Unravelling the timing of the metazoan radiation is crucial for elucidating the macroevolutionary processes associated with the Cambrian explosion. Because estimates of metazoan divergence times derived from molecular clocks range from quite shallow (Ediacaran) to very deep (Mesoproterozoic), it has been difficult to ascertain whether there is concordance or quite dramatic discordance between the genetic and geological fossil records. Here, using a range of molecular clock methods, we show that the major pulse of metazoan diver- gence times was during the Ediacaran, consistent with a synoptic reading of the Ediacaran macro- biota. These estimates are robust to changes in pri- ors, and are returned with or without the inclusion of a palaeontologically derived maximal calibra- tion point. The two historical records of life both suggest, therefore, that although the cradle of the Metazoa lies in the Cryogenian, and despite the explosion of ecology that occurs in the Cambrian, it is the emergence of bilaterian taxa in the Ediacaran that sets the tempo and mode of macroevolution for the remainder of geological time.
2.1 Introduction
Accurately and precisely elucidating the times of origin of the metazoan phyla is central to unrav- elling the causality and biological significance of the Cambrian explosion. Despite the fact that the Cambrian explosion is geologically obvious
(Darwin, 1859), it has long been argued that this same geological record, because of its incom- pleteness, might be misleading when considering metazoan origins (Runnegar, 1982b). As Runnegar recognized, a second ‘fossil record’, the genetic record written in the DNA of all living organisms (Runnegar, 1986), could be used to test hypotheses about the completeness of the geological record (Peterson et al., 2007), and initial attempts at using a molecular clock strongly suggested that meta- zoans had a deep and cryptic Precambrian history (Runnegar, 1982a, 1986; Wray et al., 1996; reviewed recently by Conway Morris, 2006). Nonetheless, several palaeontologists have cogently argued that the fossil record provides positive evidence for the absence of early Neoproterozoic and Mesoproterozoic animals, casting doubt on the veracity of these molecular clock estimates (Budd and Jensen, 2000, 2003; Jensen et al., 2005; Conway Morris, 2006; Butterfield, 2007). Comparisons between the genetic and geological fossil records of early animal evolution, as currently understood, therefore suggest that either the geological record is woefully incomplete or there is something ser- iously awry with our reading of the genetic record (Bromham, 2006).
To explore the apparent incongruity between the known fossil record and the very deep estimates of metazoan diversification suggested by molecu- lar clocks, Peterson and colleagues (Peterson et al.,
2004; Peterson and Butterfield, 2005) assembled the
largest novel data set yet, showing that the two records were remarkably concordant: metazoans originated at some time during the Cryogenian, and bilaterians arose during the Ediacaran. Part of the reason for the prior discrepancy concerned the use of vertebrate divergence times. Peterson et al. (2004) discovered that there was an approxi- mately two-fold rate reduction across the verte- brate protein-coding genome as compared with the three invertebrate lineages examined (echino- derms, molluscs, and insects), consistent with total genome comparisons between vertebrates and dip- teran insects (Zdobnov et al., 2002). However, some studies using invertebrate calibrations have also inferred divergence times consistent with a cryp- tic Precambrian history of the Metazoa (Pisani et al., 2004; Regier et al., 2005), suggesting that the two-fold rate reduction across the vertebrate gen- ome is only one of many factors influencing the estimation of divergence times (Linder et al., 2005; Peterson and Butterfield, 2005).
In addition, Peterson et al.’s (2004) estimates and explanations were called into question by several workers, notably Blair and Hedges (2005) who argued that Peterson et al. (2004) used palaeon- tologically derived calibration points as maxima as opposed to minima, which generated spuri- ously shallow estimates for metazoan divergences. Although false, as Peterson et al. (2004) stated explicitly (see also Peterson and Butterfield, 2005), this criticism highlights an important issue sur- rounding the use of molecular clocks, namely the proper way to incorporate calibration points into molecular clock analyses (Benton and Donoghue,
2007). Recent experimental analyses have shown the importance of numerous, well-constrained calibration points for returning accurate and pre- cise estimates of divergence times, and thus high- lighting the need to pay particular attention to this aspect of molecular dating (Roger and Hug, 2006; Hug and Roger, 2007). Nonetheless, difficulties arise when incorporating fossils into a molecu- lar clock analysis: unlike the establishment of a minimal divergence time for any two taxa, which is simply the first appearance of either one of the taxa, estimating the maximum divergence time is much more difficult (Benton and Donoghue, 2007). Two types of maxima have been proposed: a ‘hard’
maximum proposes an absolute value for the oldest possible date of divergence, whereas a ‘soft’ max- imum treats a divergence as having some chance of being older than a particular date, depending on a probability distribution used to describe the cali- bration point (Hedges and Kumar, 2004; Yang and Rannala, 2006; Benton and Donoghue, 2007).
Most modern molecular clock methods (e.g. Sanderson, 1997, 2002; Thorne et al., 1998; Drummond et al., 2006) allow one to constrain, as well as fix, the age of a calibration point, so that every fossil divergence can be defined using a minimum and a maximum. This is a significant improvement over older molecular clock approaches (e.g. Kumar and Hedges, 1998) because it allows the integration of palaeontological uncertainty in the estimation of divergence times. However, most existing molecu- lar clock software including r8s (Sanderson, 2004) and Multidivtime (Thorne and Kishino, 2002), do not distinguish between hard and soft maxima, instead treating all maxima as hard. The difficulty here is that divergence times estimated with uncer- tain maxima treated as if they were hard can only give minimum estimates for the true divergence time, as the soft maxima might significantly under- estimate the true age of the calibration points. Nonetheless, Drummond et al. (2006) have now implemented Bayesian relaxed molecular clock methods (in the software package BEAST) where soft maxima can be properly modelled using a probability distribution, and can thus be older than their proposed fossil date.
Here, we set out to explore the diversification of animal phyla in the Neoproterozoic using alter- native relaxed molecular clock approaches while testing the stability of our results to the choice of different priors and to the deletion of palaeontolog- ically derived maxima, and modelling soft maxima using the most appropriate probability distribu- tion. We find that although deleting or relaxing maxima tends to push divergence times toward the past (as expected), all estimates are largely congru- ent between algorithms. We conclude that a synop- tic reading of both the geological and genetic fossil records demonstrates that the Ediacaran was the time of major diversification of most higher-level animal taxa and set the stage for Phanerozoic-like macroecology and macroevolution.
2.2 Materials and methods
2.2.1 Molecular characters
All taxa are taken from Sperling et al., (2007) where a concatenated alignment of seven different house- keeping genes, for a total of 2059 amino acid pos- itions and 44 representative species (see Peterson et al., 2004, and Peterson and Butterfield, 2005), was analysed using Bayesian methods (MrBayes 3.1.2; Ronquist and Huelsenbeck, 2003). See Sperling et al., (2007) for further details.
2.2.2 Molecular clock calibration
Calibration points were taken from Peterson et al., (2004) except for the minimum estimate for crown- group Eleutherozoa, which was adjusted from 475 to 480 million years ago (Ma) in light of the dis- covery of a slightly older asterozoan (Blake and Guensberg, 2005), and the minimum and max- imum for crown-group Diptera was taken from Benton and Donoghue (2007). Several new maxima and minima were incorporated into this analysis. First, the maximum for the origin of crown-group echinoderms was set at 520 Ma, the first appear- ance of stereom in the fossil record. Because ster- eom is a highly distinctive skeletal material, and its presence in numerous stem-group taxa (Smith,
2005) demonstrates that stereom is a total-group echinoderm character, it must have evolved before the origin of the crown group. Second, this same time point also sets the minimum for Ambulacraria (Echinodermata + Hemichordata), as echinoderms appear before hemichordates in the rock record (Budd and Jensen, 2003). Third, because ambulac- rarians are characterized by the possession of four to six coeloms in each animal (Peterson et al., 2000; Smith et al., 2004), and because coeloms cannot pre-date the first appearance of bilaterian traces (Budd and Jensen, 2000, 2003), the first appearance of traces sets the maximum age for crown-group Ambulacraria at approximately 555 Ma (Martin et al., 2000; Jensen et al., 2005). Fourth, the max- imum for the origin of Gastropoda + Bivalvia is the first appearance of skeletons in the fossil record, about 542 Ma (Amthor et al., 2003; Bengtson, 1994). Fifth, the maximum for the origin of crown-group
demosponges is the first appearance of demos- ponge-specific biomarkers (McCaffrey et al., 1994; Love et al., 2006; see Peterson et al., 2007 for dis- cussion) sometime after the Sturtian, c. 657 Ma (Kendall et al., 2006). Finally, the maximum for the origin of crown-group Eumetazoa, which was only used in the BEAST analyses, is argued to be 635 Ma based on palaeoecological observations (Peterson and Butterfield, 2005).
Newly incorporated minima include the first appearance of arthropod traces 525 Ma (Budd and Jensen, 2003) as a minimum for the diver- gence between insects and the priapulids, the first appearance of medusozoans 500 Ma (Hagadorn et al., 2002) as a minimum for the origin of the crown-group Cnidaria, and the first appearance of vertebrates 520 Ma as the minimum for the origin of crown-group chordates (Benton and Donoghue,
2007).
2.2.3 Molecular estimates of divergence times
Molecular estimates of divergence times were obtained using the Bayesian methods of Thorne et al., (1998), as implemented in Multidivtime (Thorne and Kishino, 2002), and Drummond et al., (2006) as implemented in BEAST version
1.4.2 (Drummond and Rambaut, 2006). All diver- gence times were calculated assuming the tree topology of Figure 2.1, which was derived from MrBayes (see above and Sperling et al., 2007). For the Multidivtime analyses, branch lengths were estimated using the Estbranches program from the Multidivtime package, under the WAG model. For BEAST analyses, starting branch lengths were assigned arbitrarily to match the constraints imposed by the calibrations.
For the Multidivtime analyses a prior age for the root node (in our case the Fungi–Metazoa split) must be specified. We assumed a 1000 Ma prior for this node (Knoll, 1992; Douzery et al., 2004) and then tested whether this choice affected our results by performing analyses in which this age was changed to 100 Ma (standard deviation (SD) =
500 Ma), 1500 Ma (SD = 500 Ma), and 2000 Ma (SD
= 750 Ma). Other priors used in Multidivtime ana-
lyses include the mean and standard deviation of
the prior distribution at the root node, and ‘Minab’
(parameter for the beta prior on proportional node depth). The mean and standard deviation of the prior distribution of the rate at the root node were set to 0.039, as estimated from the data following the procedure outlined in the Multidivtime man- ual, and the effects that 100-fold changes to this
parameter had on the results were assessed. The Minab parameter affects the distribution of the nodes through time—Minab values greater than 1 will cause the nodes to repel each other, while values less than 1 will cause the nodes to attract each other. This parameter was set to 1 for our
2.3 Results
Molecular divergence times were estimated using the topology shown in Figure 2.1. Support for Cnidaria and Deuterostomia was low (67% and 33%, respectively), probably because of long- branch artefacts (Pisani, 2004) associated with Ciona and Obelia in particular (indeed the value for Deuterostomia goes to > 90% with the removal of Ciona), but given the clear monophyly of the phyla Chordata and Cnidaria, constraining these nodes should not generate spurious molecular divergence estimates. Most of the other nodes were strongly supported, including Calcispongia + Eumetazoa, and Eumetazoa, supporting the results of Peterson and Butterfield (2005), and contra the conclusions of Rokas and colleagues (Rokas et al., 2005; see also Baurain et al., 2007). Indeed, within Protostomia, for
example, all but one node (Stylochus + Nemertea) have posterior probability values above 80%, and both Lophotrochozoa and Ecdysozoa, as well as Annelida + Mollusca, have clade credibility values of 100%. In addition, we find strong support for the node Homoscleromorpha + Eumetazoa, which indicates that there are at least three independent extant sponge lineages (Sperling et al., 2007).
Using this topology as a constraint tree, diver- gence times were estimated using the Bayesian autocorrelated method of Thorne et al. (1998), as implemented in the software package Multidivtime (Thorne and Kishino, 2002). These Bayesian esti- mates are robust to changes in the age of the root prior as the estimates are essentially the same whether the age is 100 Ma (SD 500 Ma) or 2000
Ma (SD 750 Ma) (Table 2.1), suggesting that the age of the root prior is not biasing the analyses. Also, changing the value of Minab, or the mean rate of evolution of the root node, did not change our results (not shown). Running the analyses without data confirmed that our results were not domi- nated by our choice of priors (not shown). Because of the suggestion that fungi diverged from animals c. 1000 Ma (Knoll, 1992; Douzery et al., 2004), was confirmed by all our analyses that did not assume a particular age for the root node, and in our Bayesian analyses performed assuming different prior root ages (100, 1500, and 2000 Ma) we used the values derived from the 1000 Ma (SD = 500 Ma) prior in Figure 2.1.
The removal of the deeper calibration point, namely the maximum age of 657 Ma for the ori- gin of crown-group demosponges, resulted in increasing the estimate for the age of crown-group Metazoa by c. 18% (from 766 Ma to 904 Ma; Table 2.1). Nonetheless, the age for both crown-group Protostomia and crown-group Deuterostomia increased by only c. 4–5%, suggesting that the results derived with the use of this maximum are generally robust. Given its position in the tree, the geological depth of the divergence, and the unique nature of the evidence (biomarkers), this maximum is most likely adding both accuracy and precision to the clock estimates.
We next explored these same divergence times using the models implemented in BEAST (Drummond et al., 2006). In general, the estimates
Table 2.1 Optima (maxima, minima) in millions of years derived from Multidivtime (M) and BEAST (B) analyses for five key metazoan divergences
| Method | Metazoa | Eumetazoa | Bilateria | Protostomia | Deuterostomia |
| M-10001 | 766 (803, 731) | 676 (709, 645) | 643 (671, 617) | 619 (648, 594) | 601 (625, 579) |
| M-1002 | 760 (798, 725) | 672 (706, 642) | 641 (669, 615) | 618 (645, 592) | 600 (624, 578) |
| M-20003 | 774 (812, 739) | 679 (712, 648) | 645 (674, 619) | 622 (649, 595) | 602 (626, 580) |
| M-1000-D4 | 904 (997, 825) | 743 (798, 694) | 686 (727, 649) | 653 (689, 619) | 624 (655, 596) |
| B-UCEX Uniform5 | 815 (1621, 625) | 676 (849, 579) | 652 (764, 570) | 620 (692, 556) | 572 (614, 537) |
| B-UCEX Exp6 | 1067 (2358, 612) | 707 (985, 581) | 669 (870, 566) | 638 (784, 556) | 582 (695, 529) |
| B-UCLN Uniform7 | 891 (995, 640) | 739 (822, 607) | 699 (768, 588) | 660 (715, 572) | 640 (706, 559) |
| B-UCLN Exp8 | 953 (1093, 821) | 779 (869, 694) | 733 (808, 663) | 688 (751, 629) | 677 (746, 607) |
1Age of the root prior is 1000 Ma (SD 500 Ma).
2Age of the root prior is 100 Ma (SD 500 Ma).
3Age of the root prior is 2000 Ma (SD 750 Ma).
4Age of the root prior is 1000 Ma (SD 500 Ma) and estimates are derived without considering the demosponge maximum of 657 Ma.
5Estimates derived using an exponential rate distribution and uniform priors.
6Estimates derived using an exponential rate distribution and exponential priors.
7Estimates derived using a lognormal rate distribution and uniform priors.
8Estimates derived using a lognormal rate distribution and exponential priors.
derived from BEAST using an exponential rate distribution and uniform priors (white circles in Figure 2.1) are similar to those derived from Multidivtime (Table 2.1). The analyses that use exponential priors are somewhat deeper than those that use uniform priors (black Xs in Figure 2.1), and those using a lognormal rate distribution are deeper than those derived from an exponential rate distribution (Table 2.1), presumably because the exponential distribution on rates is leading to greater autocorrelation between rates. Analyses without data again confirmed that the priors were not dominating the data (results not shown).
2.4 Discussion
2.4.1 Concordance between the genetic and geological fossil records
Here we have shown, using a variety of analyses and appropriately testing for biases that may have been introduced by the use of palaeontologically derived maxima, that the genetic fossil record strongly supports the notion that the diversificat- ion of metazoans in general, and bilaterian meta- zoans in particular, occurred during the Ediacaran Period, 635–542 Ma (Knoll et al., 2004, 2006). How
do these molecular estimates compare with the known geological record? Macroscopic fossils of the Ediacara biota span the upper half of the Ediacaran Period, from 575–542 Ma (Grotzinger et al., 1995; Martin et al., 2000; Bowring et al., 2003; Condon et al., 2005). Because most of these fos- sils occur as soft-bodied impressions in relatively coarse-grained siliciclastic sedimentary rocks, a comprehensive array of palaeobiological inter- pretations of the Ediacara biota has been put forth. Nonetheless, a few taxa stand out as poten- tial candidates for affinities within the Metazoa. One taxon in particular, Kimberella, has generated much discussion as a possible triploblastic meta- zoan. It compares well in external form to mol- luscs (Fedonkin and Waggoner, 1997) and in a few cases an everted proboscis is preserved (Gehling et al., 2005) that is inferred to contain a radula-like organ given the association between specimens of Kimberella (Figure 2.2a, asterisk, and Plate 1) and aligned sets of paired scratch marks (Figure 2.2a, arrows) (Gehling et al., 2005). These finds sug- gest that Kimberella was preserved in place while grazing on substrate microbial mats (Seilacher,
1999; Gehling et al., 2005). Given that we estimated
the divergence between annelids and molluscs to
What about other higher-level clades? Our esti- mates suggest that arthropods diverged from priapulids c. 575 Ma, suggesting that stem-group panarthropods (Nielsen 2001) should be present in Upper Ediacaran rocks. Interestingly, several taxa compare favourably with a panarthropod interpret- ation. For example, large specimens of Parvancorina show lateral structures originating on either side of the medial ridge that might be characterised as appendages (Figure 2.2b). In fact, in external form, Parvancorina bears a striking resemblance to the unmineralized, kite-shaped Cambrian arthropod
Skania (Lin et al., 2006). Spriggina (Figure 2.2c) also preserves large numbers of appendage-like struc- tures, and still others like Marywadea (Figure 2.2d) show apparent cephalic branching structures that resemble digestive caecae in arthropods. Importantly (see below), all of these taxa were no larger than 10 cm in maximum dimension (Gehling, 1999; Fedonkin, 2003) (see Figure 2.2), and appear simultaneously with the first demonstrable trace fossils (Jensen et al., 2005). The absence of arthropod scratch marks (Seilacher, 1999), though, is not too worrisome given that such traces would demand the presence of sclerotized appendages to cut through the ubiquitously present microbial mats, a character not necessitated by the presence
of stem-group panarthropods, or even deeply nested stem-group arthropods, in Ediacaran-aged sediments.
Indeed, the distinct possibility remains that this fauna preserves numerous stem-group forms ranging from basal triploblasts up through basal ecdysozoans, spiralians, and possibly even deu- terostomes. Given the enigmatic nature of some very prominent taxa like Dickinsonia (Figure 2.2e), a taxon that appears capable of some form of lim- ited motility (Gehling et al., 2005), a position for Dickinsonia within total-group Eumetazoa is not out of the question. In fact, mobile but saprophytic feeding without the use of a gut would be com- pelling evidence that some form of ectomesoderm pre-dates the advent of endoderm.
2.4.2 Discordance between the genetic and geological fossil records
Of course, many others have addressed these ques- tions using a similar approach, and it is worth com- paring our results not only against the fossil record but also with other molecular clock estimates as well. They compares well with some molecular ana- lyses, notably Aris-Brosou and Yang (2002, 2003), Peterson et al. (2004), and Peterson and Butterfield (2005), all of whom argued that the last com- mon ancestor of protostomes and deuterostomes evolved not more than 635 Ma. However, Blair and Hedges (2005) have recently argued for much deeper divergences, based on a series of penal- ized likelihood (Sanderson, 2002) analyses using r8s (Sanderson, 2004) in which every calibration point was treated as a minimum. They suggested that the divergence between ambulacrarian and chordate deuterostomes was 896 Ma (with the 95% confidence interval spanning from 832 to 1022 Ma). They further argued that the divergence between hemichordates and echinoderms was 876 Ma (725,
1074 Ma), and the origin of crown-group echino- derms was 730 Ma. Finally, they estimated that the divergence between starfish and sea urchins was
580 Ma. Unfortunately, their results are most likely spurious because, as Sanderson (2004) pointed out, r8s cannot converge on a unique solution if only minima are used to calibrate penalized likelihood
analyses, which is supported by the fact that their estimate for the origin of a mineralized, coelom- ate taxon like crown-group Echinodermata pre- cedes their appearance in the fossil record by some
200 million years.
Of course, neither the genetic nor the geological
fossil record has a monopoly on historical accur-
acy, and as much as molecular evolutionists need
to keep in mind the relevant palaeontological data,
palaeontologists need to keep in mind estimates
derived from molecular clocks (Donoghue and
Benton, 2007). For example, Budd and Jensen (2000,
2003) argued that bilaterians could not have had
an extensive Precambrian history, as suggested
by almost all molecular clocks, as the trace fossil
record, and the inferred morphology of these ani-
mals, is not consistent with an origin much before
555 Ma. They observed that possession of coelom(s)
and a blood-vascular system (BVS) are inconsist-
ent with a meiofaunal origin, as tiny organisms
would have had no need for a transport system like
the BVS, and are only consistent with a size large
enough to be detected in the geological record. In
general, we agree with their arguments, and use
their insights to set a maximum age for crown-
group Ambulacraria (see above).
However, the same argument cannot be
extended to many other parts of the bilaterian
tree. Contra Budd and Jensen (2000), there is no
evidence for homology of coeloms either between
protostomes and deuterostomes or even within
both protostomes and deuterostomes. Because
the coelom is, by definition, just a mesodermally
lined cavity (Ruppert, 1991a; Nielsen, 2001) the
possession of the space itself cannot be used
as an argument for homology. Instead, topo-
logical similarity must be used, and when it is, it
strongly suggests homology, for example, within
Ambulacraria (Peterson et al., 2000; Smith et al.,
2004), but not homology between any other higher
taxa (Nielsen, 2001; Ruppert, 1991a). Thus, outside
of Ambulacraria, the trace fossil record cannot be
used to set a maximum for most bilaterian diver-
gences. In fact, the small size of many putative
Ediacaran bilaterians (Figure 2.2), and the fact
that acoel flatworms are now recognized as the
sister group to the remaining bilaterians (Baguñà
and Riutort, 2004; Peterson et al., 2005; Sempere et al., 2007), is consistent with an argument that small size and absence of a coelom are primitive for Bilateria. This then removes the final obstacle to a pre-555 Ma origin for Bilateria, which is con- sistent with both the appearance of many differ- ent bilaterian lineages in the Ediacaran (Figure
2.2) and the molecular clock (Figure 2.1).
But despite the presence of many different
stem-group taxa, the Ediacaran is still a transi-
tional ecology, with these organisms confined to
a two-dimensional mat-world. This stands in dra-
matic contrast to the Early Cambrian where the
multitiered food webs that typify the Phanerozoic
were established with the eumetazoan invasion
of both the pelagos and the infaunal benthos
(Butterfield, 1997, 2001; Vannier and Chen, 2000,
2005; Dzik, 2005; Peterson et al., 2005; Vannier
et al., 2007). Hence, although the Ediacaran is an
apparent quantum leap in ecological complexity
as compared with the ‘boring billions’ that char-
acterize earth before the Ediacaran, it is still rela-
tively simple when compared with the Cambrian.
The Cambrian was yet another quantum leap in
organismal and ecological evolution, and which
thus stands as the transition interval between the
‘Precambrian’ and the Phanerozoic (Butterfield,
2007). Whether it was triggered by the introduc-
tion of eumetazoans, as argued by Peterson and
Butterfield (2005), by the introduction of mobile,
macrophagous triploblasts, as is suggested by our
analyses reported here (Figure 2.1), or by some
other factor or combination of factors, remains to
be more fully studied through continued explor-
ation of the relevant rock sections throughout the
world, and continued improvements in molecular
clock methods.
2.5 Conclusions
Both the genetic and geological fossil records, each with their own inherent biases and artefacts, are largely congruent with one another, and for his- torical disciplines congruence of independent data sets is the strongest argument one can make for historical accuracy (Pisani et al., 2007). Our ana- lyses suggest that while the cradle of metazoan life occurred in the Cryogenian, and the explosion of metazoan ecology occurred in the Cambrian, it is the emergence of bilaterians in the Ediacaran that established the ecological and evolutionary rules that have largely governed earth’s macrobiota for the remainder of geological time.
2.6 Acknowledgements
KJP was supported by the National Science Foundation; JAC was supported by an Irish Research Council for Science, Engineering and Technology Post-doctoral Fellowship; JGG is supported by the Australian Research Council Discovery Project (DG0453393), the ARC Linkage Project LP0774959 including the South Australian Museum and Beach Petroleum Pty Ltd, and the SA Museum Waterhouse Club. We would like to thank P. Donoghue (University of Bristol) for his usual perspicacity, two anonymous reviewers for their helpful com- ments on an earlier version of this chapter, and T. Littlewood (Natural History Museum, London) and M. Telford (University College London) for inviting us to contribute to this volume. Finally, KJP would like to thank all of the students who have come through the lab and contributed data to this pro- ject, and the South Australian Museum for a very enlightening visit.
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