship to each other.

“It should be noted that adherence to an explosion scenario and the formalisation of uncertainty' represented in the above diagrams have no logical relationship to each other.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Balanoglossids may not be closely related to pterobranchs.

“However, the balanoglossids may not be closely related to the pterobranchs (Peterson, 1994).”

• cites Peterson, 1994Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

There are some instances where a heterochronic explanation for the origin of particular morphologies is reasonable.

“There are, however, some instances where a heterochronic explanation for the origin of particular morphologies is reasonable.”

Classified as probably not a substantial claim.

The hypothesis that the crucial phase of bauplan evolution occurred in small animals may explain the mismatch between the fossil record and evidence for Vendian cladogenesis.

“The hypothesis that the crucial phase of evolution in bauplane happened in animals of small size may explain the apparent paradox of the mismatch between the fossil record, and the evidence for Vendian cladogenesis.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

In the Lower Cambrian, the timing of developmental changes or the fixing of particular HOM/Hox expressions may reflect heterochronic change accompanying rapid body-size increase.

“In the Lower Cambrian the timing of developmental changes — or the fixing' of particular expressions of these genes — may reflect the operation of heterochronic change accompanying a rapid increase in body size.”

Probably not a substantial claim: may not pass S4 (based). May combine two claims.

The sudden appearance of metazoan phyla and classes in the Cambrian may simply reflect simultaneous size increase in many groups, possibly without fundamental genetic change.

“The sudden appearance of metazoan phyla and classes in the Cambrian may simply reflect a simultaneous size-increase in numerous groups, which, while it had profound implications for the subsequent history of life, may not have involved any fundamental genetic change.”

Probably not a substantial claim: may not pass S3 (load-bearing).

The similarity of these topologies is consistent with difficulties in relating phyla and in determining the affinities of Cambrian Problematica.

“The similarity of these topologies is consistent with the difficulties of relating phyla, and determining the affinities of Cambrian Problematica.”

May combine two claims.

There is good evidence of important cladogenesis before the Cambrian explosion, but fossils have so far poorly documented these events.

“Paradoxically, there is good evidence of important cladogenesis prior to the Cambrian explosion', but the fossils have so far provided a very poor documentation of these events.”

May combine two claims.

Heterochronic change may operate by inhibiting segmental release (paedomorphosis) or accelerating it (peramorphosis) relative to overall growth trajectory.

“Thus heterochronic change may operate either by inhibiting segmental release (paedomorphosis sensu lato) or by accelerating segmental release (peramorphosis), in relation to overall growth trajectory.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Ctenostome bryozoans are usually considered the sister group of more advanced calcified bryozoans, and include small delicate uncalcified encrusters known fossilized only as bio-immurations.

“Ctenostome bryozoans are usually considered to be the sister group of more advanced calcified bryozoans, and these, too, include small and delicate uncalcified encrusters; they are known fossilized only as bio-immurations (Taylor, 1990).”

• cites Taylor, 1990Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S2 (checkable). May combine two claims.

From published studies it is not apparent that an explosion at the base of metazoan phylogeny has better or worse explanatory power than platyhelminths or Procoelomata, nor how any one hypothesis can be falsified.

“But it is not apparent how the explanatory power of having 'explosion' at the base of metazoan phylogeny is either better or worse than having platyhelminths' or Procoelomata', nor, from the published studies, how any one hypothesis is capable of being falsified.”

Probably not a substantial claim: may not pass S2 (checkable), S4 (based).

Too literal an adoption of the explosion hypothesis may discourage attempts to place Cambrian animals into phylogenetic hypotheses.

“Too literal an adoption of the explosion' hypothesis may discourage attempts to place Cambrian animals into phylogenetic hypotheses”

Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).

Some far-reaching changes may have involved heterochrony in particular.

“Some far-reaching changes may have involved heterochrony in particular.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Collembola are minute and include interstitial species.

“They, too, are minute and include interstitial species”

Probably not a substantial claim: may not pass S3 (load-bearing).

This is not inconsistent with Valentine and Erwin's views on the lability of Cambrian genetic programs and their later fixing.

“This is not inconsistent with the views of Valentine and Erwin (1987) on the comparative lability of Cambrian genetic programs and their subsequent 'fixing' in Phanerozoic history.”

• cites Valentine and Erwin (1987)Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S4 (based). May combine two claims.

Reasonable phylogenetic trees can and have been proposed for several phyla.

“but reasonable phylogenetic trees can and have been proposed for several phyla.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Early representatives of conchiferan molluscs are already known to be small.

“Early representatives of conchiferan molluscs are already known to be small (Runnegar, 1983; Haszprunar, 1992).”

• cites Runnegar, 1983Not checked yet

  Not checked in the free run.

• cites Haszprunar, 1992Not checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

Tardigrades are usually a millimetre or less in length, and marine species live interstitially or among algae.

“Tardigrades are usually a millimetre or less in length, and the marine species live interstitially or among algae.”

Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing).

Such trees do not contradict the possibility that cladogenesis was separate from the acquisition of hard parts and the overt expression of the radiation at the base of the Cambrian.

“Certainly, such trees do nothing to contradict the possibility of cladogenesis being a separate issue from the acquisition of hard parts and overt expression of the radiation at the base of the Cambrian.”